Multifaceted Stereoselectivity in Polyatomic Reactions.

Stereoselectivity or stereorequirement refers to the enhancement of chemical reactivity resulting from the preferential alignment/orientation of the colliding reactants. This concept is deeply embodied in the pre-exponential A-factor of the Arrhenius rate expression or the entropy effect of thermal kinetics in physical chemistry textbooks. To understand its dynamical consequence and seek for its mechanistic origin, two different approaches of either selecting the rotational states of the reactant or aligning/orienting the reactant in the laboratory have traditionally been taken. Due to the experimental challenges, theory is far more advanced than experiment. However, even for the simple atom + diatom reaction, the physical interpretations of the calculated results can sometimes be ambiguous or controversial because of the entangled potential and kinematic factors. In this Feature Article, we try to experimentally tackle the problem for reactions with polyatomic reactants by adopting both approaches in parallel for the same reaction. By comparing the results from the two approaches as well as contrasting them with the analogous reactants-here, a symmetric-top CHD3 versus a spherical-top CH4, deeper physical insights are gained, which paves the road for future studies of complex systems and for establishing a more complete conceptual framework.