Structure and nonlinear electrical properties of squaric acid derivatives: a theoretical study of the conformation and deprotonation effects

Abstract

The molecular structure and nonlinear electrical properties of the three squaric acid conformers (Z,Z), (Z,E) and (E,E) and of their singly deprotonated form, the monohydrogenosquarate anion, are reported. The structures are optimized at the second-order Møller–Plesset (MP2) perturbational theory level by using the 6-311G∗∗ basis set supplemented by a set of diffuse p and d functions. The (Z,Z) conformer is more stable by 2(8)kcal/mol than the (Z,E) ((E,E)) conformer. The finite field procedure applied at the same level shows that, in the static limit, the electronic first hyperpolarizability of the anion is 40% smaller than those of the three squaric acid conformers whereas its second hyperpolarizability is more than twice as large. The origin of the first hyperpolarizability of the (Z,Z) conformer is explained by adopting the missing state analysis at the single CI level.