Minor comment: Your HTML is much improved. However, you still need TITLE, /TITLE and BODY statements, together with /BODY and /HTML at end of document - I have modified marked file to include these.

ASSIGNMENT IV

Answer to Problem I

QuestionsAnswers  
The Unit Celldue to the packing of the atom/molecule in a crystal 0 Marks
The Crystal Latticedue to the packing of the atom/molecule in a crystal 1 Mark
The origin in reciprocal latticeChosen by the crystallographer 0 Marks
The origin in the crystalChosen by the crystallographer 1 Mark
The space group symmetr of the crystalChosen by the crystallographer 0 Marks

Comment: A nice table. See model answers for correct solution.

Answer to Problem II

When a center of symmetry acts on a point at coordinates (x,y,z) it converts to a point at (-x, -y, -z). The same coordinates can be obtained if we perform 2/m operation i.e C2 axis along b and mirror along ac plane.
Similarly, in an ortho rhombic unit cell if a center of symmetry were addded to 222 groups (which is non-centrosymmetric) they become equal to a centrosymmetric 2/m2/m2/m. Measurement of a centrosymmetric property on a non-centrosymmetric crystal causes the it to rewpond as if it possessed a center of symmetry. Such classes or groups are called Laue groups.
Comment: Examples OK, but explanation could be better.
1 + 2 Marks

Crystallographic symmetry operations may be grouped into 5 distinct groups, namely, lattice translations, rotations, rotary-inversions, screw-rotations, and mirror/glide planes.
(Part of question left in accidently at this point?)

Answer to Problem III

The structure of D-glucose is shown below

the molecule does not possess any symmertry operations. This is further confirmed by its optical activity (dextro rotatory)
Comment: The question is concerned with crystal structure symmetry, and not meolcular - see model answer.

It is better to use .gif files for pictures (or .jpg for photos) since these are compressed, whereas windows .bmp files are uncompressed and very large. [I converted your .bmp file to .gif]. Please avoid the use of scanned images (which should not be required for any coursework) since there is always the risk of copyright infringement with their use. You also need to specify the correct size within the IMG tag, which I have corrected here.
0 Marks

Answer to Problem IV

The co-ordinates for the organo metallic Co compound viewing down the c-axis is worked out to ab as follows;

Co-ordinates

(0,0,0)
(1/2,0,0)
(1,0,0)
(1,0,0)
(1/2,y,1/4)
(1/2,1/2,0)
(0,1/2,0)
(0,y,1/4)
(x,1/4,z)

Since Cobalt forms predominantly Octahedral complexes with m-3m point group symmetry it is expected to occupy special positions.
Comment: Please see model answer for how to attempt this question.
0 Marks

Answer to Problem V

The diagram indicates the staggered conformation of 1,2-dichloro ethane. There is a center of inversion (i) between the two carbon atoms (seen clearly from view A) Therefore the point group symmetry for this molecule is -1.
Comment: You have missed two of the symmetry elements present, and so the end deduction is incorrect.
2 Marks

Answer to Problem VI

(i) The diagram indicates the following 3 types of symmetry elements

a. two fold axis parallel c-axis
b. ac plane
c. bc plane

Comment: You need to give more detail, e.g. plane type.
1 Mark


Comment: File seemed to end here?


Overall comments: Check that you understand the model answers. If not please email us with questions on anything that you don't understand or are unsure about.