Calculation of the Probabilities of Molecular Transitions in Modeling Interisomeric Structural Transformations

The problems of a computational procedure in modeling structural isomer–isomer transitions in multiatomic molecules are considered. A number of approximations that allow one to considerably simplify calculation of the probabilities of these transformations are suggested. In comparison with traditional calculations of potential surfaces and determination of their saddle points, in the proposed approach the relationship between the electronic and vibrational components of a general problem is virtually changed. A particular emphasis is put on the vibrational part, which seems to be more physical. It is shown that the problem of structural isomer–isomer conversions can be solved without resorting at all to explicit calculation of the contribution of an electronic matrix element to the probability of transition. There is no need to study in detail the shapes of potential surfaces; it suffices only to know the positions of their minima. The calculation of multidimensional vibrational integrals is replaced by calculation of one-dimensional integrals. The results obtained form a basis for constructing methods for modeling the dynamic spectra of complex molecules considering their structural transformations.     

Estimation of the contributions of absorption bands for overtones and composite frequencies in infrared spectra of brominated and oxygen-containing organic compounds

Intensities of fundamental, overtone, and composite absorption bands for 27 brominated hydrocarbons and 20 oxygen-containing organic compounds are calculated in an anharmonic approach. The first and second derivatives of the electric dipole moment of the molecule with respect to normal coordinates are determined using ab initio quantum-chemical MP2/6-31G(1d) calculations. For the studied compounds, the average contributions of overtones and composite frequencies to absorption in the region 100–4000 cm⁻¹ is 4.8% for brominated hydrocarbons and 3.2% for oxygen-containing compounds. The major part of the contribution of overtones and composite frequencies falls into the regions (mainly from 1600 to 2800 cm⁻¹) where fundamental transitions are observed rarely. The calculation performed well describe the positions of maxima and the intensities of fundamental, overtone, and composite absorption bands and can be used for the standardless spectrochemical analysis of compounds by their overtone spectra.     

Simulation of photochemical reactions of butadiene and calculation of the quantum yields

The photochemical reactions of butadiene-1,3 have been simulated. The calculated quantum yields of the products quantitatively agree with experimental data (deviation from the experimental data in on average less than 30%). The efficacy of the method in predicting photochemical processes of various types (isomerization, decomposition) occurring simultaneously via different pathways has been shown. It has been confirmed that the quantum beat frequency of resonating states can be used as a parameter of the method and, as such, allows obtainable quantitative estimates to be refined.     

Conference on the theory of the structure and spectra of complex molecules

Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 2, p. 203, March–April, 1995.     

This virtual molecular world

The existing rules and computing techniques are of little use for directed research dedicated to the real molecular world whose objects are enormous in number and diversity. Any progress is possible only through introducing forward-looking numerical computer experiments into the research practice. This requires changing from analysis of single objects and calculation of individual characteristics and properties to designing a virtual molecular world. Its objects, unlimited in number, can, similarly to real objects, respond to external actions and differ from one another. At the same time, by contrast to real objects, they can grow in complexity through interactions, accumulate energy, transform the initial information, etc. In this context, the prospects of designing such a world were discussed, as well as the methodological requirements on problem statement and solving as regards the theoretical and program construction aspects.