Intensities of Electronic Transitions in Molecular Spectra IV. Cyclic Dienes and Hyperconjugation

Abstract

The spectra of cyclopentadiene, 1,3-cyclohexadiene, thiophene, furan, and pyrrole are considered. Theoretical explanations of their departures from the spectra of openchain conjugated dienes are given. The cause of these departures in the cases of cyclopentadiene and cyclohexadiene is designated hyperconjugation. This is a mild sort of conjugation, apparently not recognized as such hitherto, between saturated groups and double or triple bonds or even other saturated groups. Observed displacements of spectra toward longer wave-lengths, and corresponding effects in the refractivities (cf. VI), when alkyl groups are substituted for hydrogens in ethylene, butadiene, benzene, or other double-bonded compounds, are attributed to hyperconjugation. Further, the decreasing heat of hydrogenation observed with increasing alkyl substitution in these compounds may indicate increasing stability caused by hyperconjugation. However, it is likely according to the theory that appreciable energy effects caused by hyperconjugation are usually restricted to excited states, and so should affect mainly ultraviolet spectra and refractivities. If this is correct, heats of hydrogenation ought not to be much influenced by hyperconjugation, since they depend on the energies of molecules in their normal states. Hence, it is likely that the above-noted variations in heats of hydrogenation are caused partly or mainly by other causes. Conjugation of triple bonds is briefly discussed, and it is also pointed out that in ethane there must be hyperconjugation which is analogous to the triple-bond conjugation in cyanogen. It is shown that this hyperconjugation in ethane should not appreciably hinder free rotation. Likewise, hyperconjugation in propylene should produce no barrier to free rotation of the methyl group. It is possible, however, that hyperconjugation may help to restrict free rotation in some cases. Carbon to carbon single bond distances should be slightly shortened by hyperconjugation. The unusually small C–C distance in methylacetylene as determined spectroscopically by Herzberg and co-workers may perhaps be explained in this way, although in other cases no appreciable effects are apparent. It is helpful to classify examples of hyperconjugation according to various types, which may differ in the magnitudes of their effects. Possibly only first-order hyperconjugation (between a double or triple bond and a saturated group) is important. Second-order hyperconjugation (between two saturated groups, as, for example, in ethane) very likely produces smaller effects.