MCSCF study of polaron- and bipolaron-like defects in smallall-trans conjugated polyenes

Summary A Multiconfigurational Self-Consistent Field study of neutral, singly and doubly ionized C₄H₆, C₆H₈, C₈H₁₀, and C₁₀H₁₂ as model systems for polaron and bipolaron defects in polyacetylene has been carried out. In all computations a double zeta quality basis set was used. The post-Hartree-Fock computations were carried out in the complete active subspace and the fully optimized structures are reported. Comparison with SCF results reveals that the inclusion of electron correlation attenuates the double bond/single bond alternancy in conjugated polyenes as well as the charge waves associated with the cations.Dedicated to Prof. Klaus Ruedenberg     

Comparison of the capacity of nitrogen, oxygen, sulfur, and selenium atoms to exist in onium states. quantum-chemical investigation of isomeric model systems with two heteroatoms

HF, B3LYP, and MP2 calculations with the 6-31+G(d) basis set with correction to the energy of zero-point vibrations were carried out to determine the energy characteristics of model molecules containing two heteroatoms in the sp ³- and sp ²-hybrid states; different combinations of N, O, S, and Se atoms were studied. The stability of the onium states of the nitrogen atom was found to be greater than for its chalcogen analogs and the relative stability of onium states of the chalcogen analogs was found to depend on the hybridization of these atoms. Analysis of these results permitted us to construct a stability series of onium derivatives and to interpret the positional selectivity in electrophilic substitution reactions of five-membered heterocyclic compounds with one heteroatom. To J. Stradins, an outstanding and tireless scientist, with our deep respect and sincere affection. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1801–1808, December, 2008.