Formation of dimeric parent cations in aromatic hydrocarbons

Third-order rate coefficients have been determined at 295 K for the formation of dimeric parent cations in benzene, benzene-d₆, toluene, the xylenes, mesitylene, and fluoro- and chlorobenzene using the NBS high pressure photoionization mass spectrometer. The reactions are all quite fast, the rate coefficients falling in the range 0.5–6 · 10⁻²⁵ cm⁶ molecule⁻² s⁻¹. The differences in rates observed are rationalized in terms of structural (geometric) considerations, and upper limits are established for the binding energies in (C₆H₆)₂⁺ and (C₆H₅CH₃)₂⁺. The temperature dependence for production of (C₆D₆)₂⁺ has been determined from 227 to 316 K, and the rate coefficient seems to obey a simple 1/T relationship over this range. The detection of trimeric species in benzene is also reported for the first time. Additional experiments involving benzene-nitric oxide mixtures are also reported, which include detection of an apparent equilibrium of the type NO⁺ + C₆H₆ ⇆ C₆H₆⁺ + NO.