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 approximately 7.5 angstrom when electron transfer occurs. The reaction exothermicity, 5.08 eV, is partitioned to translation of Nb+, 0.81 +/- 0.25 eV, translation of C6H6+, 1.22 +/- 0.25 eV, and internal excitation of C6H6+ to produce the la2u electronic state, which is approximately 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 = 1.4 X 10(-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) greater-than-or-equal-to 79 kcal/mol.