Long-Range Electron Binding to Quadrupolar Molecules

An excess electron can be bound to a molecule in a very diffuse orbital as a result of the long-range contributions of the molecular electrostatic field. For a molecule with a null dipole the first non-vanishing field left is the quadrupolar field. Theoretical results for quadrupole-bound anions have been rather conflicting and the few experimental reports can have alternative explanations. Following a systematic search, we report experimental evidence that such quadrupole binding occurs for the trans-succinonitrile molecule (EA = 20 ± 2 meV), while the gauche-succinonitrile conformer supports a dipole-bound anion state (EA = 108 ± 10 meV). Theoretical calculations at the DFT/B3LYP level support these interpretations and give electron affinities of 20 and 138 meV, respectively.