Kinetic energy release in thermal ion--molecule reactions: The Nb2+--(benzene) single charge--transfer reaction

We have adapted the techniques originally developed to measure ion kinetic energies in ion cyclotron resonance (ICR) spectrometry to study the single charge--transfer reaction of Nb2+ with benzene under thermal conditions in a Fourier transform ion cyclotron resonance mass spectrometer (FTICRMS). The partitioning of reaction exothermicity among the internal and translational modes available is consistent with a long-distance electron-transfer mechanism, in which the reactants approach on an ion-induced dipole attractive potential and cross to a repulsive potential at a critical separation of ∼7.5 A when electron transfer occurs. The reaction exothermicity, 5.08 eV, is partitioned to translation of Nb+ , 0.81±0.25 eV, translation of C6 H6+, 1.22±0.25 eV, and internal excitation of C6 H6+ to produce the la2u electronic state, which is ∼3 eV above the ground state of the ion. We have also studied the kinetics of the reaction of Nb2+ with benzene and determined the rate constant, k = 1.4x10-9 cm3 molecule-1 s-1, and the efficiency, 0.60, of the process. These also support the proposed charge--transfer mechanism. In addition to the charge--transfer pathway, which accounts for 95% of the reaction products, Nb2+ is observed to dehydrogenate benzene to form Nb2+ (benzyne). This process implies D(Nb2+ --benzyne)≥79 kcal/mol