Spectroscopic evidence of S∴N and S∴O hemibonds in heterodimer cations.

Abstract

Computational and condensed phase experimental evidence for the existence of S∴N and S∴O hemibonded structures has been reported previously, but no gas phase experimental evidence has been reported. To experimentally explore the existence of the S∴N and S∴O hemibonds in the gas phase, we recorded the infrared photodissociation action spectra of four cationic clusters: [CH3SH-NH3]+, [CH3SCH3-NH3]+, [CH3SCH3-H2O]+, and [CH3OCH3-H2O]+. Combined with the calculation results, it is found that the S∴N hemibonded structure is competitive with the S⋯HN H-bonded structure, though only the latter structure is actually observed in [CH3SH-NH3]+. The spectral and theoretical results show that hemibonds can form between the second- (oxygen or nitrogen) and the third-period elements (sulfur) in the heterodimer clusters of [CH3SCH3-NH3]+ and [CH3SCH3-H2O]+. However, the S∴N and S∴O hemibonded structures are found competitive with the C⋯HN and CH⋯O H-bonded structures, respectively, and both the structures coexist. On the other hand, the O∴O hemibonded structure is much less stable than other hydrogen bonded (H-bonded) structures in [CH3OCH3-H2O]+, and it shows no clear contribution to the observed spectrum. This study provides direct spectroscopic evidence for the existence of S∴N and S∴O hemibonds in the gas phase and their competition with the H-bonds, which may be also fundamentally important in biological processes.