CMY 716: L3 Exam Reported Reported Questions Well Answered.

Draw the 2D space group/ lattice p3, p4 and c2 - correct answer summaries What are point groups - correct answer Each point group represents one of the possible unique combinations of crystallographic symmetry elements How many molecular point groups are there? and why are there only 32 crystallographic ones? - correct answer 1. There are an infinite amount of molecular point groups as Cn (rotation) can go up to infinity 2. Crystallographic point groups do not have higher fold rotation axes (C6 max) as they are restricted to the shape of the unit cell. Describe and draw the point group 1 - correct answer 1. Triclinic 2. C1 (Schoenflies symbol i.e. molecular symmetry symbol) thus only identity Describe and draw the point group 1 bar - correct answer 1. Triclinic 2. One bar meaning inversion centre 3. S2 rotoreflection hence generation of an enantiomer. - NB! S2 is equivalent to an inversion centre!! 4. Schoenflies = Ci, S2 Axes for drawing triclinic systems - correct answer a = x, b = y and c comes out of page Describe and draw the point group 2 - correct answer 1. monoclinic 2. C2 rotation 3. Schoenflies = C2 Describe and draw the point group 2/m - correct answer 1. monoclinic 2. C2 rotation with a perpendicular mirror plane - NB! mirror plane generates an enantiomer 3. Schoenflies = C2h Axes for monoclinic systems - correct answer a = y and c = x because the beta angle is unique Describe and draw the point group 222 - correct answer 1. Orthorhombic 2. 3x 2-fold axes 3. Schoenflies = D2 Describe and draw the mmm point group - correct answer 1. Orthorhombic 2. 3x mirror planes i.e. mL, mt and one perpendicular with plane coming out of page. 3. Schoenflies = D2h Axes for orthorhombic systems - correct answer b = y and a = x Describe and draw the point group 4 bar - correct answer 1. tetragonal 2. bar = inversion centre 3. 4 fold rotation axis 4. Schoenflies = S4 Describe and draw the point group 4/m - correct answer 1. tetragonal 2. C4 rotation with perpendicular mirror plane 3. Schoenflies = C4h