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

Abstract

ABSTRACT: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 ⅐ mol Ϫ1 at the B3LYP/6-311ϩϩG** level. The corresponding forward/backward barrier lowerings are obtained as 24.41/24.32 kcal ⅐ mol Ϫ1 compared with the no-water-assisting isomerization barrier T (65.52/43.87 kcal ⅐ mol Ϫ1 ). But when adding one water molecule on the HNS, there is another special proton-transfer isomerization pathway with a transition state 10TЈ in which the water is out of the proton transfer loop. The corresponding forward/backward barriers are 65.89/65.89 kcal ⅐ mol Ϫ1 . 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 ⅐ mol Ϫ1 , 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 ⅐ mol Ϫ1 than the trihydration situation. In the second mode, the outersphere 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 ⅐ mol Ϫ1 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.

Nuclear quantum effect and H/D isotope effect on Cl· + (H2O)n→ HCl + OH·(H2O)n−1(n= 1–3) reactions

“…1 Thus, reactions involving water molecules have attracted a lot of attention in a wide variety of branches of chemistry, such as atmospheric chemistry, catalytic chemistry, surface chemistry, biochemistry, and so on. [2][3][4] In particular, reactions with halogen atoms have attracted special interest. For example, the reactions with Cl$, which is known as the major cause of the serious environmental problem of "ozone holes" in Antarctica, 5 have been earnestly studied by several groups.…”