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The infrared and Raman spectra (3100–50 cm −1 ) of the gas, liquid or solution, and solid have been recorded of cyanocyclopentane, c-C 5 H 9 CN. Variable temperature (−60 to −100 °C) studies of the infrared spectra (3100–400 cm −1 ) of the sample dissolved in liquid xenon have been carried out. From these data, both the envelope-equatorial (Eq) and Ax conformers have been identified and their relative stabilities obtained. The enthalpy difference has been determined to be 55 ± 12 cm −1 (0.66 ± 0.14 kJ/mol) with the Eq conformer the more stable form. The percentage of the Ax conformer is estimated to be 45 ± 1% at ambient temperature. The conformational stabilities have been predicted from ab initio calculations by utilizing several different basis sets up to aug-cc-pVTZ from both MP2(full) and density functional theory calculations by the B3LYP method. Vibrational assignments have been made for the observed bands for both conformers with initial predictions by MP2(full)/6-31G(d) ab initio calculations to obtain harmonic force constants, wavenumbers, infrared intensities, Raman activities and depolarization ratios for both conformers. The r 0 structural parameter values for the Eq[Ax] form are; for the heavy atom distances (A): C N = 1.160 [1.160] (3); C α –C = 1.463 [1.463] (3); C α –C β , C β′ = 1.543 [1.545] (3); C β –C γ , C γ′ = 1.540 [1.541] (3); C γ –C γ′ = 1.552 [1.553] (3) and angles (°): ∠C α –C N = 179.0 [178.9] (5); ∠C β C α –C = 113.1 [110.1] (5); ∠C β C α C β′ = 103.0 [102.1] (5); ∠C α C β C γ = 104.1 [104.8] (5); ∠C β C γ C γ′ = 106.3 [106.0] (5). The results are discussed and compared to the corresponding properties of some related molecules.

The microwave spectrum of isopropylisocyanate, (CH 3 ) 2 CHNCO, has been investigated from 11,000 to 21,000 MHz and 18 transitions for the more stable trans conformer were assigned and A = 6693.23(15), B = 2263.10(3), C = 1960.05(2) MHz were obtained. By utilizing these rotational constants along with ab initio MP2(full)/6-311 + G(d,p) predicted structural values, adjusted r 0 parameters have been obtained for the trans conformer along with estimated values for the gauche conformer. Variable temperature Raman and infrared spectra of xenon solutions have been recorded and a ΔH value of 115 ± 11 cm −1 (1.38 ± 0.13 kJ/mol) has been determined. The conformational stabilities have been predicted from ab initio calculations utilizing several different basis sets up to cc-PVQZ for MP2(full) and 6-311 + G(3df,3pd) for density functional theory calculations by the B3LYP method. From the MP2(full)/cc-PVQZ calculations an energy difference of 87 cm −1 (1.04 kJ/mol) is predicted between the trans and the gauche forms and 168 cm −1 (2.01 kJ/mol) for the cis form which is a transition state.

The infrared spectra (4000–400 cm−1) of the gas and Raman spectra (4000–50 cm−1) of the liquid have been recorded. Additionally, the variable temperature (−65 to −100 °C) infrared spectra of the sample dissolved in liquefied xenon was recorded from (3800–400 cm−1). The three possible conformers have been identified and their relative stabilities obtained with enthalpy difference relative to trans-Axial (t-Ax) of 58 ± 15 cm−1 for gauche-Equatorial (g-Eq) ⩾ 65 ± 12 cm−1 for trans-Equatorial (t-Eq). The percentage of the three conformers is estimated to be 31% for the t-Ax, 47 ± 2% for g-Eq and 22 ± 1% for t-Eq at ambient temperature. The conformational stabilities have been predicted from theoretical calculations by utilizing several different basis sets up to 6-311+G(2df, 2pd) from both MP2(full) and density functional theory calculations by the B3LYP method. Vibrational assignments have been provided for the observed bands for all three conformers which are predicted by MP2(full)/6-31G(d) ab initio calculations to predict harmonic force constants, wavenumbers, infrared intensities, Raman activities and depolarization ratios for all of the conformers. The results are discussed and compared to the corresponding properties of some related molecules.

Variable temperature (-60 to -100 °C) studies of ethyldichlorophosphine, CH3CH2PCl2, of the infrared spectra (4000-400 cm(-1)) dissolved in liquid xenon have been carried out. From these data, the two conformers have been identified and the enthalpy difference has been determined between the more stable trans conformer and the less stable gauche form to be 88±9 cm(-1) (1.04±0.11 kJ/mol). The percentage of abundance of the gauche conformer is estimated to be 57% at ambient temperature. The conformational stabilities have been predicted from ab initio calculations by utilizing many different basis sets up to aug-cc-pVTZ for both MP2(full) and density functional theory calculations by the B3LYP method. Vibrational assignments have been provided for both conformers which have been predicted by MP2(full)/6-31G(d) ab initio calculations to predict harmonic force fields, wavenumbers of the fundamentals, infrared intensities, Raman activities and depolarization ratios for both conformers. Estimated r0 structural parameters have been obtained from adjusted MP2(full)/6-311+G(d,p) calculations. The results are discussed and compared to the corresponding properties of some related molecules.

The FT-microwave spectrum (6000–19,800 MHz) of cyclopropylcyanosilane, c-C3H5SiH2CN has been recorded and 773 transitions for the 28Si, 29Si, 30Si, 13C and 15N isotopomers have been assigned for cis and gauche conformers. Infrared spectra (3200–220 cm−1) of gas and Raman spectra (3200–40 cm−1) of the liquid have been recorded as well as the variable temperature (−60 to −100 °C) studies of the infrared spectra of the sample dissolved in liquid xenon. The enthalpy difference between the conformers in xenon solutions has been determined to be 123 ± 13 cm−1 (1.47 ± 0.16 kJ mol−1) with the cis conformer as the more stable form. Approximately 48 ± 2% of the cis form is present at ambient temperature. By utilizing the microwave rotational constants of six isotopomers for cis and seven isotopomers for gauche combined with the structural parameters predicted from MP2(full)/6-311+G(d,p) calculations, adjusted r0 parameters have been obtained for both conformers.

The far infrared region, which is defined from 200 to 10 cm − 1 , is the region where large amplitude anharmonic vibrations are found. A relatively large amount of data has been reported for molecules in the gaseous state from which internal rotational barriers have been determined, conformational stabilities obtained, and large amounts of information on anharmonicities were discovered. A large amount of data was obtained on solid state materials in the earlier spectroscopic studies but the quantity of these studies was significantly reduced until recently. New applications of far infrared (Terahertz) have been reported which may invigorate this spectral region.

Infrared spectra (3200–220 cm−1) of gaseous and Raman spectra (3200–40 cm−1) of liquid isopropyl isocyanide ((CH3)2CHNC) have been recorded. By utilizing the microwave rotational constants combined with the structural parameters predicted from MP2(full)/6-311+G(d,p) calculations, adjusted r0 parameters have been obtained for isopropyl isocyanide. The heavy atom distances in A are: r (C1 N2) = 1.176(3), r (N2 C3) = 1.437(3), r (C3 C4,5) = 1.525(5) and the angles in (°) are ∠ C1N2C3 = 178.6(5); ∠ N2C3C4,5 = 109.4(5); ∠ C4C3C5 = 113.0(5). A complete vibrational assignment is proposed for isopropyl isocyanide based on infrared band contours, relative intensities, depolarization values, and group frequencies. The vibrational assignments were supported by normal coordinate calculation utilizing the force constants from ab inito MP2(full)/6-31G(d). The results are discussed and compared to those obtained for some similar molecules.

Fourier transform microwave spectrum of cyclopentylamine, c–C5H9NH2 has been recorded, and seven transitions have been assigned for the most abundant conformer, and the rotational constants have been determined: A = 4909.46(5), B = 3599.01(4), and C = 2932.94(4). From the determined microwave rotational constants and ab initio MP2(full)/6-311 + G(d,p) predicted structural values, adjusted r0 parameters are reported with distances (A): rCα–Cβ = 1.529(3), rCβ–Cγ = 1.544(3), rCγ–Cγ = 1.550(3), rCα–N = 1.470(3), and angles (°) ∠CCN = 108.7(5), ∠CβCαCβ = 101.4(5), and τCβCαCβ′Cγ′ = 42.0(5). The infrared spectra (4000–220 cm−1) of the gas have been recorded. Additionally, the variable temperature (−60 to −100 °C) Raman spectra of the sample dissolved in liquefied xenon was recorded from (3800–50 cm−1). The four possible conformers have been identified, and their relative stabilities obtained with enthalpy difference relative to t-Ax of 211 ± 21 cm−1 for t-Eq ≥ 227 ± 22 cm−1 for g-Eq ≥ 255 ± 25 cm−1 for g-Ax. The percentage of the four conformers is estimated to be 53% for the t-Ax, 11 ± 1% for t-Eq, 20 ± 2% for g-Ax and 16 ± 2% for g-Eq at ambient temperature. The conformational stabilities have been predicted from ab initio calculations by utilizing several different basis sets up to aug-cc-pVTZ from both MP2(full) and density functional theory calculations by the B3LYP method. Vibrational assignments have been provided for the observed bands for all four conformers, which are predicted by MP2(full)/6-31G(d) ab initio calculations to predict harmonic force constants, wavenumbers, infrared intensities, Raman activities, and depolarization ratios for all of the conformers. The results are discussed and compared to the corresponding properties of some related molecules. Copyright © 2014 John Wiley & Sons, Ltd.

