On the model dependence of kinetic shifts in unimolecular reactions: the dissociation of the cations of benzene and n-butylbenzene.

Statistical adiabatic channel model/classical trajectory (SACM/CT) calculations have been performed for transitional mode dynamics in the simple bond fission reactions of C(6)H(6)(+) --> C(6)H(5)(+) + H and n-C(6)H(5)C(4)H(9)(+) --> C(7)H(7)(+) + n-C(3)H(7). Reduced-dimensionality model potentials have been designed that take advantage of ab initio results as far as available. Average anisotropy amplitudes of the potentials were fitted by comparison of calculated specific rate constants k(E,J) with measured values. The kinetic shifts of the calculated k(E) curves and the corresponding bond energies E(0)(J=0), derived as 3.90 +/- 0.05 eV for C(6)H(6)(+) and 1.78 +/- 0.05 eV for n-C(6)H(5)C(4)H(9)(+), were in good agreement with literature values from thermochemical studies. Kinetic shifts from fixed tight activated complex Rice-Ramsperger-Kassel-Marcus (RRKM) theory, which also reproduces the measured k(E), were larger than the present SACM/CT results as well as earlier results from variational transition state theory (for C(6)H(6)(+)). The approach using RRKM theory was found to underestimate E(0)(J=0) by about 0.2-0.3 eV. A simplified SACM/CT-based method is also proposed which circumvents the trajectory calculations and allows derivation of E(0)(J=0) on the basis of measured k(E) and which provides similar accuracy as the full SACM/CT treatment.