Spontaneous Keto-Enol Tautomerization in the Crystal Lattice Visualized with the Help of Water Encapsulated in Hydrophilic Reservoirs

Abstract

Keto-enol tautomerism in the solid phase is a process that is particularly difficult to follow. In this work we demonstrate how it can be done by introducing deuterium into the crystal lattice of organic compounds which tend to form hydrates. In our studies we explored H-D exchange in the crystals stored in contact with deuterium oxide vapors. Employing barbituric acid (BA) and (+)-catechin (CAT) as model samples and by using advanced solid-state NMR spectroscopy and mass spectrometry, we revealed that not only OH and NH protons of these chemicals undergo exchange to deuterium in a crystal lattice, but also usually immobile protons, that is, (Ar)CH (in CAT) and CH₂ (in BA) are exchanged as a result of keto-enol tautomerism.

Keywords: H/D exchange; barbituric acid; catechin; solid-state NMR; solid-state tautomerism.