The Reaction between Bromine and the Water Dimer and the Highly Exothermic Reverse Reaction

Abstract

The entrance complex, transition state, and exit complex for the bromine atom plus water dimer reaction Br + (H2O)2 → HBr + (H2O)OH and its reverse reaction have been investigated using the CCSD(T) method with correlation consistent basis sets up to cc-pVQZ-PP. Based on the CCSD(T)/cc-pVQZ-PP results, the reaction is endothermic by 31.7 kcal/mol. The entrance complex Br⋯(H2O)2 is found to lie 6.5 kcal/mol below the separated reactants. The classical barrier lies 28.3 kcal/mol above the reactants. The exit complex HBr⋯(H2O)OH is bound by 6.0 kcal/mol relative to the separated products. Compared with the corresponding water monomer reaction Br + H2 O → HBr + OH, the second water molecule lowers the relative energies of the entrance complex, transition state, and exit complex by 3.0, 3.8, and 3.7 kcal/mol, respectively. Both zero-point vibrational energies and spin-orbit coupling effects make significant changes to the above classical energetics. Including both effects, the predicted energies relation to separated Br + (H2O)2 are -3.0 kcal/mol [Br···(H2O)2 ], 28.2 kcal/mol [transition state], 26.4 kcal/mol [HBr···(H2O)OH], and 30.5 kcal/mol [separated HBr + (H2O)OH]. The potential energy surface for the Br + (H2O)2 reaction is related to that for the valence isoelectronic Cl + (H2O)2 system but radically different from the F + (H2O)2 system.

I + (H2O)2 → HI + (H2O)OH Forward and Reverse Reactions. CCSD(T) Studies Including Spin–Orbit Coupling

“…It is also instructive to compare the I + (H 2 O) 2 reaction with the analogous Br + (H 2 O) 2 , Cl + (H 2 O) 2 , and F + (H 2 O) 2 reactions. − Structurally, the five stationary points for the I + (H 2 O) 2 reaction (Figure ) are qualitatively related to those for the Br + (H 2 O) 2 (ref ), Cl + (H 2 O) 2 (ref ), and F + (H 2 O) 2 reactions (ref ). Energetically, the landscape profiles for these four reactions are also shown in Figure .…”

The reaction between the bromine atom and the water trimer: high level theoretical studies

“…The computational methods used in present paper are similar to those successfully used for the reaction of bromine atom plus water dimer. 16 Briefly, the stationary points for the Br + (H 2 O) 3 reaction were first investigated with MPW1K density functional theory (DFT) method, which was recommended by Truhlar et al 19 and gave the best predictions for the F + H 2 O reaction barrier among 49 DFT functionals. 20 The basis sets used here with the MPW1K method are the Dunning correlation-consistent polarized valence basis sets (cc-pVnZ).…”

“…We also compare the Br + (H 2 O) 3 reaction with the simpler bromine reactions Br + H 2 O/(H 2 O) 2 and with the valence isoelectronic water trimer reactions F/Cl + (H 2 O) 3 . 2,16–18…”

“…15 The bromine atom plus water polymer reactions may be more important, for the water polymers detected in the atmosphere may lower the reaction barriers, as shown by our Br + (H 2 O) 2 results. 16 To understand energy trends ranging from the water monomer to water polymers, we herein investigate the reaction between the bromine atom and the water trimer, Br + (H 2 O) 3 -HBr + (H 2 O) 2 OH. This reaction was investigated using the ''gold standard'' CCSD(T) method with basis sets as large as cc-pVQZ(-PP).…”