Different catalysis role of in-loop and out-of-loop waters in assisting HNS/HSN proton transfer isomerizations: Bridging vs. surrounding effect

In this work, a density function theory (DFT) study is presented for the HNS/HSN isomerization assisted by 1–4 water molecules on the singlet state potential energy surface (PES). Two modes are considered to model the catalytic effect of these water molecules: (i) water molecule(s) participate directly in forming a proton transfer loop with HNS/HSN species, and (ii) water molecules are out of loop (referred to as out-of-loop waters) to assist the proton transfer. In the first mode, for the monohydration mechanism, the heat of reaction is 21.55 kcal · mol−1 at the B3LYP/6-311++G** level. The corresponding forward/backward barrier lowerings are obtained as 24.41/24.32 kcal · mol−1 compared with the no-water-assisting isomerization barrier T (65.52/43.87 kcal · mol−1). But when adding one water molecule on the HNS, there is another special proton-transfer isomerization pathway with a transition state 10T′ in which the water is out of the proton transfer loop. The corresponding forward/backward barriers are 65.89/65.89 kcal · mol−1. Clearly, this process is more difficult to follow than the R–T–P process. For the two-water-assisting mechanism, the heat of reaction is 19.61 kcal · mol−1, and the forward/backward barriers are 32.27/12.66 kcal · mol−1, decreased by 33.25/31.21 kcal · mol−1 compared with T. For trihydration and tetrahydration, the forward/backward barriers decrease as 32.00/12.60 (30T) and 37.38/17.26 (40T) kcal · mol−1, and the heat of reaction decreases by 19.39 and 19.23 kcal · mol−1, compared with T, respectively. But, when four water molecules are involved in the reactant loop, the corresponding energy aspects increase compared with those of the trihydration. The forward/backward barriers are increased by 5.38 and 4.66 kcal · mol−1 than the trihydration situation. In the second mode, the outer-sphere water effect from the other water molecules directly H-bonded to the loop is considered. When one to three water molecules attach to the looped water in one-water in-loop-assisting proton transfer isomerization, their effects on the three energies are small, and the deviations are not more than 3 kcal · mol−1 compared with the original monohydration-assisting case. When adding one or two water molecules on the dihydration-assisting mechanism, and increasing one water molecule on the trihydration, the corresponding energies also are not obviously changed. The results indicate that the forward/backward barriers for the three in-loop water-assisting case are the lowest, and the surrounding water molecules (out-of-loop) yield only a small effect. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006