Quadripartite bond length rule applied to two prototypical aromatic and antiaromatic molecules

TL;DR

This paper tests a simple geometric rule linking different molecular states on two prototypical aromatic and antiaromatic molecules, benzene and cyclobutadiene, showing it works well for benzene and various minima of cyclobutadiene.

Contribution

It verifies the applicability of a bond length rule to aromatic and antiaromatic molecules, expanding understanding of molecular geometry predictions across different electronic states.

Findings

Rule applies well to benzene's geometries.

Rule works for both quinoid and anti-quinoid forms of cyclobutadiene.

Radical anion species pose challenges for theoretical description.

Abstract

In 2000, a remarkably simple relationship was introduced, which connected the calculated geometries of isomolecular states of three different multiplicities. These encompass a ground singlet state, the first excited triplet state, as well as related radical anion and radical cation. The rule allows prediction of geometry of one of the species if the three remaining ones are known. Here, we verify applicability of this bond length rule for two small planar cyclic organic molecules, i.e. benzene and cyclobutadiene, which stand as prototypical examples of, respectively, aromatic and antiaromatic systems. We see that the rule works fairly well to benzene and it works independently for quinoid as well as for anti-quinoid minima, and despite the fact that radical anion species poses challenges for correct theoretical description.