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The conformational stability and structure of 2,3-dimethylpropenal, 2,3-difluoropropenal and their 3,3-dimethyl and 3,3-difluoro derivatives were investigated utilizing ab initio calculations with 3-21G and 6-31G basis sets. For 2,3-dimethylpropenal and 3,3-difluoropropenal the s-trans was predicted to be the low-energy form. In the case of 3,3-dimethylpropenal and 2,3-difluoropropenal the s-cis was predicted by both levels of calculation to be the more stable conformer. Full optimization was performed at the transition states and the barriers to internal rotation were calculated. Methyl and fluorine substitution were found to significantly increase the barrier to interconversion in propenal. The relative change in the barrier depends on the position and the type of the substituent. The trans to cis barrier in 2,3-dimethylpropenal was calculated to be about 3 kcal mol−1 greater than that in 3,3-dimethylpropenal, while the cis to trans barrier in 2,3-difluoropropenal was predicted to be about 7 kcal mol−1 higher than the corresponding one in 3,3-difluoropropenal.

The conformational stability and structure of 3-fluoro-, 3,3-difluoro- and 3,3,3-trifluoropropanal were investigated by ab initio calculations with the 3–21G, 3–21G ∗ and 6–31G ∗ basis sets. The calculated O…F and O…H equilibrium separations indicate possible dipolar interactions that would stabilize the molecules in the conformers with minimum oxygen-fluorine and maximum oxygen-hydrogen interactions. For 3-fluoropropanal, the trans-cis conformer is predicted by the 3–21G and 6–31G basis sets to be the most stable, while for 3,3-difluoropropanal, the gauche-cis conformer is predicted to be the most stable. 3,3,3-Trifluoropropanal is predicted by both basis sets to exist as a mixture of more than one conformation with a very small energy difference between the gauche, trans and cis conformers.

The conformational stability and structure of 2-methylpropenals and 2,3-dimethylpropenals and the fluoro, and chloro derivatives were investigated utilizing ab initio calculations with 3-21G, 3-21G ∗ , and 6-31G ∗ basis sets. The data indicate that the molecules exhibit an s-trans ⇌ s-cis conformational equilibrium. For the methylpropenals the s-trans conformer was predicted by all basis sets to be the lower energy form. For the methyl moieties, the energetically preferred conformation is determined to have the hydrogen atom of the 2-methyl group eclipsing the CC bond and to be in a staggered configuration with respect to the hydrogen of the 3-methyl group. In case of the halopropenals the s-cis is predicted by various levels of calculation to be the more stable conformer. The dipole moment of the s-trans conformer of halopropenals was calculated to be about 2-3 D greater than that of the s-cis, which implies a significant charge density variation with the rotation of the carbonyl group about the C-C bond. Therefore, the dipolar effects are expected to be a major factor in the determination of the conformational stability of the halides.

The structure of cyclopropylmethanol in the gauche-gauche conformation was reinvestigated by utilizing the rotational constants of the parent and three deuterated species obtained from a previous microwave study, and a better fit of the observed rotational constants was determined with torsional angles of −53° and 123° respectively for the hydrogen and oxygen atoms of the hydroxyl group. The dipole moments and rotational constants measured by microwave data were clearly found to be consistent with those predicted by ab initio calculations only for the gauche-gauche conformer of cyclopropylmethanol. The conformational stability of 1-methylcyclo-propylmethanol, 1-fluorocyclopropylmethanol and 1-chlorocyclopropylmethanol were investigated by ab initio methods and the relative stability of the gauche-gauche conformer was predicted to increase in the case of 1-methylcyclopropylmethanol and to decrease in the case of the halo derivatives.

The structural stability of phenylacetic acid and mandelic acid was investigated by the DFT-B3LYP and the ab initio MP2 calculations with the 6-311G** basis set. The two molecules were predicted at the DFT and MP2 levels of calculation to have the non-planar (Np) forms as their lowest energy structures. The observed spectral intensities of the acids were consistent with the Np conformation being the predominant form at room temperature. The vibrational wavenumbers were computed at the B3LYP level of theory and tentative vibrational assignments were provided on the basis of combined theoretical and experimental infrared and Raman data of the molecules. The sharpness of the methylenic O–H stretching mode in the IR spectrum of mandelic acid suggests the absence of intermolecular dimerization in the acid which is supported by the observation of no splitting of its C O stretching mode.