The FT-microwave spectrum of 1,3-disilacyclopentane (c-C3H6Si2H4) has been recorded and 99 transitions for five isotopologues have been assigned for the twist form. The ground state rotational constants were determined from these assignments with following values for A = 4417.6710(7), B = 2887.0548(6), C = 1938.2171(6). From the experimentally reported microwave rotational constants and ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 parameters are reported for the most stable twist form distances (A) rCα Si = 1.886(2), rSi Cβ,Cβ′ = 1.888(2), rCβ Cβ′ = 1.552(2), and angles (°) ∠ SiCαSi = 103.9(3), ∠ CαSiCβ = 102.2(3), ∠ SiCβCβ′ = 106.4(3), and τCβSiCαSi = 11.5(3), τSiCβCβ′Si = 45.6(3). The conformational stabilities have been predicted from theoretical calculations with basis sets up to aug-cc-pVTZ from both MP2(full) and density functional theory calculations by the B3LYP method. Vibrational assignments have been made for the observed bands for the twist conformer and the interpretation is utilized by ab initio calculations to predict harmonic force constants, vibrational wavenumbers, infrared intensities, Raman activities and depolarization ratios. The results are discussed and compared to the corresponding properties of some related molecules.

Variable temperature (-50 to -100 degrees C) Raman spectra (3500-136 cm(-1)) were recorded of ethynylcyclopentane, c-C5H9CCH in liquid xenon. The envelope-equatorial (Eq) conformer was determined more stable than envelope-axial (Ax) form with enthalpy difference of 94 +/- 9 cm(-1) (1.12 +/- 0.11 kJ/mol) and 39 +/- 2% Ax conformer present at ambient temperature. The conformational stabilities have been predicted from ab initio calculations with basis sets up to aug-cc-pVTZ. From previously reported microwave rotational constants and ab initio Mp2(full)/6-311+G(d,p) predicted structural values, adjusted r(0) parameters are reported Eq [Ax] distances (angstrom) rC=C = 1.211(3) [1.211(3)], rC(alpha)-C C = 1.461(3) [1.467(3)], rC(alpha)-C-beta = 1.542(3) [1.542(3)], rC(beta)-C-gamma = 1.541(3)[1.542(3)], rC(gamma)-C-y' = 1.556(3)[1.555(3)] and angles (degrees) angle C alpha-C C = 179.4(5) [179.9(5)], angle C beta C alpha-C C = 113.7(5) [111.5(5)], angle C beta C alpha C beta' = 102.6(5) [102.1(5)], angle C alpha C beta C gamma = 103.7(5) [103.7(5)], angle C beta C gamma C gamma' = 106.0(5) [105.9(5)] and tau C beta C alpha C beta'C gamma' = 40.8(5) [41.6(5)]. Vibrational assignments are reported, supported by ab initio predictions and results are discussed. (C) 2013 Elsevier B.V. All rights reserved.

The FT-microwave spectrum of 1,1,3,3-tetrafluoro-1,3-disilacyclopentane ( c -C 3 H 6 Si 2 F 4 ) has been recorded and 339 transitions for 6 isotopologues have been assigned for the twist conformation. The ground state rotational constants were determined with values for the normal species: A = 2102.74026(68), B = 751.34319(32), C = 736.51478(31). Adjusted r 0 parameters are reported with distances (A): r C α –Si = 1.859(2), r Si–C β = 1.864(2), r Si–F 1 = 1.583(3), r Si–F 2 = 1.578(3), and r C β –C β = 1.559(3), and angles (°): ∠SiC α Si = 102.8(3), ∠C α SiC β = 104.3(3), ∠SiC β C β ′ = 106.6(3), τ C β SiC α Si = 10.7(3), and τ SiC β C β ′ Si = 40.8(3). The conformational stabilities have been predicted from ab initio calculations utilizing several various basis sets. Vibrational assignments have been provided for the observed bands for the twist conformer which are supported by ab initio calculations to predict harmonic force constants, vibrational wavenumbers, infrared intensities, Raman activities and depolarization ratios. The results are discussed and compared to the corresponding properties of some related molecules.

FT-microwave, infrared spectra of gas and Raman spectra of liquid for cyclopropylfluorosilane, c-C3H5SiH2F have been recorded. 51 transitions for the 28Si, 29Si, and 30Si isotopomers have been assigned for the gauche conformer. Enthalpy differences in xenon solution by variable temperature infrared spectra between the more stable gauche and lesser stable cis form gave 109 ± 9 cm−1. From the microwave rotational constants for the three isotopomers (28Si, 29Si, 30Si) combined with structural parameters predicted from MP2(full)/6-311+G(d, p) calculations, adjusted r0 structural parameters were obtained for the gauche conformer. The heavy atom distances (A): Si–C2 = 1.836(3); C2–C4 = 1.525(3); C2–C5 = 1.519(3); C4–C5 = 1.494(3); Si–F = 1.594(3) and angles (°): ∠CSiF = 111.2(5); ∠SiC2C4 = 117.5(5); ∠SiC2C5 = 119.2(5). To support the vibrational assignments, MP2(full)/6-31G(d) calculations were carried out. Results are discussed and compared to the corresponding properties of some similar molecules.

The infrared (3500–80 cm−1) and Raman spectra (3500–40 cm−1) of gas/or liquid and solid (CH3)2PX with X = H (DMH), CN (DMCN) and Cl (DMCl) as well as (CD3)2PH have been recorded and complete vibrational assignments are given for all three molecules. To support the spectroscopic study, ab initio calculations by the Moller–Plesset perturbation method to second order MP2(full) and density functional theory calculations by the B3LYP method have been carried out. The infrared intensities, Raman activities, vibrational frequencies and band contours have been predicted from MP2(full)/6-31G(d) calculations and these theoretical quantities are compared to experimental ones when available. By utilizing the previously reported microwave rotational constants for DMH and DMCN along with the MP2(full)/6-311+G(d,p) predicted values, adjusted r0 structural parameters for DMH and DMCN have been determined. The heavy atom parameters for DMH are: r0(P C3,4) = 1.8477(30) A, ∠ CPC = 99.88(50)° and for DMCN: r0(N C) = 1.159(3), r0(C P) = 1.790(3), r0(P C4,5) = 1.841(3) A, ∠ NCP = 175.7(5), ∠ CPC4,5 = 97.9(5) and ∠ CPC = 100.7(5)°. Barriers to internal rotation are reported. The experimental values are compared to the corresponding values of some similar molecules whenever possible.

Infrared spectra (3600–500 cm −1 ) of gas, amorphous, crystalline solid and Raman spectra (3000–200 cm −1 ) of liquid and solid were recorded. Variable temperature Raman spectra (30 to −38 °C) of liquid were recorded which indicates the presence of significant amounts of six conformers at ambient temperature. Rotation about C C bond gives Trans ( T ) and Gauche ( G ) conformers and rotation around both C P bonds gives trans ( t ) and gauche ( g ) forms. Enthalpy differences for five of the lower energy conformers w.r.t. the lowest energy tTt form were determined to be 272 ± 39 cm −1 , 434 ± 40 cm −1 , 443 ± 30 cm −1 , 663 ± 60 cm −1 and 671 ± 34 cm −1 for g ′ Tt , g ′ Tg ′, g ′ Tg , tG ′ g ′ and tG ′ t conformers, respectively, which are in the increasing order of energy. Ab initio calculations with various basis sets up to aug-cc-pVTZ were performed and to support the spectroscopic studies MP2(full)/6-31G(d) was used to predict Raman activities, infrared intensities, vibrational frequencies and band contours. By utilizing the previously reported microwave rotational constants along with MP2(full)/6-311+G(d,p) predictions, adjusted r 0 parameters were obtained for two Trans and two Gauche (C C bond) conformers. The determined heavy atom structural parameters for g ′ Tt [ g ′ Tg ′] { tG ′ g ′} | tG ′ t | conformers are: distances (A) P 1 C 2 = 1.870(3) [1.866(3)] {1.858(3)} |1.859(3)|, C 2 C 3 = 1.535(3) [1.536(3)] {1.534(3)} |1.536(3) | , C 3 P 4 = 1.863(3) [1.866(3)] {1.866(3)} | 1.859(3) | , and angles (°) ∠P 1 C 2 C 3 = 110.5(5) [110.4(5)] {117.0(5)} | 118.5(5) | , ∠C 2 C 3 P 4 = 115.5(5) [110.4(5)] {113.7(5)} | 118.5(5)|. The results are discussed and compared to the corresponding properties of some similar molecules.

The infrared spectra (3500–220 cm −1 ) in the gas phase of cyclobutylsilane, c-C 4 H 7 SiH 3 has been recorded. Variable temperature (−65 to −100 °C) studies of the infrared spectra (3500–400 cm −1 ) dissolved in liquid xenon have been carried out. From these data, both the equatorial and axial conformers have been identified and their relative stability obtained. The enthalpy difference has been determined from 15 band pairs to be 67 ± 7 cm −1 (0.81 ± 0.08 kJ mol −1 ) with the equatorial conformer the more stable form. The percentage of the axial conformer is estimated to be 42 ± 1% at ambient temperature. The conformational stabilities have been predicted from ab initio calculations utilizing several different basis sets up to aug-cc-pVTZ from both MP2(full) and density functional theory calculations by the B3LYP method. By utilizing previously reported microwave rotational constants along with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r 0 parameters have been obtained for both conformers. The determined heavy atom distances (A) for the equatorial [axial] form are Si–C α = 1.879(3) [1.884(3)], C α –C β , C β ′ = 1.562(3) [1.561(3)], C γ –C β , C β ′ = 1.551(3) [1.553(3)] and angles in degrees (°) ∠SiC α C β = 118.6(5) [113.2(5)], ∠C β C α C β ′ = 88.2(5) [88.4(5)], ∠C α C β C γ = 87.8(5) [88.9(5)], ∠C β C γ C β ′ = 88.9(5) [89.0(5)] and a puckering angle of 29.0(5) [23.5(5)]. Support for the vibrational studies have been provided from MP2(full)/6-31G(d) ab initio calculations to predict harmonic force constants, fundamental wavenumbers, infrared intensities, Raman activities and depolarization ratios for both conformers. The results are discussed and compared to the corresponding properties of some related molecules.

