DEUTERIUM-INDUCED ISOTOPE EFFECTS ON C-13 CHEMICAL-SHIFTS AS A PROBE FOR TAUTOMERISM IN ENOLIC BETA-DIKETONES

Deuterium isotope effects on C-13 nuclear shielding, ''Delta C(OD), we re investigated for a series of enolic beta-diketones at different tem peratures, The investigated enolic beta-diketones cover a broad range of tautomeric equilibrium constants (K). The equilibrium constants wer e estimated from O-17 and C-13 chemical shifts, C-13 chemical shifts a nd the deuterium isotope effects show changes with temperature, which are due to a change in the tautomeric equilibrium, It is shown that th e variation of K with deuterium substitution depends on K, This has th e important consequence that the equilibrium isotope effects for a ser ies with different K may go through a maximum, If the sum of the deute rium isotope effects on C-13 chemical shifts for the carbonyl and enol ic carbons is above 0.8 ppm for five-membered and 1.2 ppm for six-memb ered ring compounds, the system is tautomeric. This statement holds fo r sterically non-hindered compounds, The intrinsic two-bond deuterium isotope effects for an intramolecular hydrogen bonded system with a ch elate six-membered ring with optimal geometry and a localized double b ond are estimated to be 1.2 ppm, Knowing the intrinsic contribution, d euterium isotope effects can be used to estimate the position of tauto meric equilibria for beta-diketones. H-1 chemical shifts of OH groups display a linear relation with the molar fraction X.