Article abstract of DOI:10.1016/0040-4020(67)85005-1

A number of new 3,5-disubstituted-N-methyl-pyridium salts were synthesized. By Na₂S₂O₄ and NaBH₄ reduction, the corresponding dihydropyridines were obtained. The structures (ratio of isomers) of the reduction products were determined by UV, IR and NMR spectroscopy. The relationship between the Z-values of several solvents of varying polarity, and the electronspectra of the dihydropyridines was established. This allowed an estimation, to what extent dipolar structures participate in the ground- and excited state of the dihydropyridine molecules. So one observes that the groundstate already exhibits to a large extent dipolar character. On the basis of the electrongas model, Stoerpotentiale were calculated from the absorption-spectra of dihydropyridines and therefrom the double bond character in the chromophore was estimated. Such values could also be calculated from bond lengths for one of the compounds whered X-ray analysis already exists. The results of the different methods used, agree with each other. The double-band UV spectrum in different disubstituted 1,6-dihydropyridines is discussed on the grounds of the existing data for the position and intensity of both maxima. The proposition is made that the absorption at longer wavelength is due to the amino-diene, 1-amino-4-cyano-chromophore resp., the one at shorter wavelength due to a β-aminoacrylic acid-like system.