Tunneling in H loss from energy selected ethanol ions

Abstract

The H/D loss and CH₃/CD₃ loss reactions from energy selected ethanol isotopologue ions C₂H₅OH⁺, C₂D₅OD⁺, CD₃CH₂OH⁺, and CH₃CD₂OH⁺ have been studied by imaging threshold photoelectron photoion coincidence (iPEPICO) spectroscopy. In the lowest energy dissociation channel, the α-carbon loses a hydrogen or a deuterium atom. Asymmetry in the daughter ion time-of-flight (TOF) peaks, an ab initio study of the reaction rates, and shifts in the phenomenological onsets between isotopologues revealed that H/D loss is slow at its onset. Tunneling through a reverse barrier along the reaction coordinate was found to play a significant role. Modeling the data with an Eckart barrier suggests that H loss from light ethanol ions proceeds via a reverse barrier of 151 meV, which agrees very well with the ab initio result of 155 meV. The higher energy methyl loss channel appears at its thermochemical threshold, but the branching ratios for methyl and H loss as a function of the ion internal energy are not entirely consistent with statistical theory. The methyl-loss signal cannot completely outcompete the hydrogen atom loss process. The shape of the photoelectron spectrum as well as our calculations indicate that the lowest energy ethanol ion structure lies considerably below the reported IE of 10.48 eV. Franck–Condon factors are favorable for ionization to a metastable ion state, which can rearrange to a more stable equilibrium structure. Combining theoretical results with previous experimental ones yields a revised ethanol adiabatic ionization energy of 10.37 eV. This applies to all isotopologues, as the isotope effect on the ionization energy is not more than a few meV.