Péter Pulay

Force field, dipole moment derivatives, and vibronic constants of benzene from a combination of experimental and ab initio quantum chemical information

The quadratic and the most important cubic force constants of benzene have been determined from ab initio Hartree–Fock calculations with a double‐zeta basis set. Some constants have also been recalculated using other basis sets, including a polarized one. A few empirical scale factors, applied to the ab initio force field, allow the reproduction of a large number of observed vibrational frequencies, isotope shifts, and Coriolis constants within the uncertainties of experiment and the harmonic model. It is shown that the simultaneous utilization of ab initio and spectroscopical information is sufficient for the conclusive resolution of the uncertainties and alternatives in previous empirical force fields. The resulting scale factors can be used directly to obtain force fields for other aromatic hydrocarbons from ab initio calculations. Reproduction of the observed infrared intensities is only moderately successful, even with the polarized basis set. The calculated vibronic coupling constants show qualitati...

Prediction of vibrational spectra by the CNDO/2 force method: II. The calculation of vibrational frequencies of cis and trans forms of glyoxal, acrolein and 1,3 butadiene

Abstract The force constants and vibrational frequencies of the trans and cis forms of glyoxal, acrolein and 1,3-butadiene and some of their deuterated analogues have been calculated by the CNDO/2 force method. Although there are systematic deviations for certain vibrations, the method reproduces the majority of the experimental fundamentals well. Reproduction of frequency shifts between different molecules, or between the two conformers of the same molecule, is particularly good. The calculations corroborate the recent reassignment of the vibrational spectra of 1,3-butadiene and indicate that four frequencies of acrolein should be reassigned. On passing from the trans to the cis form, the calculations predict the following main changes : the central C-C stretching and the totally symmetric skeletal bending decrease, the ue5fbCH 2 rocking and the antisymmetrical skeletal bending increase in frequency. Moreover, in the cis forms of butadiene and glyoxal the frequency of the antisymmetric double bond stretching mode exceeds that of the symmetric vibration.

The geometry of some amides obtained from ab initio calculations

Abstract Fully optimized geometries, obtained by a new ab initio gradient program, are reported for formamide, N-methylformamide, acetamide, and N-methylacetamide. Within the limits of comparing re with rs or rg structures, agreement with experiment is excellent. After a correction for systematic errors, true equilibrium structures are suggested for the four amides. The structural effects of N-methylation and C-methylation are discussed. It is believed that the re parameters obtained are as accurate as can be obtained by experiment and that the structural variations may be more reliable than the experimental values.

The molecular structure of toluene

Abstract Discrepancies between two recently reported electron diffraction studies of toluene have provided the incentive for an ab initio structural determination at the 4-21 level. The methyl C-H bonds are on the average 0.011 A longer than the ring C-H bonds. While the average ring C-C and C-H distances are nearly identical with those of benzene, the ring exhibits marked asymmetry, including an unexpected coupling between the ring C-C distances and the angle of methyl rotation.

Prediction of vibrational spectra by the CNDO/2 force method: Part III. In-plane vibrations of benzene

Abstract The CNDO/2 force method is applied to the study of the in-plane force field of benzene. Although, as usual, the direct calculated values differ significantly from the true ones, after scaling them with altogether 3 empirical parameters, the complete set of 26 force constants as well as the vibrational frequencies of C6H6, C6D6 and sym C6,H3D3 are well reproduced. The force field obtained seems reliable enough even to allow critical judgement of the experimental results.

Prediction of vtbrational spectra by the CNDO/2 force method: I. Out-of-plane vibrations of benzene and fluorobenzenes

Abstract The out-of-plane force constants of benzene and 12 fluorobenzenes have been calculated by the CNDO/2 force method. Two empirical scaling factors had to be applied to obtain good agreement. The mean deviation between the 88 calculated and experimental frequencies is 16 cm −1 .

Force field and vibrational assignment for cyclobutane from a combination of ab initio calculations and experimental data

Abstract The geometry, force constants and dipole moment derivatives for cyclobutane have been calculated ab initio at the Hartree—Fock level using a 4–21 Gaussian basis set and the gradient method. To account for systematic deviations, the calculated force constants are scaled first and the assignment of the fundamental frequencies of cyclobutane-d 0 and -d 8 is analysed. Besides some minor reassignments, the A 1 rocking fundamentals are assigned to significantly higher frequencies than previously. The final refined force field is determined by optimizing the scale factors on the experimental frequencies.

Calculation of force constants by semiempirical quantum chemical method

Abstract The force (gradient) method is applicable both for ab initio and semiempirical quantum chemical methods. The latter is cheaper and can be used to calculate force fields of large molecules. Therefore, it is important to check the force constants obtained by semiempirical methods. The CNDO/2 method seems to be very useful in finding the assignment of normal frequencies and in establishing the Quantum Mechanical Consistent Force Fields.

The in-plane force field of nitryl fluoride

The in-plane harmonic and some cubic force constants of nitryl fluoride (NO2F) have been determined from ab initio Hartree-Fock calculations, using 7s3p and 5s2p gaussian basis sets. The theoretical values are used to resolve the ambiguity of the experimental force constants. It is shown that of the two experimentally equivalent force fields of Mirri et al., Set I is the physically correct solution. However, as even Set I reproduces the isotope shift data poorly, a new experimental force field has been determined. The new force field agrees very well with the ab initio results and is considered the most reliable at present. It is stressed that isotope shifts are more effective in fixing the force field than centrifugal distortion constants ; the latter, although experimentally accurate, are significantly influenced by anharmonicity.