Concentration-dependent Br...O halogen bonding between carbon tetrabromide and oxygen-containing organic solvents

High concentration-dependent halogen bonding, a specific solvent effect between carbon tetrabromide and oxygen-containing organic solvents, including methanol, ethanol, acetone, dioxane, diethyl ether, and tetrahydrofuran, was found to coexist with the general solvent effect when CBr4 was over 7.8 x 10(-3) M approximately 1.6 x 10(-2) M critical concentration range. In contrary, in less than this concentration range, only general solvent effect occurred. The 1:1 stoichiometry of halogen bonding complex between CBr4 and charge donor was testified using the modified Benesi-Hildebrand method. The Mulliken correlation confirmed the charge-transfer (CT) character of the CBr4/solvent associations. The electronic coupling elements (H(DA)) showed that the C-Br...O complexes are most-likely localized, which is named as outer-type complex. Density functional theory (DFT) calculation was performed to predict geometry, surface electrostatic potential, interaction energy, and vibrational frequency in gas phase. The MP2 method was also employed to calculate the formation of the sigma-hole (sigma(h)) bonding complexes between carbon tetrabromide and oxygen-containing organic solvents. The vertical excitation energies of the sigma(h)-bonding complexes were calculated by time-dependent density functional theory (TD-DFT). The experimental and theoretical results showed that sigma(h) bonding might be a more important factor for the halogen bonding complex investigated here than the CT. The results showed that Br...O halogen bonding is a blue-shift type.