The S–H⋯N versus O–H⋯N hydrogen bonding in the ammonia complexes with CH3OH and CH3SH

The methanol and methanethiol complexes with ammonia were studied by means of infrared matrix isolation spectroscopy and ab initio MP2/6-311++G(3df,3pd) calculations. The calculations resulted in the stability of two similar structures for each of the two complexes: CH3OH–NH3, CH3SH–NH3, and in addition, in the third structure for the latter complex. In the argon matrices the most stable complexes stabilized by the O–H⋯N or S–H⋯N bond were formed. The OH, SH stretching frequencies exhibited 266, 84 cm−1 red shifts in the complexes with respect to the monomers which correlate well with the calculated frequency shifts for the most stable CH3OH–NH3, CH3SH–NH3 structures and their energies of interactions (ΔE = −18.8 kJ mol−1 for CH3OH–NH3 and ΔE = −7.1 kJ mol−1 for CH3SH–NH3). Both theoretical and experimental spectra demonstrate the large increase of the intensity of the SH stretching vibration after formation of the complex (more than 300 times), as compared to the OH stretching vibration (less than 20 times). The small intensity of the SH band in the CH3SH spectrum and its large growth in CH3SH–NH3 one are explained by atypical behavior of the dipole moment function of the SH proton donor.