Ionization energies of organic compounds can be determined by the kine
tic method by dissociation of radical cations of van der Waals complex
es. The ionized dimeric complexes of benzene and substituted benzenes,
generated in the ion source of a multiquadrupole instrument by gentle
charge exchange chemical ionization using carbon disulfide as reagent
gas, when mass-selected and allowed to undergo collision-induced diss
ociation with argon, yield only the two individual radical cations as
products. The logarithm of the ratio of their ion abundances correlate
s linearly with their ionization energies. Using this linear relations
hip, the ionization energy of 3-iodobenzonitrile was determined to be
9.39 +/- 0.05 eV, and this case serves to illustrate the application o
f the kinetic method in the determination of an unknown ionization ene
rgy. From the slope of the kinetic method plot, it is evident that the
clusters are weakly bound (effective temperature, 1670 K). Strengths
of this method are the simplicity of the procedure and its potential a
pplication to a wide variety of compounds.