COMPLEXES OF OXYGEN WITH BENZENE AND HEXAFLUOROBENZENE

The complexes of C6H6.O2, (C6H6)2O2, and C6F6.O2 were studied by photoionization using synchrotron radiation. Dissociation energies were measured to be D0(C6H6.O2) = 1.6 +/- 0.3 kcal mol-1, D0 [ (C6H6.O2) +] = 3.4 +/- 0.3 kcal mol-1, D0 (C6F6.O2) = 2.1 +/- 0.4 kcal mol-1, and D0[(C6F6.O2)+] = 3.2 +/- 0.4 kcal mol-1. We calculate from the above that D298 (C6H6.O2) = 0.4 +/- 0.4 kcal mol-1, verifying that the benzene-oxygen interaction is only a "contact" at room temperature. The dissociation energies of the heterodimer ions are much smaller than those of the homodimer ions of their constituents [viz. 15, 11, and 7 kcal mol-1 for (C6H6)2+, (O2)2+, and (C6F6)2+] Sharp onsets were observed for C6H6.O2 --> (C6H6.O2)+ and C6F6.O2 --> (C6F6.O2)+, at 9.172 +/- 0.004 and 9.856 +/- 0.003 eV, respectively, measurements made possible by autoionization in the threshold region. Surprisingly, the heterodimer ion (C6H6.O2) + is formed essentially entirely from neutral C6H6.O2, with no contribution from fragmentation of larger clusters. Production of C6H6O+ and C6F6O+ from C6H6.O2 and C6F6.O2 could not be detected, but is easily observed from mixed trimers. The first onsets occur at 14.10 +/- 0.05 and 14.10 +/- 0.09 eV, respectively, and are markedly higher than the thresholds. A second onset for C6F6O+ occurs at 14.7-15.0 eV. In addition, dips are observed near 590 angstrom in the yield spectra for both C6H6O+ and C6F6O+, where the well-known window resonances of the oxygen (c4-SIGMA(u)-)3s-sigma(g) Rydberg states occur. Thus there is evidence for the participation of two different mechanisms. The former data are consistent with a mechanism in which the organic moiety is first photoionized to produce an excited ion that then dissociates the oxygen, where one of the oxygen atoms is captured by the ion. The latter data support a mechanism in which the O2 moiety is the chromophore, where O+, formed by predissociation of [O2+]* from autoionization of a Rydberg state, is captured by a benzene molecule. In either case, the product is born excited, and to be observed must be stabilized by excitation and ejection of the third component of the original trimer.