Experimental evidence of O-H-S hydrogen bonding in supersonic jet

Experimental evidence is presented for the O-H-S hydrogen bonding in the complexes of simple model compounds of methionine (dimethyl sulphide) and tyrosine (phenol, p-cresol, and 2-naphthol). The complexes were formed in the supersonic jet and were detected using resonantly enhanced multiphoton ionization spectroscopy. In all the complexes, the band origins for the S-1-S-0 electronic transition were redshifted relative to that of their respective monomers. The resonant ion depletion IR spectra of all the complexes show redshifts of 123-140 cm(-1) in the O-H stretching frequency, indicating that the OH group acts as the hydrogen bond donor and sulfur as an acceptor. The density functional theory calculations also predict the stable structures in support of this and predict the redshifted O-H stretching frequency in the complex. The atoms-in-molecules and natural bond orbital calculations confirm the O-H-S hydrogen bonding interaction. The significant finding of this study is that the magnitudes of redshifts in the O-H stretch in the O-H-S hydrogen bonded complexes reported here are comparable to those reported for the O-H-O hydrogen bonded complexes where H2O acts as the H-bond acceptor, which suggests that the OH-S interaction is perhaps as strong as the OH-O interaction. To the best of our knowledge, this is the first such report on the O-H-S hydrogen bonded complexes.