Gábor Pongor

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

Vibrational analysis of 2-nitrophenol. A joint FT-IR, FT-Raman and scaled quantum mechanical study

Abstract FT-IR (gas, solution, solid) and FT-Raman (solution, solid) spectra of 2-nitrophenol have been recorded in the range of 4000–30 cm−1. The spectra were interpreted with the aid of normal coordinate analysis based on a scaled Becke3–Lee–Yang–Parr/6-31G* density functional force field utilising a set of scale factors introduced recently by Rauhut and Pulay (G. Rauhut, P. Pulay, J. Phys. Chem. 99 (1995) 3093). These scale factors, developed on a small training set of organic molecules containing no hydrogen bonding moieties, were found to be well transferable to 2-nitrophenol including the strong intramolecular (O)H⋯O(N) hydrogen bonding moiety as well. The scaled force field reproduced the experimental frequencies of the molecule by a weighted mean deviation of 10.5 cm−1. Based on the calculated results, 38 fundamentals from a total of 39 were identified and assigned, revising the assignments of earlier experimental studies for several fundamentals.

Extension of the density functional derived scaled quantum mechanical force field procedure.

The density functional derived scaled quantum mechanical (SQM) force field method of Rauhut and Pulay [J. Phys. Chem. 99 (1995) 3093] has been extended. The original procedure (employing B3-LYP/6-31G* computations and 11 transferable scale factors for the different kinds of internal coordinates) was capable to reproduce the vibrational fundamentals of 31 simple organic (H, C, N, O) molecules with a total mean deviation of about 13 cm(-1). The present Density Functional Theory based SQM force field method is an extension of the original one: with the help of 20 transferable scale factors can reproduce the fundamentals of 20 inorganic, organic and organosilicon molecules containing nonmetallic first and second-row atoms with a total mean deviation of 10.8 cm(-1). The transferability and reliability of the new set of scale factors are demonstrated on the examples of the a priori SQM vibrational spectra of cis and gauche-cyclopropylchlorosilane.

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

Abstract The geometry and complete harmonic force field for styrene were determined from ab initio 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 20cm −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 extremely shallowing thus justifying a vibrational treatment on the basis of planar geometry. For comparison, test calculations on cis - and gauche -butadiene are also reported.

Theoretical prediction of vibrational spectra. The a priori scaled quantum mechanical (SQM) force field and vibrational spectra of pyrimidine

Abstract The complete harmonic force field of pyrimidine has been computed at the ab initio Hartree—Fock level using a 4–21 Gaussian basis set. In order to compensate the systematic overestimations of the force constants at the aforementioned 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- h 4 and pyrimidine- d 4 . 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.

Theoretical prediction of vibrational spectra: The out-of-plane force field and vibrational spectra of pyridine

Abstract The harmonic force field for the out-of-plane vibrations of pyridine has been calculated from ab initio Hartree-Fock wavefunctions 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 sclae factors which were 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.

The reliability of scaled quantum mechanical (SQM) force fields at the MINDO/3 level as studied on nitrogen- heteroaromatics

Abstract The complete vibrational force fields of benzene, pyridine, pyrimidine, pyrazine and s-triazine have been calculated by combining 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 to 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 interpretation of the spectra.

Fitting-free curve resolution of spectroscopic data: Chemometric and physical chemical viewpoints

In this paper the authors have investigated spectroscopic data analysis according to a recent development, i.e. the Direct Inversion in the Spectral Subspace (DISS) procedure. DISS is a supervised curve resolution technique, consequently it can be used once the spectra of the potential pure components are known and the experimental spectrum of a chemical mixture is also presented; hence the task is to determine the composition of the unknown chemical mixture. In this paper, the original algorithm of DISS is re-examined and some further critical reasoning and essential developments are provided, including the detailed explanations of the constrained minimization task based on Lagrange multiplier regularization approach. The main conclusion is that the regularization used for DISS is needed because of the possible shifted spectra effect instead of collinearity; and this new property, i.e. treating the mild shifted spectra effect, of DISS can be considered as its main scientific advantage.