Ab initio study of the vibronic spectrum in the $X 2Π$ electronic state of HCCS

Potential energy surfaces for the electronic states of the HCCS radical correlating at linear nuclear arrangement with the $X 2Π$ state are calculated by means of an extensive ab initio approach. Particular attention is paid to calculating accurate three-dimensional potential surfaces involving variations of two bending and torsional coordinates, which play the central role in vibronic interactions (Renner–Teller effect), determining the structure of spectra of this radical. In the second part of this paper we use these potential surfaces and the ab initio computed spin–orbit coupling constant to calculate vibronic spectra of HCCS and DCCS in the framework of a theoretical model developed in our laboratory. The results of the present study are in excellent agreement with those derived by Tang and Saito [J. Chem. Phys. 105, 8020 (1996)] and thus strongly support the interpretation of their experimental findings.