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

Abstract

The potential energy profile for the atomic iodine plus water dimer reaction I + (H2O)2 → HI + (H2O)OH has been explored using the "Gold Standard" CCSD(T) method with quadruple-ζ correlation-consistent basis sets. The corresponding information for the reverse reaction HI + (H2O)OH → I + (H2O)2 is also derived. Both zero-point vibrational energies (ZPVEs) and spin-orbit (SO) coupling are considered, and these notably alter the classical energetics. On the basis of the CCSD(T)/cc-pVQZ-PP results, including ZPVE and SO coupling, the forward reaction is found to be endothermic by 47.4 kcal/mol, implying a significant exothermicity for the reverse reaction. The entrance complex I···(H2O)2 is bound by 1.8 kcal/mol, and this dissociation energy is significantly affected by SO coupling. The reaction barrier lies 45.1 kcal/mol higher than the reactants. The exit complex HI···(H2O)OH is bound by 3.0 kcal/mol relative to the asymptotic limit. At every level of theory, the reverse reaction HI + (H2O)OH → I + (H2O)2 proceeds without a barrier. Compared with the analogous water monomer reaction I + H2O → HI + OH, the additional water molecule reduces the relative energies of the entrance stationary point, transition state, and exit complex by 3-5 kcal/mol. The I + (H2O)2 reaction is related to the valence isoelectronic bromine and chlorine reactions but is distinctly different from the F + (H2O)2 system.

Chemical Kinetics Approves the Occurrence of C (3Pj) Reaction with H2O

“…In other words, the triplet PES is not affected by SOC, and there is no significant coupling between the angular momentum and spin eigenvectors (L–S coupling) of the involved electrons . This finding has several important implications: (i) as SOC is insignificant, the spin states can be considered independently and quantum mechanical effects on the energy levels can be ignored, in consistent with the previous results; (ii) spin relaxation is not feasible; (iii) ISC, which relies on the SOC interaction, is not possible on the triplet surface; and (iv) the dissociation energies of the species and the associated reaction paths are reliable because of lack of any SOC effect. ,, It should be noted that though some studies ,,,, have mentioned ISC between the triplet and singlet states, the SOC results claim that ISC from the triplet surface to the singlet PES is not probable while, based on the outlined studies, ISC from the singlet surface to the triplet surface is possible. A similar conclusion has been withdrawn by Lakshmanan et al, who studied the 3 CH2 + 3 O 2 reaction through direct dynamics simulations and stated that the triplet reaction does not provide a long time to permit coupling of the triplet and singlet surfaces.…”

Spin–Orbit Coupling via Four-Component Multireference Methods: Benchmarking on p-Block Elements and Tentative Recommendations

“…While this effect is small for light atoms (404 cm –1 for F), it becomes increasingly significant for heavier atoms (7603 cm –1 for I) . This is important in both astrochemistry and atmospheric chemistry when dealing with the binding of free atoms, particularly the binding of halogens. , SOC can significantly affect the dissociation energies of systems as simple as water–halogen complexes and plays a large role in their reaction pathways. − …”