Chemical Activation of Water Molecule by Collision with Spin–Orbit-State-Selected Vanadium Cation: Quantum-Electronic-State Control of Chemical Reactivity

Abstract

We have obtained absolute integral cross sections (σ's) for the reactions of spin-orbit-state-selected vanadium cations, V+[a5DJ(J = 0, 2), a5FJ(J = 1, 2), and a3FJ(J = 2, 3)], with a water molecule (H2O) in the center-of-mass collision energy range Ecm = 0.1-10.0 eV. On the basis of these state-selected σ curves (σ versus Ecm plots) observed, three reaction product channels, VO+ + H2, VH+ + OH, and VOH+ + H, from the V+ + H2O reaction are unambiguously identified. Contrary to the previous guided ion beam study of the V+(a5DJ) + D2O reaction, we have observed the formation of the VO+ + H2 channel from the V+(a5DJ) + H2O ground reactant state at low Ecm's (<3.0 eV). No spin-orbit J-state dependences for the σ curves of individual electronic states are discernible, indicating that spin-orbit interactions are weak with little effect on chemical reactivity of the titled reaction. For the three product channels identified, the triplet σ(a3FJ) values are overwhelmingly higher than the quintet σ(a5DJ) and σ(a5FJ) values, showing that the reaction is governed by a "weak quintet-triplet spin crossing" mechanism, favoring the conservation of total electron spins. The σ curves for exothermic product channels are found to exhibit a rapid decreasing profile as Ecm is increased, an observation consistent with the prediction of the charge-dipole and induced-dipole orbiting model. This experiment shows that the V+ + H2O reaction can be controlled effectively to produce predominantly the VO+ + H2 channel via the V+(a3FJ) + H2O reaction at low Ecm's (≤0.1 eV) and that the ion-molecule reaction dynamics can be altered readily by selecting the electronic state of V+ cation. On the basis of the measured Ecm thresholds for the σ(a5DJ, a5FJ, and a3FJ: VH+) and σ(a5DJ, a5FJ, and a3FJ: VOH+) curves, we have deduced upper bound values of 2.6 ± 0.2 and 4.3 ± 0.3 eV for the 0 K bond dissociation energies, D0(V+-H) and D0(V+-OH), respectively. After correcting for the kinetic energy distribution resulting from the Doppler broadening effect of the H2O molecule, we obtain D0(V+-H) = 2.2 ± 0.2 eV and D0(V+-OH) = 4.0 ± 0.3 eV, which are in agreement with D0 determinations obtained by σ curve simulations.