The known crystal structures of barbituric acid (including hydrates) have the triketo tautomer. A recent demonstration that the tautomeric polymorph of barbituric acid obtained by grinding is the thermodynamically stable form and the inference from powder diffraction data that it is the mono enol1 has been confirmed using single crystal X-ray diffraction data at 224 and 95 K. Computations at a high level show1 that the enol is higher in energy than the triketo by ca. 54 kJ/mol. This is thus a rare case in which the tautomer found in the crystal is very much higher in energy than the lowest energy tautomer. The additional OH O hydrogen bond of this tautomer with an O-O distance of 2.5 Å does not appear to be adequate to explain the relative stability of the enol crystal over the triketo crystal. Here we explore the vibrational dynamics of these two polymorphic forms as a source of this stability. This is suggested as relevant by the observation that the slurry experiments used to establish relative stability proceed to the triketo form in deuterated ethanol. This isotope effect implies a zero point or heat capacity origin for the free energy difference. 1. M. U. Schmidt, J. Bruening, J. Glinnemann, M. W. Huetzler, P. Moerschel, S. N. Ivashevskaya, J. van de Streek, D. Braga, L. Maini, M. R. Chierotti, R. Gobetto, Angew. Chemie, Int. Ed. DOI:10.1002/anie.201101040 2. S. A. Rivera, D. G. Allis, B. S. Hudson, Cryst. Growth Des. DOI:10.1021/cg800524d

Enol tautomeric polymorph of barbituric acid: The role of zero point energy in stability

251st National Meeting (2016)

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