Infrared (3200–400 cm −1 ) and Raman spectra (3050–50 cm −1 ) of 2-cyanoethylphosphine, NCCH 2 CH 2 PH 2 , were recorded. Rotation about C–C bond gives Trans ( T ) and Gauche ( G ) conformers and rotation around C–P bond gives trans ( t ) and gauche ( g ) forms. Enthalpy differences in xenon solution by variable temperature Raman spectra between Gt and Tt gives 89 ± 12 cm −1 and Tg 331 ± 18 cm −1 . Ab initio calculations by MP2(full) and DFT calculations were carried out. From previously reported microwave rotational constants and MP2(full)/6-311+G(d,p) predicted structural values, adjusted r 0 parameters were obtained for three conformers. The determined heavy atom parameters for Gt [ Tt ] { Tg }: distances (A) P 1 –C 2 = 1.860(3) [1.858(3)] {1.864(3)}, C 2 –C 3 = 1.534(3) [1.535(3)] {1.535(3)}, C 3 –C 4 = 1.464(3) [1.465(3)] {1.463(3)}, C 4 –N 5 = 1.160(3) [1.160(3)] {1.160(3)} angles (°) ∠ P 1 C 2 C 3 = 117.1(5) [114.9(5)] {110.0(5)}, ∠ C 2 C 3 C 4 = 112.5(5) [112.1(5)] {111.8(5)}, τP 1 C 2 C 3 C 4 = 63.4(5) [180.0(5)] {176.8(5)}. Vibrational assignments are reported, supported by ab initio predictions.

The infrared spectra (4000–200 cm −1 ) of the gas and solid and the Raman spectrum (4000–40 cm −1 ) of the liquid have been recorded. Vibrational fundamentals have been identified for four of the possible five stable conformers and complete vibrational assignments have been made for the Gg and Gt forms where the Capital G is for Gauche for the orientation around the C C bond and the lower cases g and t for gauche and trans orientation for the amine. These forms are the two lowest energy conformers. Vibrational assignments have been supported by MP2(full)/6-31G(d) ab initio calculations to predict harmonic force constants, fundamental frequencies, infrared intensities, Raman activities, and depolarization ratios. The conformational stabilities have been predicted from ab initio calculations utilizing several different basis sets up to aug-cc-pVTZ for both MP2(full) and density functional theory calculations by the B3LYP method. The enthalpy differences between the Gg and Gt conformers was determined to be 75 cm −1 and for the Gg to Tg form 333 cm −1 . The r 0 structural parameters have been obtained for the two most stable conformers from predicted parameters from ab initio MP2(full)/6-311+G(d,p) calculations adjusted to fit the previously reported microwave rotational constants. The determined heavy atom structural values for the Gg [ Gt ] conformer are: the distances (A) N 1 C 2 = 1.461(3) [1.453(3)], C 2 C 3 = 1.535(3) [1.545(3)], C 3 C 4 = 1.466(3) [1.463(3)], C 4 N 5 = 1.161(3) [1.161(3)] and angles (°) ∠ N 1 C 2 C 3 = 109.5(5) [116.0(5)], ∠ C 2 C 3 C 4 = 111.1(5) [111.1(5)], ∠ C 3 C 4 N 5 = 177.4(5) [177.0(5)]. These parameters are compared to those estimated from the microwave study. The results are discussed and compared to the corresponding properties of some similar molecules.

The Raman and infrared spectra (4000 to 50 cm–1) of the gas, liquid or solution, and solid have been recorded of n-propylamine, CH3CH2CH2NH2. Variable temperature (−60 to −100 °C) studies of the Raman (1175 to 625 cm–1) and far infrared (600 to 10 cm–1) spectra dissolved in liquid xenon were carried out. From these data, the five possible conformers were identified and their relative stabilities obtained with enthalpy difference relative to trans–trans (Tt) for trans–gauche (Tg) of 79 ± 9 cm–1 (0.9 ± 0.1 kJ/mol); for Gg of 91 ± 26 cm–1 (1.08 ± 0.3 kJ/mol); for Gg′ of 135 ± 21 cm–1 (1.61 ± 0.2 kJ/mol); for Gt of 143 ± 11 cm–1 (1.71 ± 0.1 kJ/mol). The percentage of the five conformers is estimated to be 18% for the Tt, 24 ± 1% for Tg, 23 ± 3% for Gg, 18 ± 1% for Gg′ and 18 ± 1% for Gt at ambient temperature. The conformational stabilities have been predicted from ab initio calculations utilizing several different basis sets up to aug-cc-pVTZ from both second-order Moller–Plesset (MP2, full) and density functional theory calculations by the Becke, three-parameter, Lee–Yang–Parr method. Vibrational assignments were provided for the observed bands for all five conformers, which are supported by MP2(full)/6-31G(d) ab initio calculations to predict harmonic force constants, wavenumbers, infrared intensities, Raman activities and depolarization ratios for both conformers. Estimated r0 structural parameters were obtained from adjusted MP2(full)/6-311+G(d,p) calculations. The results are discussed and compared with the corresponding properties of some related molecules. Copyright © 2012 John Wiley & Sons, Ltd.

The microwave spectrum of cyclobutylisocyanate, c-C4H7NCO, has been investigated from 21,000 to 11,000 MHz and 11 transitions for the more stable equatorial-trans conformer were assigned. The rotational constants of the ground vibrational state have been determined and the molecule has been shown to be a near symmetric prolate rotor (К = −0.99). The B and C rotational constants have been confidently determined to be B = 1508.68(3) and C = 1476.55(2) MHz, respectively, whereas the value for the A rotational constant of 6,891(302) MHz had a large uncertainty. Variable temperature (−100 to −55 °C) studies of the infrared spectra (3,500–400 cm−1) of cyclobutylisocyanate dissolved in liquid xenon as well as the infrared spectra of the gas and solid have been recorded. In addition, the Raman spectra (3,600–100 cm−1) of the liquid have been investigated. These spectral data indicated the present of three conformers in the fluid states which are the equatorial-trans, equatorial-gauche, and axial-trans forms. The second part of the conformational name refers to the relative position of the NCO moiety relative to the alpha hydrogen. By utilizing four conformer pairs, an enthalpy difference of 131 ± 13 cm−1 (1.57 ± 0.16 kJ/mol) was obtained with the equatorial-trans conformer the more stable form, which is in good agreement with the ab initio predicted value of 137 ± 36 cm−1 (1.64 ± 0.43 kJ/mol). To aid in the vibrational assignment, ab initio and DFT calculations have been carried out by using a variety of basis sets up to 6-311G(3df,3pd).

The Raman and infrared spectra (4000-50 cm(-1)) of the gas, liquid or solution, and solid have been recorded of isopropylamine, (CH3)(2)CHNH2. Variable temperature (-50 to -100 degrees C) studies of the Raman spectra (3500-100 cm(-1)) dissolved in liquid xenon have been carried out. From these data, both the trans and gauche conformers have been identified and their relative stability obtained. The enthalpy difference has been determined to be 113 +/- 11 cm(-1)(1.35 +/- 0.13 kJ mol(-1)) with the trans conformer the more stable form. The percentage of the gauche conformer is estimated to be 54 +/- 1% at ambient temperature. The conformational stabilities have been predicted from ab initio calculations utilizing several different basis sets up to aug-cc-pVTZ from both MP2(full) and density functional theory calculations by the B3LYP method. By utilizing previously reported microwave rotational constants along with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r(0) parameters have been obtained for the trans conformer. The determined heavy atom and NH2 distances are (angstrom) C-C = 1.530(3), C-N = 1.465(3), N-H = 1.019(3) and angles in degrees (degrees) angle NCC = 108.9(5), angle CCC = 111.0(5), angle HNC = 110.3(5). The structural parameters for the gauche conformer were estimated by using the same adjustment differences to the gauche form as those obtained for the corresponding trans parameters. Vibrational assignments have been provided for the observed bands for both conformers which are supported by MP2(full)/6-31 G(d)ab initio calculations to predict harmonic force constants, wavenumbers, infrared intensities, Raman activities and depolarization ratios for both conformers. The results are discussed and compared to the corresponding properties of some related molecules. (C) 2011 Elsevier B.V. All rights reserved.

Infrared and Raman spectra (3500–60 cm −1 ) of gas and/or liquid and solid 1-bromo-1-silacyclopentane (c-C 4 H 8 SiBrH) have been recorded and the vibrational data indicate the presence of a single conformer with no symmetry which is consistent with the twisted form. Ab initio calculations with a variety of basis sets up to MP2(full)/6-311+G(2df,2pd) predict the envelope-axial and envelope-equatorial conformers to be saddle points with nearly the same energies but approximately 900 cm −1 (5.98 kJ/mol) lower in energy than the planar conformer. Density functional theory calculations by the B3LYP method predict slightly lower energies for the two envelope forms and considerably lower energy for the planar form compared to the MP2 predictions. By utilizing the MP2(full)/6-31G(d) calculations the force constants, frequencies, infrared intensities, band contours, Raman activities, and depolarization values have been obtained to support the vibrational assignment. Estimated r 0 structural parameters have been obtained from adjusted MP2(full)/6-311+G(d,p) calculations. These experimental and theoretical results are compared to the corresponding quantities of some other five-membered rings.

