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

Molecular clusters of BBr3 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 BBr3 being solvated by neutral BBr3 molecules. Geometry optimizations on the
observed cluster ions using density functional theory (B3LYP/6-31G*) predict that fragment ions smaller
than BBr3+ undergo reactions with neutral BBr3 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 BBr3 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 BBr3 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.