N–H···S Interaction Continues To Be an Enigma: Experimental and Computational Investigations of Hydrogen-Bonded Complexes of Benzimidazole with Thioethers

The N–H···S hydrogen bond, even though classified as an unconventional hydrogen bond, is found to bear important structural implications on protein structure and folding. In this article, we report a gas-phase study of the N–H···S hydrogen bond between the model compounds of histidine (benzimidazole, denoted BIM) and methionine (dimethyl sulfide, diethyl sulfide, and tetrahydrothiophene, denoted Me2S, Et2S, and THT, respectively). A combination of laser spectroscopic methods such as laser-induced fluorescence (LIF), two-color resonant two-photon ionization (2cR2PI), and fluorescence depletion by infrared spectroscopy (FDIR) is used in conjunction with DFT and ab initio calculations to characterize the nature of this prevalent H-bonding interaction in simple bimolecular complexes. A single conformer was found to exist for the BIM–Me2S complex, whereas the BIM–Et2S and BIM–THT complexes showed the presence of three and two conformers, respectively. These conformers were characterized on the basis of IR spec...