Electron photodetachment spectroscopy of solvated anions: RO center dot HF- or ROH center dot F-?

Electron photodetachment spectra of three alkoxide ions, 2,2-dimethyl-3-pentoxide, 3,3-dimethyl-2-butoxide, trifluoroethoxide, and two complex ions, 3,3-dimethyl-2-butanol/fluoride, 2,2-dimethyl-3-pentanol/fluoride, have been measured using an ion cyclotron spectrometer to generate, trap, and detect the ions. The electron affinities of trifluoroethoxide, 2,2-dimethyl-3-pentoxide, and 3,3-dimethyl-2-butoxide were measured as 58.9+/-0.1, 47.8+/-0.3, and 45.7+/-0.1 kcal mol(-1), respectively. Using known proton affinities of the three alcohols allows us to determine the O-H bond dissociation energies. The threshold detachment energies for 2,2-dimethyl-3-pentanol/fluoride and 3,3-dimethyl-2-butanol/fluoride complex ions are 70.3+/-0.5, and 70.1+/-0.5 kcal mol(-1), respectively. The fluoride binding energies are thus on the order of 30 kcal mol(-1). We show that the observation or non-observation of photodetachment for alcohol/fluoride ions depends only on the alcohol acidity. In complex ions of the form ROHF(-), if ROH is more acidic than HF the structure is RO(-). HF, and if Roll is less acidic than HF the structure is ROH . F-. A flat potential energy surface model is consistent with the experimental results. The experimental results show no evidence for unusual structural change or stabilization when a hydrogen bond forms between two acids with similar acidities.