Keto-enol tautomerism of dimedone studied by dynamic NMR, A. Martínez-Richa, G.
Mendoza-Díaz and P. Joseph-Nathan, Appl. Spectrosc., 50, 1408-1412 (1996).
The keto-enol tautomeric equilibrium of dimedone (5,5-dimetyl-1,3-cyclohexanedione)
has been studied by high-resolution 13C NMR spectroscopy. Kinetic information of the
solution keto-enol tautomerism for dimedone in DMSO, in the temperature range of 2585°C, was derived from line shape measurements in a 75-MHz spectrometer. A value of
3.43 Kcal/mol was found for the Arrhenius activation energy Ea and of 1.07 x 106 s-1 for
the pre-exponential factor A. With the use of the observed chemical shifts in the highresolution 13C-NMR spectra of dimedone in the solid state, an estimate coalescence
temperature of 240 K for dimedone in DMSO was obtained by extrapolation of the
experimental curve. The estimated free energy of activation at the coalescence
temperature, ∆Gc≠, is 10.8 Kcal/mol. Finally, the 13C spin-lattice relaxation times,
T1(13C), in solid dimedone were measured as a function of temperature in the range of
25 to 90°C. The data are discussed in terms of the different motional environments that
result from the geometric restrictions imposed by hydrogen bonding in the crystal
structure.
O
8
1
6
5
2
3
HO
4
7