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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, we report a combined anion photoelectron spectroscopy and density functional theory (DFT) study of XB in solvated Br- anions. The XB strength between the positively-charged σ-hole on the Br atom of the bromotrichloromethane (CCl3Br) molecule and the Br- anion was found to be 0.63 eV (14.5 kcal/mol). In the neutral complexes, Br(CCl3Br)1,2, the attraction between the free Br atom and the negatively charged equatorial belt on the Br atom of CCl3Br, which is a second type of halogen bonding, was estimated to have interaction strengths of 0.15 eV (3.5 kcal/mol) and 0.12 eV (2.8 kcal/mol).