Electron Photodetachment Spectroscopy of Solvated Anions:  RO·HF^-or ROH·F^-?

Abstract

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 ROH 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.

Chemical ionization pathways of polyfluorinated chemicals—A connection to environmental atmospheric processes

“…As shown in Scheme 1, it is believed that a percentage of the alcohol molecules first undergo dissociative electron capture to produce F Ϫ , a process that is common for fluorinated materials [27]. An alcohol molecule is then observed to complex with the F Ϫ in a fashion similar to that shown by Brauman et al [28] The telomer alcohols, in comparison to the 7:1 alcohol, do not appear to undergo any appreciable dimerization. This is likely due to decreased hydrogen bonding between molecules which results from the decreased acidity of the -OH group when the CF 2 moiety is in the ␤ position rather than the ␣ position relative to it.…”

The change in hydrogen bond strength accompanying charge rearrangement: Implications for enzymatic catalysis

“…However, rate enhancement relative to the solution reaction requires that the strengthening of the H bond to enzymatic catalytic groups be greater than that to water. The steeper dependences of H bond strength on ⌬pK a generally observed in organic solvents and the gas phase than in water suggested that this is possible (10,11,(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)36). However, we were aware of no direct comparison of these dependencies in nonaqueous and aqueous media.…”

“…DMSO may therefore provide a crude mimic for the environment in an enzymatic interior, despite its high bulk dielectric constant. In general, larger Brønsted slopes for H bonds are observed in nonaqueous solutions than in water (10,11,(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)36).…”

Self-Terminating, Oxidative Radical Cyclizations

“…In a comparative order of magnitude are also the bond strengths in aliphatic ethers (340 kJ mol −1 ) [ 89 ]. The O−H bond in aliphatic alcohols is still considerably stronger (435 kJ mol −1 ) [ 91 , 92 ]. With these data in mind, comparison with the results in Table 2 revealed that the ratio of the epoxy ketones 13a/b vs. spiro ketone 14 directly reflected the energy of the O−X bond.…”