Aqueous radicals exhibit solute-solvent interactions that differ from those of analogous closed-shell molecules in solution. Yet limited experimental or theoretical data are available to deduce the electronic features and chemical properties of radical species in aqueous solution. We elucidated the nature of intermolecular binding in 14 binary complexes each containing a single water molecule complexed with either a small radical or an analogous closed-shell species, using basis set-extrapolated coupled cluster computations up to CCSDT(Q) excitations and density functional theory with natural bond orbital (NBO) analysis. Studied radicals include the electron-deficient species F●, Cl●, Br●, CH3●, and NH3●+, and also the main group metal species Li●, Na●, Be●+, and Al●+. Our results reveal that many radical-water complexes exhibit substantially increased binding energies compared to analogous closed-shell complexes. These complexes exhibit delocalization of the unpaired electron, which can be directly deduced from the visualized spin density. In the NBO framework, delocalization and increased binding energies are attributed to increased donor-acceptor interactions relative to the analogous closed-shell complexes. Our results have broad implications for modeling radicals in solution. For example, delocalization of the unpaired electron often contributes to stabilization of radical species in solution, according to our explicit solvent simulations of several organic radicals in water. The nature of the interaction between radicals and aqueous solvent deserves further study.

Modeling radical interactions with water

253rd National Meeting (2017)

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