Mechanism and Thermodynamics of Reductive Cleavage of Carbon-Halogen Bonds in the Polybrominated Aliphatic Electrophiles

Quantum-mechanical computations revealed that, despite the presence of electron-withdrawing and/or pi-acceptor substituents, the lowest unoccupied molecular orbitals (LUMO) of the polybromosubstituted aliphatic molecules R-Br (R-Br = C3Br2F6, CBr3NO2, CBr3CN, CBr3CONH2, CBr3CO2H, CHBr3, CFBr3, CBr4, CBr3COCBr3) are delocalized mostly over their bromine-containing fragments. The singly occupied molecular orbitals in the corresponding vertically excited anion radicals (R-Br center dot-)* are characterized by essentially the same shapes and show nodes in the middle of the C-Br bonds. An injection of an electron into the antibonding LUMO results in the barrierless dissociation of the anion-radical species and the concerted reductive cleavages of C-Br bonds leading to the formation of the loosely bonded {R-center dot center dot center dot center dot Br-} associates. The interaction energies between the fragments of these ion-radical pairs vary from similar to 10 to 20 kcal mol(-1) in the gas phase and from 1 to 3 kcal mol(-1) in acetonitrile. In accord with the concerted mechanism of reductive cleavage, all R-Br molecules showed completely irreversible reduction waves in the voltammograms in the whole range of the scan rates employed (from 0.05 to 5 V s(-1)). Also, the transfer coefficients alpha, established from the width of these waves and dependence of reduction peak potentials E-p on the scan rates, were significantly lower than 0.5. The standard reduction potentials of the R-Br electrophiles, ER-Br/R center dot+X-degrees and the corresponding R-center dot radicals, ER center dot/R-degrees were calculated in acetonitrile using the appropriate thermodynamic cycles. In agreement with these calculations, which indicated that the R-center dot radicals resulting from the reductive cleavage of the R-Br molecules are stronger oxidants than their parents, the reduction peaks' currents in cyclic voltammograms were consistent with the two-electron transfer processes.