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

The molecular structure and nonlinear electrical properties of the three sq uaric acid conformers (Z, Z), (Z, E) and (E, E) and of their singly deproto nated form, the monohydrogenosquarate anion. are reported. The structures a re optimized at the second-order Moller-Plesset (MP2) perturbational theory level by using the 6-3 11G** 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 t he (Z, E) ((E, E)) conformer. The finite field procedure applied at the sam e level shows that, in the static limit, the electronic first hyperpolariza bility of the anion is 40% smaller than those of the three squaric acid con formers whereas its second hyperpolarizability is more than twice as large. The origin of the first hyperpolarizability of the (Z, Z) conformer is exp lained by adopting the missing state analysis at the single CI level. (C) 2 000 Elsevier Science B.V. All rights reserved.