The Crystal Structure and Thermal Expansion of Two Modifications of N-Methylurea-Oxalie Add (2 " 1)

The crystal structures of the orthorhombic and mono-clinic modifications of N-methylurea-oxalic acid, 2[HzN. CO. NHCHa]. (COOH)2, 2C2H6N20 • C2-H204, have been determined from X-ray data collected on an automatic diffractometer. The struc-tures consist of layers of N-methylurea and oxalic acid molecules. The molecules are held together by hydro-gen bonds. The compounds are addition compounds. Orthorhombic modification: space group Pnma, a = 10.4220(2), b = 6.4274(1), c = 16.1253(5) A at 295 (1) K, D c = 1.46 Mg m-a, Z = 4, Cu Ktt radiation (2 = 1.5418 /t,), Rw = 0.059. The elements of the thermal-expansion tensor at 295 (1) K are a a = 14 (5), % = 274 (14), a c = 29 (6) x 10-6 K-k Monoclini