  Numerous experiments have demonstrated that many classes of organic reactions
exhibit increased reaction rates when performed in heterogeneous water
emulsions. Despite enormous practical importance of the observed "on-water"
catalytic effect and several mechanistic studies, its microscopic origins
remains unclear. In this work, the second generation Car-Parrinello molecular
dynamics method is extended to self-consistent charge density-functional based
tight-binding in order to study "on-water" catalysis of the Diels-Alder
reaction between dimethyl azodicarboxylate and quadricyclane. We find that the
stabilization of the transition state by dangling hydrogen bonds exposed at the
aqueous interfaces plays a significantly smaller role in "on-water" catalysis
than has been suggested previously.
