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The structural stability of mandelic acid and 3-phenyllactic acid was investigated by Density Functional B3LYP calculations with the 6-311G ∗∗ basis set. The two acids were predicted to have non-planar forms with cisoid α-hydroxy moiety with respect to the C O group as their lowest energy structures. From the SCRF = SMD calculations the stability of both acids is predicted to be significantly dependent on the dielectric constant of the solvent. The infrared and Raman spectra of l -mandelic and l -3-phenyllactic acids were compared to the corresponding solid state spectra of the dl -racemic mixtures. 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 1 H and 13 C NMR spectra of both acids were interpreted by experimental and DFT calculated chemical shifts of the two acids. The RMSD between experimental and theoretical 1 H and 13 C chemical shifts for mandelic acid is 0.961 and 6.119 ppm, whereas for 3-phenyllactic acid, it is 0.611 and 5.214 ppm, respectively.

The structure, vibrational and 1 H and 13 C NMR spectra of the local anesthetic drug lidocaine were investigated by the B3LYP/6-311G ∗∗ calculations. The molecule was predicted to have the non-planar cis (NCCN ∼ 0°) structures being about 2–6 kcal/mol lower in energy than the corresponding trans (NCCN ∼ 180°) forms. The calculated NCCN (9.6°) and CNCC (−132.2°) torsional angles were in a good qualitative agreement with the reported X-ray angles (3.1 and 13.0°, −102.67 and −77.9° , respectively, for H-bonded dimers). The Gibbs energy of solution of lidocaine in formamide, water, dimethylsulfoxide, acetonitrile, methanol, ethanol and chloroform solutions was estimated at the B3LYP level. The predicted affinity of lidocaine toward the alcohols, acetonitrile and chloroform solutions was in excellent agreement with the reported experimental solubility of the drug in organic solvents. The analysis of the observed vibrational spectra is consistent with the presence of lidocaine in only one conformation at room temperature. The 1 H and 13 C NMR spectra of lidocaine were interpreted by experimental and DFT calculated chemical shifts of the drug. The RMSD between experimental and theoretical 1 H and 13 C chemical shifts for lidocaine is 0.47 and 8.26 ppm, respectively.

The structural stability, vibrational, 1 H and 13 C NMR spectra of valeric and valproic acids were investigated by the B3LYP calculations with the 6-311G ** basis set. Valeric acid is predicted to exist predominantly in the planar cis form (80% abundance). Valproic acid is predicted to have an equilibrium mixture of 68% gauche - 1 and 32% gauche - 2 structures at 298.15 K. The spectral feature of the O H stretching mode in the infrared spectra of both acids suggests the presence of strong H-bonding in the condensed phase of valeric acid and weak H-bonding in the case of valproic acid. The harmonic and anharmonic 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. Not all of the calculated anharmonic wavenumbers showed a consistent trend with the observed wavenumbers. The 1 H and 13 C NMR spectra of both acids were interpreted by experimental and DFT calculated chemical shifts of the two acids. The RMSD between experimental and theoretical 1 H and 13 C chemical shifts for valeric acid is 1.8 and 3.8 ppm, whereas for valproic acid, it is 1.4 and 4.5 ppm, respectively.

We have calculated the vibrational spectra of isoamyl alcohol and of tert-amyl alcohol using Density Functional Theory (DFT) with the Becke-3 Lee Yang Parr (B3LYP) functional and a 6-311+G** atomic basis set. The energies of the conformers were also calculated with ab initio Perturbation Theory of second (MP2) and fourth order restricted to single, double and quadruple excitations (MP4 SDQ) with the same basis set. We found rather complicated equilibria of four conformations in each case, counting only those with appreciable abundancies. PED data were compared with GAUSSVIEW animations. The calculated wavenumbers agree rather well with the experimental ones when the gauche-trans conformer is assumed as the most important one for isoamyl alcohol, and the gauchegauche one for tert-amyl alcohol. However, some of the experimental bands had to be assigned also to other conformers, indicating their presence in the equilibrium mixture. Due to sterical reasons both the CO and the OH bonds appear to be weaker in the tertiary alcohol, considering the wavenumbers of the CO and OH bond stretching vibrations. The bond lengths point into the same direction, however, the OH bond in the tertiary alcohol is only slightly longer than that in the primary alcohol.

The complex conformational profile and the vibrational spectra of carbohydrazide (1,3-diaminourea) and thiocarbohydrazide were investigated by the DFT-B3LYP and ab initio MP2 and MP4(SDQ) levels of theory using the 6-311G ** basis set. The planar syn - cis - anti - trans structure of the two molecules with C s symmetry was predicted to have an imaginary frequency. The corresponding non-planar structure ( C 1 symmetry) was predicted to be the lowest energy structure and to have a positive real frequency as the real minima of the molecules. The stability of the non-planar structure agrees with the X-ray study of carbohydrazide where significant molecular distortions from planarity were reported for the lowest energy form as a result of strong intermolecular hydrogen bonding in the crystal. The vibrational wavenumbers of the lowest energy non-planar conformer of carbohydrazide and thiocarbohydrazide were computed at the B3LYP level and complete vibrational assignments were provided on the basis of the combined DFT-B3LYP results, normal coordinate calculations and experimental infrared and Raman data.

The possible conformations in the most stable structure of cyclophosphamide were investigated at the DFT-B3LYP and MP2/6-311G** levels of calculation. The axial structure is calculated to be lower in energy than the equatorial form of cyclophosphamide by 5 (MP2) to 6 (DFT) kcal mol-1 because of a very weak anomeric effect and weak conjugation. Further it is the same structure as the one found in the X-ray investigations both of the hydrate and of the anhydrous form. The computed DFT vibrational frequencies of the axial structure were used to analyze the infrared and Raman spectra using normal-coordinate calculations to provide reliable assignments to the normal modes of the molecule.

The structural stability of 1,3-diphenylurea and 1,3-diphenylacetone was investigated by the DFT-B3LYP and ab initio MP2 calculations using the 6-311G(**) basis set. From full energy optimization at the MP2 level of theory the near-planar cis-trans form of 1,3-diphenylurea was predicted to be about 1.5kcal/mol lower in energy than the X-ray near-planar cis-cis conformer. At the DFT-B3LYP level of theory the near-planar cis-trans and cis-cis forms were predicted to have a very comparable relative stability. The dibenzyl ketone was predicted by both levels to exist predominantly in a gauche-gauche conformation. On comparison the DFT near-planar structures of each of the two molecules were found to be consistent with the reported X-ray structures. The analysis of the infrared and Raman spectra of 1,3-diphenylurea suggests the presence of a second form at room temperature. The vibrational frequencies of the conformers most consistent with X-ray data of diphenylurea and dibenzyl ketone were computed at the B3LYP level and tentative vibrational assignments of their normal modes were provided on the basis of combined experimental and computed data.

