Water-Assisting Proton Transfer Isomerization of the HNO/HON System in the Singlet State: On the Number of the Effective Water Molecules

Abstract

A density function theory study is presented for the HNO ⇌ HON isomerization assisted by m water molecules (m = 1−4) on the singlet state potential energy surface. Two modes are considered to model the catalytic effect of m water molecules: (1) water molecule(s) directly participate in forming a proton transfer loop with HNO/HON species and (2) water molecules are out-of-loop, modeling the outer-sphere water effect from the other water molecules directly H-bonded to the loop (referred to as out-of-loop waters). Two mechanisms are proposed for the mono-water-assisting isomerizations and one for each of the other cases. The reactant and product of all groups have been characterized for all potential energy surfaces. For the monohydration mechanism, the reactant complex is connected to the product complex via two determined saddle points, and the reaction heat is 35.5 kcal·mol-1 at the B3PW91/6-311++G** level. The corresponding forward/backward barrier lowerings are obtained as 7.4/1.2 (AT2) and 34.1/28.3 (...