Spectroscopic Determination of an Anion-π Bond Strength

The anion-π bond has emerged as an important nonvalence interaction in supramolecular and biological structure. Although recognized as a strong noncovalent interaction, driven by electrostatic charge-quadrupole moment and correlation interactions, benchmark experimental and computational studies on the intrinsic anion-π bond strength are scarce. Here, we present a gas-phase photoelectron spectroscopic study on the archetypical iodide-hexafluorobenzene anion-π bonded complex. In combination with high-level electronic structure calculations, the anion-π bond strength is found to be 0.53 eV (51 kJ mol^-1). The interaction arises for a large part from correlation forces (∼40%), with electrostatic quadrupole-anion and polarization making up most of the remainder.