Pál Császár

Geometry optimization by direct inversion in the iterative subspace

Abstract A version of the direct inversion in the iterative subspace (DIIS) algorithm (ref.8) adopted to geometry optimization is presented. Comparison with the most widely used variable metric methods indicate that this novel technique is very efficient.

EQUILIBRIUM GEOMETRIES OF URACIL AND ITS C- AND N-METHYLATED DERIVATIVES

Abstract Equilibrium geometries, heats of formation and dipole moments of free uracil (U), 1-methyl-uracil (1-MeU), 3-methyl-uracil (3-MeU), 5-methyl-uracil (5-MeU, thymine), 6-methyl-uracil (6-MeU), 1,3-dimethyl-uracil (1,3-diMeU) and 5,6-dimethyl-uracil (5,6-diMeU) have been determined by semiempirical MINDO/3 calculations using Pulays force method. Monosubstitutions by the methyl group manifested themselves in mainly local changes in bond distances and angles, while effects of disubstitutions, in good approximation, proved to be additive. Additivity also holds for heats of formation and dipole moments.

Vibrational spectrum and structure of the second stable rotamer of butadiene-1,3

Abstract The dependence of the calculated vibrational frequencies of the second stable rotamer of butadiene- 1,3 on the torsional angle about the central single CC bond has been studied in the interval from 0° (planar syn -form) to 50°. The force field was described by scaled force constants obtained from CNDO/2 calculations. The best agreement between the calculated and experimental vibrational frequencies was found at 25° from the planar syn -form. The results of the vibrational frequency calculations for this configurations of some isotopomers of butadiene- 1,3 are given. In addition, in the light of the theoretical results obtained and new experimental evidence, the assignment of certain out-of-plane normal modes of the anti -form of C 4 D 6 is also discussed.

Vibrational spectra, scaled quantum-mechanical (SQM) force field and assignments for 4H-pyran-4-one

Abstract The gas phase i.r. spectrum of 4H-pyran-4-one (hereafter called γ-pyrone) has been recorded in the 4000-400 cm −1 region by a Nicolet 7199 FTIR spectrometer and interpreted using a general valence force field calculated quantum mechanically at the ab initio level with a split-valence 4–21 basis. Assignment of certain fundamentals was facilitated by information gained from the i.r. and Raman spectra of the melt and from the i.r. spectrum of the saturated solution in CCl 4 . To account for systematic computational errors, the theoretical ab initio force field was scaled using a set of constants derived by the empirical fitting of force fields computed for related molecules to their observed spectra. Either the scale factors derived for a family of open-chain molecules or, better, for benzene could be used to yield a scaled force field which gave unequivocal assignments for γ-pyrone. The method promises to be of general applicability for molecules of this complexity.

The vibrational spectra and structure of tris(ethylene)metals

Abstract The IR and Raman spectra of [Pt(C2H4)3], [Pt(C2D4)3] and IR spectra of [Ni(C2H4)3] are reported and assignments proposed. Comparisons are made with previously reported IR spectroscopic features of Pd, Co and Cu analogues. The results of a CNDO/2 geometry optimization calculation for [Ni(C2H4)3] are summarized, and reveal closely similar structural parameters for the ethylene ligand to those established for the Pt complex. Some comparisons are made between vibrational frequencies of the complexes and those of ethylene chemisorbed on single crystal metal surfaces.

Scaled quantum mechanical (SQM) force field and vibrational assignment for cyclohexane

Abstract The complete harmonic force field of cyclohexane has been calculated ab initio, using the split valence 4–21 Gaussian basis set. To account for systematic errors, the ab initio force constants were scaled empirically. Altogether seven scale factors were used in this procedure, whose values were mainly based on previous results on cyclobutane. The scaled quantum mechanical (SQM) force field obtained this way reproduces the fundamental frequencies of cyclohexane and several of its isotopomers with mean deviations of below 10 cm−1. The results are in excellent overall agreement with and support previous assignments and only a few minor modifications and additions are suggested.

Scaled quantum mechanical (SQM) ab initio force field and vibrational spectra of maleimide (1H-pyrrole-2,5-dione)

Abstract The vibrational spectrum of free maleimide has been interpreted using a theoretical general valence force field (GVFF) calculated ab initio at the Hartree-Fock level using the 4–21 split valence basis set. The directly computed GVFF was scaled with 14 empirical factors transferred from other molecules. The vibrational fundamentals predicted by this scaled GVFF enabled corrections to be suggested for several previous empirical assignments. The scale factors were also optimized directly on the experimental spectrum of maleimide producing the best GVFF obtainable from combined experimental and theoretical evidence. This scaled GVFF yielded frequencies agreeing with experimental values and with mean deviations of 6.8 and 9.6 cm−1 for in-plane and out-of-plane frequencies, respectively. The vibrational frequencies of the 2H and 15N isotopomers of C2v symmetry are also presented and discussed.

USE OF SEMIEMPIRICAL METHODS FOR THE PREDICTION OF EQUILIBRIUM GEOMETRIES AND IONIZATION POTENTIALS OF 4H-PYRAN-4-ONE AND SULPHUR ANALOGUES

Equilibrium geometries and ionization energies of 4H-pyran-4-one (I), CH-pyran-4thione (II), 4H-thiapyran-4-one (III) and 4H-thiapyran-4-thione (IV) are calculated by CNDO/P and MIND0/3 semiempirical methods. The geometries obtained are compared to the MW data available for I, II and IV. Reliability of geometry optimization and predictions based on semiempirical results are discussed. For I the vertical ionization energies estimated from ab initio, CNDO/Z and MINDO/S orbital energies are compared to each other and to those obtained by HAM/3 calculations, Tbe HAM/3 results are in the best agreement with the experimental data. For systematic HAM/3 studies on molecules for which no experimental geometries are available, the use of CND0/3 or MIND0/3 optimized geometries is suggested.

Theoretical force field and vibrational spectrum of maleimide by the CNDO/2 force method

Abstract In-plane and out-of-plane vibrations of maleimide have been studied by the CNDO/2 force method. To account for systematic errors, the direct theoretical force field was scaled by empirical factors optimized on the basis of experimental frequencies. The calculations suggest some new assignments in the vibrational frequencies.

Studies on the unimolecular decomposition processes of organometallic ions

Abstract The ionic decomposition processes of dimethyl derivatives of the Main Group IV elements (Me 2 SiH 2 , Me 2 GeH 2 and Me 2 SnH 2 ) were studied by means of mass spectrometry and He(I) photoelectron spectroscopy. The second derivatives of the total ion currents obtained by numerical differentiation were compared to the He(I) photoelectron spectra in the region of low energy excitation. QET calculations of 70 eV mass spectra were also performed. The results obtained do not show inconsistencies with the statistical theory.