Variable temperature (−55 to −100 °C) studies of the infrared spectra (3500–400 cm −1 ) of fluorocyclobutane, c-C 4 H 7 F, dissolved in liquid xenon have been carried out as well as the infrared spectra of the gas. By utilizing eight pairs of conformers at 10 different temperatures, the enthalpy difference between the more stable equatorial conformer and the axial form has been determined to be 496 ± 40 cm −1 (5.93 ± 0.48 kJ/mol). The percentage of the axial conformer present at ambient temperature is estimated to be 8 ± 1%. The ab initio MP2(full) average predicted energy difference from a variety of basis sets is 732 ± 47 cm −1 (9.04 ± 0.44 kJ/mol) and the average value of 602 ± 20 cm −1 from density functional theory predictions by the B3LYP method are significantly larger than the experimentally determined enthalpy value. By utilizing previously reported microwave rotational constants for the equatorial and axial conformers combined with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r 0 parameters have been obtained. The determined heavy atom structural parameters for the equatorial [axial] conformer are: distances (A) C–F = 1.383(3) [1.407(3)], C α –C β = 1.543(3) [1.546(3)], C β –C γ = 1.554(3) [1.554(3)] and angles (°) ∠C α C β C γ = 85.0(5) [89.2(5)], ∠C β C α C β = 89.3(5) [89.2(5)], ∠F–(C β C α C β ) = 117.4(5) [109.2(5)] and a puckering angle of 37.4(5) [20.7(5)]. The conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational frequencies have been obtained for both conformers from MP2(full)/6-31G(d) ab initio calculations and compared to experimental values where available. The results are discussed and compared to the corresponding properties of some other monosubstituted cyclobutanes with halogen and pseudo-halogen substituents.

The Raman and infrared spectra (3500–50 cm−1) of the gas, liquid or solution, and solid were recorded of 2,2,3,3,3-pentafluoropropylamine (CF3CF2CH2NH2) and the ND2 isotopomer. Variable temperature (−55 to − 100 °C) studies of the infrared spectra (3600–400 cm−1) of samples dissolved in liquid xenon have been carried out. From these data, two of the five possible conformers have been identified and their relative stabilities obtained. The enthalpy difference was determined between the more stable Tt conformer and the less stable Tg form to be 280 ± 14 cm−1 (3.35 ± 0.17 kJ/mol). The first indicator is the NCCC dihedral angle (G = gauche or T = trans), and the second one (g = gauche or t = trans) is the relative position of the lone pair of electrons on nitrogen with respect to the β-carbon. The percentage of the Tg conformer at ambient temperature is estimated to be 34 ± 2%. The conformational stabilities have been predicted from ab initio calculations utilizing several different basis sets up to aug-cc-pVTZ for both MP2(full) and density functional theory calculations by the B3LYP method. Vibrational assignments have been provided for most of the observed bands for both isotopomers, supported by MP2(full)/6-31G(d) ab initio calculations to predict the harmonic force fields, wavenumbers, infrared intensities, Raman activities, and depolarization ratios for both conformers. Estimated r0 structural parameters were obtained from adjusted MP2(full)/6-311 + G(d,p) calculations. The results are discussed and compared with the corresponding properties of some related molecules. Copyright © 2011 John Wiley & Sons, Ltd.

The infrared spectra of gaseous and variable-temperature liquid xenon solutions of pyrrolidine have been recorded. The enthalpy difference has been determined to be 109 ± 11 cm(-1) (1.30 ± 0.13 kJ mol(-1)) with the envelope-equatorial conformer more stable than the twist form with 37 ± 3% present at ambient temperature. Ab initio calculations utilizing various basis sets up to MP2(full)/aug-cc-pVTZ have been used to predict the conformational stabilities, energy at the equatorial-axial saddle point, and barriers to planarity. From previously reported microwave rotational constants along with MP2(full)/6-311+G(d,p) predicted structural values, adjusted r(0) parameters have been obtained for both conformers. Heavy atom distances (Å) of equatorial[twist] conformer are as follows: N(1)-C(2) = 1.469(3)[1.476(3)], N(1)-C(3) = 1.469(3)[1.479(3)], C(2)-C(4) = 1.541(3)[1.556(3)], C(3)-C(5) = 1.541(3)[1.544(3)], C(4)-C(5) = 1.556(3)[1.543(3)]; and angles (deg)∠N(1)C(2)C(4) = 102.5(5)[107.6(5)], ∠N(1)C(3)C(5) = 102.5(5)[105.4(5)], ∠C(2)C(4)C(5) = 104.3(5)[104.6(5)], ∠C(3)C(5)C(4) = 104.3(5)[103.7(5)], ∠C(2)N(1)C(3) = 104.1(5)[103.9(5)], τC(2)C(4)C(5)C(3) = 0.0(5)[13.5(5)]. A complete vibrational assignment is proposed for both conformers.

Variable temperature (−55 to −100 °C) studies of the infrared spectra (4000–400 cm−1) of 2,2-difluoroethylamine, F2CHCH2NH2 dissolved in liquid xenon have been carried out. From these data three of the five possible conformers have been identified and their relative stabilities obtained. The enthalpy differences have been determined among the most stable Tt conformer and the second stable conformer, Gg, to be 83 ± 8 cm−1 (0.99 ± 0.10 kJ/mol), the third stable conformer, Gt, to be 235 ± 11 cm−1 (2.81 ± 0.13 kJ/mol). The percentage of each conformer at ambient temperature is estimated to be with Tt (34 ± 1%), Gg (45 ± 1%), Gt (22 ± 1%). The first indicator is the NCCH dihedral angle (G = gauche or T = trans) and the second one (g = gauche or t = trans) is the relative position of the lone pair of electrons on the nitrogen atom with respect to the β-carbon atom. The conformational stabilities have been predicted from ab initio calculations utilizing several different basis sets up to aug-cc-pVTZ for both MP2(full) and density functional theory calculations by the B3LYP method. By utilizing previously reported microwave rotational constants along with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 parameters have been obtained for the two most stable conformers. The determined heavy atom structural parameters for the Tt[Gg]{Gt} conformers are: the distances (A) C1–C2 = 1.514(3) [1.507(3)] {1.512(3)}, C2–N3 = 1.452(3) [1.458(3)] {1.453(3)}, C1–F4 = 1.368(3) [1.370(3)] {1.369(3)}, C1–F6 = 1.368(3)[1.365(3)]{1.366(3)} and angles in (°) ∠N3C2C1 = 115.0(5)[108.8(5)]{114.8(5)}, ∠F4C1C2 = 110.3(5) [109.3(5)] {109.7(5)}, ∠F6C1C2 = 110.3(5) [110.0(5)] {109.9(5)}, ∠F4C1F6 = 106.5(5) [107.0(5)] {107.1(5)} and τF4C1C2N3 = 58.6(5) [64.5(5)] {61.2(5)}. Vibrational assignments have been provided for most of the observed bands which have been supported by MP2(full)/6-31G(d) ab initio calculations to predict harmonic force fields, frequencies, infrared intensities, Raman activities and depolarization ratios for the three conformers. The results are discussed and compared to the corresponding properties of some similar molecules.

Infrared and Raman spectra (3500–60 cm −1 ) of gas and/or liquid and solid 1-chloro-1-silacyclopentane (c-C 4 H 8 SiClH) have been recorded and the vibrational data indicate the presence of a single conformer with no symmetry which is consistent with the twisted form. Ab initio calculations with a variety of basis sets up to MP2(full)/aug-cc-pVTZ predict the envelope-axial and envelope-equatorial conformers are saddle points with nearly the same energies but much lower in energy than the planar conformer. Density functional theory calculations by the B3LYP method predicts slightly lower energies for the two envelope forms and considerably lower for the planar form. By utilizing the MP2(full)/6–31G(d) calculations the force constants, frequencies, infrared intensities, band contours, Raman activities, and depolarization values have been obtained to support the vibrational assignment. Estimated r 0 structural parameters have been obtained from adjusted MP2(full)/6–311 + G(d, p) calculations. These experimental and theoretical results are compared to the corresponding quantities of some other five-membered rings.

The infrared spectrum (4000–400 cm −1 ) of the gas and variable temperature (−55 to −100 °C) spectra of xenon solutions of ethanol (ethyl alcohol), CH 3 CH 2 OH, have been recorded. From these data complete vibrational assignments have been made for both the trans (anti) and gauche conformers. By utilizing the conformer pair of the CH 3 rock fundamental modes in the xenon solution the enthalpy difference has been determined to be 62 ± 12 cm −1 (0.75 ± 0.14 kJ mol −1 ) with the trans form the more stable conformer from variable temperature spectra of xenon solutions. It is estimated that there is approximately 40 ± 1% of the trans form present at ambient temperature. By using the previously reported fifteen rational constants of the five heavy atom isotopomers for the trans conformer and the three rotational constants for the gauche form combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, the adjusted- r 0 parameters have been obtained. The heavy atom structural parameters for the trans [ gauche ] are: the distance (A), C–O = 1.433(3) [1.430(3)], C–C = 1.518(3) [1.523(3)]; and angle (°), CCO = 107.6(5) [112.5(5)], HOCC = 180.0 [56.9(5)]. The Δ J , Δ JK , Δ K , δ J , δ K centrifugal distortion constants have been predicted from ab initio and density functional theory calculations which are compared to the corresponding experimentally determined values. The harmonic force field, infrared intensities, band contours, and vibrational frequencies have been obtained for both conformers from ab initio MP2 (full)/6-31G(d) calculations and the results are compared to the experimental values when appropriate. The results are discussed and compared to the corresponding quantities of some similar molecules.

