Ion–Molecule Reactions in Aromatic Systems. I. Secondary Ions and Reaction Rates in Benzene

Abstract

The reactions between benzene and the ions obtained by electron bombardment of benzene have been investigated in a pulsed ion source mass spectrometer. The major secondary ions were found to be C₁₂H₁₁, C₁₂H₁₀⁺, C₁₂H₉⁺, C₁₂H₈⁺, C₁₁H₇⁺, C₁₀H₉⁺, C₁₀H₈⁺, C₁₀H₇⁺, C₉H₇⁺, and C₈H₇⁺. Ionization‐efficiency curves were measured for parent and product ions, employing the retarding potential difference (RPD) method. The parent ions leading to most of the product ions were identified. The secondary ions appear similar to ions formed by electron bombardment of biphenyl and related compounds. The ion–molecule reaction rate constants were evaluated for benzene and hexadeuterobenzene. The rate constants varied between 2% and 30% of the values predicted by the Stevenson–Gioumousis theory of ion–molecule reactions. Inverse isotope effects were observed for “condensation” reactions, while normal effects were obtained for most of the other reactions. Variation of the translational energy of the reactant ions in a conventional mass spectrometer caused the expected decrease in cross section with increasing energy. The ions formed by “condensation” processes were affected the most. The results are discussed in the light of the various existing theories of ion–molecule reactions. In ion–molecule reactions of this complexity, the low rate constants may be due to the interference of back reactions. The isotope effects agree qualitatively with prediction.