ИСТИНА

A precise prediction of vibrational frequencies of isolated molecules assists in verification of their dynamics. Building quantum-mechanical models that solve the problem with “spectroscopic accuracy” of approximately 1 cm–1 is a long-standing goals of quantum chemistry. In this study, we predict fundamental frequencies of CH2F2, CH2Cl2 and CH2FCl molecules. Our theoretical model rests upon a rectilinear normal coordinate hybrid sextic PES (four independent variables), obtained at the MP2/cc-pVTZ level for the anharmonic part and at the CCSD(T)/aug-cc-pVQZ level for the harmonic part. The key feature of our approach in solving the vibrational Schrödinger equation is using a numerical-analytic canonical Van Vleck operator perturbation theory in second and fourth order (CVPT2&4).[1] In comparison, most vibrational studies of this kind are performed with a “semi-diagonal” quartic force field and are limited to second order because this approach depends on closed expressions derived for a general case.[2] The CVPT approach allows detection and treatment of vibrational resonances at any order, which, in turn, assists in identifying vibrational polyads. We have shown that employing the fourth order of CVPT is critically important when a highly accurate prediction of fundamental frequencies is sought. It was previously shown that CVPT4 can produce corrections of the order of 3-4 cm–1 in comparison with the second order (Δ4-2).[3] For the halogenated methylenes studied by us, we have found that Δ4-2 can be even larger, of the order of 10 cm–1. In comparison with the recent study of the CH2FCl molecule,[2] conducted with a CCSD(T)/CBS extrapolation, our hybrid non-extrapolated PES results show about three times better correlation between observed and predicted fundamentals, with an average deviation of the order of 1.5 cm–1. Analogous results were achieved for the other molecules. Literature: [1] Krasnoshchekov, S. V.; Isayeva, E. V.; Stepanov, N. F. Numerical-Analytic Implementation of the Higher-Order Canonical Van Vleck Perturbation Theory for the Interpretation of Medium-Sized Molecule Vibrational Spectra. J. Phys. Chem. A, 2012, 116, 3691–3709. [2] Charmet, A. P.; Stoppa, P.; Tasinato, N.; et al. An integrated experimental and quantum-chemical investigation on the vibrational spectra of chlorofluoromethane, J. Chem. Phys., 2013, 139, 164302, 1-15. [3] Krasnoshchekov, S. V.; Craig, N. C.; Stepanov, N. F. Anharmonic Vibrational Analysis of the Gas-Phase Infrared Spectrum of 1,1-Difluoroethylene Using the Operator Van Vleck Canonical Perturbation Theory. J. Phys. Chem. A, 2013, 117, 3041–3056.