| Abstract
| - Clusters consisting of two and three methanethiol molecules were subjected to HF, DFT/B3LYP, and MP2calculations using the aug-cc-pvdz/cc-pvdz basis set. Overall, five structures corresponding to minima on thepotential surface for methanethiol dimer were located that allowed the identification of interactions of theS−H···S and C−H···S types. While the latter type of interaction prevails in the dimer, the former is onlyobserved in two of the five minima. Overall, the S−H···S contacts present angles that depart considerablyfrom linearity. The use of a method that considers intermolecular electron correlation is indispensable witha view to obtaining accurate results. In fact, the HF and DFT/B3LYP methods provided significantly longerintermolecular 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 associatedwith the two interactions between the sulfur atom and the hydrogen atoms in the methyl group. Five possibleminima for methanethiol trimer were also examined. The predominating interaction in these structures wasof the S−H···S type, which was found to occur in all five minima and was accompanied by interactions withthe methyl groups that contributed to stabilizing the clusters. The interaction energy for the most stable structurewas −28.1 kJ/mol. The contribution of nonadditive pairwise terms to the interaction was fairly low, butsignificant (ca. 6% of the overall interaction energy). The analysis of vibration modes revealed the dimer toexhibit no specially significant frequency shifts, which suggests that no S−H···S hydrogen bonds areestablished. However, the structures that present S−H···S contacts exhibited red shifts of ca. 60 cm-1. Thesituation with the trimer was different: all structures exhibited S−H···S interactions, which resulted in redshifts of ca. 80−90 cm-1, suggesting the presence of a cooperative phenomenon. Other vibration modesexhibited virtually no shifts; by exception, the frequency of the Hs-S−C−Hp torsion underwent a markedblue shift (about 130 cm-1 for the dimer and up to 200 cm-1 for the trimer).
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