We have studied the structural properties and vibrational spectra of Bisphenol S (4,4′-sulfonyldi-phenol) with the help of Density Functional Calculations using a 6-311G∗∗ basis set. In addition we recorded experimental spectra in our laboratory. With the help of the GaussView program we could provide a complete assignment of the vibrational lines to the calculated normal modes of the molecule. The calculated and experimental vibrational spectra agree rather well with each other and we found the properties of Bisphenol S to be rather similar to the sulfa drug dapsone studied earlier. Interestingly, the electron donating power of an OH aubstituent at a phenyl ring seems to be less effective than that of an NH2 (in dapsone) substituent, because in Bisphenol S the relative intensity of the ring breathing line in the Raman spectrum appears to be much larger than in dapsone. We reach this conclusion about the electron donating powers of susbtituents, because in an early study on chlorine substituted anilines we have found that the larger the electron density in the ring becomes, the more the relative intensity of that line diminishes.

The structural stability of 3,4-dihydroxyphenylacetic acid and 4-hydroxy-3-methoxyphenylacetic acid were investigated using the DFT-B3LYP and the ab initio MP2 methods together 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 the lowest energy conformer of the acids. Another very low lying conformer of the former acid in the gas phase seems to play no role in the condensed phase. 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 vibrational assignments were provided on the basis of combined theroretical and experimental infrared and Raman data of the molecules. As reported earlier in the case of chlorine substituted anilines, the ring breathing mode, which is expected to have a very high intensity in the Raman spectra of substituted benzenes, diminishes in intensity with increasing electron density in the ring (activation for electrophilic attacks).

The complex conformational behavior of 3-phenylprop-2-en-1-ol (cinnamyl alcohol) and its saturated analogue 3-phenyl-1-propanol were investigated at the DFT-B3LYP/6-311G ** , MP2 and MP4(SDQ) levels of theory. The unsaturated 3-phenylprop-2-en-1-ol was predicted to exist in Cg and Gg1 conformational mixture as a result of competitive conjugation and hyperconjugation interactions in the molecule. The saturated 3-phenyl-1-propanol was predicted to exist predominantly in a Ggg structure as a result of predominant steric hindrances in the alcohol. Only the one predominant form was identified in the infrared and Raman spectra of both alcohols. The excellent agreement between the calculated wavenumbers and the observed ones in the infrared and Raman spectra supports the conclusion that each of the two alcohols is present in one predominant form in the condensed phases. The vibrational frequencies of 3-phenylprop-2-en-1-ol and 3-phenyl-1-propanol in their lowest energy forms were computed at the B3LYP level and tentative vibrational assignments were provided on the basis of combined calculated and experimental data.

We have performed density functional calculations with the B3LYP functional and a 6-311G** basis set to obtain the vibrational spectra in harmonic approximation of the anti-leprosy drug Dapsone and the parent compound diphenylsulfone. Although the chemical difference between the two molecules is not that pronounced (Dapsone has amino groups in the para positions in the phenyl rings), Dapsone is an active drug, while to our knowledge diphenylsulfone shows no medical activity. We compared the theoretical results to experimental vibrational spectra found in the literature. With the help of the program GAUSSVIEW we were able to assign the experimentally found spectral lines to specific atomic motions. The remarkable difference between the two molecules, regarding their structural behavior, is that the drug Dapsone has a more flexible structure of the phenyl ring than the parent molecule has. This might contribute to a greater ability of the drug to fit into receptor sites in a cell membrane although one has to be well aware that this plays most propably only a minor role in the drug activity of Dapsone

Our interest in conjugation effects in substituted phenyl compounds has turned our attention to the highly reactive compounds phenyl ketene, azide, and isocyanate, which due to their reactivity are of utmost importance in organic synthesis. We performed local density functional theory (DFT) calculations using a 6-311G** basis set to calculate the structures and potential functions of the internal rotation. Further for the minimum structures we computed the vibrational infrared and Raman spectra of the three molecules. In order to confirm that DFT works rather well in these systems we performed the geometry optimizations also using ab initio Moeller–Plesset perturbation theory of second order (MP2) in the same basis set. As expected there exist just two minimum structures for each of the molecules which both correspond to planar structures and are identical due to the symmetry of the phenyl ring. The transitions states (TS) of the internal rotations are the perpendicular ones. We expect conjugation to play no major role in these molecules since extensive conjugation effects would imply a large reduction of the aromatic character of the phenyl ring which in turn would greatly destabilize the systems. However, although the rotational barriers appear to be rather small in these systems conjugation must play at least some role in stabilizing the planar ground state. As detailed later, the relative heights of the rotational barriers can all be explained naturally. Experimental vibrational spectra could be obtained only for phenyl isocyanate and azide, but not for the ketene because of the high reactivity of this molecule. Since in the former cases the calculated spectra agree fairly well with the measured ones, we present those of the other molecule as theoretical prediction, which could be useful to detect spectroscopically in a reaction mixture residual reactant. On the basis of potential energy distribution (PED) calculations we present a complete assignment of the vibrational lines to symmetry coordinates, where, for example, ring breathing must show up with rather large intensities in the Raman spectra of the molecules.

The structural stability of aniline, 2,4,6-trichloroaniline and 2,3,5,6-tetrachloroaniline was investigated by DFT-B3LYP and ab initio MP2 and MP4(SDQ) calculations with the 6-311G∗∗ basis set. From the calculations the three molecules were predicted to exist predominantly in a symmetric near-planar structure. The NH2 inversion barrier was estimated from the MP2/6-311G∗∗ level of theory to be about 9.6 kJ/mol for aniline, 5.9 kJ/mol for the trichloro and 4.6 kJ/mol for the tetrachloro derivatives. The line intensities of the ring breathing and the C Cl stretching modes were shown to have a great dependence on the number of chlorine atoms on the benzene ring. The relative change in Raman line intensity of the C Cl stretching mode was explained on the basis of the inductive effect of the chlorine atoms on the benzene ring. The vibrational frequencies were computed at the DFT-B3LYP level and the infrared and Raman spectra for each molecule were calculated. Complete vibrational assignments were made on the basis of normal coordinate analyses and potential energy distributions for the two chloroanilines.

