Roles of Water Molecules and Counterion on HS Sensing Reaction Utilizing a Pyrylium Derivative: A Computational Study.

In this paper, we present a comprehensive computational study on the hydrogen sulfide ion (HS) sensing mechanism in aqueous solution using pyrylium-thiopyrylium transformation. Explicit water molecules up to three water molecules are considered using supramolecular models. The effect of water bulk solvent is also taken into account according to the polarizable continuum model. Our results demonstrate that water molecules are directly involved in the sensing reactions by altering reaction mechanisms and dramatically lower the activation energies. The most favorable HS sensing mechanism involves a 10-membered ring transition structure formed by three water molecules and one hydronium. The catalytic effects of water molecule(s) due to the alleviation of ring strain and the stabilization from deprotonated hydronium significantly lower the activation energy. The activation energies in aqueous solution decrease from 40.2 kcal/mol for the hydronium-only-catalyzed reaction to 15.7, 14.8, and 7.4 kcal/mol for one-water-, two-water-, and three-water-catalyzed mechanisms, respectively. In addition, the effect of the counterion tetrafluoroborate (BF) on the reaction mechanisms was also investigated. Our results demonstrate that the counterion BF most likely behaves as a spectator and has minor influence on the reaction mechanism.