Theoretical study of the structure and unimolecular decomposition pathways of ethyloxonium, [CH 3 CH 2 OH 2 ] +

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 ([CH 3 CH 2 OH 2 ] + ) 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 , CH 4 , H 2 O or C 2 H 4 . 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 CH 4 loss, respectively. On the other hand, elimination of either C 2 H 4 or H 2 O 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 C 2 H 4 elimination reaction, acts as an intermediate in the proton‐transfer reaction connecting [C 2 H 5 ] + +H 2 O and C 2 H 4 + [H 3 O] + and plays an important role in the isotopic scrambling that has been observed experimentally in the elimination of either H 2 O or C 2 H 4 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 .