As a first step in testing the possibility of correlation calculations of extended systems using localized orbitals, localization of the canonical Hartree-Fock orbitals of the four nucleotide bases was performed. The results show an excellent localization of the occupied orbitals and a rather good one for the virtuals (both for the σ- and π-orbitals). Interaction energy calculations of the stacked nucleotide bases show a few kcals/mol repulsion at the Hartree-Fock level, but they become attractive (again a few kcals/mol) if dispersion energy computed with the aid of London's formula is added. The internal charge transfer of ˜0.2 e from sugar to the nucleotide base (C, T, or A) found in large-scale ab initio homopolynucleotide calculations raises the question of whether there are free charge carriers in DNA B. Though in the Hartree-Fock level the answer is negative, one cannot rule out this possibility if, using a high-quality basis set, correlated band structures (using an unproved version of the electronic polaron model) will be computed. Interaction energy calculations between a polyglycine chain in different conformation and polynucleotide chains show a few kcals/mol attraction per base and glycine unit and a very small (a fraction of a kcal/mol) additional attraction due to the dispersion correction.

The structural stability and ring inversion of 2-chloro-1,3,2-dioxaphospholane 2-oxide and 2-sulfide were investigated by the DFT-B3LYP and the ab initio MP2 calculations with 6-311+G ∗∗ basis set. From the calculations the two molecules were predicted at the two levels of theory to exist predominantly in the twist conformation. The effect of the mutual interactions between the two oxygen atoms in the ring and the -PXCl moiety on the inversion of the five-membered hetero-ring of the two molecules and hence on determining their conformational stability was thoroughly explained. The repulsive interaction between the lone-pairs at the two in-ring oxygen atoms was concluded to be superior to the non-bonded overlap between the occupied lone-pair orbitals at the in-ring O atoms with the empty, antibonding orbitals of both the P Cl and the P X (X is O or S) bonds. The vibrational wavenumbers were calculated 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 structures and conformational stability of vinylphosphonic and vinylthiophosphonic acids were investigated using calculations mostly at DFT/6-311G** level and ab initio ones at MP2/6-311G** level. From the calculations the molecules were predicted to exist in a nonplanar near-cis (nc) to trans -gauche (tg) conformational equilibrium with nc (phosphonic oxygen or sulfur nearly eclipses the vinyl group) being the predominant conformer at ambient temperature, at least on the basis of Gibbs free energy (in the case of the thio compound the potential energy of tg is slightly lower than that of nc). The antisymmetric potential function for the internal rotation was determined for each one of the molecules. However, when starting optimizations from a structure too close to the full cis symmetry, the cis form results as an extremum on the potential hyper-surface. Consequently cis was reported in the literature several times as minimum. However, when we calculated the vibrational frequencies the cis form turned out to have an imaginary frequency, and thus cis is a local maximum along one of the normal coordinates. The true minimum, with only real frequencies, actually is an nc one with a very low barrier to rotation. The rotational angle out of cis is rather small; however, HOPX dihedral angles are quite away from real cis. This holds true not only with DFT but also with MP2. The vibrational frequencies were computed and the spectra were plotted as a prediction. Normal coordinate calculations were carried out and potential energy distributions were calculated for the molecules in the nc and the tg conformations. From our results and their analysis, we conclude, in agreement with results from the literature based on localized orbitals, that conjugation effects are absent – or at least negligible – as compared with electrostatic or steric ones in determining the structures of the stable conformers in the vinyl derivatives.

The conformational stability of 2,2,3,3,3-pentafluoro-1-propanol was investigated by the DFT-B3LYP/6-311+G** and the ab initio MP2/6-311+G** calculations. The calculated potential energy curves of 2,2,3,3,3-pentafluoro-1-propanol at both levels of theory were consistent with three distinct minima that correspond to Trans-gauche-gauche (Tgg), trans-trans-gauche (Ttg) and trans-gauche-gauche(-) (Tgg1) conformers in the order of decreasing relative stability. The equilibrium constants for the conformational interconversion of 2,2,3,3,3-pentafluoro-1-propanol were calculated and found to correspond to an equilibrium mixture of about 46% Tgg, 43% Ttg and 11% Tgg1 conformations at 298.15K. The calculated (%Ttg/%Tgg) ratio of 0.93 is consistent with the 0.85 ratio of the observed intensities of the 772 and 794 cm(-1) lines in the Raman spectrum of the liquid. The nature of the high energy Ttg conformation was verified by solvent experiments using formamide (epsilon = 109.5) and acetonitrile (epsilon = 37). The vibrational frequencies of the molecule in its 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 ring puckering in the four-membered 3-chloro-1,3-thiaphosphetane 3-oxide, its 3-sulfide derivative and 1,3-dithietane 1,1-dioxide hetero rings were investigated by DFT and ab initio MP2 calculations using a 6-311+G ∗∗ basis set. From the calculations at both levels of theory the asymmetric ring puckering potential energy curves in the two phosphetanes were consistent with an almost single minimum that corresponds to a pseudo - axial configuration (P Cl bond is in axial position) with a puckering angle of about 15–20°. In the case of the dithietane dioxide the symmetric ring puckering potential was consistent at the B3LYP level with a flat minimum that corresponds to a planar ring but at the MP2 level with a double minimum with a very low barrier of about 185 cal/mol to ring planarity. The potential functions that describe the ring puckering in the three molecules were derived at the B3LYP/6-311+G ∗∗ and the MP2/6-311+G ∗∗ levels of calculations. The vibrational wavenumbers were calculated and the potential energy distributions PED among the symmetry coordinates of the normal modes were computed for the stable conformers of each 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 phosphetane molecules were simulated and compared to the calculated ones.

The structure and conformational stability of dichloro(vinyl)phosphine and dichloro(phenyl)- phosphine oxides and sulfides were investigated using calculations at the DFT/6-311G** and ab initio ones at the MP2/6-311G** level. We know from our previous results that the addition of diffuse functions to a valence triple zeta basis with polarization functions might lead to an unbalanced basis, which performs even worse than the smaller basis without diffuse functions, as it is the case for the 6-311++G** basis set in the Gaussian program. For large energy differences between conformers, DFT works very well, in some cases even better than MP3 or MP4. The vinyl derivatives were predicted to exist in a cis/gauche conformational equilibrium with cis (the PX bond, X being oxygen or sulfur eclipses the vinyl groups) being the predominant conformer at ambient temperature. In the phenyl case case the two planar forms are equivalent minima. The asymmetric potential function for the internal rotation was determined for each of the molecules. The vibrational frequencies were computed and the spectra, where possible, were compared with the experimental ones. Normal coordinate calculations were carried out and potential energy distributions were calculated for the molecules in the cis and gauche conformations (in the vinyl case, planar one for phenyl), providing a complete assignment of the vibrational lines to symmetry coordinates in the molecules. From our results and their analysis we conclude, in agreement with literature results based on localized orbitals, that conjugation effects are absent - or at least negligible - as compared with electrostatic ones in determining the structures of the stable conformers in both the vinyl and the phenyl derivatives. The P-O bond should be a highly polarized triple bond, as confirmed by analysis of Mulliken populations. The polarization turned out to be much less in the sufides due to the much smaller electronegativity of sulfur as compared with oxygen.