The conformational equilibria and stability of 2,3-butadienal and 2,3-butadienoyl fluoride and chloride were investigated using ab initio calculations with the extended 4-31G and 6-31G∗ basis sets. The results indicate that the molecules show s-trans ⇌ s-cis conformational equilibria similar to the corresponding allylic molecules. The conformational stability of these molecules was found to be more dependent on the nature of the substituent and less dependent on the type of π system. The replacement of the allylic group by an allenic group slightly influenced the conformational equilibrium between the two stable conformers of the molecules. For 2,3-butadienal the s-trans conformer was predicted to be predominantly more stable than the s-cis conformer at ambient temperatures. For 2,3-butadienoyl halides the energy difference between the two conformers was calculated to be very small with slight thermodynamic stability favouring the s-trans form at room temperature.

The utility of ab initio calculations for the determination of structural parameters for allyl halides and ethers is demonstrated. It is shown that calculations with the 6-31G* basis set can be utilized with off-sets for the CH distances of 0.011 A which will give CH distances that agree with the experimental values obtained from infrared frequencies within 0.003 A. When conformers are present, the differences between the structural parameters for the two conformers can be used as constraints for the electron diffraction determination of the structure or for the microwave determinations by utilizing rotational constants from both conformers. Examples are given which show the utility of ab initio calculations for structural determinations, force constants, and frequency predictions.

The conformational stability and the three rotor internal rotations in 2,3-dichloro-1-propanol were investigated at DFT-B3LYP/6-311 + G**, MP2/6-311 + G** and MP4(SDQ) levels of theory. From the calculated potential energy surface, ten distinct minima were located all of which were predicted to have real frequencies at the B3LYP level of theory. The calculated lowest energy minima in the potential curves of the molecule were predicted to correspond to the Ggg and Gtg1 structures. The observed broad and very intense infrared band centered at about 3370 cm −1 supports the existence of the strong intermolecular H-bonding in 2,3-dichloro-1-propanol. The equilibrium constants for the conformational interconversion in the molecule were estimated from the calculated Gibb's energies at the B3LYP/6-311 + G** level of calculation and found to correspond to an equilibrium mixture of about 49% Ggg , 27 % Gtg1 , 5% Ggt and 5% Tgg conformations at 298.15 K.

The conformational stability and the C–O and O–H internal rotations in oxiranemethanol were investigated at the DFT-B3LYP/6-311G**, MP2/6-311G** and MP4(SDQ)/6-311G** levels of theory. Three minima were predicted in the CCOH potential energy scans of the molecule to have relative energies of about 2 kcal/mol or less and all were calculated to have real frequencies upon full optimization of structural parameters at the DFT and the MP2 levels of calculations. The Cg1 (H bond inner) conformation was predicted to be the lowest energy conformation for oxiranemethanol in excellent agreement with an earlier microwave study. The equilibrium mixture was calculated from Gibb's free-energy changes to be about 79% Cg1, 17% G1g and 3% Gg1 at the B3LYP/6-311G** level and about 87% Cg1, 11% G1g and 2% Gg1 at the MP2/6-311G** level for oxiranemethanol at 298.15 K. No conclusive evidence was obtained for the presence of high-energy form in the liquid phase of oxiranemethanol. The vibrational frequencies of oxiranemethanol in its three stable forms were computed at the B3LYP level and complete vibrational assignments were made for the lowest energy Cg1 form on basis of calculated and experimental data of the molecule.

