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)

Use diagrams to show the location of all symmetry elements:

2. pentafluorobromine (BrF5)

Use diagrams to show the location of all symmetry elements:

3. sulfur (S8)

Use diagrams to show the location of all symmetry elements:

4. ethylene-BrCl or hydrogen peroxide (H2O2)

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)

Use diagrams to show the location of all symmetry elements:

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

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