The conformational stability and the three rotor internal rotations in 3-fluoro-1-propanol were investigated by the DFT-B3LYP/6-311 + G** and the ab initio MP2/6-311 + G** levels of theory. The calculated potential energy curves of the molecule at both levels of theory were consistent with complex conformational equilibria of about 12 minima, all of which were predicted to have real frequencies at both the B3LYP and the MP2 levels. The lowest energy minimum in the potential curves of 3-fluoro-1-propanol was predicted to correspond to the Gauche–gauche–trans (Ggt) conformer in excellent agreement with microwave and electron diffraction results. The equilibrium constants for the conformational interconversion of the molecule were calculated and found to correspond to an equilibrium mixture of about 33% Ggt, 14% Ggg1 and 13% Gg1g and about 43% Ggt, 12% Ggg1 and 10% Gg1g distribution by the B3LYP/6-311 + G** and the MP2/6-311 + G** calculations, respectively, at 298.15 K. The vibrational frequencies of each molecule in its three 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.

In recent literature it was reported that the valence triple zeta basis set augmented by polarization functions is not too reliable for vinyl monohalo- and dihalomethanes and -silanes, the halogen being fluorine and chlorine. The major conclusion was that a valence triple zeta basis is too small to be augmented by polarization functions in a balanced way, at least on vinylmonofluoromethane. Thus we decided to apply the 6-311++G** basis set to the complete series of methanes, silanes and germanes (the latter ones are just added for completeness because no experimental data are available for them and, moreover, we published them already previously) and to compare the results to experimental data available in the literature to see whether the failures of this basis set show up in the complete series of molecules. In the literature we found five such molecules and the information which of the conformers is the most stable. Indeed we found that predictions on the relative stability of conformers in those systems with this basis set and MP2 as well as DFT are with a 60:40 chance, three being correct predictions and two being incorrect ones out of the five. However, since the energy differences are rather small in these systems and due to the fact that - as a consequence of twofold degeneracy of the gauche conformer on the potential curve of the torsional vibration - the abundances of the conformers in equilibrium do not change too much, we decided to calculate also vibrational spectra for three examples and to compare them also to experiment. It is reported that besides the failures in total energy (we have chosen two examples where predictions of the nature of the stable conformer are correct, and one where it is not), the vibrational spectra are rather well reproduced, especially when experimental energies are used to calculate abundances in equilibrium in the case where the prediction of the stable conformer failed.

The structure, vibrational and NMR spectra of the local anesthetic drug lidocaine hydrochloride monohydrate salt were investigated by B3LYP/6-311G(∗∗) calculations. The lidocaine·HCl·H2O salt is predicted to have the gauche structure as the predominant form at ambient temperature with NCCN and CNCC torsional angles of 110° and -123° as compared to 10° and -64°, respectively in the base lidocaine. The repulsive interaction between the two N-H bonds destabilized the gauche structure of lidocaine·HCl·H2O salt. The analysis of the observed vibrational spectra is consistent with the presence of the lidocaine salt in only one gauche conformation at room temperature. The (1)H and (13)C NMR spectra of lidocaine·HCl·H2O were interpreted by experimental and DFT calculated chemical shifts of the lidocaine salt. The RMSD between experimental and theoretical (1)H and (13)C chemical shifts for lidocaine·HCl·H2O is 2.32 and 8.21ppm, respectively.

We studied the ring puckering potential in thietane-1,1-dioxide with different methods, using a suitable basis set, 6-311+G**. We obtained a barrier to ring puckering of 153 cal/mol with the DFT/B3LYP (Becke3 exchange–Lee, Yang, Parr correlation functional) method, ∼60% too small compared with experiment. However, using MP2, MP3, and MP4 we obtained values around 200% too large. The MP series turned out to converge far too slowly to the experimental barrier value, showing no sign of convergence even at MP4, while higher orders are out of our reach for such a system. Obviously, of all methods used, DFT worked best despite some shortcomings. The barrier corresponds to 77 K; thus, there should be rapid interconversion over the barrier at room temperature, a phenomenon actually observed at room temperature, the measured barrier corresponding to 201 K. Thus, we decided to use the DFT/6-311+G** calculations to predict reasonable vibrational spectra and assignments of the lines found. The ring puckering mode is found at 80 cm−1 in DFT in good agreement with the experimental value of 78.3 cm−1 for this vibration. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Wolfgang Fo rner*, Hassan M. Badawi, and Zaki S. Seddigi Department of Chemistry, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia *Author to whom correspondence should be addressed; e-mail: forner@kfupm.edu.sa Received: 7 March 2005 / Accepted: 27 March 2005 / Published: 4 October 2005 Abstract: The structural stability of haloselenonyl azides was investigated by quantum mechanical Moller-Plesset perturbation theory of second order and density functional theory calculations. The 6-311+G** basis set was used to include polarization and diffuse functions in the calculations at the DFT-B3LYP level. The potential scans for the rotation of the -NNN rotor were calculated and found to be consistent with a single minimum that corresponds to a gauche conformation (-NNN moiety nearly eclipses one of the two selenonyl Se=O bonds) for the three halogens at ambient temperature. The structural parameters for the minima calculated by MP2 and DFT turned out to be very similar. The vibrational modes, infrared and Raman intensities as well as depolarization ratios were calculated at DFT-B3LYP/6-311+G** level for the three molecules in their gauche conformations. The potential energy distributions among symmetry coordinates of the normal modes of the mo-lecules in their gauche conformation were then computed from normal coordinate analyses. Keywords: MP2 and DFT calculations; structural stability; vibrational modes and assign-ments; haloselenonyl azides. Introduction The conformational behavior of fluorosulfonyl isocyanate F-SO

