A low energy crossed beam study of the proton transfer reactions of H3O+ with CH3OH and C2H5OH

Abstract

We present a study of the title reactions over the range from 0.76 to 2.75 eV for proton transfer to methanol and at 0.99 and 1.84 eV for the ethanol system. The dynamics of proton transfer are direct at all collision energies, with the ionic products scattered in the backward direction relative to the incoming ion. The average fraction of the available energy appearing in product translation increases from 0.25 for CH3OH2+ production at 0.76 eV to 0.44 at 2.75 eV. At 0.99 eV, the corresponding fraction for protonation of ethanol is 0.26 and increases to 0.49 at 1.84 eV. The translational energy distributions show depletion of intensity at low values of ET′, suggestive of unimolecular decay of the protonated alcohols via C–O bond cleavage and elimination of H2 across the C–O bond. Isotope effect measurements for protonation of CH3OD and subsequent elimination of H2 vs HD indicate the importance of a large hydride transfer contribution to the isotope effect. These results, along with statistical calculations, are employed to estimate the exit channel barrier with elimination process. Anomalously large elimination isotope effects (kH2/kHD=7) arise because of the near confluence of the CO bond cleavage and HD elimination thresholds. We find that the H2 elimination barrier lies between 2.8 and 2.9 eV, within experimental error equal to the endothermicity of the C–O bond cleavage reaction. This result is in agreement with recent calculations of Nobes and Radom and explains the nonoccurrence of the condensation reaction between CH3+ and H2O.