Towards numerical vibrational spectroscopy

From the mathematical standpoint, it is possible to reduce a spectral study to the solution of the so-called inverse spectral problems. In practice, the inverse problem is solved through consistent comparison of the spectrum of the model with the experimental spectrum, the degree of closeness between theoretical and experimental spectra providing a quality criterion for the solution. The model may be characterized by different levels of complexity. This paper discusses the course of recent developments in theoretical spectroscopy, the present state-of-the-art, and forecasts future union of experimental and theoretical methods.     

Method for estimating the influence of an external electric field on the probability of photochemical transformations

We propose an approach to estimating the influence of a strong external electric field on the probability of photochemical transformations. The approach is based on an analysis of the appearance or disappearance of the resonance levels of interacting subsystems that are necessary for the chemical transformation and the change of overlap integral of the corresponding wave functions. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 4, pp. 543–545, July–August, 2007.     

Adjacency matrices and chemical transformation graphs

The vast numbers of molecular objects and chemical transformations of a substrate cannot be evaluated even intuitively, which necessitates the development of simple reaction monitoring methods. Here this problem is formulated for the first time, and possible solutions are substantiated.     

Determination of electrooptical parameters of molecules with the use of nonempirical quantum-chemical calculations of absolute intensities of absorption bands in the IR region

The authors verified the possibility of more accurate determination of electrooptical parameters (EOP) obtained in quantum-chemical calculations by solution of the inverse spectral problem for different molecular species. Intensities calculated with the ab initio method are taken as reference quantities. V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry of the Russian Academy of Sciences, 19 Kosygin St., 117975, Moscow, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 66, No. 1, pp. 19–24, January–February, 1999.     

Intramolecular resonance as a catalytic factor

It was shown that the resonance mixing of states on large molecules can increase the probability of reaction. This effect is similar to a decrease in the activation threshold or the action of an external catalyst.     

Complementarity of experiment and the theory of molecular spectra as a basis for standard-free quantitative substance analyses

The previously proposed approach to quantitative spectrochemical substance analysis without certified reference samples (standard-free spectroscopy) based on experimental data and the theoretical evaluation of transition probabilities between energy levels in polyatomic molecules upon electromagnetic excitation is developed. It is shown that analyses by luminescence and Raman can be performed similarly.     

Solution of a Generalized Inverse Spectral Problem for Weakly Resolved Spectra Using a Coefficient of Correlation

We consider in a new statement the inverse problem of the theory of vibrational spectra of molecules, in which a theoretical spectral curve directly gets closer to the experimental one. A coefficient of correlation between the curves is used as a criterion of the closeness of the curves and as a functional of the inverse problem. The procedure developed for solving the inverse problem makes it possible to dispense with the preliminary referencing of frequencies in an experimental spectrum and with its expansion into elementary absorption bands.     

Estimation of the contribution of absorption bands for overtones and composite frequencies in IR spectra of olefins and heteroatomic organic compounds

Intensities of fundamental, overtone, and composite absorption bands for 11 olefins, 17 nitrogen- and oxygen-containing organic compounds, and 12 sulfur-containing organic compounds are calculated in the anharmonic approximation. The first and second derivatives of the electric dipole moment of a molecule were calculated by the quantum-chemical ab initio MP2/6-31G(1d) approach. It is shown that, for the studied compounds, the average contribution of overtones and composite frequencies to absorption in the region from 100 to 4000 cm⁻¹ is of about 10%. The major contribution (on the average 80%) of overtones and composite frequencies falls in the regions (mainly from 1600 to 2800 cm⁻¹) where fundamental transitions are rarely observed. The calculations well describe the centers and intensities of the fundamental, overtone, and composite absorption bands and can be used for the standard-free spectrochemical determination of the compounds of interest by their overtone spectra.