Variational calculations of HBN energy levels in the X 2Π and A 2Σ+ states

Abstract

A theoretical study of the HBN radical in the X 2Π and A 2Σ+ states, taking into account vibronic coupling effects, is reported. The lowest (1 2A′,1 2A″,2 2A′) potential energy surfaces (PES) of the HBN–BNH system have been studied to identify all stationary points. The HBN minimum was found to be 20.0 kcal/mol above BNH, with an isomerization barrier of ≈11 000 cm−1 on the 1 2A′ surface. For the HBN isomer, accurate near-equilibrium three-dimensional diabatic PESs for the ground X 2Π (1 2A′,1 2A″) and first excited A 2Σ+ (2 2A′) electronic states have been calculated at the multireference configuration interaction level of theory, with extended basis set. A vibronic coupling between bend and BN stretch, analogous to that found in the isoelectronic C2H and HCN+ radicals, has been found to take place due to the crossing of the X 2Π and A 2Σ+ states at energies close to 11 000 cm−1. Vibronic energy levels of HBN and DBN have been calculated variationally using a previously developed method [Carter et al., Mol. Phys. 98, 1967 (2000)] suitable for three-atomic molecules showing three-state vibronic interactions. Energy levels of Σ and Π symmetry up to 10 000 cm−1 for HBN, and 8800 cm−1 for DBN, are reported. It is shown that due to the high-energy surface crossing, the vibronic interaction becomes non-negligible only for levels above 8500 cm−1. For all levels, Renner–Teller effects and Fermi resonances are analyzed.