THEORETICAL-STUDY OF THE STRUCTURE AND UNIMOLECULAR DECOMPOSITION PATHWAYS OF ETHYLOXONIUM, [CH3CH2OH2]+

Ab initio molecular orbital calculations with split-valence plus polarization basis sets and incorporating valence-electron correlation have been performed to determine the equilibrium structure of ethyloxonium ([CH3CH2OH2]+) and examine its modes of unimolecular dissociation. An asymmetric structure (1) is predicted to be the most stable form of ethyloxonium, but a second conformational isomer of C(s) symmetry lies only 1.4 kJ mol-1 higher in energy than 1. Four unimolecular decomposition pathways for 1 have been examined involving loss of H-2, CH4, H2O or C2H4. The most stable fragmentation products, lying 65 kJ mol-1 above 1, are associated with the H-2 elimination reaction. However, large barriers of 257 and 223 kJ mol-1 have to be surmounted for H-2 and CH4 loss, respectively. On the other hand, elimination of either C2H4 or H2O from ethyloxonium can proceed without a barrier to the reverse associations and, with total endothermicities of 130 and 160 kJ mol-1, respectively, these reactions are expected to dominate at lower energies. A second important equilibrium structure on the surface is a hydrogen-bridged complex, lying 53 kJ mol-1 above 1. This complex is involved in the C2H4 elimination reaction, acts as an intermediate in the proton-transfer reaction connecting [C2H5]+ + H2O and C2H4 + [H3O]+ and plays an important role in the isotopic scrambling that has been observed experimentally in the elimination of either H2O or C2H4 from ethyloxonium. The proton affinity of ethanol was calculated as 799 kJ mol-1, in close agreement with the experimental value of 794 kJ mol-1.