Spectroscopic Measurement of a Halogen Bond Energy

Halogen bonding (XB) has emerged as an important bonding motif in supramolecules and biological systems. Although regarded as a strong noncovalent interaction, benchmark measurements of the halogen bond energy are scarce. Here, a combined anion photoelectron spectroscopy and density functional theory (DFT) study of XB in solvated Br^- anions is reported. The XB strength between the positively-charged σ-hole on the Br atom of the bromotrichloromethane (CCl₃ Br) molecule and the Br^- anion was found to be 0.63 eV (14.5 kcal mol^-1 ). In the neutral complexes, Br(CCl₃ Br)_1,2 , the attraction between the free Br atom and the negatively charged equatorial belt on the Br atom of CCl₃ Br, which is a second type of halogen bonding, was estimated to have interaction strengths of 0.15 eV (3.5 kcal mol^-1 ) and 0.12 eV (2.8 kcal mol^-1 ).