Dissociation Kinetics of Energy-Selected (C₆H₆)₂Cr⁺Ions:  Benzene−Chromium Neutral and Ionic Bond Energies

Abstract

Threshold photoelectron−photoion coincidence spectroscopy has been used to investigate the dissociation kinetics of the bis(benzene) chromium ion, (C 6 H 6 ) 2 Cr + . The dissociation of the (C 6 H 6 ) 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 C 6 H 6 Cr + and Cr + were determined to be 7.91 ± 0.15 and 9.61 ± 0.18 eV, respectively. The two C 6 H 6 −Cr bond energies in the (C 6 H 6 ) 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, C 6 H 6 and Cr +, the 298 K heats of formation of (C 6 H 6 ) 2 Cr, (C 6 H 6 ) 2 Cr +, and C 6 H 6 Cr + were obtained to be 286 ± 17, 804 ± 18, and 968 ± 15 kJ/mol, respectively. Neutral C 6 H 6 −Cr bond energies of 2.78 and 0.06 eV were determined for the first and second losses of the benzene ligand, respectively.