Showing papers by "Sanjay Wategaonkar published in 2016"

Water bridges anchored by a C–H⋯O hydrogen bond: the role of weak interactions in molecular solvation

Abstract: The imidazole group, characterized by an activated C(2)-H bond sandwiched between two N atoms, occurs in several biomolecules including alkaloids, amino acids, and nucleobases. The speculated role of this potential hydrogen bond donor in shaping the solvation shell around the neutral imidazole moiety, however, remains unidentified. In contrast, hydrogen bonding and electrostatic interactions are commonly observed in the imidazolium cation where the acidity of the C(2)-H bond is markedly enhanced. Here, we show direct evidence of the weakly activated C(2)-H bond in shaping the solvation shell of neutral imidazole, via spectroscopic characterization of the water clusters (Wn, n = 2-4) of two model compounds, benzimidazole (BIM) and N-methylbenzimidazole (MBIM). Infrared spectra in the OH, NH, and CH stretching regions allow unambiguous detection of a N-WW-C(2)H binding motif in the doubly hydrated cluster of both molecules. Remarkably, H-bonded water bridges between the N atom and N-H bond in BIM-W3,4 clusters are switched to the C(2)-H bond in MBIM-W3,4 with comparable binding strength, indicating that the weakly activated C(2)-H bond in the neutral imidazole moiety can serve as a potent H-bond donor.

Acid–Base Formalism Extended to Excited State for O–H···S Hydrogen Bonding Interaction

Abstract: Hydrogen bond can be regarded as an interaction between a base and a proton covalently bound to another base. In this context the strength of hydrogen bond scales with the proton affinity of the acceptor base and the pKa of the donor, i.e., it follows the acid–base formalism. This has been amply demonstrated in conventional hydrogen bonds. Is this also true for the unconventional hydrogen bonds involving lesser electronegative elements such as sulfur atom? In our previous work, we had established that the strength of O–H···S hydrogen bonding (HB) interaction scales with the proton affinity (PA) of the acceptor. In this work, we have investigated the other counterpart, i.e., the H-bonding interaction between the photoacids with different pKa values with a common base such as the H2O and H2S. The 1:1 complexes of five para substituted phenols p-aminophenol, p-cresol, p-fluorophenol, p-chlorophenol, and p-cyanophenol with H2O and H2S were investigated experimentally and computationally. The investigations we...