A theoretical study of BrCN+ in the 2Î electronic ground state: Large amplitude bending motion

We have calculated the three-dimensional potential energy surfaces for the 1 2A′ and 1 2A″ states of BrCN+ at the MR-SDCI_DK+Q/[QZP-ANO-RCC (Br, C, N)] level of theory, where MR-SDCI_DK means 'multi-reference single and double excitation configuration interaction calculation with Douglas-Kroll Hamiltonian.' These ab initio potential energy surfaces have a common minimum (corresponding to the X˜2Π state) at a linear equilibrium structure with re(Br-C) = 1.735 Å and re(C-N) = 1.199 Å. Variational RENNER calculations yield a zero-point averaged structure (with the structural parameters calculated as expectation values over rovibrational wavefunctions) with 〈r(Br-C)〉0 = 1.739 Å, 〈r(C-N)〉0 = 1.204 Å, and 〈∠(Br-C-N)〉0 = 172(4)°. A severe Fermi resonance between 2ν2 and ν3 has been found theoretically for the 2A″ potential energy surface. Comparing the ab initio zero-point averaged structure with a recent, experimentally derived r0 structure, it is concluded that the effects of large-amplitude bending motion should be taken into account explicitly in the process of deriving the r0 structure from the experimental values of the rotational constants. The electronic structure of X˜2Π BrCN+ has also been discussed.