Pongor, Gábor: selected publications

G. Pongor: On the application of Saunders' level shifting technique to CNDO/2 calculations; Chem. Phys. Letters, 24, 603-605 (1974).

Abstract:

A method, recently applied to ab initio calculations for overcoming convergence difficulties, was introduced into CNDO/2 calculations for the same purpose. It was stated that the method works excellently and a new, more advantageous formalism is given, which reduces the store requirements and the computer time.

P. Pulay, G. Fogarasi, G. Pongor, J.E. Boggs and A. Vargha: Combination of Theoretical ab Initio and Experimental Information to obtain Reliable Harmonic Force Constants. Scaled Quantum Mechanical (SQM) Force Fields for Glyoxal, Acrolein, Butadiene, Formaldehyde, and Ethylene; J. Am. Chem. Soc. 105, 7037-7047 (1983).

Abstract:

Fully optimized geometries, complete in- and out-of-plane force fields, and dipole moment derivatives have been calculated for the title compounds at the ab initio Hartree-Fock level using the 4-21 Gaussian basis set. The theoretical information is combined with experimental data by fitting the calculated force constants through a few parameters to the observed frequencies to obtain th final, scaled quantum mechanical (SQM) force fields. recommendations for a standard procedure of this type are given. The SQM force fields give excellent reproduction of the fundamental frequencies and are considered as approaching the best accuracy which can be achieved in a harmonic treatment. The infrared intensities obtained at this level of theory are only qualitative estimates, but they are still useful for making assignments more reliable.

G. Pongor, P. Pulay, G. Fogarasi and J.E. Boggs: Theoretical Prediction of Vibrational Spectra. I. The In-Plane Force Field and Vibrational Spectra of Pyridine; J. Am. Chem. Soc. 106, 2765-2769 (1984).

Abstract:

The complete harmonic vibrational force field and the diagonal and semidiagonal cubic force fireld elements have been computed for pyridine at the ab initio Hartree-Fock level. The harmonic constants were scaled by using scale factors previously derived by fitting the computed force field of benzene to the observed benzene vibrational spectrum. The scaled force field was then used to predict the vibrational spectrum of pyridine. Comparison with experimental spectra, after a few corrections in the experimental assignments, shows a mean deviation of 5.7 cm-1 for the non-CH frequencies. Computed intensities are qualitatively in agreement with experiment. This succesful a priori accurate prediction of the vibratioanl spectrum of a moderately complex molecule offers hope that completely unknown spectra of such molecules can be computed to harmonic oscillator accuracy if the spectrum of a related substance is known.

G. Pongor, G. Fogarasi, J.E. Boggs and P. Pulay: Theoretical Prediction of Vibrational Spectra: The Out-of-Plane Force Field and Vibrational Spectra of Pyridine; J. Mol. Spectrosc. 114, 445-453 (1985).

Abstract:

The harmonic force field for the out-of-plane vibrations of pyridine has been calculated from ab initio Hartree-Fock wavefunction obtained with a 4-21 basis set of contracted Gaussians. To account for systematic errors, the calculated force constants were scaled, using only two independent scale factors which wer transferred unchanged from benzene. The resulting scaled quantum mechanical force field, which is strictly a priori in that it is not based on any experimental data on pyridine, predicts the 64 out-of-plane fundamental frequencies of pyridine and its deuterated isotopomers of C2v symmetry with a mean deviation from experiment of only 8.5 cm-1. Addition of polarization functions to the basis set for the nitrogen atom and refinement of the two scale factors by fitting them to the observed pyridine spectra produce no significant improvement in the fit. assignments of the vibrational spectra are discussed.

V.I. Pupyshev, Yu.N. Panchenko, C.W. Bock and G. Pongor: Harmonic force field: An approximate relationship between the exact nonrelativistic and the hartree-Fock limit values of the force constants; J. Chem. Phys. 94, 1247-1252 (1991).

