Insight from energy decomposition analysis on a hydrogen-bond-mediated mechanism for on-water catalysis

Abstract

Many classes of organic reactions exhibit a remarkable increase in reaction rates when they occur at the water–organic interface. Although this observed ‘on-water’ catalysis has been extensively studied, the suggested mechanisms still do not explain some of the experimental findings. The mechanism proposed by Jung and Marcus (JACS 129, 5492 (2007)) involves stabilising the transition-state (TS) complex via H-bonds to ‘dangling’ interfacial water molecules. Although the reactants also experience H-bonding to interfacial water molecules in the reactant configuration, it has been argued that the H-bonds are enhanced, in terms of number and strength, in the TS. Therefore, the observed decrease in activation energy has been attributed to this preferential enhancement of H-bonds which leads to a more pronounced TS stabilisation. We employ energy decomposition analysis using the method of absolutely localised molecular orbitals to study this proposition. We find that H-bonds to interfacial water molecules are equivalent in the TS and reactant configurations. Nevertheless, these H-bonds result in significantly enhanced charge-transfer between the reactants in the TS complex, which rationalises the decrease in activation energy.