Prototropic tautomerism in 2- and 4-hydrexypyridines. Halogen substituent effects in the gas phase calculated by semiempirical (AM1) method

Tautomeric equilibrium constants (pK(T)) were calculated in the gas phase f or unsubstituted, mono- and tetrahalogenated 2-hydroxypyridines and for uns ubstituted, mono-, di- and tetrahalogenated 4-hydroxypyridines using semiem pirical method (AM1). Influence of position of halogen (F, Cl, Br) on the p K(T) is studied and compared with that found previously in solution. Observ ed differences in the pK(T) between the gas phase and solution are explaine d by differences in dipole moments calculated for the hydroxy- and oxo-form s. Influence of intramolecular hydrogen bonding on the pK(T) is also discus sed.