Spectroscopic evidence of S therefore N and S therefore O hemibonds in heterodimer cations

Computational and condensed phase experimental evidence for the existence of S therefore N and S therefore O hemibonded structures has been reported previously, but no gas phase experimental evidence has been reported. To experimentally explore the existence of the S therefore N and S therefore 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 therefore N hemibonded structure is competitive with the SMIDLINE HORIZONTAL ELLIPSISHN 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 therefore N and S therefore O hemibonded structures are found competitive with the CMIDLINE HORIZONTAL ELLIPSISHN and CHMIDLINE HORIZONTAL ELLIPSISO H-bonded structures, respectively, and both the structures coexist. On the other hand, the O therefore 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 therefore N and S therefore O hemibonds in the gas phase and their competition with the H-bonds, which may be also fundamentally important in biological processes.