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S. V. Krasnoshchekov, V. V. Nechayev, E. V. Isayeva, N. F. Stepanov
Calculation of
anharmonic intensities in the Vibrational Raman Spectrum and a Full
Interpretation of the Vibrational spectrum of trans-1,3-butadiene
Abstract
An anharmonic model of vibrations of a
polyatomic molecule allows, using second-order perturbation theory, a detailed
interpretation of vibrational spectra with resonances taken into account and
the calculation of intensities of spectral bands of fundamentals, overtones and
combination bands. For molecules possessing a center of symmetry (for example,
trans-1,3-butadiene), some vibrations have zero intensity in the infrared
spectrum due to the principle
of mutual exclusion. For a rigorous analysis
of such molecules it is necessary to measure Raman spectra and to have a
corresponding theoretical model for calculation of anharmonic intensities. In
this work it is shown that perturbation theory (PT) in the form of contact
transformations (CT) is applicable for calculations of anharmonic intensities
in Raman spectra. The Fortran computer program ANCO has been developed that
allows calculation of vibrational frequencies and infrared/Raman-intensities of
fundamental vibrations, overtones and combination bands on the basis of
second-order PT in the form of CT with polynomial representations of potential
energy, dipole moments and polarizability surfaces. With Using those
properties, calculated by the B3LYP/6–31+G(d,p) method, frequencies and forms
of anharmonic vibrations have been obtained, and the interpretation of the
experimental spectrum of trans-1,3-butadiene molecule has been given, as an
example. The method of calculation of scale factors of anharmonic force field
was proposed. It has been shown, that in the framework of anharmonic model
these factors are close to unity.
Copyright (C) Chemistry Dept., Moscow State University, 2002
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