Stabilizing Otherwise Unstable Anions with Halogen Bonding.

Both hydrogen bonding (HB) and halogen bonding (XB) are essentially electrostatic interactions, but whereas hydrogen bonding has a well-documented record of stabilizing unstable anions, little is known about halogen bonding's ability to do so. Herein, we present a combined anion photoelectron spectroscopic and density functional theory study of the halogen bond-stabilization of the pyrazine (Pz) anion, an unstable anion in isolation due to its neutral counterpart having a negative electron affinity (EA). The halogen bond formed between the σ-hole on bromobenzene (BrPh) and the lone pair(s) of Pz significantly lowers the energies of the Pz(BrPh) and Pz(BrPh) anions relative to the neutral molecule, resulting in the emergence of a positive EA for the neutral complexes. As seen through its charge distribution and electrostatic potential analyses, the negative charge on Pz is diluted due to the XB. Thermodynamics reveals that the low temperature of the supersonic expansion plays a key role in forming these complexes.