High-Level ab Initio Calculation of the Rotation−Vibration Energies in the c̃ 1A1 State of Methylene, CH2

Abstract

For the third excited electronic state (c- {sup 1}A{sub 1}) of the methylene radical, CH{sub 2}, we calculate the electronic potential energy surface using a high-level ab initio method and the rotation-vibration energies using a variational technique with a large rotation-vibration basis set. The potential energy surface is calculated at a carefully selected grid of 48 nuclear geometries that cover all types of combination of stretching and bending deformations to energies more than 20000 cm{sup -1} above that the equilibrium configuration. We fit an analytical function, in which we vary 23 parameters, through the points and find that the state is almost linear with an equilibrium angle of 172.7{degree} and a barrier to linearity of only 6 cm{sup -1}. It is well-known that theoretical treatments of higher lying states in the same symmetry are substantially tedious and complicated. The basis set used [TZ3P(2f,2d)+2diff] was triple-{zeta} plus triple polarization with two sets of higher angular momentum functions and two sets of diffuse functions. We have used the variational MORBID procedure to calculate the rotation-vibration energies. Because of the peculiar shape of the bending part of the potential surface, some very large bending force constants f{sub 0}{sup (i)} are obtained, and this more » has necessitated the use of very large basis sets in the MORBID calculation in order to achieve acceptable convergence. 17 refs., 2 figs., 4 tabs. « less