Single-parameter quantification of the sensitivity of a molecular collision to molecular polarization

This article introduces the concept of intrinsic entropy, S, of a molecular collision. Defined in rigorously quantum mechanical terms as the von Neumann entropy of the intrinsic density matrices of reagents and products, the intrinsic entropy is a dimensionless number in the 0 < or = S < or = 1 range. Its limits are associated with situations where the collision cross section is due to a single combination of reagent and product polarizations (S = 0) or where there is absolutely no selectivity with respect to the molecular polarizations (S = 1). The usefulness of the intrinsic entropy as a quantifier of the sensitivity of a molecular collision to molecular polarizations is demonstrated with examples for the benchmark H + D(2) reaction.