Variable temperature (−55 to −145 °C) studies of the infrared spectra (400050 cm−1) of allyl alcohol (3-hydroxypropene), CH2CHCH2OH, dissolved in liquid krypton and/or liquid xenon have been carried out. The infrared spectra of the gas and solid have also been recorded. From these data four of the five possible stable conformers have been identified and their relative stabilities determined. The order of the conformers stabilities has been obtained experimentally where the first indicator is for the relative position of the OH group (C = cis or G = gauche) to the double bond (rotation around the CC bond) and the second one (t = trans, g = gauche, g′ = gauche′) is the relative position of the hydroxyl rotor, i.e. rotation around the CO bond. The enthalpy differences have been determined between the most stable Gg conformer and the second most stable rotamer, Cg, to be 135 ± 14 cm−1 (1.62 ± 0.1 kJ/mol), the third most stable conformer Ct, 260 ± 46 cm−1 (3.11 ± 0.6 kJ/mol), and the fourth most stable conformer Gt 337 ± 75 cm−1 (4.03 ± 0.9 kJ/mol). This experimentally determined order is consistent with the order of stability predicted by ab initio calculations Gg > Cg > Ct > Gt > Gg′. No evidence was obtained for the fifth conformer Gg′ which is predicted by most of the ab initio calculations to be less stable by an enthalpy difference more than 500 cm−1 than the Gg form. The percentage of each conformer at ambient temperature is estimated to be Gg(54 ± 2%), Cg(28 ± 2%), Ct(8 ± 2%) and Gt(11 ± 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 ab initio MP2(full)/6-31G(d) 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 Gg and Cg conformers along with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r0 parameters have been obtained for these two conformers. The determined heavy atom structural parameters for the Gg [Cg] conformer are: the distances(Å) CC = 1.343(3)[1.340(5)], CC = 1.499(3)[1.504(5)], CO = 1.428(3)[1.419(5)] and angles(o) CCC = 122.8(5)[124.7(5)], CCO = 112.1(5)[114.9(5)] and CCCO = 122.7(10)[5.63(5)]. Several of the CH distances are significantly different from the values previously reported from the earlier microwave data. The results are discussed and compared to the corresponding properties of some similar molecules.

The conformational equilibria of cyclopropylcarboxaldehyde and its 1-methyl, 1-fluoro and 1-chloro derivatives were investigated using ab initio calculations with 4–31G and 6–31G basis sets. The results indicate that cyclopropylcarboxaldehyde exhibits a cis←trans conformational equilibrium, with the cis form being the more stable conformer at room temperature. The conformational stability of the molecule was found to be dependent on the nature of the α-group substituent of the three-membered ring. For 1-halocyclopropylcarboxaldehyde the equilibrium is found to shift more towards the cis conformation, in which the dipolar repulsion between the C0 group and the halogen destabilizes the molecule in the trans conformation. In the case of 1-methylcyclopropylcarboxaldehyde, the trans conformation is predicted to be more stable than the cis conformer at ambient temperature. For the methyl group the energetically preferred conformation is predicted to have the hydrogen atom eclipsing the cyclopropane ring in a staggered configuration.

The conformational stability and the three rotor internal rotations in 3-chloro- and 3-bromo-1-propanols were investigated by DFT-B3LYP/6-311+G and ab initio MP2/6-311+G, MP3/6-311+G and MP4(SDTQ)//MP3/6-311+G levels of theory. On the calculated potential energy surface twelve distinct minima were located all of which were not predicted to have imaginary frequencies at the B3LYP level of theory. The calculated lowest energy minimum in the potential curves of both molecules was predicted to correspond to the Gauche-gauche-trans (Ggt) conformer in excellent agreement with earlier microwave and electron diffraction results. The equilibrium constants for the conformational interconversion of the two 3-halo-1-propanols were calculated at the B3LYP/6-311+G level of calculation and found to correspond to an equilibrium mixture of about 32% Ggt, 18% Ggg1, 13% Tgt, 8% Tgg and 8% Gtt conformations for 3-chloro-1-propanol and 34% Ggt, 15% Tgt, 13% Ggg1, 9% Tgg and 7% Gtt conformations for 3-bromo-1-propanol at 298.15K. The nature of the high energy conformations was verified by carrying out solvent experiments using formamide ( epsilon=109.5) and MP3 and MP4//MP3 calculations. The vibrational frequencies of each molecule in its three most stable forms were computed at the B3LYP level and complete vibrational assignments were made based on normal coordinate calculations and comparison with experimental data of the molecules.