The conformational and structural stabilities of nitrosoethylene CH2=CH–N=O, chloronitrosoethylene CH2=CCl–N=O, and Dichloronitrosoethylene CCl2=CH–N=O were investigated by ab initio Moeller–Plesset perturbation theory of second order (MP2) calculations using the 6−311+G** basis set to include electron correlation. From the calculations all three were predicted to exist predominantly in the planar trans structure (C=C and N=O bonds are trans to each other) with high trans-cis rotational barriers of about 9 kcal mol−1 as a result of pronounced conjugation between C=C and N=O bonds. The vibrational frequencies were computed for the three molecules, and also the d 1 and d 2 deuterated variants for the parent molecule at the MP2 level. Normal coordinate analyses were carried out and the potential energy distributions (PED), among the symmetry coordinates of the normal modes of the molecule were computed. Complete vibrational assignments were made on the basis of normal coordinate analyses for the molecules. The two chlorinated derivatives of nitrosoethylene were also investigated in the same way. As expected, we then find high Raman and infrared intensities in all modes that contain a high content of chlorine movements because vibrations of C–Cl bonds lead to large changes in polarizability, as well as to a large change in dipole moment. However, modes involving double bonds also have quite large intensities. An appreciable number of modes in these molecules are more or less pure symmetry coordinates.

The conformational and structural stability of monofluoro-nitrosoethylene CH 2 CF–N O, and difluoro-nitrosoethylene CF 2 CH–N O were investigated by ab initio Moller-Plesset Perturbation Theory of second order (MP2) calculations using the 6-311+G** basis set to include electron correlation. From the calculations the two molecules were predicted to exist predominantly in the planar trans structure with high trans – cis rotational barriers of about 10–11 kcal/mol as a result of pronounced conjugation between C C and N O bonds. The differences in the rotational barriers when going from the parent molecule, nitrosoethylene, to its fluorinated derivatives are explained on the basis of electronegativity. The vibrational frequencies were computed for the two molecules at the MP2 level. Normal coordinate analyses were carried out and the potential energy distributions, PED, among the symmetry coordinates of the normal modes of the molecules were computed. Complete vibrational assignments were made on the basis of normal coordinate analyses for the molecules. As expected we find there low Raman and appreciable infrared intensities in all modes which contain a high content of fluorine movements because vibrations of C–F bonds lead to a small change in polarizability, but to a large change in dipole moment. However, also modes involving double bonds have quite large intensities in both spectra. An appreciable number of modes in these molecules are composed of relatively large numbers of symmetry coordinates, i.e. they are highly mixed.

We have derived earlier the rather lengthy formalism for time simulations in poly(p-phenylene), treating the rings as semirigid rotors. To this end the Euler-Lagrange formalism had to be used. As a first step we intended to parametrize the simplified Hückel-type hamiltonian on the basis of density functional theory (DFT) calculations on some dimeric model systems. The results of this attempt are reported here. However, calculations on much longer chains, containing up to 200 rings, show a clear tendency of our model to favor the quinoid B-phase structure over the aromatic one. Further, in doubly charged chains, the charge tends to remain unseparated and to be completely delocalized over virtually the complete part of the chain, that is in B-phase conformation. The bipolaron width turns out to be extremely small, of about 10 rings in a chain having 200 rings. This is rather unexpected and interpreted as a shortcoming of the Hückel-type nature of the hamiltonian. The reason is that in the Hückel-type model the two electrons, taken away to charge the chain, are from the same orbital, and thus charge separation is more difficult, leading, in this case, only to a delocalization, keeping the bipolaron small. We assume, that in line with Prof. Paldus' work, the inclusion of direct electron-electron interactions in the form of a Pariser-Parr-Pople (PPP) type model could overcome this difficulty. The treatment has to be done, probably, in an open shell form to make possible spin separation, if necessary. Care has to be taken for spin contaminations in such treatments and possibly even the explicit inclusion of electron correlation might be necessary. In this paper we report our model which was derived in detail in a previous paper. Then we discuss the parametrization attempts and our results on longer chains. In conclusion our suggestion is that a PPP type model must be used at least to allow for bipolaron calculations and confinement of the two like charges. Such calculations would be the content of a forthcoming paper.

The conformational stability of azidoketene OCCH–NNN was investigated by Density Functional B3LYP and ab initio second-, third- and fourth-order Moller Plesset calculations with the 6-311+G** basis set. Azidoketene was predicted at the DFT-B3LYP level to have the s-cis⇔s-trans conformational equilibrium with the s-trans (O=C=C and NNN moieties are trans to each other) being the predominant low energy form, while at the MP2, MP3 and MP4 levels to have the gauche (O=C=C and NNN moieties are near trans to each other) form as the minimum in the potential curve of the molecule. From full structural optimization of the ground and transition states at the DFT-B3LYP level, the trans–cis barrier was calculated to be about 2 kcal/mol. The vibrational frequencies for the cis and trans conformations were computed for the gauche form at the MP2 level. The calculated infrared and Raman spectra of the molecule were plotted. Complete vibrational assignments were provided on the basis of normal coordinate calculations for the molecule.

We have derived the rather lengthy formalism for time simulations in polyparaphenylene, treating the rings as semi-rigid rotors. To this end an Euler–Lagrange formalism was used. As one of the model systems to parametrize a Pariser–Parr–Pople (PPP) Hamiltonian, we have chosen an asymmetric cationic dimeric system in our model of the geometry, which allows the rings to move between aromatic A-phase and quinoidal B-phase structures. A cation was chosen, in order to avoid the presence of an unpaired spin in the model. Further, anionic models would require more extended basis sets in our Density Functional (DFT) calculations. The asymmetric structure of the model allows to scan the total potential surface between aromatic and quinoidal structures. The cation shows only one minimum on the quinoidal side. To make sure that this is not only due to the charge, around the expected minima, we also performed calculations on a quinoidal B-phase model dimer and on an aromatic A-phase model dimer (biphenyle). However, since our geometrical model is based on the central unit of the trimer, we do not find double minimum potentials in dimers, which consist of just two terminal units. But the potentials still can serve as the basis of a parameterization of the model Hamiltonian for the π-electrons, while in longer chains one has to replace the terminal units by ones which have a more appropriate geometry. The calculated DFT potentials will be compared in a forthcoming paper with the corresponding PPP potentials in order to allow a parameterization of the PPP Hamiltonian and subsequently bipolaron structure optimizations as well as simulations of the dynamics of the system. The formalism needed to this end is derived and outlined in this paper in detail. It is currently under programming.

