Crystal structure of cephalexin monohydrate, C 16 H 17 N 3 O 4 S(H 2 O)

The crystal structure of cephalexin monohydrate has been solved and refined using synchrotron X-ray powder diffraction data and optimized using density functional techniques. Cephalexin monohydrate crystallizes in space group C 2 (#5) with a = 27.32290(17), b = 11.92850(4), c = 16.75355(8) Å, β = 108.8661(4)°, V = 5166.99(3) Å 3 , and Z = 12. Although the general arrangement of molecules is similar to that in cephalexin dihydrate, the structural differences result in very different powder patterns. The crystal structure is characterized by alternating layers of hydrogen bonds and van der Waals contacts parallel to the bc -plane. The water molecules occur in clusters. Five of the six protons in the water molecules act as donors in O–H⋯O hydrogen bonds. The sixth hydrogen atom acts as a donor to two different phenyl ring carbon atoms to form bifurcated O–H⋯C hydrogen bonds. Each cephalexin molecule is a zwitterion, containing ammonium and carboxylate groups. The ammonium ions form N–H⋯O hydrogen bonds to carboxylate groups and water molecules, as well as to carbonyl groups. The powder pattern is included in the Powder Diffraction File™ as entry 00-065-1417.