Hydrogen-Bonding Mediated Reactions of Criegee Intermediates in the Gas Phase: Competition between Bimolecular and Termolecular Reactions and the Catalytic Role of Water.

Criegee intermediates have substantial Zwitterionic character and interact strongly with hydrogen-bonding molecules like H2O, NH3, CH3OH, etc. Some of the observed reactions between Criegee intermediates and hydrogen-bonding molecules exhibit third-order kinetics. The experimental data indicate that these termolecular reactions involve one Criegee intermediate and two hydrogen-bonding molecules; quantum chemistry calculation shows that one of the hydrogen-bonding molecules acts as a catalytic bridge, which receives a hydrogen atom and donates another one. In this Feature Article, we will discuss the roles of the hydrogen-bonding molecules and the trend of the reactivity for the title reactions. To better predict the competition between a catalyzed reaction (a termolecular process) and its bare reaction (a bimolecular process), we analyzed the free energy landscape of the competing reaction paths under pseudo-first-order conditions. The results indicate that the entropy reduction in the translational degrees of freedom is the main cause to hinder a catalyzed termolecular process under typical experimental concentrations at near ambient temperatures. For such a termolecular process to be significant, its energy gain (barrier lowering) by adding the catalytic molecule has to be large enough to compensate the corresponding entropy cost. One great advantage of this analysis is that the translational entropy only depends on simple parameters like temperature, reactant masses, and concentrations and thus can be easily estimated.