Influence of water in the tautomerism of 2,2'-bipyridine-3,3'-diol

Excited-state proton transfer processes (ESPT) have been widely studied in recent years. Two protons can be transferred via an ESPT process in [2,2'-bipyridine]-3,3'-diol (BPD). Experimental studies have been carried out to explain the spectroscopic behaviour and its relationship with tautomerization. For analogous species, it has been established that the solvent may participate in the interconversion mechanism or that it may influence the relative stabilities of the tautomeric species. We have studied, with ab initio and semiempirical methods, the influence of water in the tautomerization process. We have found that water stabilizes both tautomeric forms in a different amount. This effect is not observed in other polar or non-polar solvents. Additionally, it has been found that discrete interactions of two water molecules on each side of the molecule are needed for predicting this behaviour.