Intramolecular hydrogen bonds in 1,4-dihydropyridine derivatives

1,4- dihydropyridine (1,4-DHP) derivatives have been synthesized and characterized by 1H, 13C, 15N NMR spectroscopy, secondary H/D 13C isotope shifts, variable temperature 1H 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 15N NMR signals, the shift to the higher wave numbers of the NH stretching vibration in the IR spectra and the increase of the 1J(15N,1H) values may indicate the shortening of the N-H bond length upon intramolecular NH•••O=C hydrogen bond formation.