The conformational equilibria of isopropylcarboxylic acid fluoride and chloride and isopropyl-carboxaldehyde were investigated by ab initio methods. The results indicate that the molecules exhibit trans ⇌ gauche conformational equilibrium. The gauche conformation was determined by all basis sets to be the most stable conformation for the molecules at room temperature. The structure optimization of the lowest-energy gauche conformer was carried out at the 3-21G, 4-31G and 6-31G levels of calculation. The calculated structural parameters of the molecules are compared with those obtained from microwave data for the corresponding cyclobutanes. The calculated differences in bond distances and angles are consistent with the open-chain analogue in isopropyl molecules.

The conformational stability and structure of cyclopentylcarboxaldehyde was studied using the ab initio method at the 4-31G and 6-31G levels. Energy optimization of all possible conformers of the molecule predicted the twist-gauche and the twist-trans to be the lowest energy conformers with both basis sets, with the twist-gauche being the most stable form at room temperature. Structure optimization of the twist-gauche conformer was carried out using the two extended basis sets, and the calculated geometrical parameters are compared with microwave data obtained for cyclobutylcarboxaldehyde in the equatorial-gauche conformation. The calculated differences in bond distances and angles are consistent with the less strained five-membered ring in the molecule.

The conformational analysis of cyclopentylcarboxylic acid fluoride was investigated by ab initio methods. The results indicate that, the molecule may exist in one of nine possible conformers, determined by three low-frequency anharmonic vibrations. The twist-gauche and the twist-trans conformers were determined, by all basis sets, to be the most stable conformations for the molecules with the twist-gauche being the lowest-energy conformers at room temperature. The structure optimization of the lowest energy twist-gauche conformer was carried out at the 3-21G, 4-31G and 6-31G levels of calculation. The calculated structural parameters are compared to those obtained from microwave data for cyclobutylcarboxylic acid fluoride in the equatorial -gauche conformation. The calculated differences in bond distances and angles are consistent with the lessstrained five-membered ring in cyclopentylcarboxylic acid fluoride.

The structural stability of sulfolane (tetrahydrothiophene1,1-dioxide) and 3-sulfolene (dihydrothiophene1,1-dioxide) was investigated by DFT-B3LYP and ab initio MP2 calculations with 6-311+G**) basis set. The calculated symmetric ring-puckering potential of 3-sulfolene at the B3LYP level is consistent with a flat minimum that corresponds to a planar ring but at the MP2 level with a double minimum with a low barrier of about 193 cal mol(-1) to ring planarity in reasonable agreement with experimental results. From the calculations at the two levels of theory sulfolane was predicted to exist predominantly in the twist conformation. The vibrational wavenumbers were calculated at the MP2/6-31G** level of theory and the potential energy distributions PED among the symmetry coordinates of the normal modes were computed for the low-energy structure of the molecules. Complete vibrational assignments were provided on the basis of the calculated PED values. The experimental infrared and Raman spectra of the two molecules were compared to the calculated ones.

The internal rotations in acrylic and methacrylic acids CH2=CX-COOH and their amides CH2=CX-CONH2 (X is H or CH3) were investigated by DFT-B3LYP calculations with 6-311+G** basis set. The potential energy curves were consistent with two minima that correspond to planar cis and trans conformation in the case of the acids (or cis and near-trans forms in the case of the amides). Acrylic acid and acrylamide were predicted to have the cis form as the low and predominant conformation of the molecules. In the case of the methacrylic acid and methacrylamide, the conformational relative stability was predicted to reverse as going from the acrylic to the metha compounds. The trans conformer in methacrylic acid or the near-trans in methacrylamide were predicted to be thermodynamically low energy structures of the molecules. The CCCO rotational barrier was calculated to vary from 4 to 6kcal/mol in the four molecules. The OCOH and OCNH torsional barriers were calculated to be about 13 and 22kcal/mol in the acids and the amides, respectively. The vibrational frequencies of methacrylic acid and methacrylamide were computed at the DFT-B3LYP/6-311+G** level and reliable vibrational assignments were made on the basis of normal coordinate analyses and comparison with experimental data of both molecules in their low energy conformations.

