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 (C(6)H5(C)l.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.