Thermochromic nitroxide: A nitroxide radical (AZADO) exhibits thermochromism with a structural change from a dimer at low temperature (4 °C) to paired monomers with large antiferromagnetic interaction at high temperature (35 °C). An hysteresis loop is observed in the magnetic data of this compound, revealing it to exhibit a first‐order phase transition with room‐temperature magnetic bistability.

In this study, the tautomeric equilibrium between the keto and enol forms has been studied for five typical ketones and aldehydes: i-butanal, acetaldehyde, acetone, acetylacetone, and dimedone. The level of theory used in the gasphase calculation was Becke, three-parameter, Lee-Yang-Parr/6-311G(d,p)//Becke, three-parameter, Lee-Yang-Parr/ 6-31G(d). The free energies of solvation were included in the calculation by using the free-energy perturbation method based on Monte Carlo simulation, that is, the quantum mechanical/Monte Carlo/free-energy perturbation method. Three different models, incorporating no-water, one-water, and two-waters, were adopted. The results showed that in the gas phase the addition of water molecules to the reaction mechanism caused the activation barriers (ΔG { gas ) to decrease by half relative to the water-free mechanism, but there was no effect on the relative difference in free energy, ΔG gas . The solvation effects (ΔG sol ), based on quantum mechanical/Monte Carlo/freeenergy perturbation calculations, were added to those of the gas-phase results of the one-water and two-waters models. The two-waters model produced values that were very consistent with the experimental data for all of the tautomers. The differences in the relative Gibbs free energy (ΔG rxn ) were less than 1.0 kcal mol -1 . In summary, the inclusion of solvent molecules in gas-phase calculations plays a very important role in producing results consistent with experimental data.

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