Nitroalkane anomaly: computational study with cluster and continuum modeling

Abstract

The origin of anomalous relation between rates and equilibria for the proton-transfer reactions of nitroalkanes, known as nitroalkane anomaly, was investigated by theoretical calculations with a cluster model, in which three water molecules are explicitly considered in the system, as well as the PCM solvent continuum model. For the CH3NO2/CH3CH2NO2 system, B3LYP and MP2 computations reproduced the anomaly, and the imbalance in the charge distribution at the transition state (TS) was observed. In contrast, although the TS imbalance was detected for the substituted phenylnitromethane system, the Bronsted plots did not show any anomaly. The experimentally observed abnormally large Bronsted coefficient (α ~ 1.4) for this system does not arise from the charge imbalance at the saddle point structure, but is likely due to the effect of the reaction dynamics.