The keto-enol tautomeric equilibrium of dimedone has been investigated by high-resolution <sup>13</sup>C NMR spectroscopy. Kinetic information of the solution keto-enol tautomerism for dimedone in DMSO, in the temperature range of 25-85 °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 <i>E</i><sub>a</sub> and of 1.07 x 10<sup>6</sup> s<sup>-1</sup> for the pre-exponential factor <i>A.</i> With the use of the observed chemical shifts in the high-resolution <sup>13</sup>C-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, Δ<i>G</i><sub>c</sub>, is 10.8 Kcal/mol. Finally, the <sup>13</sup>C spin-lattice relaxation times, <i>T</i><sub>1</sub>(<sup>13</sup>C), 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.
Antonio Martínez-Richa, Guillermo Mendoza-Díaz, and Pedro Joseph-Nathan, "Keto-Enol Tautomerism of Dimedone Studied by Dynamic NMR," Appl. Spectrosc. 50, 1408-1412 (1996)