Unimolecular Reactions of Proton-Bound Cluster Ions: Competition between Dissociation and Isomerization in the Ethanol−Acetonitrile Dimer

The proton-bound dimer of acetonitrile and ethanol, (CH 3 CN)(CH 3 CH 2 OH)H +, exhibits three unimolecular reactions on the microsecond time scale: two simple bond cleavage reactions to form CH 3 CNH + + CH 3 CH 2 OH and CH 3 CH 2 OH 2 + + CH 3 CN, and the loss of water to form CH 3 CNCH 2 CH 3 + . The latter process is preceded by the isomerization of the proton-bound dimer to a second isomer, (CH 3 CNCH 2 CH 3 )(H 2 O) + . The competition between the simple dissociation reactions and the isomerization reaction was modeled with ab initio calculations and RRKM theory to obtain relative energies for the reaction surface. The 0 K binding energy of the (CH 3 CN)(CH 3 CH 2 OH)H + complex was calculated to be 152 kJ mol -1 at the G2(MP2,SVP) level of theory (relative to the dissociation products CH 3 CNH + and CH 3 CH 2 OH). The isomerization barrier for the proton-bound dimer was estimated to be 22 kJ mol -1 lower than CH 3 CNH + + CH 3 CH 2 OH. The greater polarizability of the ethyl group is believed to stabilize this transition structure over that found for the (CH 3 CN)(CH 3 OH)H + ion (which was previously estimated to lie 6 kJ mol -1 below CH 3 CNH + and CH 3 OH).