The electronic structure of some polyenes and aromatic molecules VI-Phenylethylene, stilbene, tolane and the phenylmethyl radical.

Abstract Observation and theory agree that the internuclear separation of two carbon atoms linked together in a molecule may be anything between 1·54 A, a distance characteristic of the single bond, and 1·20 A, the triple bond distance. Deviations from the single, double and triple bond distances are especially to be expected when neighbouring carbon-carbon links in the molecule are of different order. An approximate method of calculating lengths of linkages in such molecules was given by Pauling, Brockway and Beach (1935), who also applied their theory to links involving atoms other than carbon. Lennard-Jones (1937a), Lennard-Jones and Turkevitch (1937b), and Penney (1937), in papers I, II and III of the present series, have gone into the particular matter of carbon bonds more thoroughly, and have considered in detail the molecules of benzene, butadiene, hexatriene, biphenyl and napthalene, and the crystal graphite. Unfortunately, extensive comparison with experiment was not possible because of the lack of measurements, but in those cases where it could be made, namely benzene and graphite, the agreement was excellent. Very recently, Robertson and Woodward (1937) have made careful X-ray analyses of crystals of stilbene and tolane, and have succeeded in determining with great accuracy the dimensions of the stilbene and tolane molecules. Their results are very interesting from a theoretical point of view because they offer the first possibility of a stringent test for the theory.