Predominance of (E)-1,2-Di(pyridin-2-yl)ethene-1,2-diol over 2-Hydroxy-1,2-di(pyridin-2-yl)ethanone in Solution

1H, 13C and 15N NMR spectra, supported by the GIAO/DFT calculated (B3LYP/6- 311G//RHF/3-21G) 13C and 15N (B3LYP/6-31++G**//RHF/3-21G) NMR chemical shifts, show that (E)-1,2-di(pyridin-2-yl)ethene-1,2-diol (OO3) is the only tautomer present in chloroform solution. MP2/6-31G**//RHF/6-31G** and MP2/6-31G** ab initio calculations confirm that this perfectly planar form is really more stable than 2-hydroxy-1,2-di(pyridin-2-yl)ethanone (OK1, OK2 and OK3) and other isomeric dimers of pyridine-2-carboxaldehyde. The strong intramolecular hydrogen bonds are responsible for high stability of (E)-1,2-di(pyridin-2-yl)ethene-1,2-diol (the conjugation in the molecule is of minor importance).