Binding Energies for Al Atom Association Complexes with Simple Alkenes and Arenes

Article abstract of DOI:10.1021/j100317a053

Al atom association reactions with simple alkenes and arenes in the gas phase are investigated by time-resolved resonance fluorescence excitation of ground-state Al atoms following pulsed visible laser photolysis of trimethylaluminum in a gas cell.Ar buffer gas pressure effects on the reaction rates are observed and interpreted in terms of collision-complex lifetimes intermolecular reactions.The limiting high Ar pressure bimolecular rate constants are near the gas kinetic values, implying negligible activation energies and large Arrhenius preexponential factors for these reactions.For reactions involving trans-2-butene, tetramethylethylene, benzene, toluene, and o-xylene, an equilibration is observed between free Al atoms and Al atoms bound in complexes with the reactant molecules.Equilibrium constants for the association reactions are obtained from an analysis of kinetic data at different pressures of reactant.Binding energies are derived from observations of the temperature dependence of the equilibrium constant in the range 283-333 K, or by estimating the standard enthropy change for the association reaction.Evidence is presented which indicates that monoligand complexes are formed in all cases.Al atom binding energies (kcal mol^-1) are reported for C2H2 (>13), C2H4 (>16), 1-butene (>15), trans-2-butene (14.2+/-1), tetramethylethylene (13.5+/-1), 1,4-cyclohexadiene (>14), benzene (11.7+/-1), toluene (14.1+/-1), and o-xylene (14.3+/-1).Kinetic and thermochemical results are discussed in terms of alternate bonding schemes for Al-alkene and Al-arene complexes.