Photoionization-induced intracluster reactions of chlorobenzene/ammonia mixed complexes

Complexes of chlorobenzene and ammonia, C₆H₅Cl·NH₃, C₆H₅Cl(NH₃)₂, and C₆H₅Cl(NH₃)₃, were studied by single-photon ionization. The ionization potentials of these three complexes were measured to be 8.744 ± 0.022, 8.652 ± 0.013, and 8.555 ± 0.012 eV, respectively. The appearance potential of C₆H₅NH₃⁺ from C₆H₅Cl·NH₃ was found at 8.935 ± 0.004 eV, which, with the known heat of formation of anilinium ion, gives the dissociation energy D(C₆H₅Cl·NH₃) = 2.9 ± 0.5 kcal mol^-1 (12.0 ± 2.2 kJ mol^-1). Then, from its onset energy from C₆H₅Cl·NH₃, the dissociation energy of (C₆H₅Cl·NH₃)⁺ is calculated to be D[(C₆H₅Cl·NH₃)⁺] = 10.4 ± 0.7 kcal mol^-1 (43.5 ± 2.9 kJ mol^-1), unusually large for a heterodimer ion. No production of C₆H₅NH₃⁺ from trimers could be detected in the onset region. The ion C₆H₅NH₂⁺ has onsets of 8.849 ± 0.009 and 8.855 ± 0.029 eV from C₆H₅Cl·NH₃ and C₆H₅Cl(NH₃)₂, respectively. These energies are below the onset for C₆H₅NH₃⁺ but far above the thermochemical thresholds for aniline ion, which are near 7.6 eV. Evidently, C₆H₅NH₂⁺ is not produced by dissociative ionization of the excited neutral complex. Instead, the complex must first be ionized and excited to at least 0.1 eV. Since the onsets for (C₆H₅Cl·NH₃)⁺ and C₆H₅NH₂⁺ are lower than for C₆H₅NH₃⁺, then (C₆H₅Cl·NH₃)⁺ in its ground state does not spontaneously form C₆H₅NH₃⁺. Kinetic energy release distributions measured for C₆H₅NH₂⁺ and C₆H₅NH₃⁺ indicate that the formation of both ions is consistent with statistical processes; i.e., no evidence for nonstatistical mechanisms was found, even for photon energies as large as 17.7 eV.