DEUTERIUM-ISOTOPE EFFECTS ON C-13 CHEMICAL-SHIFTS OF INTRAMOLECULARLYHYDROGEN-BONDED OLEFINS

A series of intramolecularly hydrogen-bonded enamines, enols and eneth iols with ester carbonylic, ketonic carbonylic, thioester carbonylic, nitro and sulphoxide accepters were investigated to obtain C-13 chemic al shifts and deuterium isotope effects. Results from 33 new compounds and six remeasurements are compared with already existing data. An im portant aim was to show that isotope effects on chemical shifts are us eful descriptors of hydrogen-bonded systems and not only a parameter p roportional to the C-13 chemical shifts. Substituent effects were stud ied and the donors and accepters ranked according to their abilities t o support hydrogen bonding. Steric effects strengthen the hydrogen bon ding in cyclic five-membered beta-diketones. Plots of two-bond [(2) De lta C(OD)] vs. four-bond isotope effects [(4) Delta C(OD)] show that ( 4) Delta C(OD) increases with increasing hydrogen bond strength and th at large deviations from this relationship can be an indicator of taut omerism.