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 ,Π_1/2,Π_3/2 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.

Acknowledgement

This work was supported by the MIUR of Italy, the University of Bologna and the US Office of Naval Research (grant No. N00014-01-1-0809). SC would like to acknowledge the Project HPC-EUROPA (R113-CT-2003-506079) with the support of the European Community-Research Infrastructure Action under the FP6 ‘Structuring the European Research Area’ Programme. RT would like to acknowledge P. J. Knowles and H.-J. Werner for providing him with access to the MOLPRO code. The authors also thank T. J. Sears and H.-G. Yu for reading the manuscript.