Estimation of free radical ionization energies by the kinetic method and the relationship between the kinetic method and the Hammett equation

Substituted 1,2-diphenylethanes undergo competitive dissociations upon electron ionization (EI) to generate substituted benzyl cation and benzyl radical pairs. Application of the kinetic method to the previous reported EI mass spectra of these covalently bound precursor ions (data are taken from McLafferty et al. J. Am. Chem. Sec. 1970, 92, 6867)) is used to estimate the ionization energies of substituted benzyl free radicals, A correlation is observed between the Hammett a constant of the substituents and the kinetic method parameter, In(k(x)/k(H)), where k(x) is the rate of fragmentation to give the substituted product ion and k(H) is the rate to give the benzyl ion itself, Systems involving weakly bound cluster ions, including proton-bound dimers of meta-and para-substituted pyridines and meta-and para-substituted anilines, and electron-bound dimers of meta-and para-substituted nitrobenzenes, also show good correlations between the kinetic method parameter and the Hammett a constant.