Abstract
The structures of the uracil and thiouracils were examined using NMR spectroscopy and crystal structure data when available. The relationships between the extent of polarization and the C5-C6 bond length as well as the H5-H6 coupling constants were probed. It was found that the bond length and coupling constants correlate well with the proton affinities at the carbonyl or thiocarbonyl groups at C4 but not C2. The possible implication in the tighter binding of thiouracil based nucleotides to orotidine-5'-monophosphate decarboxylase was discussed.