X-ray diffraction and ENDOR investigation of hydrogen bonding in malonic acid-urea 1 : 1 adduct I. X-ray diffraction

The crystal structure of a 1 : 1 stoichiometric adduct of urea and malonic acid, has been investigated in connection with an ENDOR study of the paramagnetic centre which is formed by ?-irradiation of a single crystal. It is found that the crystal structure consists of a two-dimensional network of hydrogen bonded urea and malonic acid molecules. Interesting features are the planarity of the acid molecule induced by the presence of an intramolecular hydrogen bond, and the presence of a very short hydrogen bond between the carboxylic group and an urea molecule. The position of the proton in this bond is discussed.