Intramolecular hydrogen bonds in 1,4-dihydropyridine derivatives.

1,4-Dihydropyridine (1,4-DHP) derivatives have been synthesized and characterized by H, C, N nuclear magnetic resonance (NMR) spectroscopy, secondary proton/deuterium C isotope shifts, variable temperature H NMR experiments and quantum-chemical calculation. The intramolecular hydrogen bonds NH⋯O=C and CH⋯O=C in these compounds were established by NMR and quantum-chemical studies The downfield shift of the NH proton accompanied by the upfield shift of the N nuclear magnetic resonance signals, the shift to the higher wavenumbers of the NH stretching vibration in the infrared spectra and the increase of the J(N,H) values may indicate the shortening of the N-H bond length upon intramolecular NH⋯O=C hydrogen bond formation.