The complex character of molecular vibrations as a source for errors in theoretical modeling of spectra of molecules with internal rotationтезисы доклада
Аннотация:In the experimental studies of the structure and conformational dynamics of non-rigid molecules based on the methods of vibrational or electron-vibrational spectroscopy, the heights of barriers to conformational transitions are obtained by solving inverse vibrational problems.In this case, the vibrational Schrödinger equation is solved using the variational principle, and the desired structural information (conformer energy differences and conformational transition barriers) is obtained from the optimal effective potential functions. The described problems are mathematically incorrect: they usually do not have a unique solution and, moreover, their solutions are often unstable with respect to small variations in the initial data.The described approach remains productive only if the set of coordinates used (in practice it is usually one coordinate — a one-dimensional approximation) for constructing the PES (Potential Energy Surface) sections properly represents the selected nuclear motions of the molecular system. Analysis of the shape of the PES often allows one to qualitatively evaluate the adequacy of the chosen coordinate to the problem being solved, for example, by the degree of curvature of the minimal energy path without much effort. The data on the shape of the PES are obtained from quantum-chemical calculations, and largely due to this, the use of quantum chemistry methods in the processing of experimental data from molecular spectroscopy has become the modern standard for such studies.However, the kinetic part of the Hamiltonian does not lend itself to such a simple analysis. An example of insufficient consideration of the features of the kinetic part of the Hamiltonian is the almost twofold discrepancy in the estimates of the barrier to internal rotation in the benzaldehyde molecule, obtained by various methods of quantum chemistry and from three sets of experimental data [1].Using the example of molecules containing planar fragments (benzene, furan, and others rings), the report discusses the structural features of molecular systems in which internal rotation can have a multimode character due to the relatively large off-diagonal elements of the matrix of kinematic coefficients B. The author presents an attempt, by comparing the results of one- and two-dimensional vibrational problems based on quantum-chemically calculated PES cross sections, to evaluate possible errors in the one-dimensional model and the stability of the inverse problem for internal rotation, which has a complex character.
