Selective bond fission in methyl mercaptan at 193 nm via radial derivative coupling between the 2 1A″ and 1 1A″ adiabatic electronic states

Abstract

We investigate the origin of the observed fission of the stronger S-H bond over the weaker C-S bond in CH3SH excited at 193 nm using the complementary techniques of mass-resolved photofragment time-of-flight spectroscopy and emission spectroscopy. The velocities and angular distributions of the CH3S and SH photofragments show that both C-S and S-H bond fission occur on a subpicosecond timescale and impart considerable energy to relative product translation. The dispersed emission from photoexcited CH 3SH molecules in a flow cell evidences a progression in the CH 3 umbrella mode and combination bands with one quantum in the C-S stretch, but no progression with S-H stretch. Examination of the results with reference to previous ab initio calculations of the excited state surfaces reveals the importance of nonadiabatic coupling in the dissociation dynamics. This is a clear example of selective bond fission upon excitation of an electronic state that is not repulsive in the bond that breaks. We discuss the implication of the work with respect to using the Born-Oppenheimer approximation in reactive collisions near a saddle point along the reaction coordinate. © 1992 American Institute of Physics.