Géza Fogarasi

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...

Geometry optimization in redundant internal coordinates

The gradient geometry‐optimization procedure is reformulated in terms of redundant internal coordinates. By replacing the matrix inverse with the generalized inverse, the usual Newton–Raphson–type algorithms can be formulated in exactly the same way for redundant and nonredundant coordinates. Optimization in redundant coordinates is particularly useful for bridged polycyclic compounds and cage structures where it is difficult to define physically reasonable redundancy‐free internal coordinates. This procedure, already used for the geometry optimization of porphine, C20N4H14, is illustrated here at the ab initio self‐consistent‐field level for the four‐membered ring azetidine, for bicyclo[2.2.2]octane, and for the four‐ring system C16O2H22, the skeleton of taxol.

THEORETICAL PREDICTION OF VIBRATIONAL SPECTRA. 1. THE In-PLANE FORCE FIELD AND VIBRATIONAL SPECTRA OF PYRIDINE.

Calcul du champ de force de vibration harmonique complet et des elements du champ de force cubique diagonal et semidiagonal pour la pyridine, au niveau Hartree-Fock 4-21

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.

Tautomers of cytosine and their excited electronic states: a matrix isolation spectroscopic and quantum chemical study

We have measured the IR and UV spectra of cytosine in a low-temperature argon matrix. An attempt was made to determine the tautomeric ratios existing in the matrix, making use of the matrix-isolation IR spectrum and computed IR intensities of the tautomers in a least squares fitting procedure. The mole fractions are about 0.22 for oxo(-amino) form, 0.26 and 0.44 for the two rotamers, respectively, of the hydroxy(-amino) form and 0.08 for the (oxo-)imino tautomer. These ratios were then used to simulate the matrix-isolation UV spectrum as a composite of the individual spectra, the latter calculated ab initio at high levels of electron correlation theory. The agreement between simulated and experimental UV spectra seems satisfactory. This indicates that, in contrast to the solid state and solution spectra described up to now by the oxo(-amino) form alone, the reproduction of the matrix-isolation UV spectrum needs at least the hydroxy(-amino) and oxo(-amino) forms, and probably also the (oxo-)imino form.

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.

The molecular structure of cyclic methylsiloxanes

Abstract A series of methylcyclosiloxanes from the trimer to the hexamer has been investigated by gas phase electron diffraction. The Si-O bond distance varies between 1.635 and 1.620 A, the SiOSi angle has values from 132° to 149° within the series. The trimer is substantially planar, the best agreement for the tetramer was obtained with a model of S 4 . symmetry. As a consequence of large amplitude vibrations, the larger rings do not possess a well-defined conformation. The results are discussed in relation to previous spectroscopic investigations.

Scaled quantum mechanical (SQM) force field and theoretical vibrational spectrum for benzonitrile

Abstract The complete harmonic force field of benzonitrile has been determined by ab initio Hartree—Fock calculations using a 4–21 Gaussian basis set. As force constants are systematically over-estimated at this level, the directly calculated force field was scaled by empirical factors previously optimized for benzene and HCN. Frequencies calculated from this scaled quantum mechanical (SQM) force field confirm the published experimental assignments for benzonitrile, benzonitrile-p-d and benzonitrile-d5. Aside from the CH (and CD) stretching frequencies, which are strongly affected by anharmonicity, the mean deviation between the observed and calculated frequencies is below 9 cm−1 for each isotopomer. Theoretical i.r. intensities reproduce the main features of the spectra semiquantitatively.

An efficient direct method for geometry optimization of large molecules in internal coordinates

A new efficient procedure has been developed for geometry optimization of large molecules using internal coordinates. The method stores only the nonzero elements of the large transformation matrices, in the spirit of direct methods in electronic structure theory. Matrix inversion has been replaced by iterative solution of linear systems of equations by the preconditioned conjugate gradient method. A new incomplete Cholesky preconditioner proved essential to accelerate the conjugate gradient procedure. The geometries of several alpha helical alanine polypeptides, up to 50 alanine units, have been optimized by the new method, using the SYBYL force field. For larger systems, the number of energy/gradient evaluations is reduced by a factor of 6–10, compared to Cartesian optimization, and the cost of the optimization is small compared to the energy calculation. We expect this method to be useful in molecular mechanics and in mixed quantum mechanics/molecular mechanics calculations.