The structural stability of acetohydrazide CH3–CO–NH–NH2 was investigated by DFT-B3LYP and ab initio MP2 calculations with 6-311+G** basis set. The C–N rotational barrier in the molecule was calculated to be about 26 kcal/mol that suggested the planar sp2 nature of the nitrogen atom of the central NH moiety. The N atom of the terminal NH2 group was predicted to highly prefer the pyramidal sp3 structure with an inversion barrier of about 7–8 kcal/mol. The molecule was predicted to have a trans-syn (N–H bond is trans with respect to C O bond and NH2 moiety is syn to C–N bond) conformation as the lowest energy structure. The vibrational frequencies were computed at B3LYP level of theory and normal coordinate calculations were carried out for the trans-syn acetohydrazide. Complete vibrational assignments were made on the basis of normal coordinate analyses and experimental infrared and Raman data.

The conformational stability of 2,2,3,3-tetrafluoro-1-propanol was investigated by DFT-B3LYP/6-311+G** and ab initio MP2/6-311+G** calculations. The calculated potential energy curves of the molecule at DFT-B3LYP level were consistent with five distinct minima that correspond to gauche (−)– gauche – gauche ( G1gg ), trans – trans – gauche ( Ttg ), trans – gauche – gauche ( Tgg ), trans – gauche – gauche (−) ( Tgg1 ) and gauche (−)– gauche – trans ( G1gt ) conformers in the order of decreasing relative stability. The equilibrium constants for the conformational interconversion of 2,2,3,3-tetrafluoro-1-propanol were calculated and found to correspond to an equilibrium mixture of about 38% G1gg , 28% Ttg , 13% Tgg , 11% Tggt and 10% G1gt conformations at 298.15 K. The vibrational frequencies of 2,2,3,3,-tetrafluoro-1-propanol in its five stable forms were computed at B3LYP level and complete vibrational assignments were made based on normal coordinate calculations and comparison with experimental data of the molecule.

The structure and conformational stability of vinylsulfonamide CH2 CH SO2NH2 were investigated by DFT-B3LYP/6-311 + G** and ab initio MP2/6-311 + G** calculations. From the calculations the molecule was predicted to exist predominantly in the gauche-syn (vinyl group nearly eclipses one of the S O bonds and the NH2 and the SO2 moieties eclipse each other) conformation with the possibility of low abundance of the cis-syn and the gauche-anti forms. The asymmetric potential function for the internal rotation about C S bond was determined for the molecule. The vibrational frequencies were computed at DFT-B3LYP level for the gauche-syn conformer of the molecule and its d2(C2H3 SO2ND2) and d3(C2D3 SO2NH2) deuterated species. Normal coordinate calculations were then carried out and the potential energy distributions were calculated for the molecule.

The structure and conformational stability of vinyl selenonyl fluoride, chloride and bromide CH2=CH–SeO2X (X is F, Cl and Br) were investigated using density functional B3LYP/6-311+G** and ab initio MP2/6-311+G** calculations. From the calculations the molecules were predicted to exist only in the non-planar gauche conformation with the vinyl C=C group almost eclipsing one of the selenonyl Se=O bonds as a result of conjugation between the two moieties. Single-minimum potential scans were calculated at the DFT level for the molecules. The vibrational frequencies were computed using B3LYP/6-311+G**. Normal coordinate calculations were then carried out and potential energy distributions were calculated for the three molecules in the gauche conformation. Figure Potential function for the asymmetric torsion in vinyl selenonyl fluoride (dotted line), chloride (dashed line) and bromide (solid line) as determined at the DFT-B3LYP/6-311+G** level

The structural stability of (halocarbonyl)sulfenyl halides XCO-SX (X is F, Cl, and Br) was investigated by DFT-B3LYP and ab initio MP2 calculations using 6–311 + G** basis set. From the calculations the molecules were found to exist predominantly in the trans conformation (two halogen atoms are trans to each other). Full energy optimizations were carried out for the minima and the transition states (TS) at the two levels, from which the rotational barriers about CS bond in the three molecules were calculated to be about 12–13 kcal mol−1. The vibrational frequencies of (fluorocarbonyl)sulfenyl fluoride (FCO-SF), (chlorocarbonyl)-sulfenyl chloride (ClCO-SCl), and (bromocarbonyl)-sulfenyl bromide (BrCO-SBr) were computed at the DFT-B3LYP level and the vibrational assignments for the normal modes of the stable forms of the compounds were made on the basis of normal coordinate calculations and experimental data of the chloride.

