Deuterium-Induced Isotope Effects on13C Chemical Shifts as a Probe for Tautomerism in Enolic β-Diketones

Deuterium isotope effects on 13C nuclear shielding, nΔC(OD), were investigated for a series of enolic β-diketones at different temperatures. The investigated enolic β-diketones cover a broad range of tautomeric equilibrium constants (K). The equilibrium constants were estimated from 17O and 13C chemical shifts. 13C 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 equlibrium isotope effects for a series with different K may go through a maximum. If the sum of the deuterium isotope effects on 13C 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 β-diketones. 1H chemical shifts of OH groups display a linear relation with the molar fraction X.