From Fragile to Firm: Reinforcement of Excess Electron Binding in Dipole-Bound Anions through Sigma-Hole and Hydrogen-Bond Interactions

The stability of dipole-bound anions formed by small polar molecules and their noncovalent complexes was investigated using highly correlated <i>ab initio</i> methods and flexible basis sets. The HCN, HNC, and ClCN species were found to form weakly bound anions of dipole-bound nature, with excess electron binding energies not exceeding 50 cm^-1. When ClCN forms noncovalent complexes with either HCN or HNC, two isomeric structures become possible, stabilized either by a hydrogen bond or by a σ-hole interaction. All noncovalent complexes exhibit dipole moments increased by more than a factor of 2 compared to the isolated components, which in turn facilitates significantly stronger excess electron binding. Among them, hydrogen-bonded complexes display stronger intermolecular interactions and, consequently, substantially enhanced binding of the excess electron. The predicted excess electron binding energies span from 413 to 1265 cm^-1 for the four resulting dipole-bound anions, namely, (HCN···ClCN)^-, (ClCN···HCN)^-, (HNC···ClCN)^-, and (ClCN···HNC)^-, with hydrogen-bonded species being approximately twice as strongly electronically bound as their σ-hole-stabilized counterparts. These results demonstrate that excess electron binding can increase by an order of magnitude upon the formation of a noncovalent complex, even when its individual components exhibit only marginal ability to accommodate an excess electron.