The structural stability of halocarbonyl azides CXO-NNN (X=F, Cl and Br) was investigated by DFT and MP2 calculations using the 6-311++G** basis set. From the calculations, the molecules were found to have an s-cis--s-trans conformational equilibrium with cis being the lower -energy form. Full energy optimizations were carried out for the transition states and the minima at the B3LYP/6 -311++G** and MP2/6 -311++G** levels, from which the rotational barriers were calculated to be of the order 8-10 kcal x mol(-1). The vibrational frequencies were computed at the DFT -B3LYP level and the vibrational assignments for the normal modes of the stable conformers were made on the basis of normal coordinate calculations.

The conformational behavior and the structural stability of formyl fluoroketene, formyl chloroketene and formyl methylketene were investigated by utilizing quantum mechanical DFT calculations at B3LYP/6-31I + + G** and ab initio calculations at MP2/6-311 + + G** levels. The three molecules were predicted to have a planar s-cis s-trans conformational equilibrium. From the calculations, the direction of the conformational equilibrium was found to be dependent on the nature of the substituting group. In formyl haloketenes, the cis conformation, where the C=O group eclipses the ketenic group, was expected to be of lower energy than the trans conformer. In the case of formyl methylketene the conformational stability was reversed and the trans form (the aldehydic hydrogen eclipsing the ketenic group) was calculated to be about 2 kcal mol(-1) lower in energy than the cis form. The calculated cis-trans energy barrier was found to be in the order: fluoride (15.3 kcal mol(-1)) > chloride (13.1 kcal mol(-1)) > methyl (11.7 kcal mol(-1). Full optimization was performed at the ground and the transition states of the molecules. The vibrational frequencies for the stable conformers of the three ketenic systems were computed at the DFT-B3LYP level, and the zero-point corrections were included into the calculated rotational barriers. Complete vibrational assignments were made on the basis of both normal coordinate calculations and comparison with experimental results of similar molecules.

The structural stability of indole-3-acetic acid was investigated by DFT-B3LYP calculations with the 6-311G** basis set. From the calculations the gauche–gauche (gg) structure was predicted to be the second lowest energy minimum for the acid. It is energetically only 0.57 kcal/mol above the lowest conformer which is the trans–cis (tc) structure. A further stable conformer, however, highest in energy, is the trans–trans (tt) one, which is by 2.68 kcal/mol higher in energy than tc. The tc conformer upon full optimization turned a little bit away from real tc to a near tc (ntc) structure (defining torsional angles only changed by a few decigrades). However, the X-ray data indicate a structure in the solid, which is most similar to gg, stabilized by intermolecular eight ring hydrogen bonds. In the present DFT calculations such stabilizations cannot be accounted for, because the calculations treat only isolated molecules. To take such interactions into account at least dimers would have to be optimized. Therefore the vibrational frequencies of the gg conformer were computed at the B3LYP level of theory and tentative vibrational assignments were provided on the basis of normal coordinate analysis, normal mode calculations and experimental infrared and Raman data. However, some of the observed lines are obviously due to a small amount of the tc conformer present.

The conformational stability and vibrational infrared and Raman spectra of trichloroacetyl isocyanate (CCl3CONCO) were investigated by ab initio MP2 and density functional B3LYP calculations using the 6-311++G** basis set. From the potential energy scans of the internal rotations in both the halomethyl and the isocyanate rotors, the molecule was predicted to exist predominantly in the cis–cis conformation. The steric hindrance between the halomethyl group and the nitrogen lone-pair was found to favor the staggered configuration for the chlorine atom, while conjugation effects favor the planar configuration for the C=O and the NCO groups. Vibrational wavenumbers were computed for the molecule at the DFT-B3LYP/6-311++G** level. Normal coordinate calculations were carried out to obtain the potential energy distributions (PED) among the symmetry coordinates of the normal modes for the molecule. The theoretical vibrational assignments were compared with experimental ones and ratios of observed to calculated wavenumbers of about 0.97–1.04 were obtained.

