B3LYP and CCSD(T) studies of the mechanisms of unimolecular reactions of HXCS (X = H and F)

The potential energy surface for the decomposition of singlet HXCS (X = H a nd F) has been explored by B3LYP and CCSD(T) calculations. Five different t ypes of reaction are proposed: (A) 1,1-HX elimination, (B) 1,2-H shift, (C) 1,2-X shift, (D) H . and XCS . radical formation, and (E) X . and HCS . ra dical formation. These results show interesting trends for the HXCS isomers . Our theoretical investigations suggest that the doubly bonded species HXC =S is the lowest energy structure among the isomers from both kinetic and t hermodynamic viewpoints. We also report theoretical predictions of molecula r parameters and vibrational IR spectra of the monohalogen substituted thio formaldehyde, which should be useful for future experimental observations.