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 metal−ligand 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 CH 3 CN and (CH 3 ) 2 O 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.