Abstract:

A theoretical background of the scaling procedure used for correcting molecular force constants computed at the Hartree-Fock (HF) level is presented, in which scaling is considered as an empirical simulation of the effect of electron correlation. Using a variational formalism for the analytical first and second derivatives, it is shown that a succesful scaling requires (i) relatively large exact excitation energies; (ii) a singlet-stable solution of the HF groung state; and (iii) molecular orbitals that can be well localized for the ground electronic state. The relationship between the exact nonrelativistic and HF limit values of the quadratic force constants have been investigated when the above conditions are satisfied. a single multiplicative (scale) factor is required at this limit and its value is approximately (Co)**2 near the "exact" equilibrium geometry, where Co is the coefficient of the HF determinant in a complete configuration interaction expansion. This approach requires a moderate size of the molecules investigated. A specific numerical example is considered.

G. Pongor, G. Fogarasi, I. Magdó, J.E. Boggs, G. Keresztury and I. Ignatyev: Theoretical prediction of vibrational spectra. The a priori scaled quantum mechanical (SQM) force field and vibrational spectra of pyrimidine; Spectrochim. Acta 48A, 111-119 (1992).

Abstract:

The complete harmonic force field of pyrimidine has been computed at the ab intio Hartree-Fock level using a 4-21 Gaussian basis set. In order to compensate the systematic overestimations of the force constants at the aformenetioned level of quantum mechanical approximation, the theoretical force constants were empirically scaled by using nine scale factors. (The values of all these scale factors were previously determined by fitting the theoretical force field of benzene to the observed vibrational spectra of benzene.) The resulting a priori scaled quantum mechanical (SQM) force field is regarded as the most accurate and physically the most correct harmonic force field for pyrimidine. This force field was then used to predict the vibrational spectra of pyrimidine-h4 and pyrimidine-d4. On the basis of these a priori vibrational spectra uncertain assignments have been confidently resolved. After a few reassignments, the mean deviations between the experimental and calculated frequencies are below 9 and 18 cm-1 for the non-CH stretching in plane and the out-of-plane vibrations, respectively. Computed IR intensities are generally in agreement with experiments at a qualitative level.

A.R. Bérces, P.G. Szalay, I. Magdó, G. Fogarasi and G. Pongor*: A Priori Results for Molecular Geometry, Scaled Quantum Mechanical (SQM) Force Field, and Vibrational Spectra of Pyridazine; J. Phys. Chem. 97, 1356-1363 (1993).

Abstract:

For the ground electronic state of pyridazine, two kinds of molecular data, the molecular geometry and the harmonic force field, have been determined theoretically at the ab initio Hartree-Fock level using the 4-21G basis set. In order to compensate for the systematic errors of both sets of computed molecular data, empirical corrections were used with the help of a few parameters whose values have been calibrated previously on model molecule(s). Thus, the resulting corrected data are a priori for pyridazine. These a priori molecular data are regarded as good estimates of the "true" values of the corresponding data. The a priori molecular geometry of pyridazine [r(N1N2) = 133.8 pm, r(N2C3) = 132.6 pm, r(C3C4) = 140.4 pm, r(C4C5) = 137.9 pm, r(C3H3) = 107.4 pm, r(C4H4) = 107.5 pm,

(NNC) = 119.8°,

(NCC) = 123.2°,

(CCC) = 117.1°,

(NCH) = 115.7°, and

(C3C4H4) = 120.6°] is presently the best equilibrium structure for the gaseous molecule and shows less aromatic character for this molecule than expected; this is manifested by the significant alternation of bond lengths around the ring. Our results are in excellent agreement with the ring structure of a recent X-ray analysis od pyridazine at 100 K. The a priori scaled quantum mechanical (SQM) force field of pyridazine (transferring all the required scale factors from the benzene molecule) is regarded as physically the most correct and the most accurate harmonic force field of this molecule. On the basis of this force field, the a priori vibrational spectra of pyridazine-h4 and pyridazine-d4 have been determined as if the experimental vibrational spectra of both molecules were completely unknown. Comparison with experimental spectra, after a few reassignments, shows 15.6 and 30 cm-1 for the mean and the maximal individual deviations, respectively. (Only the non-CH/CD stretching frequencies were considered.) Computed IR intesities are generally in agreement with experiments at a qualitative level.

