Dipole-bound anions and relations

In most chemistry textbooks you will find that the electron acceptor properties of a molecule are determined by its empty valence orbitals. Yet, many molecules can bind electrons through long-range interactions such as a dipole or polarization potential. The prototypical example is the acetonitrile molecule H3C-CN; it has a dipole moment of 3.9 Debye, and it can bind an electron in an extended orbital off the methyl group:
This Figure shows the dipole-bound molecular orbital of CH3CN. The molecule itself is dwarfed by the diffuse cloud of the excess electron. The inner iso-contour encloses 20%, the outer iso-contour encloses 80% of the excess electron density.

Electron correlation is crucial

Whereas for many neutral ground states electron correlation is not of particular importance (depending quite a lot on whom you ask), it is absolutely crucial for weakly attached electrons. This seems counterintuitive as electrons occupying the relative compact valence region should be (and are) more strongly correlated with each other than with a diffuse electron off the nuclear framework. However, correlation is the difference between a one-particle and a many-particle description, and "correlation is crucial" simply implies that a one-particle description of the excess electron is bad.
This Figure shows the excess electron orbital of CH3CN computed with and without electron correlation. The outer iso-contour encloses 50% of the excess electron density computed at the Hartree-Fock level, the inner iso-contour encloses 50% of the natural orbital describing the excess electron computed at the equation-of-motion coupled-cluster level of theory. With electron correlation the density of the excess electron is far more compact, and the computed electron binding energy is increased from 6 to 18 meV.

Excess electrons bound to real molecules

There is a lot of work done on electrons bound to dipole, quadrupole, and polarization model potentials represented by point-multipoles, point-charge distributions, and point-polarizable sites. I am interested in electrons bound by molecules, and to what extent the binding to a molecule can be characterized (and modeled) as dipole-bound or polarization-bound.

Here is an example, the anti conformer of succinonitrile (NC-CH2-CH2-CN). This conformer binds an electron with roughly 10 meV binding energy, and it has been called a quadrupole-bound state. Yet, the quadrupole moment is too small to do the job, and only when, in addition to the electrostatic potential, electron correlation is included, the electron becomes bound.
12% contour 30% contour 50% contour

In the picture the natural orbital from an equation-of-motion coupled-cluster calculation is shown. Close to the molecule the orbital is roughly donut shaped and wraps around the four H atoms, but it is so diffuse that this shape can only be seen when contours containing a small portion of the total density are plotted. About 2% of the excess electron's density resides within the van der Waals volume of the molecule.

Coupling between dipole-bound and valence states

If a molecule has both, a diffuse dipole-bound state and a compact valence anion state, the two states will couple and an the excess electron can be transferred from the molecular periphery on to the nuclear framework (or vice versa). The diffuse state can be though of as a doorway allowing a molecule to capture a near zero-energy electron even though its unoccupied valence orbitals does have a much higher energy.
Nitromethane is a prototypical system that has both: A dipole-bound state (lhs) and a valence anion (rhs). Note the different scales of the figures.


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