Ab initio prediction of the infrared absorption spectrum of the C2Br radical

The first three electronic states (12A', 22A', 12A'') of the C2Br radical, correlating at linear geometries with 2Pi and 2Sigma states, have been studied ab initio, using Multi Reference Configuration Interaction techniques. The electronic ground state is found to have a bent equilibrium geometry, RCC=1.2621 A , RCBr=1.7967 A , CCBr =156.1 deg, with a very low barrier to linearity. Similarly to the valence isoelectronic radicals C2F and C2Cl, this anomalous behaviour is attributed to a strong three-state non-adiabatic electronic interaction. The ,1=2,3=2 vibronic energy levels and their absolute infrared absorption intensities at a temperature of 5K have been calculated for the 12C12C79Br isotopomer, to an upper limit of 2000 cm-1, using ab initio diabatic potential energy and dipole moment surfaces and a recently developed variational method.