A THEORETICAL-STUDY OF THE STRUCTURES AND STABILITIES OF SC2H2+ SPECIES - THE REACTION OF S+ WITH ACETYLENE

A theoretical study of the SC2H2+ species on both the doublet and quartet surfaces has been carried out. Predictions have been made for some of the molecular properties (geometries, dipole moments, and harmonic vibrational frequencies) which could help in their possible experimental detection. A vinylidene-type structure, (SCCH2+(B1))-B-2, has been shown to be the global minimum, with a cyclic (SC2H2+(B1))-B-2 species lying about 19 kcal/mol higher in energy at correlated levels of theory. The lowest-lying quartet state is a cis-SCHCH+ structure corresponding to the sigma-interaction of S+ with acetylene (which lies about 30 kcal/mol below reactants), and the pi-interaction gives a long-distance ion-molecule complex (A-4(2) symmetry) which lies about 15 kcal/mol below (S+(S)-S-4 + C2H2(1-SIGMA(g)+). The reaction of S+ with acetytene to give SCC+ + H-2 is found to be clearly endothermic, whereas the production of SCCH+ is shown to be only slightly endothermic. However, we have found for the latter an activation barrier of approximately 11 kcal/mol which could be even higher at more refined levels of theory. Therefore, this barrier could hinder the reaction of S+ with acetylene under low-temperature conditions, such as in interstellar media, severely limiting the role of this process in the production of interstellar SC2.