Experimental and computational studies of deuterated ethanols: gas-phase acidities, electron affinities and bond dissociation energies

Abstract

The tandem flowing afterglow-selected ion flow tube has been employed to measure the gas-phase acidities of a family of deuterated ethanols. We find that both α and β deuterated ethanols are weaker acids than undeuterated ethanol with α deuteration having a more pronounced effect. The acidities, relative to undeuterated ethanol [Δ H acid,298 K (CH 3CH 2OH) = 377.5 ± 2kcal mol −1] are ΔΔ H acid(CD 3CH 2OH) = 0.20 ± 0.15kcal mol −1, ΔΔ H acid(CH 3CD 2OH) = 0.35 ± 0.15 kcal mol −1, and ΔΔ H acid0(CD 3CD 2OH) = 0.50 ± 0.15 kcal mol −1. In a separate set of measurements, we have studied the negative ion photoelectron spectroscopy of a set of ethoxide ions to determine the electron affinities of the corresponding ethoxy radicals. The electron affinities decrease with increasing deuterium substitution. We find: EA(CH 3CH 2O) = 39.55 ± 0.16kcal mol −1, EA(CH 3CD 2O) = 39.48 ± 0.16kcal mol −1, EA(CD 3CH 2O) = 39.41 ± 0.16kcal mol −1, and EA(CD 3CD 2O) = 39.27 ± 0.16 kcal mol −1; the relative error bars are ±0.05 kcal mol −1. The corresponding O-H bond strengths of the deuterated ethanols can be extracted from these acidities and electron affinities. We find that α deuteration increases the OH bond strength by 0.3 ± 0.2 kcal mol −1 while β deuteration has a negligible effect. The bond dissociation energies, relative to undeuterated ethanol [ D 0,OK (CH 3CH 2OH) = 102.2kcal mol −1] are D 0,OK(CD 3CH 2OH) = 102.3 ±0.2kcal mol −1, D 0,OK(CH 3CD 2OH) = 102.5 ± 0.2kcal mol −1, and D 0,OK(CD 3CD 2OH) = 102.5 ± 0.2kcal mol −1. Indicated error bars are relative errors; absolute errors are ±2 kcal mol −1. Hartree-Fock-SCF calculations were performed on the various deuterated and undeuterated ethanols, ethoxy radicals, and ethoxide anions to calculate the relative acidities, electron affinities and bond dissociation energies. The results are in good agreement with our experimental values.