The energetics of the unimolecular reactions C2H+4sdot;C2H4rarr;C3H+5+CH3and C4H+7+H have been reinvestigated by the molecular beam photoionization method. At nozzle expansion conditions where the concentrations of (C2H4)3and heavier clusters produced in the beam are high, the appearance energies (AE) for C3H+5and C4H+7were found to shift to energies lower than the expected thermochemical thresholds for the formation of C3H+5and C4H+7from ethylene dimers. This indicates that ethylene trimers and heavier clusters can give rise to the same product ions as ethylene dimers. The AErsquo;s for C3H+5and C4H+7from (C2H4)2measured in an ethylene beam, which mainly consists of C2H4and (C2H4)2, are 10.21plusmn;0.04 eV (1214plusmn;5 Aring;) and 10.05plusmn;0.04 eV (1234plusmn;5 Aring;), respectively. Taking into account the error estimates of the thermochemical thresholds for the formation of C3H+5and C4H+7from (C2H4)2, the latter values are in accordance with the conclusion of previous studies that the barriers for the reverse reactions of the ionndash;molecule reactions C2H+4+C2H4are negligible. From the observed ionization energies of (C2H4)2(9.84plusmn;0.04 eV) and C2H4(10.505plusmn;0.004 eV) and the estimated binding energy of (C2H4)2(0.02 eV), the bond dissociation energy for C2H+4sdot;C2H4is calculated to be 15.8plusmn;1 kcal/mol.
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