Reactive scattering from double minimum potentials: Energetics and mechanism of the gas phase dehydration reaction of lithium ion with t e r t-butyl alcohol

Abstract

We present a crossed beam study of the reactions of Li+ with tert-butanol, a reactive system hypothesized to proceed on a double minimum potential energy surface, over the collision energy range from 0.85 to 1.80 eV. We observe product energy and angular distributions for the dehydration products as well as for Li+ which is nonreactively scattered from a transient collision complex, thereby probing both wells on the surface. The direct observation of such nonreactive flux provides confirmation of the Brauman model for nonunity reaction efficiency as arising from significant nonreactive decay of the initial encounter complex back to reagents. We also measure the branching ratios for nonreactive to reactive scattering, and the branching ratios for the two dehydration products relative to one another, over the entire kinetic energy range of the experiments. The product angular distributions indicate that the collision complex lifetimes are under 1 ps. The product kinetic energy distributions are in reasonably good agreement with statistical phase space theory predictions, with larger deviations occurring for the nonreactive channel, and with the deviations for the dehydration channels increasing with increasing collision energy. The branching ratio for nonreactive to reactive scattering, in conjunction with statistical calculations, indicates that the intermediate barrier on the double minimum surface is equal to the energy of the asymptotic reagents within ±0.05 eV. The Li(H2O)+/Li(C4H8)+ branching ratios, when compared with statistical calculations including product dissociation, are consistent with products formed in statistical equilibrium. The data suggest that the reaction dynamics in both wells are in substantial agreement with statistical theories, with a reduced number of vibrational modes required to effect the dehydration process at high energies, resulting in significant energy transfer, but a dramatically reduced decay lifetime.