The principle of nonperfect synchronization: recent developments

This chapter describes the developments regarding the principle of nonperfect synchronization (PNS). The foundations of the PNS rest mainly on a relation between two fundamental concepts of physical organic chemistry: the concept of intrinsic barriers and the concept of transition state imbalances. The benefit of determining intrinsic barriers or intrinsic rate constants as the measures of chemical reactivity is that they can be used to describe the reactivity of an entire reaction family, irrespective of the thermodynamic driving force of a particular member of that family, and to make comparisons between different families. Because the observed trends in the intrinsic barriers can plausibly be explained by assuming that charge delocalization lags behind proton transfer, this may be taken as evidence for the existence of imbalance. The higher intrinsic rate constant for 1,1-dinitroethane compared to that for CH3NO2 is open to two interpretations, but both are related to the steric hindrance of the coplanarity of the two nitro groups in the anion. Even though the intrinsic barriers of the gas-phase reactions depend on the same factors as solution-phase proton transfers—such as resonance, polar, and polarizability effects—the electrostatic effects involving the proton-in-flight constitute an important additional factor. © 2010, Elsevier Ltd. All rights reserved.