Substituent and temperature controlled tautomerism of 2-phenacylpyridine: the hydrogen bond as a configurational lock of (Z)-2-(2-hydroxy-2-phenylvinyl)pyridine

2-Phenacylpyridines substituted in the benzene ring are in equilibrium with (Z)-2-(2-hydroxy-2-phenylvinyl)pyridines when dissolved in chloroform. The substituent affects significantly the tautomeric equilibrium [the amount o f the enolimine form stabilized by the intramolecular hydrogen bond is 1 an d 92% for R=p-N(CH2)(4) and p-NO2, respectively]. The negative logarithm of the tautomeric equilibrium constant, K-T, is linearly dependent on the Ham mett sigma substituent constants. The dependence of K-T vs. temperature is exponential in character: the more electron-withdrawing is the substituent, the more distinct is the influence of temperature. Unexpectedly, the tauto mer present in the crystalline state is not the same for all compounds stud ied (it is the ketimine one for those carrying strong electron-donor groups ). Among the different ab initio methods used to calculate the enthalpy of the proton transfer in chloroform solution, MP2/6-31G** gives the best resu lts.