The structure of 3,5-dichlorophenylcyanamide c-C6H3Cl2–NHCN was investigated by DFT-B3LYP and ab initio MP2 calculations with the 6-311+G** basis set. The planar to perpendicular rotational barrier was calculated to be of about 5 kcal mol−1 at both levels of calculation. The stability of the planar structure of the molecule was explained on the basis of conjugation effects between the cyanamide–NHCN moiety and the phenyl c-C6H5 ring in agreement with earlier NMR results. The CNC and the HNC bond angles were calculated to be about 120° especially by MP2 calculation, which is consistent with sp 2 (planar –NH–CN group) and not sp 3 (pyramidal –NH–CN group) structure. The vibrational frequencies of the d0, d1 and d3 species of 3,5-dichlorophenylcyanamide and the potential energy distributions among symmetry coordinates of the normal modes of the parent molecule were computed at the DFT-B3LYP level. The calculated infrared and Raman spectra of the molecule were plotted. Complete vibrational assignments were made on the basis of isotopic substitution and normal coordinate calculations. Figure Potential curves for the internal rotation in 3,5-dichlorophenylcyanamide as determined by DFT-B3LYP/6-311+G** (solid) and MP2/6-311+G** (dotted) calculations

We have calculated infrared and Raman spectra of biphenyle, the aromatic A-phase dimer and of a quinoidal B-phase dimer of polyparaphenylene at Density Functional Theory (DFT) level using a Becke-3 exchange and a Lee-Yang-Parr correlation functional (B3LYP). We found in both types of spectra features typical for the two phases, which offer the possibility to detect the presence of a quinoidal chain segment in conducting samples of the polymer. Together with more sophisticated future calculations on longer oligomers and on higher level of calculation, it should be possible to gauge intensity ratios in the spectra theoretically so that bipolarons could be found and their width be estimated spectroscopically. This is due to the fact that usually only within a bipolaron the B-phase is fully formed, which is not the case in a singly charged polaron. Calculations on neutral model molecules, while bipolarons are doubly charged, can be used for the purpose because in bipolarons the two like charges are usually quite localized in the parts of the bipolaron where the phase is changed from A to B on one side of the bipolaron or from B to A on the other side. Further, we found very interesting features useful for bipolaron detection, which are connected with mostly intra-ring motions of the atoms and thus the use of dimers, as model seems to be justified. For example, the Raman spectra of the B-phase show lines ten times more intense than the aromatic phase, which are due to ring breathing and ring deformations. To obtain quantitative informations about the atomic motions involved in each line in the spectra, in addition the Potential Energy Distribution of each normal mode among symmetry coordinates was calculated.

The structure of phenylcyanamide c-C 6 H 5 –NHCN was investigated by DFT-B3LYP and ab initio MP2 calculations with 6-311+G** basis set. The planar to the perpendicular rotational barrier was calculated to be of about 4 kcal/mol by both levels of calculation. The stability of the planar structure of phenylcyanamide was explained on the basis of conjugation effects between the cyanamide–NHCN moiety and the phenyl c-C 6 H 5 ring in agreement with earlier NMR results. The CNC and the HNC bond angles were calculated to be about 120° especially by MP2 calculation that is consistent with sp2 ( planar –NH–CN group) and not sp3 ( pyramidal –NH–CN group) structure. The vibrational frequencies of the d 0 -, d 1 - and d 5 -phenylcyanamide and the potential energy distributions among symmetry coordinates of the normal modes of the parent molecule were computed at the DFT-B3LYP level. The calculated infrared and Raman spectra of the molecule were plotted. Complete vibrational assignments were made on the basis of isotopic substitution and normal coordinate calculations.

The conformational and structural stability of nitrosoazide NNN-N=O and nitroazide NNN-NO2 were investigated by DFT-B3LYP and ab initio MP2 calculations with 6-311++G** basis set. From the calculations, nitrosoazide was predicted to exist predominantly in the planar trans (NNN and N=O groups are trans to each other) structure with high trans-cis rotational barrier of about 11 kcal mol-1 as a result of pronounced conjugation between the azide group and the N=O bond. The NO2 rotational barrier in nitroazide was predicted from the symmetric potential function to be of about 7 kcal mol-1. The vibrational frequencies were calculated at the DFT-B3LYP level and the infrared and Raman spectra of the cis-trans mixture were plotted. Complete vibrational assignments were made on the basis of normal coordinate calculations for the stable conformers of both molecules. For nitrosoazide, the calculated wavenumbers were compared to the corresponding experimental values obtained from early reported Raman spectrum of the molecule.

We have studied vibrational infrared (IR) and Raman spectra of several organic ring compounds. These include first cyclobutanecarboxaldehyde (CBA) and cyclohexanecarboxaldehyde (CHA) using density functional theory (DFT) together with a Becke-3 exchange and Lee–Yang–Parr correlation (B3LYP) functional. DFT in these systems gives a stable and low-energy gauche-axial (g-ax) conformer for both of the molecules together with the experimentally found equatorial conformations. However, on the basis of comparison of the theoretical vibrational spectra of the conformational mixture at 300 K with experimental spectra, the presence of the g-ax conformer could be ruled out because it would yield extra lines not present in the corresponding experimental spectra. Thus, we concluded that DFT yields properties related to energy derivatives such as minimum geometries and vibrational spectra in fair agreement with experiment, while total energy differencies are not trustworthy when they are small, especially if small linear dependencies in the atomic basis sets applied appear. However, in that case we found that Moller–Plesset perturbation theory of second order (MP2) applied to CBA has the same problems and even does not predict the absolute minimum on the potential hypersurface correctly. Therefore, we applied DFT also to biphenyle (BP) and terphenyle (TP). Again, in BP and TP DFT turned out to yield reliable structures that as far as the important torsional angle between the rings is concerned are even better than MP2. Also, the vibrational spectra are in fair agreement with experimental ones. In BP restricted Hartree–Fock (RHF), DFT and experimental IR spectra all show two intense absorptions around 700 cm−1, while MP2 gives only one intense absorption band in this region. DFT also reproduces the position of these two lines well. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004

The conformational behavior of 1-halovinyl azides CH2=CX-NNN (X=F, Cl and Br) were investigated by DFT-B3LYP and ab initio MP2 calculations with the 6-311++G** basis set. The molecules were predicted to exist predominantly in the trans (the vinyl CH2=CH- and the azide -NNN groups are trans to each other) conformation. The relative energy between cis and trans were calculated to decrease in order: bromidechloridefluoride. Full optimization was performed at the ground and transition states in the molecule at both MP2 and B3LYP levels. The barrier to internal rotation around the C-N single bond in the three molecules was calculated to be about 4-5 kcal mol(-1). The vibrational frequencies were computed at the DFT-B3LYP level and the calculated infrared and Raman spectra of the cis- trans mixture of the three molecules were plotted. Complete vibrational assignments were made on the basis of normal coordinate calculations for both stable conformers of the three molecules.

