Excluded-volume descriptors for dipole-bound anions: Amine N-oxides as a test case

Dipole-bound anions can be theoretically characterized at three fundamentally different levels. The highest are ab initio calculations, which themselves range from fairly approximate, say, Koopmans's Theorem (KT) or second-order Møller-Plesset perturbation theory, to highly sophisticated, say, the electron affinity equation-of-motion couple-cluster with single, double, and perturbative triple substitutions, which rivals experiments in reliability. The next level down is represented by one-electron model Hamiltonians. Again, one-electron model Hamiltonians can be fairly approximate, especially if the molecular system is modeled by a simple point-dipole and point-polarizable site; however, very reliable models have been developed for specific systems, for example, water clusters. At the lowest level, one can qualitatively explain trends in classes of dipole-bound anions in terms of the dipole moment, μ, the polarizability, α, and the so-called excluded volume, V_x. This project aims at the qualitative level. While the dipole moment and the polarizability possess clear-cut definitions, the excluded volume must-similar to all molecular volumes-remain a rather vaguely defined term, and so far, we are unaware of any quantitative definition in the literature. Here, we introduce and investigate three descriptors for V_x. To this end, we first establish a dataset with consistent ab initio results for 25 amine N-oxides structures. Then, we demonstrate that the descriptors are indeed able to explain trends for sets of isomers and conformers and investigate to what extent the descriptors are able to predict electron binding energy of dipole-bound states using simple quantitative structure-property relationship-like models. It turns out that μ and V_x provide a reasonably accurate prediction of the electrostatic part of the electron bind energy (the KT value) and that the polarizability α provides an acceptable prediction of the electron correlation contribution.