Variable temperature (−55 to −100 °C) studies of the infrared spectra (4000–400 cm−1) of ethylenediamine, NH2CH2CH2NH2, dissolved in liquid xenon and the infrared spectra in the gaseous phase have been carried out. From these data, three of the possible ten conformers have been identified, and the enthalpy differences have been determined among the most stable g′G′g′ conformer and the second stable conformer, g′G′t, to be 64 ± 6 cm−1 (0.77 ± 0.07 kJ mol−1) and the third conformer, gG′g′, to be 210 ± 19 cm−1 (2.51 ± 0.23 kJ mol−1). The first indicator is the lone pair of electrons on the nitrogen-1 atom with respect to the NCC angle (g = gauche or t = trans) and the second indicator is the NCCN dihedral angle (G = gauche or T = trans) and the third one similar to the first one but with respect to nitrogen-2. The percentage of each conformer at ambient temperature is estimated to be 47 ± 1% of g′G′g′, 36 ± 2% of g′G′t and 17 ± 2% of gG′g′. The conformational stabilities have been predicted from ab initio calculations by utilizing several different basis sets up to aug-cc-PVTZ for both MP2(full) and density functional theory calculations by the B3LYP method. By utilizing previously reported microwave rotational constants along with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 parameters have been obtained for the two most stable conformers. The determined heavy atom structural parameters for the g′G′g′ [g′G′t] conformer are: the distances (A) N1–C2 = 1.472(3) [1.470(3)], C2–C3 = 1.526(3) [1.532(3)], N4–C3 = 1.464(3) [1.463(3)] and the angles (°)∠N1C2C3 = 109.7(5) [109.7(5)], ∠N4C3C2 = 109.5(5) [115.3(5)] and τN1C2C3N4 = 63.5(5) [59.7(5)]. Vibrational assignments have been provided for most of the observed bands which have been supported by MP2(full)/6-31G(d) ab initio calculations to predict harmonic force fields, frequencies and infrared intensities for all three conformers. The results are discussed and compared to the corresponding properties of some similar molecules.

Rotational spectra have been measured for five naturally occurring isotopologues of germanium in the gauche conformer of the cyclopropylmethylgermane molecule by Fourier-transform microwave spectroscopy. Internal rotation of the methyl group caused all rotational transitions to be doubled and analysis of these A and E states with an internal rotation Hamiltonian furnished a barrier to rotation of approximately 4.7 kJ mol−1 for each isotopic species. In addition, the quadrupolar nature of the I = 9/2 73Ge nucleus caused nuclear quadrupole hyperfine structure to be observed for all transitions belonging to this isotopologue, allowing a determination of nuclear quadrupole coupling constants for the 73Ge nucleus. Dipole moment components have also been measured via the Stark effect. Ab initio results will be presented and compared with experimental rotational constants, dipole moment components and electric field gradients at the Ge nucleus.

The infrared spectra (3500–300 cm−1) of the gas and xenon solutions of fluorocyclohexane have been recorded. By the utilization of nine different temperatures of nine conformer pairs the enthalpy difference between the more stable chair-equatorial conformer and the chair-axial form was determined to be 48 ± 5 cm−1 (137 ± 14 cal/mol). The ab initio predicted values ranged from 121 cm−1 with the chair-axial conformer more stable from the MP2(full)/6-311G(d,p) calculations to 24 cm−1 with the chair-equatorial form more stable with diffuse functions. The r0 structural parameters have been obtained from predicted parameters from ab initio MP2(full)/6-311+G(d,p) calculations adjusted to fit the previously reported microwave determined rotational constants. The determined heavy atom structural parameters for the equatorial[axial] conformer are: the distances (A) C1–C7,8 = 1.533(3)[1.531(3)], C7,8–C13,14 = 1.543(3)[1.538(3)], C4–C13,14 = 1.518(3)[1.520(3)], C4–F6 = 1.404(3)[1.410(3)] and angles in degrees ∠C1C7,8C13,14 = 111.0(5)[111.2(5)], ∠F6C4C13,14 = 108.7(5)[107.9(5)] and τC8C1C7C13 = 55.9(10)[55.9(10)]. The harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational frequencies have been obtained for both conformers from MP2(full)/6-31G(d) ab initio calculations. With two scaling factors of 0.88 for the C–H stretches and 0.9 for the remaining ones, the fundamental wavenumbers have been predicted and along with the depolarization values and infrared band contours (B-type for A″ modes) a complete vibrational assignment has been made for both conformers. The results are discussed and compared to the corresponding properties of some similar molecules.

The FT-microwave spectrum of ethynylcyclohexane, c-C 6 H 11 CCH has been investigated from 11,000 to 21,000 MHz and 32 transitions for the chair-equatorial conformer and 22 transitions for the chair-axial form have been observed and assigned from which the rotational constants and five centrifugal distortion constants have been obtained. By utilizing the six microwave rotational constants along with ab initio MP2(full)/6-311 + G(d,p) predicted structural values, adjusted r 0 parameters have been obtained for the two conformers. The determined heavy atom structural parameters for the equatorial[axial] conformer are: the distances (A) C 1 C 9,10 = 1.532(3)[1.537(3)], C 9,10 C 15,16 = 1.544(3)[1.534(3)], C 4 C 15,16 = 1.541(3)[1.545(3)], C 6 C 7 = 1.210(3)[1.212(3)] and angles in degrees: ∠C 1 C 9,10 C 15,16 = 111.0(5)[111.2(5)], ∠C 6 C 4 C 15,16 = 110.7(5)[110.3(5)] and τ C 1 C 9,10 C 15,16 C 4 = 56.4(10)[55.3(10)]. Variable temperature (−60 to −100 °C) studies of a xenon solution were carried out and by using 10 conformer pairs an enthalpy difference of 124 ± 13 cm −1 (1.48 ± 0.16 kJ/mol) was obtained with the more stable form the chair-equatorial conformer. At ambient temperature, the abundance of the chair-equatorial conformer is 55 ± 3%. By measuring the temperature dependency of the Raman spectrum of the liquid of the conformer pair at 822 (axial)/842 (equatorial) cm −1 , a standard enthalpy difference of 172 ± 32 cm −1 (2.06 ± 0.38 kJ/mol) was obtained again with the chair-equatorial conformer the more stable form. From MP2(full)/6-311 + G(d,p) ab initio calculations the chair-axial conformer is predicted to be more stable by 135 cm −1 (1.62 kJ/mol) whereas in contrast the density functional theory calculations by the B3LYP method with the same basis set gives the chair-equatorial conformer more stable by 289 cm −1 (3.46 kJ/mol). A complete vibrational assignment is given for the chair-equatorial conformer and most of the fundamentals for the chair-axial form have been assigned. To support the vibrational assignments, normal coordinate calculations with two scaling factors for the force constants from MP2(full)/6-31G(d) calculations were carried out to predict the fundamental vibrational frequencies, infrared intensities, Raman activities, depolarization values and infrared band contours. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.

Variable temperature (−55 to −155 °C) studies of the infrared spectra (400–3500 cm−1) of ethylisocyanate, CH3CH2NCO, dissolved in liquid xenon and krypton have been recorded. Additionally the infrared spectra of the gas and solid have been re-investigated. These spectroscopic data indicate two conformers in the fluid states which are the cis and trans forms with a large proportion of molecules in the gas phase at ambient temperature in the excited states of the NCO torsional mode which has a very low barrier to conformational interchange. Variable temperature (−110 to −155 °C) studies of krypton solutions were carried out and by using two conformer pairs, an enthalpy difference of 100 ± 4 cm−1 (1.20 ± 0.05 kJ/mol) was obtained with the cis conformer the more stable form. To aid in the analyses of the vibrational and rotational spectra, ab initio calculations have been carried out by the perturbation method to the second order (MP2) with full electron correlation using a variety of basis sets up to 6-311+G(2df,2pd) and cc-PVQZ. With the basis sets 6-311+G(2d,2p) and larger, the barrier at the cis position ranged from a low value of 11 cm−1 to a high value of 31 cm−1 with a value of 19 cm−1 from the largest basis set of cc-PVQZ. Thus, the gauche well is probably so shallow that it does not contain a bound vibrational state. This results in the cis conformer as the most stable form which is consistent with the experimental rotational and vibrational data. The predicted energy difference from these calculations between the cis conformer and the transitional-state skew form is ∼100 cm−1 which is consistent with the assigned microwave lines for four excited states of the NCO torsion. Density functional theory calculation by the B3LYP method with many of the same basis sets provided little information. By utilizing the previously reported microwave rotational constants with the structural parameters predicted by the ab initio MP2(full)/6-311+G(d,p) calculations, adjusted r0 structural parameters have been obtained for the cis form. The determined heavy atom parameters are: r(C N) = 1.211(5), r(C O) = 1.167(5), r(C N) = 1.448(5), r(C C) = 1.516(5) A for the distances and angles of ∠CCN = 112.6(5), ∠CNC = 137.5(5), ∠NCO = 172.9(5)°. The centrifugal distortion constants, dipole moments, conformational stability, vibrational frequencies, infrared intensities and Raman activities have been predicted from ab initio calculations and compared to experimental quantities when available. These results are compared to the corresponding quantities of some similar molecules.

