Renner-teller effect, spin-orbit coupling, and fermi resonance in BrCN(+) (X̃(2)Π): a combined experimental and theoretical study.

The spin-vibronic energy levels of BrCN(+) (X̃(2)Π) up to 7700 cm(-1) above the vibrational ground state have been measured using zero-kinetic energy photoelectron spectroscopy (ZEKE) and tunable coherent extreme ultraviolet (XUV) light. The fundamental bands for C≡N stretching, Br-C≡N bending, and C-Br stretching vibrations have been measured, and a strong bending excitation was observed. The Renner-Teller effect and the Fermi interaction between the C-Br stretching and the Br-C≡N bending have been observed experimentally. To characterize the spin-vibronic interaction in BrCN(+) (X̃(2)Π), an effective diabatic model Hamiltonian with spectroscopic parameters describing these interactions was used to calculate the vibronic energy levels. The spectroscopic parameters have been determined by fitting the experimental data. Theoretical calculations based on the diabatic model were also performed. The theoretical spectroscopic parameters have been calculated using the potential energy surfaces reported by Biczysko and Tarroni (Chem. Phys. Lett. 2005, 415, 223). The calculated vibronic energy levels and spectroscopic parameters have been compared with those from the experimental data. For the diabatic potential energy matrix, the ab initio calculations provide good description of the diagonal elements, however, the off-diagonal elements deviate appreciably from those determined by experimental data. By analyzing the ZEKE spectrum of the ground vibrational band, the first adiabatic ionization energy for BrCN has been determined as 95 675.5 ± 2.0 cm(-1).