Exploring flat multi-minima potential energy surface by matrix isolation IR spectroscopy: Revisiting methanol-H2S complex.

An extensive potential energy surface search of the methanol-H2S binary complex reveals four distinct conformers (two each of S-H⋯O and O-H⋯S H-bonded) within a small energy window (binding energies within -2.9 to -3.3 kcal mol-1). The four conformers are interconvertible via rotation along the C-O bond of methanol and the S-H bond of H2S, with very small rotational barriers (<0.5 kcal mol-1). All four stable methanol-H2S complexes have been experimentally identified for the first time in cold and solid argon and nitrogen matrices via spectral signatures in νO-H and νS-H regions, while an earlier study in a molecular beam could identify only the global minimum structure. To further investigate matrix effects, quantum chemical calculations were carried out for the methanol-H2S complex embedded within an argon environment modeled as a face-centered cubic lattice, mimicking the solid matrix conditions. The rotational barriers were found to increase appreciably inside the model (>1.5 kcal mol-1). Rate constants for conformational interconversions revealed that populations in local minima rapidly transfer to the global minimum at low temperatures, while the same become prohibitively slow inside the model. This prediction aligns well with the trapping of hitherto unidentified local minima conformers in a matrix environment.