Magnetism and Bond Order in Diatomic Molecules Described by Semiclassical Electrons

Abstract

The past decade has seen the first attempts at quantifying a semiclassical description of electrons in molecules. The challenge in this endeavor is to find potentials for electron interactions that adequately capture quantum effects. As has been the case for density functionals, the challenge is particularly great for the effects that follow from the requirement for wave function antisymmetry. Here we extend our empirical inquiry into effective potentials, from prior work on the monatomic atoms and ions of nonmetals, to diatomic molecules and ions formed by these elements. Newly adjusted and trained for the longer distances relevant to diatomics, pairwise potentials are able to fit the bond orders and magnetic properties of homonuclear species. These potentials are then found to do an excellent job of predicting the magnetism of heteronuclear species. In these molecules the predicted distribution of electrons also correctly reflects increasing ionic character with increasing difference in the electronegativities of the participating atoms. The distinctive features of the current potential are discussed, along with issues calling for further improvements.

SOMO–HOMO Level Inversion in Biologically Important Radicals

“… 31 The electronic configuration of O 2 4H 2 O in its triplet state is shown in Figure 1 b, and its spin density and MO plots are shown in Figure 2 . In addition, we, also, considered 19 other diatomics (B 2 , Al 2 , Si 2 , S 2 , NH, PH, CH – , OH + , SiH – , SH + , BN, NF, PCl, NO – , NCl, AlN, PF, SO, and PO – ), all of which have triplet ground states, 32 , 33 and calculated the electronic configurations of their corresponding anions using the B3LYP/6-31++G** method. The B3LYP/6-31++G**-calculated electronic configurations of these anions show SOMO–HOMO level switching, see Table S4 in the Supporting Information.…”

Bending the Curve: Molecular Manifestations of Electron Antisymmetry

“…[8] In hindsight, hints of the additional importance of exchange for the distribution of electrons around atoms were present when the heuristically chosen exchange potential had negative curvature at short range: while this potential could account for the ionization and spin excitation energies of monatomic and diatomic species of the 2p elements, overall it produced a very rough potential energy surface with deep local minima for various uneven electron distributions. [8c] Preliminary calculations show that replacing these exchange potentials with ones that have positive curvature at short range provides a smoother potential energy surface. In other work with sub‐atomistic force fields, preliminary calculations show that using exchange potentials with negative curvature at interatomic range can predict a torsion barrier at the staggered conformation of N 2 H 4 .…”

“…In earlier work, we focused on the importance of exchange for stabilizing high spin states, such as the triplet ground states of the oxygen atom and dioxygen [8] . In hindsight, hints of the additional importance of exchange for the distribution of electrons around atoms were present when the heuristically chosen exchange potential had negative curvature at short range: while this potential could account for the ionization and spin excitation energies of monatomic and diatomic species of the 2p elements, overall it produced a very rough potential energy surface with deep local minima for various uneven electron distributions [8c] . Preliminary calculations show that replacing these exchange potentials with ones that have positive curvature at short range provides a smoother potential energy surface.…”

Chemistry with semi-classical electrons: reaction trajectories auto-generated by sub-atomistic force fields

“…In our group, the first generation force field is LEWIS 4,5 and the second generation is LEWIS˙. 6–8 In the Caltech group the first generation force field is eFF 9,10 and the second generation is eFF–ECP 11,12 (where eFF stands for “electron force field” and ECP for “effective core potential”). Until well into these projects, neither of these groups was aware of the work of the other and the models from the two groups differ significantly.…”