Computer Lab - Visualization of molecules - Symmetry elements and operations
Introduction:
Symmetry is an organizing principle that provides a means for determining both invariance and change. Symmetry analysis may result in the identification of a fundamental region (motif) that is the smallest element required to explain the repetition that forms a pattern. Molecular symmetry (point symmetry) is the description of the symmetry of an isolated molecule, i.e. a set of operations transforming a system about a common point, which usually turns out to be the centre of gravity of a molecule. Crystallographic symmetry - point symmetry + translational symmetry
In this laboratory students are asked to draw molecules, identify symmetry elements and operations.

What is a symmetry element? A symmetry element is a point, line or plane that leaves the molecule unchanged when a symmetry operation is applied to it.

What is a symmetry operation? A symmetry operation is a permutation (transformation) that leaves the object (molecule or crystal) unchanged, i.e. indistinguishable from the starting state.

Symmetry elements:
  1. E -identity - unity -does nothing
  2. Axis of rotation (Cn): is an axis in which rotation by 360/n leaves the molecule unchanged. n is the order of rotation.
  3. Reflection plane (mirror plane) - reflection through the plane leaves the molecule unchanged - types
    1. Vertical reflection plane (sv)- the plane is parallel to the axis of rotation.
    2. Horizontal reflection plane (sh) - the plane is perpendicular to the rotation axis
    3. Diagonal plane (sd) - vertical plane of symmetry that bisects the axis.
  4. Improper rotation axis (Sn) - Combination of rotation axis (Cn) and horizontal plane of symmetry.
  5. Centre of inversion (i) - is a point in which an atom in position (x,y,z) if moved in a straight line through (i) will have its equivalent at (-x,-y,-z).
Symmetry Operations:
Types:
  1. Identity
  2. Rotation
  3. Reflection
  4. Inversion
  5. Improper rotation-reflection

In this laboratory students will use available software (e.g Arguslab, BIOVIA/Accelrys Discovery Studio, Jmol) to draw molecules, and identify symmetry elements and operations. For each molecule; use the software to draw the molecule; identify its symmetry elements and operations:


1. 1,4 (p)-dibromobenzene (C6H4Br2)

Additional Symmetry Operations:

Use diagrams to show the location of all symmetry elements:





2. pentafluorobromine (BrF5)

Additional Symmetry Operations:

Use diagrams to show the location of all symmetry elements:





3. sulfur (S8)

Additional Symmetry Operations:

Use diagrams to show the location of all symmetry elements:





4. ethylene-BrCl or hydrogen peroxide (H2O2)

Additional Symmetry Operations:

Use diagrams to show the location of all symmetry elements:





5. cis- or trans-1,2-dichloroethylene OR cis- or trans- [NiBr2Cl2]2- (ignore charge on Ni(II) anion)

Additional Symmetry Operations:

Use diagrams to show the location of all symmetry elements:





6. ferrocene Fe(C5H5)2 (eclipsed or staggered)

Additional Symmetry Operations:

Use diagrams to show the location of all symmetry elements:





URL for symmetry web site pages:
http://symmetry.otterbein.edu/gallery/
http://www.stolaf.edu/depts/chemistry/mo/struc/
http://wwwchem.uwimona.edu.jm/spectra/jsmol/demos

Copyright © 2013-2016 by Robert John Lancashire, all rights reserved.

Created and maintained by Prof. Robert J. Lancashire,
The Department of Chemistry, University of the West Indies,
Mona Campus, Kingston 7, Jamaica.

URL http://wwwchem.uwimona.edu.jm/lab_manuals/CHEM2111_Lab2.html