The Raman spectra (3200–100 cm−1) of epifluorohydrin, OCH2CH(CH2F), in variable solvents, as well as that of the gas have been recorded and several of the bands due to the two less stable conformers have been identified. The variable solvent studies were inconclusive on the relative conformer stabilities. The conformational energy differences and optimized geometries for all three conformers have been obtained from ab initio calculations with the 3–21G, 4–31G and 6–31G* basis sets. A normal coordinate analysis has also been performed for each conformer with a force field determined from the 3–21G basis set. Assignment of the vibrational fundamentals observed in the Raman spectra of the fluid phases is proposed based on the normal coordinate calculations. In the liquid phase, one of the conformers with a large dipole moment predominates and it appears to be the gauche-I form which is the only one found in the solid. Utilizing the three rotational constants previously reported for each conformer, along with restricted relative distances for several of the structural parameters among the conformers from ab initio calculations, r0 structural parameters for the heavy atoms have been determined.

The structures and conformational stabilities of phenylphosphonic acid and phenylthiophosphonic acid were investigated using calculations mostly at DFT/6-311G** and ab initio MP2/6-311G** level. From the calculations the molecules were predicted to exist in a conformational equilibrium consisting of two conformers which as enantiomers have the same energy, but rather unexpected dihedral angles XPCC (X being O or S) which are not equal to zero. The antisymmetric potential function for the internal rotation was determined for each one of the molecules. In these functions the conformers with zero dihedral angles appear to be stable minima (also optimization converges to this), but the vibrational frequency for the torsion turned out to be imaginary, indicating that they are maxima with respect to this symmetry coordinate. Only optimization without any restrictions and starting from a non-zero torsional angle converged to a real minimum with such a geometry (“non-planar”). For that minimum structure infrared and Raman spectra were calculated, and those for phenylphosphonic acid were compared to experimental data, showing satisfactory agreement. This gives confidence to present the spectra of phenylthiophosphonic acid as a prediction. The rather low intensity of the OH bands in the experimental infrared spectrum (as compared to normal organic acids) indicates rather weak hydrogen bonding. Normal coordinate calculations were carried out, and potential energy distributions were calculated for the molecules in the non (near)-planar conformations providing a complete assignment of the vibrational modes to atomic motions in the molecules. From the rather low rotational barriers we conclude, in agreement with results from the literature (for other P=O compounds) based on localized orbitals that conjugation effects are absent - or at least negligible - as compared to electrostatic and steric ones in determining the structures of the stable conformers in the phenyl derivatives. The P=O (and also the P=S) bond is highly polarized according to our analysis of Mulliken populations. The polarization turned out to be smaller in the thiophosphonic acid due to the smaller electronegativity of sulfur as compared to oxygen.

The conformational stability and structure of cyclopentylcarboxaldehyde was studied using the ab initio method at the 4-31G and 6-31G levels. Energy optimization of all possible conformers of the molecule predicted the twist-gauche and the twist-trans to be the lowest energy conformers with both basis sets, with the twist-gauche being the most stable form at room temperature. Structure optimization of the twist-gauche conformer was carried out using the two extended basis sets, and the calculated geometrical parameters are compared with microwave data obtained for cyclobutylcarboxaldehyde in the equatorial-gauche conformation. The calculated differences in bond distances and angles are consistent with the less strained five-membered ring in the molecule.

The microwave spectrum of isopropyl carboxaldehyde has been investigated from 26.5 to 40 GHz. The a-type R-branch transitions have been assigned for both the low energy gauche and high energy trans conformers. For the ground vibrational state of the gauche conformer the rotational constants are A = 7494.39 ± 0.21, B = 4107.45 ± 0.01, and C = 2980.74 ± 0.01 MHz. The ground state rotational constants for the trans conformer are A = 7706.75 ± 0.35, B = 3736.52 ± 0.02, and C = 2815.15 ± 0.01 MHz. From an analysis of the Stark effect the respective dipole moment components were determined for each conformer. From a temperature study of the Raman spectrum of the gas, the value of ΔH has been experimentally determined.

