title: Commensurate and incommensurate hydrogen bonds. An exercise in crystal engineering.
abstract: Ureas characteristically form one-dimensional hydrogen-bonded alpha-networks with a repeat distance of about 4.60 A. Oxamides form similar alpha-networks with a longer 5.05 A repeat distance. The urea of glycine and the oxamide of glycine were each cocrystallized with a series of four bipyridines, including two urea derivatives and two oxamide derivatives. This series of eight cocrystals was studied by X-ray diffraction in order to see what would happen when molecules that would normally form alpha-networks with incommensurate distances were forced into the same crystal. The two all-urea crystals and the two all-oxamide crystals contained the expected alpha-networks with repeat distances in accordance with normal urea or oxamide values. Four of the crystals were mixed, containing both oxamide and urea molecules. Three consisted of two-dimensional beta-networks with alternating parallel urea and oxamide subnetworks. The repeat distances averaged 4.87 A, a value close to the value expected for oxamides, but shorter than any previously observed examples. In the fourth mixed crystal, the urea alpha-network formed with a normal urea repeat distance, but the oxamide network did not form, the oxamide adopting an unusual molecular conformation that maximizes intramolecular hydrogen bonds instead.
authors.affiliation: Department of Chemistry, State University of New York, Stony Brook, NY 11794, USA.
authors.forename: T L
authors.initials: TL
authors.lastname: Nguyen
authors.forename: F W
authors.initials: FW
authors.lastname: Fowler
authors.forename: J W
authors.initials: JW
authors.lastname: Lauher
mesh_terms: D003460:Crystallization; D005998:Glycine / Q000737:chemistry; D006860:Hydrogen Bonding; D008958:Models, Molecular; D010072:Oxamic Acid / Q000031:analogs & derivatives* / Q000737:chemistry; D011725:Pyridines / Q000737:chemistry; D014508:Urea / Q000737:chemistry*
chemical_list: D011725:Pyridines; D014508:Urea; D010072:Oxamic Acid; C008728:oxamide; D005998:Glycine