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), were investigated for a series of enolic beta-diketones at different temperatures, The investigated enolic beta-diketones cover a broad range of tautomeric equilibrium constants (K). The equilibrium constants were estimated from O-17 and C-13 chemical shifts, C-13 chemical shifts and the deuterium isotope effects show changes with temperature, which are due to a change in the tautomeric equilibrium, It is shown that the variation of K with deuterium substitution depends on K, This has the important consequence that the equilibrium isotope effects for a series with different K may go through a maximum, If the sum of the deuterium isotope effects on C-13 chemical shifts for the carbonyl and enolic carbons is above 0.8 ppm for five-membered and 1.2 ppm for six-membered ring compounds, the system is tautomeric. This statement holds for sterically non-hindered compounds, The intrinsic two-bond deuterium isotope effects for an intramolecular hydrogen bonded system with a chelate six-membered ring with optimal geometry and a localized double bond are estimated to be 1.2 ppm, Knowing the intrinsic contribution, deuterium isotope effects can be used to estimate the position of tautomeric equilibria for beta-diketones. H-1 chemical shifts of OH groups display a linear relation with the molar fraction X.