The X-ray crystal Structure of the adduct formed between 4,6-dimethylpyrimidin-2-one and urea (DMP·U) shoes a compact unit joined through two Nâ H â ¯ O.dbd6;C hydrogen bonds. One of these is remarkably short with RN â ¯ O 2.735 à (RH â ¯ O 1.705 à ), and is believed to be among the shortest example of an NHO hydrogen bond between neutral molecules. Its hydrogen-bond energy, estimated from IR data, is ca. 75 kJ mol^{â 1}