The structure of histamine free base has been determined from three-dimensional X-ray data, collected by counter methods. The tautomer obtained in the solid state by recrystallization from benzene is 5-(2-aminoethyl)imidazole, with no evidence for the coexistence of 4-(2-aminoethyl)imidazole. The imidazole and NH2 groups are trans to one another across the C-C bond of the ethyl group. The nitrogen and carbon atoms of the imidazole ring are coplanar. The dihedral angle between this plane and the plane of the carbon and nitrogen atoms of the aminoethyl side chain is 66.3°. Distances within the molecule are in good agreement with recent X-ray and neutron diffraction results on the orthorhombic form of L-histidine, despite the fact that orthorhombic l-histidine is a derivative of 4-ethylimidiazole, rather than 5-ethylimidazole. In histamine free base there is an N-H…N hydrogen bond involving the N-H function of the imidazole ring with the NH2 group on the side chain of an adjacent molecule. In this way infinite zig-zag chains of imidazole molecules are formed in the crystal. Histamine free base cry stallizesin space group C22-P21 of the monoclinic system, in a cell of dimensions a = 7.249 (2), b = 7.634 (3), c = 5.698 (2) á, and β = 104.96 (2)°. A density of 1.218 g/cm3 calculated for two molecules in the unit cell agrees with that of 1.20 (1) g/cm3 observed by flotation in an ethyl benzoate-iodobenzene medium. A total of 548 unique reflections was collected using Cu Ka X-radiation, prefiltered with Ni foil. The structure was solved by direct methods and refined by least-squares methods to a final agreement index on F of 3.1 %.
ASJC Scopus subject areas
- Colloid and Surface Chemistry