The belt-like polyphenylenes, [0 n ]paracyclophanes, (n = 5 and 6), have been investigated using semi-empirical, ab initio and DFT methods. The molecular structure, rotational barrier on twisting a single phenyl ring and the aromatic character within each ring as well as in the whole molecule have been evaluated. [0 5 ]Paracyclophane is predicted to have a quinonoid structure. In contrast, the equatorial pentaphenyl fragment found in C 70 as well as the hexagons of the less strained [0 6 ]paracyclophane have benzenoid character. Approximate band structures have been derived for larger cycles of [0 n ] paracyclophanes.

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At elevated temperatures bis(alkynyl)(1,5-cyclooctadiene)platinum complexes catalyze the cross-linking of polyorganosiloxanes containing Si−H and vinyl groups. Density functional calculations with medium-size basis sets and effective core potentials are reported for reactions that may activate these precatalysts for hydrosilylation. For a model system consisting of the bis(ethynyl) complex, trimethylsilane, and ethylene, the computations provide two plausible pathways for gaining access to the Chalk−Harrod cycle. The first one involves a sequence of four oxidative additions and reductive eliminations, while the second one requires a reductive coupling that is induced by olefin coordination. In both cases, the initial step is rate-determining, with a computed barrier of 27 kcal/mol. Experiments for polysiloxane systems of industrial interest favor the first pathway and yield barriers of 25−30 kcal/mol. Substituents in the alkynyl groups affect the measured barriers and the barriers computed for the rate-determining initial step of the first pathway in a qualitatively similar manner. We propose that the activation of the precatalysts is initiated by oxidative addition of Si−H.

Density functional calculations have been performed on different conformers and isomers of the propyl group in alkyl cations [L2Zr−Pr]+ (L = Cp, Cp*; Pr = n-propyl), corresponding to two catalysts with different observed rate orders (n) for ethylene polymerization, to explore the single-center, two-state kinetic model for olefin polymerization. For L = Cp (n ≈ 1), the β-agostic conformer is found to be the most stable structure and also the most reactive with respect to ethylene coordination, which is commensurate with unity rate order. For L = Cp* (n ≈ 1.4), the favored propagation route involves the γ- and α-agostic conformations of the alkyl complex, with coordination taking place to an α-agostic conformation in order to minimize the steric hindrance experienced by the incoming ethylene. The barriers to rearrangement from the α- and γ-agostic conformers to the more stable β-agostic structure are significantly lower than those of propagation. Moreover, no structure was found to be of lower energy than the β-agostic conformation, and the latter thus takes the role of the resting state for both catalysts in the present study. Assuming that the single-center, two-state model applies to zirconocene-catalyzed ethylene polymerization, our calculations thus suggest that the corresponding fast and slow, or dormant, states do not originate from equilibria of the alkyl group as investigated here.