Hydrogen Abstraction Reaction H2Se + OH → H2O + SeH: Comparison with the Analogous Hydrogen Sulfide and Water Reactions

The mechanism for the reaction of hydrogen selenide (H2Se) with the OH radical is examined using the "gold standard" CCSD(T) method along with the correlation consistent basis sets up to aug-cc-pV5Z. For the Se atom, the corresponding basis sets are combined with the core relativistic pseudopotential, designated as aug-cc-pVnZ-PP (n = D, T, Q, and 5). The predicted geometries and vibrational frequencies for reactants and products agree well with the available experimental results. For the entrance complex, the two-center three-electron hemibonded H2Se···OH structure (RC-B) is predicted to be the lowest. This structure lies 5.1 kcal/mol below the separated reactants H2Se + OH without zero-point energy correction. For the analogous H2S and H2O reactions, the entrance complexes lie 3.3 and 5.7 kcal/mol, respectively, below the reactants. The relative energies of the transition states are -1.7 (H2Se), +0.1 (H2S), and +9.5 (H2O) kcal/mol with respect to the appropriate reactants. The three reaction exoergicities are 40.8 (H2Se), 29.5 (H2S), and 0.0 (H2O) kcal/mol. Compared to the separated products XH + H2O (X = O, S, and Se), the exit well depths are 2.1 (Se), 2.9 (S), and 5.7 (O) kcal/mol. Intrinsic reaction coordinate analysis indicates that all stationary points were correctly identified. Several other stationary points were identified and analyzed. Finally, 29 density functional theory methods were tested systematically to explore their ability in describing the potential energy surface of the H2Se + OH reaction. The hemibonded H2Se···OH entrance complex should be observable, initially via matrix isolation spectroscopy.