Binding energy of Al(C6H6)(+) from analysis of radiative association kinetics

Analysis of radiative association kinetics is a new and promising approach to estimating absolute metal-ligand bond energies for gas-phase metal ions. The method is illustrated using previously published data to estimate the binding energy of aluminum cation to benzene and several deuterium-substituted benzenes. A formulation of radiative association theory is applied which is valid at low association efficiency, and is independent of assumptions about the transition state. Photon emission rates from the complex are derived from McMahon-type analysis of collisional and radiative association data, and alternatively from ab initio calculations of IR radiative intensities, with excellent agreement for all four isotopomers. Analysis of the radiative association data gives-a binding energy of 1.53 +/- 0.10 eV (35.2 +/- 2 kcal mol(-1)), which is concordant with, but has a smaller estimated uncertainty than, an interpolated thermochemical estimate based on data from other methods. For this system the semiquantitative ''standard hydrocarbon'' estimate of photon emission rate is a good approximation, but it is shown that in order to give valid predictions of the radiative association rate this scheme requires a correction for the fact that one of the reactants is an atomic ion.