The infrared spectra (3200–400 cm −1 ) of the gas, xenon solutions, and solid and the Raman spectrum (3200–100 cm −1 ) of liquid cyanocyclopentane, c-C 5 H 9 CN, have been recorded. From infrared spectra of xenon solutions at 10 different temperatures, the enthalpy difference between the more stable axial conformer and the equatorial form has been determined to be 109 ± 37 cm −1 (312 ± 106 cal/mol). The percentage of the equatorial conformer present at ambient temperatures is estimated to be 41 ± 7%. The ab initio MP2(full) average predicted energy difference from a variety of basis sets is 182 ± 23 cm −1 (520 ± 66 cal/mol) whereas the energy difference obtained from the corresponding density functional theory calculations by the B3LYP method predict the equatorial conformer to be more stable by 164 ± 11 cm −1 (469 ± 31 cal/mol). By utilizing previously reported microwave determined rotational constants for both conformers combined with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r 0 parameters have been obtained. The determined heavy atom structural parameters for the axial[equatorial] conformer are: the distances C 1 –C 2 = 1.542(5)[1.543(5)], C 1 –C 3 = 1.542(5)[1.543(5)], C 2 –C 4 = 1.542(5)[1.544(5)], C 3 –C 5 = 1.542(5)[1.544(5)], C 4 –C 5 = 1.559(5)[1.564(5)], C 1 –C 6 N 8 = 1.467(5)[1.462(5)], C 6 N 8 = 1.156(5)[1.159(5)] (A) and angles in degrees ∠C 3 C 1 C 2 = 102.0(5)[102.3(5)], ∠C 1 C 2 C 4 = 103.6(5)[103.2(5)], ∠C 1 C 3 C 5 = 103.6(5)[103.2(5)], ∠C 2 C 4 C 5 = 105.7(5)[105.8(5)], ∠C 3 C 5 C 4 = 105.7(5)[105.8(5)], ∠C 6 C 1 C 2 = 111.5(5)[113.1(5)], ∠C 6 C 1 C 3 = 111.5(5)[113.1(5)] (°) and τ C 1 C 2 C 4 C 5 = 25.8(5)[−26.1(5)], τ C 1 C 2 C 4 C 5 = 0.0(5)[0.0(5)] (°). Vibrational assignments have been provided for most of the observed bands which have been supported by MP2(full)/6-31G(d) ab initio calculations to predict harmonic force fields, frequencies, infrared intensities, Raman activities and depolarization ratios for both conformers. The results are discussed and compared to the corresponding properties of some similar molecules.

Ab initio calculations with full electron correlation by the perturbation method to second order and hybrid density functional theory calculations by the B3LYP method utilizing a variety of basis sets have been carried out for the F(CN)C NX (X = H, F, Cl, Br) and CH 3 (Y)C NH (Y = H, CN) molecules. From these calculations, force constants, vibrational frequencies, infrared intensities, Raman activities, depolarization ratios, and structural parameters have been determined and compared to the experimental quantities when available. By combining previously reported rotational constants for F(CN)C NBr with the ab initio MP2/6-311+G(d) predicted parameters, adjusted r 0 parameters have been obtained. The structural parameters are: distances (A) (N−Br) = 1.865(3); (C N) = 1.270(3); (C−F) = 1.327(3); (C−C) = 1.437(3); (C N) = 1.160(3); and angles (°) ∠ BrNC = 114.4(5); ∠ NCF = 127.4(5); ∠ NCC = 119.7(5). These reported parameters have much lower uncertainties than those previously reported from the microwave data. Similar theoretical calculations have been carried out for CH 3 (Y)C NH (Y = H, CN) and r 0 structural parameters have been obtained for both the cis and trans conformers of ethylidenimine, CH 3 (H)C NH, and for cis iminopyruvonitrile, CH 3 (CN)C NH utilizing previously reported microwave rotational constants. For CH 3 (H)C NH the determined r 0 heavy atom parameters for the cis [trans] form are: distances (A) (C N) = 1.276(3) [1.275(3)]; (C−C) = 1.500(3) [1.508(3)]; angles (°) ∠ (HN C) = 109.4(5) [108.8(5)]; ∠ (N CC) = 121.2(5) [127.2(5)]; ∠ (HCC) = 116.2(5) [116.4(5)]. Centrifugal distortion constants have been obtained for all of the molecules from the ab initio predicted force constants and the values are compared to the experimentally determined values when available. The results of these studies are compared to the corresponding quantities of some other similar molecules.

The FT-microwave spectrum of cyanocyclohexane, c-C 6 H 11 CN has been investigated from 11,000 to 21,000 MHz and 99 transitions for the chair-equatorial conformer and 76 transitions for the chair-axial form have been observed and assigned from which the rotational constants and five centrifugal distortion constants have been obtained. By utilizing the six microwave rotational constants along with ab initio MP2(full)/6-311 + G(d,p) predicted structural values, adjusted r 0 parameters have been obtained for the two conformers. The determined heavy atom structural parameters for the axial[equatorial] conformer are: the distances (A) C 1 –C 8,9 = 1.536(3)[1.536(3)], C 8,9 –C 14,15 = 1.532(3)[1.532(3)], C 4 –C 14,15 = 1.544(3)[1.544(3)], C 6 N 7 = 1.162(3)[1.161(3)] and angles in degrees: ∠C 1 C 8,9 C 14,15 = 111.6(5)[110.8(5)], ∠C 6 C 4 C 14,15 = 110.0(5)[110.8(5)] and τ C 1 C 8,9 C 14,15 C 4 = 55.6(10)[56.9(10)]. Variable temperature (−55 to −100 °C) studies of a xenon solution were carried out and by using two conformer pairs from which an enthalpy difference of 63 ± 9 cm −1 (0.75 ± 0.11 kJ/mol) was obtained with the more stable form the chair-axial conformer. At ambient temperature, the abundance of the chair-axial conformer is 58 ± 8%. By measuring the temperature dependency of the Raman spectrum of the liquid of the conformer pair at 819 (axial)/842 (equatorial) cm −1 , a standard enthalpy difference of 46 ± 16 cm −1 (0.54 ± 0.19 kJ/mol) was obtained again with the chair-axial conformer the more stable form. From MP2(full)/6-311 + G(d,p) ab initio calculations the chair-axial conformer is predicted to be more stable by 120 cm −1 (1.44 kJ/mol) whereas in contrast the density functional theory calculations by the B3LYP method with the same basis sets gives the chair-equatorial conformer more stable by 180 cm −1 (2.16 kJ/mol). A complete vibrational assignment is given for the chair-axial conformer and most of the fundamentals for the chair-equatorial form have been assigned. To support the vibrational assignments, normal coordinate calculations with two scaling factors for the force constants from MP2(full)/6-31G(d) calculations were carried out to predict the fundamental vibrational frequencies, infrared intensities, Raman activities, depolarization values and infrared band contours. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.

The microwave spectra of seven isotopomers of fluoromethylsilane, CH(2)FSiH(3), in the ground vibrational state were measured and analyzed in the frequency range 18-40 GHz. The rotational and centrifugal distortion constants were evaluated by the least-squares treatment of the observed frequencies of a- and b-type R- and b-type Q-transitions. The values for the components of the dipole moment were obtained from the measurements of Stark effects from both a- and b-type transitions and the determined values are: |mu(a)| = 1.041(5), |mu(b)| = 1.311(6), and |mu(t)| = 1.674(4) D. Structural parameters have been determined and the heavy atom distances (r(0)) in Angstroms are: Si-C = 1.8942(57) and C-F = 1.4035(55) and the angle in degree, angleSiCF = 109.58(14). A semi-experimental r(e) structure was also determined from experimental ground state rotational constants and vibration-rotation constants derived from ab initio force fields. The internal torsional fundamental, SiH(3), was observed at 149.2 cm(-1) with two accompanying hot bands at 138.8 and 127.5 cm(-1). The barrier to internal rotation was obtained as 717.3(16) cm(-1) (2.051(46) kcal mol(-1)) by combining the analysis of the microwave A and E splittings and the torsional fundamental and hot band frequencies. Ab initio calculations have been carried out with full electron correlation by the second-order perturbation method with several different basis sets up to MP2/6-311+G(d,p) to obtain geometrical parameters, barriers to internal rotation, and centrifugal distortion constants. Adjusted r(0) structural parameters have been obtained by combining the ab initio MP2/6-311+G(d,p) predicted values with the determined rotational constants for the fluoride as well as with the previously reported microwave data for the chloro- and bromo- compounds. These experimental results are compared to the corresponding parameters for the carbon analogues.

The Raman spectra (3500–50 cm−1) of the liquid and solid methylcyclohexane and the infrared spectra of the gas and solid methylcyclohexane have been recorded. The Raman band at 754 cm−1 in the liquid has been confidently assigned to the less stable axial conformer and its intensity was recorded as a function of temperature from 25 to −95 °C. By the utilization of 15 different temperatures, the enthalpy difference between the more stable chair-equatorial conformer and the chair-axial form was determined to be 712 ± 71 cm−1 (8.50 ± 0.84 kJ/mol). The ab initio predicted value of 710 cm−1 (8.50 kJ/mol) from the MP2(full)/6-311G(2d,2p) calculations with and without diffuse functions is in excellent agreement. The harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational wavenumbers have been obtained for both conformers from MP2(full)/6-31G(d) ab initio calculations. With two scaling factors of 0.88 for the C-H stretches and 0.9 for the remaining ones, the fundamental wavenumbers have been predicted and along with the depolarization values and infrared band contours (B-type for A″ modes) a complete vibrational assignment has been made for the chair-equatorial conformer. Predicted r0 structural parameters have been provided from adjusted parameters from ab initio MP2(full)/6-311+G(d,p) calculations. The results are discussed and compared with the corresponding properties of some similar molecules. Copyright © 2009 John Wiley & Sons, Ltd.

