Noreen Hanif
ASSIGNMENT 4

1.(a) The unit cell
(ii) chosen by the crystallographer. The unit cell is a repeat unit that can be any size which when repeated produces the crystal but it is chosen by the crystallographer to contain the least number of lattice points and the highes symmetry.

(b) The crystal lattice
(i) packing of the atoms/molecules in the crystal. The crystal lattice is the repeat of atoms/molecules in the crsytal therefore depends on how they pack to produce the lattice.

(c) The origin in reciprocal space
(iii) neither. This is a natural origin that corresponds to the X-ray beam.

(d) The origin in the crystal
(ii) chosen by the crystallographer. The origin is chosen so that there is high symmetry in the unit cell of the crystal.

(e) The space group symmetry of the crystal
(i) packing of the atoms/molecules in the crystal. The space group is the classification of the symmetry of the crystal which depends upon how the atoms/molecules are packed in the crystal to give the particular symmetry operators.

2. A Laue class is one of the 11 centrosymmetric point groups which describe 3-dimensional diffraction symmetry. Each crystal system has a corresponding point group but the higher symmetry crystal systems, e.g. cubic, belong to 2 Laue groups. The Laue class for the triclinic system is -1 and the monoclinic system belongs to the 2/m class.

3. Rotary inversions cannot be found in an enantiomorphic molecule because there will be a centre of inversion in the molecule hence a centrosymmetric molecule which cannot be chiral. Enantiomorphic molecules do not contain mirror/glide planes.

4.Other coordinates:
(0.271, 0.25, 0.958) (0.729, -0.25, 0.542) (0.729, 0.75, 0.042)
No the Co atom is not sited on a special position in this space group.

5. I'm a bit confused, in the question the molecule is called 1,2-dichloroethylene but the diagram of the molecule is of 1,2-dichloroethane, I will assume the question is about 1,2-dichloroethane.

The symmetry element present is a centre of inversion through the centre of the C-C single bond. The molecular point symmetry will be -1.

6.(i) Symmetry elements present are mirro planes, glide planes and 2 fold rotation axes.
(ii) Red object related to magenta objects:
Z - mirror plane
J - glide
K - 2 fold rotation axis
(iii) K - no change of handedness, same as the red object - 2 fold rotation does not involve any change of handedness.
Z - change in handedness from the red object due to the mirror plane.
J - chnage in handedness from the red object due to glide plane.
(iv) Space group Pma2 therefore crystal system mm2.