ИСТИНА

The eight-atom molecule of diborane (B2H6) is of considerable theoretical interest because of its unusual electronic structure and the presence of strong vibrational resonances. Its anharmonic vibrational states were studied by various methods, including the second-order perturbation theory (VPT2) and the matrix variational method (VCI) [1,2]. The numerical-analytical operator implementation of the canonical vibrational Van Vleck perturbation theory (CVPT) allows extending the usual second-order approach (VPT2) to the fourth (CVPT4) and even higher orders depending on the size of the molecule.[3] Using the halogenated methanes (CH2X2) [4] and 1,1-difluoromethane [5] as examples, it was shown that the fourth order corrections can be up to ∼3–10 cm−1 and should be taken into account in precise calculations using high-quality ab initio sextic force fields. We employed the hybrid quantum mechanical sextic force field using the models MP2/aug-cc-pVTZ (anharmonic) and CCSD(T)/aug-cc-pVQZ (harmonic part) and numerical-analytic implementation of the fourth-order operator perturbation theory (CVPT4) for predicting the vibrational states of 11B2H6 and 10B2H6. The vibrational resonances have been accounted for by solving a small size variational problem (CVPTn/VCI). A good accuracy (∼5 cm−1) of predicting the fundamental frequencies has been achieved, and the importance of the fourth-order corrections has been shown. The systematic nature of errors in the quantum mechanical force field has been demonstrated. [1] doi:10.1063/1.476417, J.F. Stanton and J. Gauss, J.Chem.Phys., 108, 9218-9220 (1998). [2] doi:10.1007/s00214-012-1122-1, D. Begue, C. Pouchan, J.-C. Guillemin and A. Benidar, Theor. Chem. Acc., 131, 1122 (2012). [3] doi:10.1021/jp211400w, S. V. Krasnoshchekov, E. V. Isayeva and N. F. Stepanov, J. Phys. Chem. A, 116, 3691–3709 (2012). [4] doi:10.1063/1.5020295, S. V. Krasnoshchekov, R. S. Schutski, N. C. Craig, M. Sibaev and D. L. Crittenden, J. Chem. Phys., 148, 084102 (2018). [5] doi:10.1021/jp211400w, S. V. Krasnoshchekov, N. C. Craig and N. F. Stepanov, J. Phys. Chem. A, 117, 3041-3056 (2013).