I. Magdó, G. Pongor and G. Fogarasi: The Reliability od Scaled Quantum Mechanical (SQM) Force Fields at the MINDO/3 Level as Studied on Nitrogen-Hteroaromatics; J. Mol. Struct. (THEOCHEM) 303, 243-253 (1994).

Abstract:

The complete vibrational force fields of benzene, pyridine, pyrimidine, pyrazine and s-triazine have been calculated by combining the MINDO/3 semiempirical quantum chemistry with a further empirical correction procedure, the method of Scaled Quantum Mechanical (SQM) force fields. The reproduction of vibrational frequencies is rather modest, with mean deviations of 30 - 40 cm-1 and maximum deviations of up to 100 cm-1. These MINDO/3 SQM force fields can be useful for experimental spectroscopists if no other theoretical information is available to assist the inetrpretation of the spectra.

R. Hargitai, P.G. Szalay, G. Pongor and G. Fogarasi: Scaled quantum mechanical (SQM) force field and vibrational assignment for styrene; J. Mol. Struct. (THEOCHEM) 306, 293-311 (1994).

Abstract:

The geometry and complete harmonic force field for styrene were determined from ab intio Hartree-Fock calculations using the 4-21 gaussian basis set. Following the method of scaled quantum mechanical (SQM) force fields, systematic errors characteristic of this level of theory were corrected empirically. The scale factors were taken over fixed from benzene and butadiene so that the final SQM force field represents pure a priori results. Frequencies calculated from this force field reproduce the experimental values with a mean deviation of 13.9 and maximum deviations within 20 cm-1. The torsional potential around the C-C bond between the ring and the vinyl group was also investigated, also using larger basis sets. All calculations give a gauche form as the minimum, but the potential is extremeley shallowing thus justifying a vibrational treatment on the basis of planar geometry. For comparison, test calculations on cis- and gauche-butadiene are also reported.

A. Kovács, K.B. Borisenko, and G. Pongor: Investigation of the Density Functional Theory (DFT)-Derived Scaled Quantum Mechanical (SQM) Force Field Method for a Weakly Bonded System: A Priori SQM DFT-Derived Force Field and Vibrational Spectra for N2O4; J. Phys. Chem. (submitted).

Abstract:

The adequacy of the density functional derived scaled quantum mechanical (SQM) force field method of Rauhut and Pulay [Rauhut, G.; Pulay, P. J. Phys. Chem. 1995, 99, 3093] has been tested for a weakly bound dimer, N2O4, which serves as a good example for the compounds containing non-common structural motifs. According to the aforementioned method, the equilibrium geometry and the harmonic force field of the molecule have been calculated with the Becke3-Lee-Yang-Parr (B3-LYP) hybrid functional using the small 6-31G* basis set. Our DFT-derived molecular geometry is one of the best equilibrium structures for N2O4 at present. The harmonic force field has been scaled with the transferable scale factors of Rauhut and Pulay, developed originally for compounds containing common structural motifs without reference for the N-O bond. On the basis of the resulting force field, without any use of experiment, the a priori vibrational spectra of N2O4 have been determined, and the experimental assignments have been confirmed basically. Comparison with experimental spectra shows 20.7 and 38 cm-1 for the mean and the maximal individual deviations, respectively. Computed IR intensities are satisfactory, the Raman intensities are at a qualitative level. Linear mean-square amplitudes could be determined using our a priori SQM DFT-derived force field in a good agreement with the experiments. Consequently, the DFT-derived SQM force field method of Rauhut and Pulay may give reliable results for non-common compounds as well.