The Raman (3200-30 cm−1) and/or infrared spectra (3500 to 400 cm−1) of gaseous, liquid and solid dimethylsilylisocyanate, (CH3)2 Si(H)NCO, have been recorded. The MP2(full) calculations, employing a variety of basis sets with and without diffusion functions, have been used to predict the structural parameters, conformational stability, vibrational fundamental wavenumbers, Raman activities, depolarization values and infrared intensities to support the vibrational assignment. The low wavenumber Raman spectrum of the gas with a significant number of Q-branches for the SiNC(O) bend is consistent with an essentially linear SiNCO moiety. The ab initio calculations supported this conclusion as all possible orientations of the NCO moiety lead to nearly the same energy. This result is at variance with the conclusion from the electron diffraction study that the heavy atom skeleton was bent with an angle of 152(5)° with one stable cis conformer. It is believed that this reported angle difference from 180° is due to the shrinkage effect. The SiH distance of 1.486 A has been obtained from the isolated SiH stretching wavenumber. From the adjustment of the ab initio MP2(full)/6-311+G(d,p) predicted structural parameters, a proposed structure is reported, which is expected to give rotational constants within a few megahertz of the actual ones. These experimental and theoretical results are compared with the corresponding quantities of similar molecules. Copyright © 2009 John Wiley & Sons, Ltd.

The infrared spectra (3200-50 cm(-1)) of the gas and solid and the Raman spectrum (3200-30 cm(-1)) of liquid and solid fluorocyclopentane, c-C5H9F, have been recorded. Additionally the infrared spectra (3200-400 cm(-1)) of liquid xenon solutions have been recorded at -65 and -95 degrees C. In all of the physical states, only the twisted C(1) conformer was detected. Ab initio calculations utilizing various basis sets up to MP2(full)/6-311+G(2df,2pd) with and without diffuse functions have been used to predict the conformational stabilities. These calculations predict only the twisted C1 conformer as the stable form. The two envelope (C(s) symmetry) forms with axial and equatorial structures were predicted to be first order saddle points with average higher energies of 75 +/- 33 and 683 +/- 44 cm(-1), respectively, from the C1 conformer but lower energies of 2442 and 1812 cm(-1), respectively, than the planar form by MP2 calculations. Similar values were obtained from the corresponding density functional theory calculations by the B3LYP method. A complete vibrational assignment is given for the twisted (C1) conformer which is supported by normal coordinate calculations with scaled force constants from MP2(full)/6-31G(d) calculations. The adjusted r0 structural parameters have been obtained by systematically fitting the MP2(full)/6-311+G(d,p) predicted values with the rotational constants obtained from a microwave study. The determined heavy atom r0 distances in A are (C1C2) = 1.531(3), (C1C3) = 1.519(3), (C2C4) = 1.553(3), (C3C5) = 1.533(3), (C4C5) = 1.540(3), and (C1F6) = 1.411(3) and the angles in degrees are angle C3C1C2 = 105.5(5), angle C1C2C4 = 106.2(5), angle C1C3C5 = 102.9(5), angle F6C1C2 = 108.9(5), and angle F6C1C3 = 107.6(5) with a dihedral angle angle C2C4C5C3 = 25.3(3). These experimental and theoretical results are compared to the corresponding quantities of some similar molecules.

Variable temperature (−55 to −100 °C) studies of the infrared spectra (3500–400 cm−1) of bromocyclobutane, c-C4H7Br, dissolved in liquid xenon have been carried out as well as the infrared spectra of the gas and solid and Raman spectrum of the liquid. By utilizing eight pairs of conformers at ten different temperatures, the enthalpy difference between the more stable equatorial conformer and the axial form has been determined to be 291 ± 22 cm−1 (3.48 ± 0.26 kJ/mol). The percentage of the axial conformer present at ambient temperature is estimated to be 20 ± 1%. The ab initio MP2(full) average predicted energy difference from a variety of basis sets is 560 ± 46 cm−1 (6.70 ± 0.55 kJ/mol) and the average value of 569 ± 47 cm−1 from density functional theory predictions by the B3LYP method are significantly larger than the experimentally determined enthalpy differences. By utilizing previously reported microwave rotational constants for the equatorial conformer combined with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 parameters have been obtained. The determined heavy atom structural parameters for the equatorial conformer are: distances (A) C–Br = 1.942(3), Cα–Cβ = 1.541(3), Cβ–Cγ = 1.552(3) and angles (°) ∠CαCβCγ = 86.8(5), ∠CβCαCβ = 89.7(5), ∠Br–(CβCαCβ) = 132.1(5) and a puckering angle of 29.8(5). The conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational frequencies have been obtained for both conformers from MP2(full)/6-31G(d) ab initio calculations and compared to experimental values where available. The results are discussed and compared to the corresponding properties of some similar molecules.

The infrared (3500–300 cm −1 ) and/or Raman (3500–50 cm −1 ) spectra of gaseous, liquid and solid of methylisocyanate, CH 3 NCO, have been recorded. Fine structures of the nearly free internal rotation of the methyl rotor has been observed for the pseudodegenerate CH 3 stretch and deformation from which the band centers and Coriolis coupling constants have been determined. Ab initio calculations with full electron correlation by the perturbation method to second order and hybrid density functional theory calculations by the B3LYP method utilizing the 6-31G(d), 6-311+G(d,p), and 6-311+G(2d,2p) basis sets have been carried out. From these calculations, force constants, vibrational frequencies, infrared intensities, Raman activities, depolarization ratios, and structural parameters have been determined and compared to the experimental quantities. Several differences are noted between the predicted and experimental values. By combining the three previously reported rotational constants for CH 3 NCO with the ab initio MP2/6-311+G(d,p) predicted structural values, adjusted r 0 parameters have been obtained. The r 0 values for the distance (A) are: r (C N) = 1.447(3); r (N C) = 1.215(3); r (C O) = 1.166(3); r (C H a ) = 1.089(2); r (C H s ) = 1.093(2), and for the angles (degrees): ∠ CNC = 135.8 ( 5 ) ; ∠ NCO = 172.6 ( 5 ) ; ∠ NCH a = 108.6 ( 5 ) ; ∠ NCH s = 110.8 ( 5 ) . Several of these parameters are significantly different from those previously reported from the latest microwave study. Predicted barriers to internal rotation and linearity as well as values of the centrifugal distortion constants are given and compared to experimental values when available. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results of some similar molecules.

The infrared spectra (3100–40 cm−1) of gaseous and solid and Raman spectra (3200–20 cm−1) of liquid and solid c-C3H5SiH2CH3, methylsilylcyclopropane, have been recorded. Additionally, variable temperature (−55 to −100 °C) studies of the infrared spectra of the sample dissolved in xenon have been recorded. From these spectral data, two conformers have been identified with one the cis form where the methyl group is over the three-membered ring and the other the gauche form. By utilizing four conformer pairs of vibrational bands the enthalpy difference of the sample dissolved in xenon has been determined to be 98 ± 13 cm−1 (1.17 ± 0.16 kJ mol−1) with gauche the more stable conformer. It is estimated that there is approximately 23 ± 6% of the cis form present at ambient temperature. The enthalpy difference of the liquid was obtained from the Raman spectra to be 101 ± 37 cm−1 (1.21 ± 0.44 kJ mol−1) again with gauche the more stable conformer. The SiH distances of 1.488 and 1.489 A have been determined from their stretching frequencies. By utilizing the microwave rotational constants for three isotopomers (28Si, 29Si, 30Si) combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, adjusted r0 structural parameters have been obtained for the gauche conformer. The heavy atom distances in A are: r0(SiC2) = 1.852(5); r0(C2C4) = 1.518(3); r0(C2C5) = 1.519(3); r0(C4C5) = 1.500(3); r0(SiC6) = 1.871(5) and the angles in degrees are: ∠CSiC = 111.5(5); ∠SiC2C4 = 119.9(5); ∠SiC2C5 = 119.2(5). For the cis form very small differences of 0.003 and 0.002 A from those of the gauche form are predicted for the SiC bonds whereas the other distances are predicted to be the same or differ by 0.001 A. A complete vibrational assignment is given for both the conformers. To support the vibrational assignments, normal coordinate calculations with scaled force constants from MP2(full)/6-31G(d) calculations were carried out to predict the fundamental vibrational frequencies, infrared intensities, Raman activities, depolarization values and infrared band contours. Barriers to internal rotation are reported. The results are discussed and compared to the corresponding properties of some similar molecules.

The infrared spectra (4,000–30 cm−1) of the gas and solid and the Raman spectrum of liquid 2,2-difluoroethanol as well as variable temperature infrared spectra of krypton/xenon solutions have been recorded. From all these data, two (Gg and Tg) out of the five possible stable conformers have been confidently identified. The order of the stabilities has been predicted to be Gg > Tg > Gt > Gg′ > Tt by utilizing ab initio MP2 (full) and DFT (B3LYP method) calculations, where the first indicator (capital letter) is in reference to rotation around the C–C bond (G = gauche or T = trans) and the second one (small letter) refers to the orientation of the hydroxyl group. The percentage of the minor conformer Tg, at ambient temperature, is estimated to be (16 ± 3%). The optimized geometries, fundamental frequencies, infrared intensities, Raman activities, and depolarization values as well as centrifugal distortion constants have been obtained from ab initio and density functional theory calculations by utilizing a variety of basis sets as well as those with diffuse functions. By utilizing the previously reported microwave rotational constants for two isotopomers of the Gg conformer combined with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r 0 parameters have been obtained. The determined heavy atom distances (A) for the Gg conformer are: C1–C2 = 1.510(3), C2–F4 = 1.371(3), C2–F5 = 1.362(3), C1–O3 = 1.412(3) A and angles ∠O3C1C2 = 111.0(5), ∠F4C2C1 = 108.8(5), ∠F5C2C1 = 109.8(5), τF4C2C1O3 = 63.5(5), τF5C2C1O3 = 179.1(5)°. Barriers of internal rotation have been obtained and vibrational assignments for the Gg and Tg conformers are given. The five predicted centrifugal distortion constants compared to the experimental values are in reasonable agreement except for ∆K, which appears to be in error. The results are discussed and the structural parameters compared to the corresponding ones for 2-fluoroethanol and 2,2,2-trifluoroethanol where those for the latter molecule have been redetermined. The currently determined heavy atom parameters are quite different from the earlier assumed values, which led to poor values of the six adjusted parameters.

