title: From charge transfer to electron transfer in halogen-bonded complexes of electrophilic bromocarbons with halide anions.
abstract: Experimental and computational studies of the halogen-bonded complexes, [R-Br, X(-)], of bromosubstituted electrophiles, R-Br, and halide anions, X(-), revealed that decrease of a gap between the frontier orbitals of interacting species led to reduction of the energy of the optical charge-transfer transition and to increase in the ground-state charge transfer (X(-) → R-Br) in their associates. These variations were accompanied by weakening of the intramolecular, C-Br, and strengthening of the intermolecular, BrX(-), bonds. In the limit of the strongest electron donor-acceptor pairs, formation of the halogen-bonded complexes was followed by the oxidation of iodide to triiodide, which took place despite the fact that the I(-) → R-Br electron-transfer step was highly endergonic and the calculated outer-sphere rate constant was negligibly small. However, the calculated barrier for the inner-sphere electron transfer accompanied by the halogen transfer, R-BrI(-) → R˙Br-I(-)˙, was nearly 24 kcal mol(-1) lower as compared to that calculated for the outer-sphere process and the rate constant of such reaction was consistent with the experimental kinetics. A dramatic decrease of the electron-transfer barriers (leading to 18-orders of magnitude increase of the rate constant) was related to the strong electronic coupling of the donor and acceptor within the halogen-bonded precursor complex, as well as to the lower solvent reorganization energy and the successor-complex stabilization.
authors.affiliation: Department of Biological, Chemical and Physical Sciences, Roosevelt University, Chicago, IL 60605, USA. srosokha@roosevelt.edu.
authors.forename: Sergiy V
authors.initials: SV
authors.lastname: Rosokha
authors.forename: Alfredo
authors.initials: A
authors.lastname: Traversa
mesh_terms: D000838:Anions / Q000737:chemistry; D017605:Bromine Compounds / Q000737:chemistry*; D017554:Carbon Compounds, Inorganic / Q000737:chemistry*; D004579:Electron Transport; D004583:Electrons; D006219:Halogens / Q000737:chemistry*; D008958:Models, Molecular; D013816:Thermodynamics
chemical_list: D000838:Anions; D017605:Bromine Compounds; D017554:Carbon Compounds, Inorganic; D006219:Halogens