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 ab initio 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.