Competition between C-H⋯S and S-H⋯Cl H-bonds in a CHCl3-H2S complex: a combined matrix isolation IR spectroscopic and quantum chemical investigation.

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H-bonded complexes between CHCl3 and H2S have been studied in a cold and inert argon matrix using IR spectroscopy. Both molecules were found to act as both a H-bond donor and acceptor, resulting in two different conformers. The more stable one (binding energy 3.25 kcal mol-1) was bound by a C-H⋯S H-bond, while the less stable one (1.90 kcal mol-1) by a S-H⋯Cl H-bond. The H-bonded complex formation has been confirmed by monitoring the spectral changes in νC-H and νS-H fundamental vibrations. The νC-H mode exhibited red shifts by 22.7 cm-1 upon C-H⋯S H-bond formation, while the formation of a S-H⋯Cl H-bond resulted in 24.2 and 25.4 cm-1 red shifts in the νS-H modes. The barrier for conversion of the less stable conformer to the more stable one was found to be 0.6 kcal mol-1. The rigid matrix environment prevented any detectable population transfer that required significant relative movement of the monomeric moieties. Dispersion interaction was found to significantly contribute to the overall stabilization of both conformers, more so for the S-H⋯Cl H-bonded one.