Enol−Enamine Tautomerism in Crystals of 1,3-Bis(pyridin-2-yl) Propan-2-one:  A Combined Crystallographic and Quantum-Chemical Investigation of the Effect of Packing on Tautomerization Processes

The enolpyridine, <b>OH</b>−ketoenamime, <b>NH</b> equilibrium in crystals of 1,3-bis(pyridin-2-yl)propan-2-one was studied using temperature-dependent single-crystal X-ray diffraction. The relative population of the different tautomers was found to be sensitive to the temperature in the range of 100−300 K, illustrating the small thermodynamic difference between these two tautomers. This energy resemblance is partially attributed to the molecular packing in the crystal, where the molecules are arranged in the form of dimers. Ab initio electronic energy calculations (HF/6-31G** and MP2/6-31G**) reveal the effect of dimerization in the crystal on the electronic energy levels. Several tautomeric states were identified in the dimer of 1,3-bis(pyridin-2-yl)propan-2-one. A model is proposed in which four of these dimer states are populated in the crystal at ambient temperatures. The crystallographic data were treated according to this four-state dimer model, suggesting that the free energy of the OH−NH dimers is higher than that of the OH−OH dimers by 120 ± 10 cal mol<sup>-1</sup> and that the NH−NH dimers are yet higher in free energy by 50 ± 10 cal mol<sup>-1</sup>.