The infrared (3500 to 30 cm−1) and Raman (3200 to 30 cm−1 spectra have been recorded for the gaseous and solid states of cyclobutylmethyl ketone, c-C4H7C(CH3)O. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. Evidence is presented based on the band contours in the infrared spectrum that the molecule exists predominantly in the equatorial-gauche conformation (the C(CH3)O group is in the equatorial position relative to the four-membered ring with the CO nearly eclipsing one of the CC bonds of the ring) in the gaseous and liquid states and exclusively in this conformation in the solid state. However, there is clear evidence in the Raman spectra of the fluid phases that a second conformer is present at ambient temperature and this conformer is believed to be the equatorial-trans where the CO is eclipsing the α-CH bond of the four-membered ring. The asymmetric torsion was observed as a very broad, structureless band centered at about 51 cm−1. From the relative intensities of the Raman lines of the liquid at 386 (equatorial-gauche) and 442 cm−1 (equatorial-trans) as a function of temperature, the enthalpy difference was found to be 480 ± 62 cm−1 (1.37 kcal mol−1) with the equatorial-gauche being more stable. A complete vibrational assignment is proposed based on infrared band contours, depolarization values, and group frequencies. The methyl torsion was tentatively assigned at 112 cm−1 from which a threefold barrier to internal rotation was calculated to have a value of 0.94 kcal mol−1 (331 cm−1). A few of the normal modes were observed to be split in the infrared spectrum of the solid which indicates that there are at least two molecules per primitive cell. These results are compared to similar quantities in some related molecules.

The microwave spectrum of cyclobutylcarboxylic acid fluoride, c‐C4H7CFO, has been recorded from 18.0 to 40.0 GHz. The a‐type R‐branch transitions have been observed and assigned for the ground and two vibrationally excited states of the asymmetric torsional mode as well as for the first vibrationally excited state of the ring puckering fundamental. The rotational constants were determined from the frequency fit to a rigid rotor model for the ground vibrational state to be: A=5467.35±0.03, B=1884.76±0.01, and C=1558.64±0.01 MHz. These constants are shown to be consistent with an equatorial‐gauche conformation (i.e., the CFO group is in the equatorial position relative to the ring and the C=O bond is eclipsing, or nearly so, one of the C–C bonds of the ring). From relative intensity measurements the frequency for the asymmetric torsion for this conformer is estimated to be 72±10 cm−1. From the Stark effect the dipole moment components were determined to be: ‖μa‖ =2.97±0.02, ‖μb‖ =0.84±0.01, ‖μc‖ =0.35±0.01,...

The microwave spectrum of cyclobutylcarboxylic acid chloride, c-C 4 H 7 C(O)Cl, has been recorded from 26.5 to 39.0 GHz and only a -type transitions were observed. The R -branch assignments have been made for the ground and first two excited states of the asymmetric torsional mode of the 35 Cl species. The ground state rotational constants determined from these assignments are consistent with an equatorial- gauche conformation and the values are: A = 4349.86 ± 0.17, B = 1414.78 ± 0.01, and C = 1148.24 ± 0.01 MHz. These values when used with a set of reasonably assumed heavy atom structural parameters are consistent with a dihedral angle of puckering of 20°. From the relative intensity measurements of the excited state transitions, the frequency of the asymmetric torsion was estimated to be 54 ± 10 cm −1 . From a comparison of the infrared spectrum of the gas (3500 to 30 cm −1 ) and the Raman spectrum of the liquid (3200 to 100 cm −1 ) to the corresponding spectra obtained for the solid a second conformation can be shown to be present in the fluid phases at ambient temperature. From variable temperature studies of the Raman spectrum of the liquid the energy difference between the preferred equatorial- gauche and high energy (assumed to be equatorial- trans ) conformations is determined to be 1.4 kcal mol −1 . The ring puckering fundamental has been assigned to a weak band observed in the far infrared spectrum of the solid at 153 cm −1 and this assignment in addition to the assignments made for the remainder of the vibrational spectra are compared to those for some similar molecules.