ELECTRON-AFFINITIES OF P-BENZOQUINONE, P-BENZOQUINONE IMINE AND P-BENZOQUINONE DIIMINE, AND SPIN-DENSITIES OF THEIR P-BENZOSEMIQUINONES COMPUTED BY SEVERAL QUANTUM-CHEMICAL MODELS

Restricted and unrestricted (U) Hartree-Fock (HF), second-order Moller -Plesset perturbation (MP2), density functional (DF), hybrid HF/DF and semiempirical (half-electron (HE) method) models have been used to ca lculate adiabatic electron affinities (EA(ad) values) of p-benzoquinon e (I), p-benzoquinone imine (VI) and p-benzoquinone diimine (XI), as w ell as expectation values ([S-2]) and spin density distributions in th e radical anions of I, VI and XI. The AM1/AM1-HE and ab initio calcula ted structures are found to be in accord with each other. The ROHF/6-3 1G(d) method gave the poorest EA(ad) result. The UHF and UMP2 wave fun ctions were found to be substantially spin contaminated (for the radic als) and the accuracies of the EA(ad) values calculated were also poor . The use of molecular energies obtained after spin annihilation did n ot lead to significant improvement of the UHF and UMP2 results. In con trast to the ROHF, UHF and UMP2 results, the DF(USVWN, UBVWN, UBLYP) a nd hybrid HF/DF(UB3LYP) methods, as well as the AM1-HE, gave much bett er results. The calculated EA(ad) values decreased, as predicted by mo st of the models, in the order EA(ad)(I)>EA(ad)(VI)>EA(ad)(XI). The di fferences in the EAs, EA(ad)(I)-EA(ad)(VI) and EA(ad)(I)-EA(ad)(XI), w ere consistently predicted to be about 8-9 and 17-18 kcal/mol, respect ively, by the DF, B3LYP and AM1-HE models. The performance of the PM3 and SAM1 models was not as good as the AMI model. Of all the methods t ested, the B3LYP/6-311G(d,p) model is concluded to give the most accur ate quantitative trend (I(42.6)>VI(33.1)>XI(23.7)) in EA(ad). The pred icted trend in EA can satisfactorily be rationalized by the calculated LUMO orbital energies, atomic charges and spin density distributions. Analysis of the spin density data predicts that phenoxyl-and anilino- type radical anions predominate in the p-benzosemiquinones of I and XI , respectively, while both phenoxyl-and anilino-type radicals contribu te to the structure of the p-benzosemiquinone of VI, with the anilino- type predominating.