The Quantum Chemistry of Loosely-Bound Electrons

By some measure, the title of this chapter could have been “The quantum chemistry of weakly-bound anions,” because much of it will focus on how to describe the weak binding of an “extra” electron to a stable, neutral molecule using electronic structure theory. Electron binding energies in such cases may be quite small, typically less than the largest atomic electron affinities (EAs)1 (3.4 eV for F and 3.6 eV for Cl), and even less than 0.1 eV in some cases. Unlike the case where a neutral molecule, M, is ionized, an electron separated from the anion M− does not experience an attractive −1∕r potential at large separations,2,3 but rather only charge–dipole and or higher order charge–multipole interactions. Cases where the electron affinity of M is ≲ 0.5 eV are the signature of a short-range valence potential that is weakly attractive at best, such that electron binding in M− results primarily from long-range electron–molecule, charge–multipole interactions. In such cases, one expects to find an unpaired electron in M− that is radially diffuse, much more so than in F− or Cl−, for example. It is in this sense that the odd electron in M− is “loosely-bound.” Moreover, the preceding discussion assumes that M− is bound at all, but in fact we will also consider cases in which the electron