ИСТИНА

Harmonic vibrational analysis, enhanced by the empirical correction of a force field in the form of Pulay’s scale factors, is widely used for interpretation and prediction of vibrational spectra of polyatomic molecules but has many limitations due to the simplicity of the physical model. Since the calculation of anharmonic molecular properties became available through progress in quantum chemistry, the application of previous methods of anharmonic analysis has become a practical need. In this work, we used second-order perturbation theory (VPT2) as a link between molecular properties and spectroscopic constants that are necessary for full interpretation of vibrational spectra. In comparison to the variational method, VPT2 is more efficient for bigger molecules and allows solving an inverse problem in a straightforward manner. We have developed an original software implementation of VPT2 that allows the full anharmonic vibrational analysis of polyatomic molecules using quartic force fields and cubic surfaces of dipole moment and of polarizability in Cartesian or normal coordinates, obtained by double numerical differentiation of molecular properties available from Gaussian ’03. Evaluation of anharmonic intensities is essential for a proper interpretation of bands in experimental vibrational spectra. The method for calculation of infrared intensities in the double anharmonic approximation was implemented in the existing software SPECTRO.1 To date, there are very few works on evaluation of anharmonic intensities in Raman spectra, and, to the best of our knowledge, VPT2 was never used for such a purpose. We extended the general method for infrared intensities1 and applied it to evaluating Raman intensities. As the accuracy of predicting fundamental frequencies within VPT2 may not be always sufficient for unambiguous interpretation of experimental spectra, a suitable form of fitting observed frequencies is very helpful. For this purpose, we implemented the method that was used earlier for anharmonic vibrational analysis of benzene.2 It is based on fitting theoretical anharmonic frequencies to experimental fundamentals using variation of harmonic frequencies. Afterwards, the frequencies of two-quantum transitions are also corrected. In addition, it is possible to evaluate the scale factors of the harmonic part of the force field that could be transferred to other isotopomers or molecules. The efficiency of the developed extensions of theoretical methods and of the software implementation is demonstrated by full interpretation of the vibrational spectra of 1,3-butadiene and its fully deuterated isotopomer in the region up to 3500 cm–1. ACKNOWLEDGEMENT. We are grateful to Prof. N. C. Craig for providing us with experimental spectra and helpful discussions. 1 A. Willetts et al., J. Phys. Chem., 1990, 94:5608. 2 A. Miani et al., J. Chem. Phys., 2000, 112:248.