Evaluation of the Arrhenius Parameters for the Thermal Isomerization of a Gaseous Ion. The Temperature Dependence of the Cyclohexylium Ion Unimolecular Rearrangement

Article abstract of DOI:10.1021/ja00198a005

Gas-phase isomerization of cyclohexylium ion (1), obtained via protonation of cyclohexene and of bicyclo<3.1.0>hexane and via hydride-ion abstraction from cyclohexane by CnH5(1+) (n = 1,2) and by s-C3H7(1+) ions, has been investigated by a radiolytic technique in CH4 and C3H8 in the pressure range from 106 to 1480 Torr.Temperature-dependence studies in the temperature interval from 310 to 353 K have provided the activation parameters of the thermal rearrangement of 1 into 1-methylcyclopentylium ion (2) in CH4 at 750 Torr and in C3H8 at 1480 Torr.Least-squares analysis of the two mutually consistent sets of data leads to an activation energy of 7.4 +/- 1 kcal mol^-1 and to a preexponential factor of 1E12+/-1.3 s^-1.These values represent the first estimates of the activation parameters of a unimolecular isomerization of a gaseous ion thermally equilibrated with the bath gas, i.e., under conditions which allow meaningful comparison with solution kinetics.The results support previous evidence for the existence of gaseous 1, providing a quantitative evaluation of the energy barrier for its rearrangement into 2.The more extensive formation of 2 from the protonation of bicyclo<3.1.0>hexane than that of cyclohexene is consistent with the long-postulated protonated-cyclopropane route in the branching rearrangement of cycloalkylium ions.