The conformational behavior of 2,2-dihalovinyl aizdes CX 2 CH–NNN (X is F and Cl) were investigated by DFT-B3LYP and ab initio MP2 calculations with 6-311++G** basis set. The two molecules were predicted to exist predominantly in the trans (the vinyl CX 2 CH– and the azide –NNN groups are trans to each other) conformation. The relative energy between cis and trans were calculated to decrease in the order: dichloride>difluoride. Full optimization was performed at the ground and transition states in the molecules at both MP2 and B3LYP levels. The barrier to internal rotation around the C–N single bond was calculated to be 3.719 and 5.171 kcal/mol in the difluoride and the dichloride, respectively. The vibrational frequencies were computed at the DFT-B3LYP level and the calculated infrared and Raman spectra of the cis – trans mixture of the two molecules were plotted. Complete vibrational assignments were made on the basis of normal coordinate calculations for both stable conformers of the two molecules.

The structural stability of halosulfonyl isocyanate X–SO 2 –NCO and haloselenonyl isocyanate X–SeO 2 –NCO (X is F and Cl) was investigated by density functional DFT-B3LYP/6-311+G** and ab initio MP2/6-311+G** calculations. The potential scans for the rotation of the –NCO rotor were calculated and found to be consistent with a single minimum that corresponds to a gauche conformation (–NCO moiety nearly eclipses one of the two MO bonds). The vibrational frequencies, infrared intensities as well as depolarization ratios were calculated at DFT-B3LYP/6-311+G** level for the two selenides at their gauche conformation. The potential energy distributions among symmetry coordinates of the normal modes of both F–SeO 2 –NCO and Cl–SeO 2 –NCO in their gauche conformation were then computed from normal coordinate analyses.

The structure of the CH 2 CH–NCX (X=O, S and Se) series of compounds was optimized at ab initio MP2/6-311+G** level of calculation. Vinyl isocyanate, vinyl isothiocyanate and vinyl isoselenocyanate were predicted to exist in a mixture of the cis (the NCX group eclipses the vinyl group) and the trans conformations with the latter being the predominant form at ambient temperature. The size of the rotational barrier of the internal rotations around C–N bond was predicted to decrease in the order: isocyante>isothiocyanate>isoselenocyanate. The vibrational wavenumbers, IR intensities, and Raman activities were calculated at MP2/6-311+G** level for the molecules in their cis and trans conformations. Normal coordinate calculations were then carried out and potential energy distributions were calculated for each of the three molecules in their stable forms. The vibrational infrared and Raman spectra were plotted for the mixture of two stable conformations of the molecules.

The structural stability of fluoro- and chlorosulfonyl azides was investigated by quantum mechanical DFT and ab initio MP2 calculations. The 6-311++G** basis set was employed to include polarization and diffuse functions in the calculation at DFT-B3LYP and MP2 levels. The potential scans for the rotation of the –NNN rotor were calculated and found to be consistent with single minimum that corresponds to gauche conformation (–NNN moiety nearly eclipse one of the two sulfonyl SO bonds) for both the fluoride and the chloride at ambient temperature. The vibrational frequencies, infrared intensities as well as depolarization ratios were calculated at DFT-B3LYP/6-311++G** level for the two molecules at their gauche conformation. The potential energy distributions among symmetry coordinates of the normal modes of the molecules in their gauche conformation were then computed from normal coordinate analyses.

The conformational stability and vibrational infrared and Raman spectra of chloroacetyl isocyanate (CH2ClCONCO) 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 of both the halomethyl and the isocyanate rotors, chloroacetyl isocyanate was predicted to exist predominantly in a mixture of the cis-cis (chlorine atom and NCO group eclipse C=O bond) and the gauche-cis (one hydrogen atom and NCO group eclipse C=O bond) conformations with a comparable relative stability. The vibrational wavenumbers of each of the two conformers of the molecule were computed at 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 each of the stable conformers of chloroacetyl isocyanate. The theoretical vibrational assignments are compared with experimental ones and a ratio of observed/calculated wavenumbers of about 0.97-1.04 was obtained.

We applied density functional theory (DFT) to biphenyle (BP) and terphenyle (TP) and also MP2 (Moller–Plesset perturbation theory of 2nd order) to BP. Further, we also investigated the corresponding model molecules for the quinoidal B phase. These are the molecules of interest for us, because they are oligomers of polyparaphenylene (PP) in its aromatic (A-phase) and quinoidal (B-phase) form. This polymer is of importance because of its blue light emission. In BP and TP, DFT turned out to yield reliable structures, which as far as the important torsional angle between the rings is concerned, even better than MP2. Also the vibrational spectra are in fair agreement with experimental ones. In BP, restricted Hartree Fock (RHF), DFT and experimental IR spectra all show two very intense absorptions around 700 cm−1, while MP2 gives only one intense absorption band in this region. DFT also reproduces the position of these two lines rather well. Superposition of the vibrational IR spectra of terphenyle and of the B-phase model molecule reveals that features could be detected in the spectra of the doped polymer which would indicate the presence of B-phase, and therefore the presence of bipolarons in the doped material. Further, this might lead the way to an experimental estimate of the width of such bipolarons, if present at all.

We present explicitly and in detail the LSD (local spin density approximation) and LSD-SIC (self interaction correction) formalisms for the Hubbard and the PPP (Pariser–Parr–Pople) model for the π-electrons. The basic formalism (besides the actual expressions for the matrix elements of the potentials) can be used also in ab initio applications. Our numerical results for polyparaphenylene indicate that the SIC method meets severe convergency problems for these models, because due to the ZDO (zero differential overlap) approximation the matrix elements of the potential appear only on the diagonal of the LDA (local density approximation) matrix. Since due to symmetry in polyparaphenylene the densities at each site are equal (in LDA for the Hubbard case) this has the consequence that the band structure is the same as the one obtained from the one-electron part of the matrix. The same behaviour occurs in PPP-LDA, because there in the course of the iterations the densities converge in such a way, that again the electron–electron interaction contributions are the same for all diagonal elements. However, the LDA-SIC total energies as function of a geometry parameter are in good agreement with correlated Hubbard and PPP results.

