抄録
Refractive indices of the hexagonal urea adducts were measured by the polarizing microscope, and they were compared with that of the tetragonal crystal of urea.
In the tetragonal crystal of urea, the planes of the Y-shaped molecules are perpendicular to each other but parallel to the c axis; consequently this crystal is uniaxially positive (ω=1.483, ε=1.593). In the hexagonal urea adducts, although the oxygen atoms of urea molecules are superimposed along a vertical edges axis, the urea molecules are oriented 120° with respect to one another forming a helix spiral. The guest molecules are packed along c axis. As a result, the urea adducts are also uniaxially positive, and ω were constant (=1.492), but ε were changed by the forms of guest molecules (paraffins=1.566, saturated fatty acids =1.577, unsaturated fatty acids=1.5827).
For the birefringence, therfore, the tetragonal urea was stronger than the hexagonal urea adducts.
