Experimental and Theoretical Studies of the F^• + H-F Transition-State Region by Photodetachment of [F-H-F]

The transition-state (TS) region of the simplest heavy-light-heavy type of reaction, F^• + H-F → F-H + F^•, is investigated in this work by a joint experimental and theoretical approach. Photodetaching the bifluoride anion, [F···H···F]^-, generates a negative ion photoelectron (NIPE) spectrum with three partially resolved bands in the electron binding energy (eBE) range of 5.4-7.0 eV. These bands correspond to the transition from the ground state of the anion to the electronic ground state of [F-H-F]^• neutral, with associated vibrational excitations. The significant increase of eBE of the bifluoride anion, relative to that of F^-, reflects a hydrogen bond energy between F^- and HF of ∼46 kcal/mol. Theoretical modeling reveals that the antisymmetric motion of H between the two F atoms, near the TS on the neutral [F-H-F]^• surface, dominates the observed three bands, while the F-H-F bending, F-F symmetric stretching modes, and the couplings between them are calculated to account for the breadth of the observed spectrum. From the NIPE spectrum, a lower limit on the activation enthalpy for F^• + H-F → F-H + F^• can be estimated to be ΔH^‡ = 12 ± 2 kcal/mol, a value below that of ΔH^‡ = 14.9 kcal/mol, given by our G4 calculations.