The crystal structure of the urea–hydrocarbon complexes

The structure of the urea-normal hydrocarbon complex has been determined. The unit cell is hexagonal, a 0 ---8.230, c o ----11.005 /k, space group C6:2-D~, six urea molecules per unit cell. The general features of the structure and the nitrogen positions of the urea were obtained directly from an implication diagram or Patterson-Harker section P(x, y, 16) which, for this space group, is essentially equivalent to an electron-density projection along the c axis. The urea molecules form a hollow channel structure in which the n-hydrocarbon molecules are enclosed. The hydrocarbons are in an extended planar zigzag configuration with their long axis parallel to the c axis. The electron-density projection and the implication diagram indicate that the time average of the positions of the plane of the hydrocarbon molecule are randomly disposed over positions perpendicular to the a axis and at multiples of 60 ° to this position. Diffuse bands observed in most of the complexes are attributed to the hydrocarbon molecules which behave as a system of linear gratings regularly arrayed in the x y plane but with random z coordinates.