This paper is dedicated to Professor Camille Sandor-on the occasiotz of his 65th birthday SHIRO KOSEKI, TAKESHI NAKAJIMA, and AZUMAO TOYOTA. Can. J. Chem. 63, 1572Chem. 63, (1985. Violation of Hund's multiplicity rule in the electronically excited states of conjugated hydrocarbons is studied by using the Pariser-Pam-Pople type SCF MO method and the ab initio MO method with STO-3G basis set, both methods being augmented by CI-type treatments. It is shown that for symmetrical structures (Dzl,) of the nonalternant hydrocarbons, propalene, pentalene, and heptalene, the lowest excited singlet state is energetically lower than the corresponding triplet state. This is mainly due to the spin polarization (SP) effects. For D2/, structures of pentalene and heptalene the open-shell excited singlet state is predicted to be lower in energy than the closed-shell state, with the result that the former is really the ground state. Further, calculations made by including electron correlation effects reveal that in linear polyenes and polyacenes, the lowest excited singlet "minus" state (using Pariser's classification of the alternancy symmetry species) is lower in energy than the corresponding triplet state. The energy lowering of the singlet "minus" state in linear polyenes is due mostly to the mixing with the doubly excited configurations (tnm + nn), while the considerable part of it in polyacenes is due to the SP effects. [Traduit par le journal]

To elucidate the nature of the pseudo-Jahn-Teller (JT) effect, an energy component analysis has been carried out for the ground and electronically excited states of the titled cyclic polyenes by using the MCSCF method with 6-31G(d) basis set. Examination of the energy components comprised in the total energy reveals that in the ground state of planar cyclobutadiene and cyclooctatetraene molecules, the stability of a bond-alternated structure is largely attributable to a decrease in the internuclear repulsion energy and the interelectronic repulsion energy due to σ electrons. These observations are consistent with a totally symmetric expansion of the carbon skeleton brought about by the pseudo-JT distortion. Concomitantly, a contraction of the π electron cloud takes place by polarization of the bond charges, and the nuclear-electron attraction energy of π electrons also plays an important role in the pseudo-JT stabilization. Further, the stability of a nonplanar tub structure in cyclooctatetraene results from a lowering of the nuclear-electron attraction energy. In the excited states examined, the situation differs from molecule to molecule. In the lowest excited singlet state of cyclobutadiene, the stability of a rhombic structure originates from the energy lowerings of the nuclear-electron attractive term of σ electrons and the kinetic term of π electrons. While in the lowest excited triplet state of benzene, the stability of a quinoid structure arises from the energy lowerings of the internuclear repulsive term, the interelectronic repulsive term of σ electrons, and the nuclear-electron attractive term of π electrons.

To gain insight into the nature of the pseudo-Jahn−Teller (JT) effect, an energy component analysis has been carried out for the bicyclic nonalternant hydrocarbons termed pentalenoid and heptalenoid systems by using the ab initio RHF method with 6-31G(d) basis set. Inspection of the energy component comprised in the total energy reveals that the stability of a less symmetrical nuclear configuration is largely responsible for the decrease in the internuclear repulsion energy and the interelectronic repulsion energy due to σ electrons. These observations are consistent with an expansion of the carbon skeleton brought about by the pseudo-JT distortion. Another energy component also plays an essential role in the pseudo-JT stabilization:  For the examples, the preference for the C 2h structure rather than the D 2h one for the pentalene arises from a decrease in the interelectronic repulsion energy due to π electrons, while that for heptalene results from a lowering of the nuclear−electron attraction energy due to π electrons. This sharp distinction between the pentalenoid and heptalenoid systems is accounted for in terms of an electrostatic interaction combined with a charge relaxation attributed to π electrons.

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A symmetry rule for predicting molecular shapes which is based on the second-order Jahn-Teller effect has been applied to the predictions of bond distortions in a number of nonalternant hydrocarbons in their ground and electronically excited states. In spite of the very crude approximation that only the lowest-lying excited state plays a dominant role in determining the energetically most favorable nuclear displacement, a fairly clear-cut criterion for molecular-symmetry reduction was obtained. The actual types of the most soft bond distortions, determined by examining the two-center components of transition densities, are in good agreement with the results of variable bond-length SCF MO calculations and available experimental results. The problem of symmetry reductions possible in large cyclic polyenes C4n+2H4n+2 is briefly discussed.

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tert-Butyl thymidylate 3 was prepared from thymidine 1 in six steps and 67% overall yield. When the lithium trianion of 3 (prepared by treatment of 3 with excess LDA and then excess tert-butyllithum) is reacted with electrophiles, trapping occurs stereoselectively from either the alpha- or beta-face depending on the electrophile (Scheme 1). Deuterioacetic acid in deuteriomethanol affords mainly the alpha-deuterated product (4a/4b = 2.4:1) while all other electrophiles, e.g., phenylselenenyl chloride, allyl bromide, and N-fluorobenzenesulfonimide (NFSI), give predominately (or completely) the products of attack from the beta-face (5bcd/4bcd = 3.7:1 to 100:0). The structures of the products were determined by coupling constant analysis of both the initial compounds and the diols 6bcd prepared by ester reduction and by formation of the acetonides 7bc. The methyl ester of the 3'-epimer of thymidylic acid 9 was also prepared from thymidine 1 in nine steps and 74% overall yield. When the lithium trianion of 9 (prepared by treatment of 9 with excess LDA and then excess tert-butyllithum) is reacted with electrophiles, trapping also occurs stereoselectively with attack on either the alpha- or beta-face depending on the electrophile (Scheme 2). Again, deuterioacetic acid in deuteriomethanol affords mainly the beta-deuterated product (11a/10a = 1.6:1) while all other electrophiles, e.g., phenylselenenyl chloride, methyl iodide, allyl bromide, and NFSI, gave predominately (or completely) the product of attack from the alpha-face (8.7:1 to 100: 0). Again, the structures of the products were determined by coupling constant analysis of both the initial compounds, and the diols 12b-e were prepared by reduction of the ester and by formation of the acetonides 13bcd. A rationale has been developed using molecular mechanics calculations to explain the diastereoselectivity that involves staggered axial attack on the sp(2) carbon opposite to the pseudoaxial alkoxy group in the most stable half-chair conformation of the enolates, as shown in Schemes 3-5.