Analysis and predictions of the vibronic spectrum of the ethynyl radical C2H by ab initio 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 theX ²Σ⁺ andA ² II systems, is presented. The potential surfaces for the three lowest-lying electronic states 1² A′, 2² A′ and 1² A″ correlating withX ²Σ⁺ andA ² II 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.