NMR studies on intramolecular mobility of ortho-substituted push-pull phenyl butadienes†
Abstract
The 1H and 13C NMR spectra of a series of 3-aryl-2-cyano-5,5-bis(methylthio)penta-2,4-dienenitriles (3) and 3-aryl-2-cyano-5-dimethylamino-5-methylthiopenta-2,4-dienenitriles (4) with different ortho-phenyl substituents were recorded. The NMR data are compared with those of the corresponding para-substituted compounds 1 and 2. Dynamic 1H 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 free energies of activation ΔG c‡ 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.