Stability, reactivity and dynamic study of switchable intermolecular hydrogen bonding in dinitrobenzene isomers: A theoretical – Experimental approach

Abstract

Among several non-covalent weak interaction(s), hydrogen bonding is predominantly considered as a key factor in some biological events i.e. information transfer at intra cellular and/or intra/inter organelle level. Dinitroaromatics contain –NO2 group(s) which strongly facilities electron and/or proton transfer, could be envisaged to be used as an active group in a drug molecule. We have decided to investigate dinitroaromatics as model substances for the following reasons (a) low reduction potential, (b) multi-electron accepting ability of NO2 groups, (c) formation of stable anion radicals and dianions. Linear scan and cyclic voltammetry were used for study. It was found that DNBs in the presence of HB-agents exhibit switchable H-bonding via electro-reduction steps and formation of DNB−./2-⋯(HB)n or m. HB strength significantly perturbs chemical equilibrium and/or kinetic process, hence, an array of strong, intermediate and weak type interactions was considered for investigation. Experimental values of stability constant, Keq, rate constant of formation of H-bond, k, and number of bonds formed were evaluated for corresponding process(es) (i) DNB−.+n[HB]⇄DNB−.(HB)n and (ii) DNB2−+(m−n)[HB]⇄DNB2−(HB)m−n. Values of Keq for processes (i) and (ii) were found to be within the range of 1.9–9.0 M-n and 7.0–9.4 × 104 M-m, whereas, k values were moderate to high as 0.23–26.1 M-n.s−1 and 3.3–6.5 × 106 M-m.s−1, respectively. A more plausible assumption is the existence of 1:1 or 1:2 associates between (X)DNB2− (where, X = 1,2-,1,3-,1,4-) and HB-agents. Quantum chemical calculations were carried out for (X)DNB−./2-⋯(HB)n or m, vis-a-vis geometry optimization leading to better understanding of structural array in three dimensional space.