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Vibrational transition moment directions of s-trans-1,3-butadiene were deduced from measurements of its IR linear dichroism on samples partially oriented (i) by photoselective photoiosomerization of s-cis-1,3-butadiene isolated in inert-gas matrices with linearly polarized light, (ii) in stretched polyethylene, and (iii) in stretched poly(ethylene-d 4 ). The moment directions are in quite good agreement with those predicted by theory at the MP2/6-311G** level of approximation. For two plausible mechanisms of s-cis to s-trans isomerization, it predicts the observed near coincidence of the C 1 -C 4 directions. For another reaction path, it predicts a 30 o angle between them. However, at present, it is unknown whether matrix photoisomerizations indeed proceed without exertion of a net torque on the environment

Information on the potential-energy surfaces of the ground and 1σσ* excited states of oligosilane chains, collected from experimental and theoretical sources, is discussed in terms of electronic structure and the mechanisms of bond and excitation delocalization.

A survey is given of recent progress in our laboratory in the preparation and spectroscopic characterization of compounds containing highly twisted bridgehead CN and CC double bonds.