Spectroscopic investigations on butadiene derivatives - Part 9. NMR studies on intramolecular mobility of ortho-substituted push-pull phenyl butadienes

The H-1 and C-13 NMR spectra of a series of 3-aryl-2-cyano-5,5-bis(methylth io)penta-2,4-dienenitriles (3) and 3-aryl-2-cyano-5-dimethylamino-5-methylt hiopenta-2,4-dienenitrile (4) with different ortho-phenyl substituents were recorded. The NMR data are compared with those of the corresponding para-s ubstituted compounds 1 and 2. Dynamic H-1 NMR measurements showed-rotation processes about the C-2, C-3, C-3, C-4, C-4, C-5, and C-5, N bonds. The fre e energies of activation Delta G(c)double dagger are discussed with respect to electronic and steric effects of the substituents. The magnitude of the rotation barriers correlates with the variation in bond lengths and angles as determined by X-ray structure analyses.