Methanethiol dimer and trimer.: An ab initio and DFT study of the interaction

Clusters consisting of two and three methanethiol molecules were subjected to HF, DFT/B3LYP, and MP2 calculations using the aug-cc-pvdz/cc-pvdz basis set. Overall, five structures corresponding to minima on the potential surface for methanethiol dimer were located that allowed the identification of interactions of the S-H...S and C-H...S types. While the latter type of interaction prevails in the dimer, the former is only observed in two of the five minima. Overall, the S-H...S contacts present angles that depart considerably from linearity. The use of a method that considers intermolecular electron correlation is indispensable with a view to obtaining accurate results. In fact, the HF and DFT/B3LYP methods provided significantly longer intermolecular distances than the MP2 method and underestimated interaction energies by more than 50%. The interaction energy of the most stable minimum was - 11.2 kJ/mol with the MP2 method and was associated with the two interactions between the sulfur atom and the hydrogen atoms in the methyl group. Five possible minima for methanethiol trimer were also examined. The predominating interaction in these structures was of the S-H...S type, which was found to occur in all five minima and was accompanied by interactions with the methyl groups that contributed to stabilizing the clusters. The interaction energy for the most stable structure was -28.1 kJ/mol. The contribution of nonadditive pairwise terms to the interaction was fairly low, but significant (ca. 6% of the overall interaction energy). The analysis of vibration modes revealed the dimer to exhibit no specially significant frequency shifts, which suggests that no S-H...S hydrogen bonds are established. However, the structures that present S-H...S contacts exhibited red shifts of ca. 60 cm(-1). The situation with the trimer was different: all structures exhibited S-H...S interactions, which resulted in red shifts of ca. 80-90 cm(-1), suggesting the presence of a cooperative phenomenon. Other vibration modes exhibited virtually no shifts; by exception, the frequency of the H-s-S-C-H-p torsion underwent a marked blue shift (about 130 cm(-1) for the dimer and up to 200 cm(-1) for the trimer).