The structural parameters and energies of the trans -equatorial and gauche -equatorial conformers of cyclobutanecarboxaldehyde c -C 4 H 7 –CHO were investigated by quantum mechanical DFT-B3LYP (Density Functional Theory-Becke 3 exchange and Lee-Yang–Parr correlation functional) calculations using 6-311G ∗∗ basis set. The potential functions for the CHO asymmetric torsion in the equatorial molecule and for the ring puckering inversion were derived. The vibrational wavenumbers were calculated and the potential energy distributions PED among the symmetry coordinates of the normal modes were computed for the two stable conformers of the molecule. The vibrational assignments on the basis of the calculated PED values were compared to the reported ones from experimental data. The vibrational infrared and Raman spectra of the mixture of the trans -equatorial and gauche -equatorial were plotted and the line intensities were compared to the corresponding experimental ones.

The conformational behavior and structural stability of chloro- and fluoromethylsulfonyl isocyanates were investigated by quantum mechanical DFT and ab initio MP2 calculations. The 6-311++G ∗∗ basis set was employed to include polarization and diffuse functions in the calculations. The molecules were found to exist in a mixture of two stable gauche conformations. The potential scans were calculated from which the rotational barriers could be estimated. The vibrational frequencies and spectra were computed at B3LYP/6-311++G ∗∗ level. The potential energy distributions were then calculated to provide tentative vibrational assignment for the normal modes of the stable conformers of both molecules.

The conformational behavior and structural stability of dichloro and difluoromethyl-sulfonyl isocyanates were investigated by quantum mechanical DFT and ab initio calculations. The 6-311++G** basis set was employed to include polarization and diffuse functions in the calculation at B3LYP and MP2 levels. The molecules were found to exist in a mixture of trans–gauche and gauche–gauche conformations at ambient temperatures. From the calculations the isocyanate NCO moiety was predicted to nearly eclipse one of the sulfony SO bonds in the two stable conformers of both molecules. The potential scans for the rotations of the two NCO and CX2H rotors were calculated from which the rotational barriers could be estimated. The vibrational frequencies, potential energy distributions, IR intensities as well as depolarization ratios were calculated.

The molecular structure of isobutyranilide and flutamide were investigated by DFT-B3LYP/6-311G(**) and MP2/6-311G(**) calculations. Isobutyranilide was predicted to exist predominantly in a planar cis conformation, while flutamide in non-planar structures with the CF3 and the NO2 groups adopting an out of the phenyl-plane configuration. The vibrational frequencies of the low energy structures of the two molecules were computed at the DFT-B3LYP level of theory. From the calculated Gibb's free energies, isobutyranilide is estimated to have an equilibrium mixture of 91% cis and 9% trans structures, while flutamide is calculated to have a mixture of 65% cis-cis and 28% trans-cis structures at 298.15K. The analysis of the observed vibrational spectra supports the presence of isobutyranilide in only one conformation at room temperature. From a 1:1 acetonitrile solvent experiment flutamide is determined to exist in more than one conformation at ambient temperature. Complete vibrational assignments of the normal modes of isobutyranilide and flutamide were provided on the basis of combined normal coordinate calculations and experimental Infrared and Raman spectra. The (1)H and (13)C NMR spectra of isobutyranilide were measured and their chemical shifts were compared to the corresponding ones of flutamide.

The structures of 3,3,3-trifluoromethyl ketene and 3,3-difluoromethyl ketene were studied by utilizing ab initio calculations with the 6-311++G** basis set at the (B3LYP) Density Functional level. Full optimization was performed for both molecules in their ground and transition states. Energy optimization of the systems under investigation shows that trifluoromethyl ketene exists only in the cis conformation (fluorine atom eclipses the ketene group). Difluoromethyl ketene was predicted to have two stable conformations: the cis (hydrogen atom eclipses the ketene group) and the gauche (fluorine atom eclipses the ketene group) form. The conformational stability of the molecules was found to be governed mainly by electrostatic and molecular orbital interactions. The vibrational frequencies were computed and complete assignments were provided on the basis of normal coordinate calculations and comparison with similar molecules. The potential energy distributions (PED) among symmetry coordinates were derived for the stable conformations of the two molecules.

Vibrational spectra of cyclohexanecarboxaldehyde are calculated with density functional theory using the B3LYP functional together with a 6-311++G** basis set and presented. The results in the case of the infrared spectrum of the mixture of conformers at 300 K are compared with the experimental spectrum and, apart from the intensities of the CH and CO stretches, reasonable agreement is found. These deficiencies can be traced back to the well-known nonlinearities in case of CH stretches and the CO stretch. Potential energy distributions among symmetry coordinates in each normal mode are presented and used to assign specific atomic movements to each of the modes. Potential energy scans for the CHO rotor in both the axial and equatorial conformers are presented and barrier heights are compared with previous Hartree–Fock calculations and experimental data. It is reported that there could be three stable conformers, namely equatorial-gauche (eg), which is the most stable, equatorial-trans (et) and axial-gauche (ag). The optimized energies of all the minima and of the transition states are presented. However, comparison of the calculated spectrum with the experimental one indicates that total energies are slightly in error and that in the mixture of conformers no ag is present and thus the et to eg ratio is also different. Using experimental values for relative energies of conformers, we could obtain spectra in fair agreement with experiment. This indicates that when only total energy differences are calculated, slight errors in them play a role because of the very small relative energies in this case, while properties like geometries and spectra, which depend not on energy differences but on analytically calculated energy derivatives, are not affected.

The conformational stability and structure of trichloromethylsulfonyl isocyanate were investigated by utilizing ab intio calculations. The computations were carried out at DFT-B3LYP/6-311++G∗∗ and MP2/6-311++G∗∗ levels. The calculation predicted the stability of a single gauche conformer for this molecule at ambient temperature. The vibrational frequencies of the gauche conformer were computed at DFT-B3LYP/6-311++G∗∗ level with diffuse and polarization functions in a valence triple-zeta basis set. Normal coordinate calculations were carried out and the potential energy distributions PED were calculated for the stable gauche conformer of the molecule. The calculated vibrational frequencies were compared to those obtained experimentally for similar molecules.

The conformational behavior and structural stability of 2,2,3,3,3-pentafluoropropanal were investigated by ab initio calculations. The 6-311++G ∗∗ basis set was employed to include polarization and diffuse functions in the calculations at B3LYP and MP2 levels. From the calculation, the molecule was predicted to exist in gauche ⇔ cis conformational equilibrium. The potential function scans were calculated out at B3LYP/6-311++G ∗∗ level for the molecule, from which the rotational barriers were estimated. The vibrational frequencies were computed at B3LYP level and complete vibrational assignments were made based on normal coordinate calculations for the two stable conformers of 2,2,3,3,3-pentafluoropropanal. Vibrational Raman and infrared spectra of the mixture of the gauche and cis conformers were computed at 300 K.