Ion−Molecule Chemistry within Boron Tribromide Clusters: Experiment and Theory

Molecular clusters of BBr 3 were subjected to electron ionization and mass analysis in a reflectron time-of-flight mass spectrometer. Five series of cluster ions were observed, with formulas corresponding to each of the possible fragment ions of BBr 3 being solvated by neutral BBr 3 molecules. Geometry optimizations on the observed cluster ions using density functional theory (B3LYP/6-31G*) predict that fragment ions smaller than BBr 3 + undergo reactions with neutral BBr 3 molecules to form covalently bound adduct species that function as core ions within the clusters. Once all boron atoms are saturated, the reactions cease, and larger cluster ions consist of BBr 3 molecules loosely bound to the core ions. Divalent bromine atoms are present in at least three of the cluster ions, and most of the intermolecular contact within the clusters is between Br atoms. Enthalpies of formation, addition reactions, and BBr 3 elimination from the cluster ions were derived from B3LYP and MP2 calculations at the B3LYP/6-31G* geometries using both the 6-31G* and the 6-311++G(2df,2p) basis sets. The results are compared to limiting expectations based on known bulk thermochemistry.