ANALYSIS AND PREDICTIONS OF THE VIBRONIC SPECTRUM OF THE ETHYNYL RADICAL C2H BY ABINITIO METHODS

A purely ab initio study of the vibronic structure of the C2H spectrum in the region up to 7000 cm-1, which is complicated by the coupling of the X2SIGMA+ and A2PI systems, is presented. The potential surfaces for the three lowest-lying electronic states 1(2)A', 2(2)A' and 1(2)A" correlating with X2SIGMA+ and A2PI at the linear molecular geometry are calculated for the various geometrical distortions by means of the multireference configuration interaction (MRD-CI) method. These adiabatic surfaces are transformed into suitable diabatic counterparts. An approach is developed for a simultaneous treatment of three electronic states coupled via the bending and C-C stretching vibrations. Spin-orbit splitting of the vibronic levels and the vibronically averaged values for the hyperfine coupling constants are computed. The results obtained in this study enable a reliable explanation of the available experimental findings of the C2H spectrum and predict a number of features to be verified by future experiments.