Aditi Bhattacherjee, Sanjay Wategaonkar

Although the hydrogen bonding properties of the protic solvents are comparable, a comparison of the gas-phase structures of water with those of the alcohols reveals certain similarities as well as differences. In this work we report the microsolvation studies of imidazole derivatives, Benzimidazole (BIM) and N-methylbenzimidazole (MBIM) by methanol (M) and ethanol (E) in supersonic jet using electronic and vibrational spectroscopy. The cluster sizes up to 1:2 of BIM/MBIM with methanol and ethanol, and up to 1:3 in the case of MBIM-methanol were observed. Both the N-H···O and O-H···N bound structures were observed for the BIM-M1 and BIM-E1 complexes. The O-H···N bound structures of 1:1 complexes of MBIM were relatively more stable than their BIM counterparts by about ~ 0.25 kcal mol(-1). Three distinct conformations (anti, gauche and gauche') were identified for the OH···N bound complexes of BIM-E1 and MBIM-E1. IR spectroscopy of the doubly and triply-solvated clusters (namely BIM-M2, MBIM-M2,3 and MBIM-E2) gives unequivocal proof of H-bonded bridges that originate from the N acceptor and terminate at the C(2)-H group, similar to the analogous water clusters. These studies confirm that the C(2)-H in imidazole plays an important role in its solvation, particularly in the case of polar solvents. Quantum chemical calculations performed at the DFT (B3LYP as well as dispersion-corrected functionals) and MP2 levels corroborate the experimental findings. Comparison of the QTAIM and NBO parameters for the pairwise solvent interactions in the clusters with those of methanol and ethanol homodimers reflects the co-operative nature of these H-bonding interactions.