Dissociation kinetics of energy-selected (C6H6)2Cr+ ions:: Benzene-chromium neutral and ionic bond energies

Threshold photoelectron-photoion coincidence spectroscopy has been used to investigate the dissociation kinetics of the bis(benzene) chromium ion, (C6H6)(2)Cr+. The dissociation of the (C6H6)(2)Cr+ ion proceeds by the sequential loss of the two benzene ligands. The two benzene-loss reactions were found to be metastable (lifetimes in the microsecond range) at photon energies close to the dissociation limits of the precursor ions. By simulating the resulting asymmetric time-of-flight peak shapes and breakdown diagram, the 0 K appearance energies of the product ions C6H6Cr+ and Cr+ were determined to be 7.91 +/- 0.15 and 9.61 +/- 0.18 eV, respectively. The two C6H6-Cr bond energies in the (C6H6)(2)Cr+ ion were determined to be 2.51 +/- 0.15 and 1.70 +/- 0.15 eV, respectively. Using the heats of formation of the fully dissociated products, C6H6 and Cr+, the 298 K heats of formation of (C6H6)(2)Cr, (C6H6)(2)Cr+, and C6H6Cr+ were obtained to be 286 +/- 17, 804 +/- 18, and 968 +/- 15 kJ/mol, respectively. Neutral C6H6-Cr bond energies of 2.78 and 0.06 eV were determined for the first and second losses of the benzene ligand, respectively.