Elucidation of Electron-Transfer/S_N2 Borderline Mechanism : Toward the Development of a New Reaction Theory

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Anomalous relation between rates and equilibria for the proton-transfer reactions of nitroalkanes is known as nitroalkane anomaly. In a typical example, the pKa value of RCH_2NO_2 decreases in the order CH_3NO_2>CH_3CH_2NO_2>(CH_3)_2CHNO_2 in water, whereas the rate of proton abstraction by hydroxide ion decreases in the same order (reaction 1). The reactions of XC_6H_4CHRNO_2 with a base are another well-known abnormal case with an abnormal Bronsted coefficient (α > 1.0) (reaction 2). Questions arise in several ways. Does the nitroalkane anomaly arise from the inherent nature of the substrates, or does the anomaly exist in the gas phase? In what way solvent molecules play the role in the anomaly? What is the origin of experimentally observed anomaly? Ab initio calculations were carried out at HF/6-31+G^*, B3LYP/6-31+G^*, MP2/6-31+G^*, and MP2/6-311+G^<**> for reactions 1 and 2. In the gas phase the reaction with OH^- gave no transition state. In contrast, the proton transfer to CN^- gave well-characterized TSs for both reactions. These reactions, however, showed no anomaly at all ; EtNO_2 is more acidic and the barrier to the proton transfer for EtNO_2 is lower than for MeNO_2. Thus, the experimentally observed anomaly was not reproduced in the gas phase. Reactions in solution were examined in two steps, first with a supermolecule model and then incorporating bulk solvent effect by using a continuum model. Transition states of the deprotonations of nitroalkanes were observed when (H_2O)_2OH^- was used as a base instead of OH^-. Activation energy-reaction energy correlation revealed that the experimental anomaly was reproduced for reaction 1. The origin of the anomaly was considered to be due to transition state imbalance.