Mechanically Induced Phase Change in Barbituric Acid

By grinding a commercial sample of barbituric acid in its trioxo form (polymorph II, 99%) for 24 h, a new compound has been isolated. The new phase has been identified as the trihydroxyl isomer. The characterization of the new isomer has been carried out by means of X-ray powder diffraction, solid-state NMR ( 1 H MAS, 13 C and 15 N CPMAS, 2D PASS, and FSLG-HETCOR), IR and Raman spectroscopies. The conversion results from the complete tautomeric shift of a methylene and of two N−H hydrogen atoms. 1 H MAS spectra allow the characterization of the hydrogen bond interactions on the basis of their strength in the starting compound and in the new isomer. In solution, the trihydroxyl isomer immediately converts to the trioxo form as demonstrated by 1 H NMR experiments in protic, aprotic, and amphiprotic solvents (D 2 O, MeOH-d 4, DMSO-d 6, acetone-d 6 ).