Investigation of the structure and 13C NMR spectrum of 5-phenylbarbituric acid

Crystals of 5-phenylbarbituric acid, 2 , are monoclinic, space group C2/c. There are 8 molecules in the unit cell of dimensions a = 20.218(3), b = 7.477(1), c, = 12.443(2), β = 111.54° and V = 1749.6(4) ų. Full-matrix least-squares refinement has reached R = 0.039. This is the first barbiturate reported to exist in the enol form in the solid state. There is extensive hydrogen bonding between adjacent molecules which gives rise to very dense crystals. The ¹³C-nmr spectroscopy shows that 2 exists in both enol and keto forms in DMSO solution.