Cooperative effects and energy barriers to bromonium ion transfer in cyclic (BrX)_n halogen-bonded clusters (X=F, OH, or NH₂; n=3-5): An ab initio study

Cooperative effects on the Br-X⋯ Br-X halogen bond interactions present in cyclic (BrX)n clusters, (n=3-5, X=F, OH or NH2) are investigated using theoretical calculations at the B3LYP and MP2 level using the aug-cc-pvtz basis set unless indicated otherwise. Pertinent geometrical, vibrational, energetic and topological parameters are used as primary indicators of halogen bond strength, and consequently for assessment of cooperativity. Trimer formation weakens, relative to the dimer, the halogen bond interaction while formation of the larger cluster strengthens it. Cooperative enhancement of the Br⋯ X halogen bond depends on X, and follows the order X=F>X=OH>X=NH2. Thus, the weakest dimer interaction exhibits the largest cooperative enhancement in these halogen bonded systems. A linear relationship is found between the average dissociation energies of the clusters with electron density at the halogen bond critical point, while a second order polynomial relationship is found between the average dissociation energies of the clusters and the average amount of charge transfer, 〈 qT〉 , from the acceptor X lone pairs into the antibonding Br-X orbitals, nX→σ*Br-X. It is found that multiple Br transfer exhibits relatively high energy barriers, and follows a concerted mechanism with transition state geometries of Dnh symmetry for BrF and BrNH2 cyclic clusters, and with geometries of Cn symmetry for the transition states of the corresponding BrOH clusters.