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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.

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.

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.

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 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.

Variable temperature (−55 to −100 °C) studies of the infrared spectra (4,000–400 cm−1) of chlorocyclobutane, c-C4H7Cl, dissolved in liquid xenon have been carried out. The infrared spectrum (4,000–100 cm–1) of the gas has also been recorded. For this puckered ring molecule the enthalpy difference between the more stable equatorial conformer and the axial form, has been determined to be 361 ± 17 cm−1 (4.32 ± 0.20 kJ/mol). This stability order is consistent with that predicted by ab initio calculations but the ∆H is much lower than the average energy value of 646 ± 73 cm−1 obtained from the MP2 ab initio calculations or 611 ± 28 cm−1 from the B3LYP density functional theory calculations. The percentage of the axial conformer present at ambient temperature is estimated to be 15 ± 1%. By utilizing previously reported microwave 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 equatorial conformer are: the distances C–Cl = 1.783(5), C1–C4 = 1.539(3), C4–C6 = 1.558(3) A, and angles ∠C6C4C1 = 86.9(5), ∠C4C1C5 = 89.7(5)°, and for the axial conformer are: the distances C–Cl = 1.803(5), C1–C4 = 1.547(3), C4–C6 = 1.557(3) A, and angles ∠C6C4C1 = 86.3(5), ∠C4C1C5 = 88.9(5) and the puckering angles for the equatorial and axial conformers are 30.7(5)° and 22.3(5)°, respectively. 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.

Variable temperature (−55 to −131 °C) studies of the infrared spectra (4000–50 cm −1 ) of 2-fluoroethylamine, FCH 2 CH 2 NH 2 dissolved in liquid xenon and the far infrared in liquid krypton have been carried out. From these data all five possible conformers have been identified and their relative stabilities obtained. The enthalpy differences have been determined among the most stable Gg′ conformer and the second stable conformer, Gt, to be 62 ± 8 cm −1 (0.74 ± 0.10 kJ/mol), the third stable conformer, Tg, to be 262 ± 26 cm −1 (3.14 ± 0.3 kJ/mol), the fourth most stable conformer, Tt, to be 289 ± 45 cm −1 (3.46 ± 0.5 kJ/mol) and the fifth most stable conformer, Gg to be 520 ± 50 cm −1 (6.24 ± 0.6 kJ/mol). The percentage of each conformer at ambient temperature is estimated to be with Gg′ (42 ± 4%), Gt (32 ± 1%), Tg (13 ± 1%), Tt (5 ± 1%) and Gg (3 ± 1%). The first indicator is the NCCF 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 fluorine 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 r 0 parameters have been obtained for the two most stable conformers. The determined heavy atom structural parameters for the Gg′[Gt] conformer are: the distances (A) C 1 –C 2 = 1.509(3) [1.516(3)], C 2 –N 3 = 1.466(3)[1.461(3)], C 1 –F 4 = 1.400(3)[1.398(3)] and angles in degrees ∠N 3 C 2 C 1 = 109.8(5) [115.5(5)], ∠F 4 C 1 C 2 = 109.2(5)[109.3(5)], ∠H 10 N 3 H 9 = 107.1(5) [107.1(5)](°) and τ F 4 C 1 C 2 N 3 = 65.3(5) [60.9(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 all five conformers. The results are discussed and compared to the corresponding properties of some similar molecules.

The infrared spectra (3600–100 cm −1 ) of gaseous and solid cyclobutylamine, c-C 4 H 7 NH 2 have been recorded, as well as, the infrared (3600–60 cm −1 ) spectra in xenon or krypton solutions at variable temperatures. From the latter studies, the enthalpy differences have been determined to be 225 ± 26 cm −1 (2.69 ± 0.31 kJ/mol) between the most stable equatorial- gauche (Eq-g) conformer and the equatorial- trans (Eq-t) form and 417 ± 42 cm −1 (4.99 ± 0.50 kJ/mol) with the axial- trans (Ax-t) conformer. At ambient temperature there is only 13 ± 1% of the Eq-t form present and 5 ± 1% of the Ax-t form. The potential function governing the conformational interchange of the NH 2 moiety of the equatorial conformer has been determined with the following potential coefficients (cm −1 ): V 1 = −176 ± 20, V 2 = −199 ± 25, V 3 = 721 ± 5 and V 4 = 65 ± 10 with barriers of 784 cm −1 for gauche to gauche and 834 cm −1 for gauche to trans form. By utilizing previously reported microwave rotational constants for the Eq-g conformer along with the ab initio predicted structural parameters from the MP2(full)/6-311 + G(d,p) calculations, r 0 structural parameters have been obtained for this conformer. The determined r 0 heavy atom distances in A are: (C 1 N) = 1.453(5); (C 1 C 4 ) = 1.560(5); (C 1 C 5 ) = 1.547(5); (C 4 C 6 ) = 1.557(5); (C 5 C 6 ) = 1.555(5) and the angles in degrees ∠NC 1 C 4 = 122.8(5)°; ∠NC 1 C 5 = 116.8(5)° with the four ∠CCC ranging from 87.5° to 88.1°. A complete vibrational assignment is proposed for the (Eq-g) conformer based on the infrared band contours, infrared intensities, Raman activities and group frequencies which is supported by normal coordinate calculations utilizing the force constants from ab initio MP2(full)/6-31G(d) calculations. Proposed assignments are also made for several fundamentals of the (Eq-t) form and a few for the Ax-t conformer. The results are discussed and compared to the corresponding properties of some similar molecules.

Variable temperature (-55 to -100 degrees C) studies of the infrared spectra (4000-400 cm(-1)) of cyclobutanol, c-C4H7OH dissolved in liquid xenon have been carried out. The infrared spectrum (4000-100 cm(-1)) of the gas has also been recorded. From these data two of the four possible stable conformers have been confidently identified and their order of stabilities has been experimentally determined where the first indicator is for the position of attachment of the hydroxyl group on the bent cyclobutyl ring (Eq=equatorial or Ax=axial) and the second one (t=trans, g=gauche) is the relative position of the hydroxyl rotor, i.e. rotation around the ring C-O bond. The enthalpy difference between the most stable Eq-t conformer and the second most stable rotamer, Eq-g, has been determined to be 200+/-50 cm(-1) (2.39+/-0.60 kJ/mol). This experimentally determined order is consistent with the order of stability predicted by ab initio calculations Eq-tEq-gAx-gAx-t. Evidence was obtained for the third conformer Ax-g which is predicted by ab initio calculations to be less stable by more than 650cm(-1) than the Eq-t form. The percentage of each conformer at ambient temperature is estimated to be Eq-t (50%), Eq-g (47%) and Ax-g (3%). The conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios and vibrational frequencies have been obtained for all of the conformers from MP2(full)/6-31G(d) ab initio calculations. The optimized geometries and conformational stabilities have been obtained from ab initio calculations utilizing several different basis sets up to MP2(full)/aug-cc-pVTZ and from density functional theory calculations by the B3LYP method. By utilizing previously reported microwave rotational constants for the Eq-t 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 Eq-t conformer are: the distances C1-C4=1.547(5) angstroms, C4-C6=1.552(5)angstroms, C-O=1.416(5) angstroms and angles angleC6C4C1=86.6(5) degrees , angleC4C1C5=88.9(5) degrees and angleC6C5C1C4=22.8(5) degrees . The results are discussed and compared to the corresponding properties of some similar molecules.