The infrared (3200–400 cm −1 ) and Raman spectra (3500–40 cm −1 ) of gas/or liquid and solid chloromethylsilane, ClCH 2 SiH 3 , dichloromethylsilane, Cl 2 CHSiH 3 , chloromethyltrifluorosilane, ClCH 2 SiF 3 , and dichloromethyltrifluorosilane, Cl 2 CHSiF 3 , have been recorded and complete vibrational assignments are given for all four molecules. To support the spectroscopic study ab initio calculations by the Moller–Plesset perturbation method to second order MP2(full) and density functional theory calculations by the B3LYP method have been carried out. The Raman activities, infrared intensities vibrational frequencies and barriers to internal rotation have been predicted from MP2(full)/6-31G(d) calculations and these theoretical quantities are compared to the experimental ones when available. The r 0 Si–H distances for chloromethylsilane were determined to be 1.484 and 1.478 A for the in-plane and out-of-plane atoms, respectively, from the Si–H stretching frequencies. The adjusted r 0 parameters for ClCH 2 SiH 3 have been obtained by combining the ab initio MP2(full)/6-311+G(d,p) predicted values with the 12 previously reported microwave rotational constants. The heavy atom structural parameters for this molecule are: r 0 (Si–C) = 1.886(3), r 0 (Cl–C) = 1.791(3) A with ∠ ClCSi of 109.3(5)°. Estimated r 0 parameters are given for the other three molecules. Barriers to internal rotation have been obtained from the torsional frequencies and the values are compared to the predicted values as well as the corresponding ones for the carbon analogues.

The microwave spectra of the three singly substituted 13 C isotopomers and 18 O isotopomer of equatorial- trans cyclobutanol, c-C 4 H 7 OH, have been observed in natural abundance by a pulsed-jet Fourier transform microwave spectrometer. The fit for the normal species was improved from the previous study. The rotational constants for the α- 13 C, β- 13 C, γ- 13 C and 18 O isotopic species were determined. The five quartic centrifugal distortion constants were determined for the first time. These experimental values are compared to those obtained from ab initio and density functional theory calculations. By utilizing the previously reported microwave rotational constants for the O–D species along with the 15 constants determined from this study combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, adjusted r 0 parameters have been obtained. The determined heavy atom distances in A are: r 0 (C 1 –C 4,5 ) = 1.547(3), r 0 (C 6 –C 4,5 ) = 1.556(3), r 0 (C–O) = 1.412(3) A and angles O 2 C 1 C 4 , 5 = 120.2 ( 5 ) , C 4 C 1 C 5 = 88.9 ( 5 ) and puckering angle τC 6 C 5 C 4 C 1 = 31.3(10)°. The parameters are compared to the similar ones for some other monosubstituted cyclobutanes as well as to those for cyclobutane.

The FT-microwave spectrum of silylcyclohexane, c-C 6 H 11 SiH 3 has been investigated from 11,000 to 21,000 MHz and 43 transitions for the 28 Si isotopomer have been observed and assigned for the chair-equatorial conformer and likewise for 23 and 18 transitions for the 29 Si and 30 Si isotopomers, respectively. The SiH a and SiH s distances have been determined from their stretching frequencies with values of 1.486 and 1.487 A, respectively. With these parameters and by utilizing ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r 0 have been obtained. The determined heavy atom structural parameters for the most stable chair-equatorial conformer are: the distances (A) r 0 (C 1 –C 10 , 11 ) = 1.534(3); r 0 (C 10 , 11 –C 16 , 17 ) = 1.530(3); r 0 (C 4 –C 16 , 17 ) = 1.544(3); r 0 (C 4 –Si 6 ) = 1.880(5) and the angles in degrees: C 1 C 10 , 11 C 16 , 17 = 111.1(5)°; Si 6 C 4 C 16 , 17 = 111.6(5)° with the dihedral angle C 1 C 10,11 C 16,17 C 4 = 56.0(10)°. Variable temperature (−55 to −100 °C) studies of the infrared spectra (4000 to 400 cm −1 ) of silylcyclohexane dissolved in liquefied xenon have been carried out as well as the infrared spectra of the gas and solid. By measuring the temperature dependency of three conformer pairs from the spectra of xenon solutions, a standard enthalpy difference of 414 ± 19 cm −1 (1.18 ± 0.05 kcal/mol) was obtained with the chair-equatorial conformer the more stable form. At ambient temperature, the abundance of the axial conformer is 12.7 ± 1%. From MP2 ab initio calculations utilizing various basis sets, the equatorial conformer is predicted to be more stable by 525 ± 10 cm −1 from the four largest basis set calculations, which is consistent with experimental results. However, the average from the corresponding B3LYP density functional theory calculations is 671 ± 5 cm −1 which is certainly too large. A few of the previously reported vibrational assignments have been corrected. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.

Ab initio calculations with full electron correlation by the perturbation method to second order and hybrid density functional theory calculations by the B3LYP method utilizing the 6-31G(d) and 6-311+G(d,p) basis sets have been carried out for the F 2 C NX (X = H, F, Cl, Br) molecules. From these calculations, force constants, vibrational frequencies, infrared intensities, Raman activities, and structural parameters have been determined and compared to the experimental quantities when available. By combining previously reported rotational constants for all four F 2 C NX molecules with the ab initio MP2/6-311+G(d,p) predicted structural values, adjusted r 0 parameters have been obtained. The r 0 values for F 2 C NH and F 2 C NF are in excellent agreement with the previously determined values except for the difference in the two CF distances of F 2 C NH which differ by 0.017 A instead of the 0.008 A estimated distances. For F 2 C NCl [Br] the values are: distances (A) (ClN) = 1.713(5) [1.867(5)], (N C) = 1.260(3) [1.259(3)], (F t –C) = 1.304(3) [1.307(3)], (F c –C) = 1.307(3) [1.308(3)] and angles (°) ClNC = 113.2(5) [113.5(5)], F t CN = 120.3(5) [120.4(5)], F c CN = 129.7(5) [130.4(5)]. These reported values have much lower uncertainties than those previously reported from the microwave data alone but considering the relatively large uncertainties in the values given initially the two results are in near agreement. The centrifugal distortion constants have been calculated and are compared to the experimentally F 2 C NX (X = H, F, Cl, Br) determined values. These results are compared to several of the corresponding parameters of related molecules.

The infrared spectra (3200–30 cm −1 ) of the gas, amorphous and annealed solid and Raman spectra (3200–30 cm −1 ) of liquid and solid bromocyclohexane ( c -C 6 H 11 Br) have been recorded. Variable temperature (−55 to −100 o C) studies of the infrared spectra (3200–400 cm −1 ) of bromocyclohexane ( c -C 6 H 11 Br) dissolved in liquefied xenon have been carried out. From these data, the enthalpy difference has been determined to be 239 ± 24 cm −1 (2.86 ± 0.29 kJ mol −1 ), with the chair-equatorial conformer the most stable form. At ambient temperature, the abundance of the chair-axial conformer is 24% ± 2. From MP2 ab initio calculations utilizing various basis sets with and without diffuse functions, the equatorial conformer is predicted to be more stable by 168 ± 22 cm −1 from the four largest basis set calculations. However, the average from the corresponding B3LYP density functional theory calculations is 300 ± 16 cm −1 which, considering the error, is consistent with the experimental value. By utilizing the previously reported microwave rotational constants for two isotopomers ( 79 Br, 81 Br) combined with the structural parameters predicted from the MP2(full)/6-311+G(d,p) calculations, adjusted r 0 structural parameters have been obtained. The determined heavy atom distances for the most stable chair-equatorial conformer in A are r 0 (C 1 –C 7 , 8 ) = 1.532(3); r 0 (C 7 , 8 –C 13 , 14 ) = 1.539(3); r 0 (C 4 –C 13 , 14 ) = 1.524(3); and r 0 (C 4 –Br 6 ) = 1.966(5) and the angles in degrees: ∠C 1 C 7 , 8 C 13 , 14 = 111.3(5)°; ∠Br 6 C 4 C 13 , 14 = 109.8(5)° with the two dihedral angles ∠C 8 C 1 C 7 C 13 = 55.9(10)° and ∠C 14 C 4 C 13 C 7 = 57.7(10)° .For the axial form the major differences in distances are obtained for the r 0 (C 7,8 –C 13,14 ) = 1.531(3) and r 0 (C–Br) = 1.975(3) A. These parameters are in good agreement with those reported earlier from microwave and electron diffraction studies where the CC and CH distances were all assumed to be equal. A few of the previously reported vibrational assignments have been corrected. The results of these spectroscopic and theoretical studies are discussed and compared to the corresponding results for some similar molecules.