Ionization energy determination by the kinetic method.

Ionization energies of organic compounds can be determined by the kinetic method by dissociation of radical cations of van der Waals complexes. 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 dissociation with argon, yield only the two individual radical cations as products. The logarithm of the ratio of their ion abundances correlates linearly with their ionization energies. Using this linear relationship, the ionization energy of 3-iodobenzonitrile was determined to be 9.39 ± 0.05 eV, and this case serves to illustrate the application of the kinetic method in the determination of an unknown ionization energy. 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 application to a wide variety of compounds.