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

Abstract

The first three electronic states of the C2Br radical, correlating at linear geometries with 2Σ+ and 2Π 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=1.2621Å, R CBr=1.7967Å, ∠ CCBr=156.1°, 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 Σ ,Π1/2,Π3/2 vibronic energy levels and their absolute infrared absorption intensities at a temperature of 5 K have been calculated for the 12 C12 C79Br 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.