Electron paramagnetic resonance free induction decay attenuation and muon spin rotation measurements were performed in the temperature range of liquid water for the reactions of the hydrogen isotopes D, H, and Mu with undeuterated and deuterated formate ions. Accurate rate constants were determined, and excellent Arrhenius behavior represented by
was found in all cases. Ab initio calculations at the MP2 and the QCISD level with the aug-cc-pvDZ basis set
reveal that the reaction has no electronic barrier in the gas phase. This contrasts with quite sizeable activation
energies observed in aqueous solution, and it suggests that the barrier is entirely solvent induced. Calculations
at the above mentioned ab initio level using a polarized dielectric continuum for the solvated reaction system
restore a realistic barrier and confirm this interpretation. It is shown that the solvent effect is a consequence of
a pronounced change of polarization of the system along the reaction path. It may be more appropriate to
describe the reaction as a consecutive electron–proton transfer rather than an H atom abstraction.
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