The ground-state geometries and excited singlet and lowest triplet energies of polyacenes from benzene through nonacene are predicted with B3LYP/6-31G* calculations and compared to experimental data where available. The results are compared to these data for cyclacenes and polyenes. The polyacenes and cyclacenes have geometries consisting of two fully delocalized nonalternating ribbons joined by relatively long bonds. Polyacenes are predicted to have smaller band gaps than the corresponding polyenes and triplet ground states for nine or more benzene rings. The fully delocalized nonalternating nature of polyacenes differs from the bond alternation resulting from Peierls distortion in polyenes. The differences are rationalized in terms of a simple MO model, and the results are compared to extensive prior theoretical work in the literature. Predictions about the electronic structure of analogues containing polyacene units are made.

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The mechanism and origins of Z-selectivity in olefin metathesis with chelated Ru catalysts were explored using density functional theory. The olefin approaches from the "side" position of the chelated Ru catalysts, in contrast to reactions with previous unchelated Ru catalysts that favor the bottom-bound pathway. Steric repulsions between the substituents on the olefin and the N-substituent on the N-heterocyclic carbene ligand lead to highly selective formation of the Z product.

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Transition-metal-catalyzed alkene oligomerizations usually yield mixtures of alkenes in the C 4 -C 26 range.[1] It is tedious to separate these mixtures into pure compounds. Consequently, much effort from both industrial and academic communities has been devoted to the search for more efficient catalysts that will produce terminal alkenes of specific lengths. The trimerization of ethene to 1-hexene is of special industrial significance since 1-hexene is an important comonomer in the preparation of linear low-density polyethylene.[2] Most catalysts known today are based on chromium compounds. [3,4] Recently, Sen and co-workers discovered that [TaCl 3 R 2 ], generated in situ by the reaction of TaCl 5 with alkyl metal compounds such as CH 3 Li, can also function as trimerization catalysts under mild conditions (47.6 atm, 40-60 8C).[5] It has been postulated that metallacycles are involved in alkene trimerizations, [3e,f, 6-8] but there has not been detailed information about the potential energy surfaces (PESs) and the structures of the transition states (TSs) and intermediates. Here we report MP2 and B3LYP calculations on the detailed mechanism of the olefin trimerization catalyzed by Sen's catalyst [TaCl 3 (CH 3 ) 2 ]. [9,15] We also explored why this catalyst selectively trimerizes rather than dimerizes or polymerizes alkenes, and have uncovered a novel mechanism for the decomposition of dialkyl complexes of tantalum.Dialkyltantalum complexes [TaCl 3 (R 1 )(R 2 )] (R 1 and R 2 are alkyl groups) are most stable as trigonal bipyramids with two electronegative Cl ligands in the axial positions (TB-1 conformation, see 1 in Figure 1 and Supporting Information). The TSs and intermediates (except 9, see below) involved in the trimerization process adopt similar TB-1 conformations.The insertion of ethene into 1 proceeds directly via TS2 to give 3 (Figure 1). No complex of ethene with 1 was found, in contrast to the usual Cossee mechanism.[16] The lack of ethene complexation may be explained by the crowded TB-1 conformation, the coordinately saturated Ta center of 1, and the lone-pair-p repulsion between Cl ligands and the incoming ethene. Subsequently, 3 undergoes b elimination to afford 5 with liberation of methane. This step is a novel agosticassisted hydride shift (see below), and no minimum for a metal-hydride intermediate could be found. A similar mechanism was proposed by Negishi et al. for the transformation of dialkylzirconocenes to zirconocene-alkene complexes. [17a,b] The catalytic cycle shown in Figure 2 starts with the coordination of a second ethene molecule to the tantalumethylene complex 6, producing 7. Subsequently, facile ring closure via TS8 transforms 7 to a tantalacyclopentane intermediate, which initially has the TB-1 conformation of 9* but easily converts into the more stable 9 with a pseudosquare-pyramidal structure. Intermediate 9 is transformed to tantalacycloheptane 11 by ethene insertion via TS10. The activation energy (in terms of DE 0 ) of this step is 13.0 kcal mol À1 (25.6 kcal mol À1 ...

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