Energetics and structure of complexes of Al⁺ with small organic molecules in the gas phase

A new experimental apparatus is described which permits the determination of binding energetics for metalligand complexes. This technique mates laser ablation for the generation of atomic metal ions with the environment of a high-pressure ion source which leads to rapid termolecular stabilization of metal ion-ligand complexes containing one or more ligands. The time resolved capability of the detection system allows equilibrium to be studied quantitatively for binding energies in the range of 5-30 kcal mol^-1. Such measurements of the absolute binding energy of formaldehyde to Al+ combined with existing bimolecular Al⁺ exchange equilibrium data leads to an absolute Al⁺ affinity scale. For two ligand complexes an extensive ab initio search of the potential energy surfaces shows that the experimentally observed species involving CH₃CN and (CH₃)₂O involve simple ligand complexation with an acute L-Al-L bond angle (L = ligand) rather than hydrogen bonded or inserted structures. In one instance a third ligand complexation has been experimentally investigated likely leading to a species of pyramidal geometry.