Content Board exercise: How to draw 3D models (molecules / crystals) Group work: each group gets a molecules name -> draw the structure, build a model, find symmetry elements Point Groups for molecules and crystals Flow Chart to find a Point Group Practise with the molecules before myphayao.com: online quiz and tutorials
Symmetry Elements E - the identity operation C n - rotation by 2π/n angle * S n - improper rotation (rotation by 2π/n angle and reflection in the plane perpendicular to the axis) σ h - horizontal reflection plane (perpendicular to the principal axis) ** σ v - vertical reflection plane (contains the principal axis) σ d - diagonal reflection plane (contains the principal axis and bisect the angle between two C 2 axes perpendicular to the principal axis) * - n is an integer ** - principal axis is a C n axis with the biggest n. Molecule belongs to a symmetry point group if it is unchanged under all the symmetry operations of this group. http://www.webqc.org/symmetry.php
Example Cyclopropane has TWO C 3 axes: C 3 and C 3 2
Order of Symmetry = the number h of symmetry elements for one point group ! For example: Ammonia = C3v has order 6 (E + 2 C3 + 3 sv) The higher the order, the higher the symmetry ! Which one has higher symmetry: C4v or D2h ?
End of Part 1 about Symmetry Point Groups For a molecule (or any structure) we can find symmetry operations which leave the molecule unchanged. These operations are: Identity E (for each molecule) Rotations Cn Mirrors σ Inversion i... And the combination Sn (combine Cn + σ ) The sum of all possible operations for a molecule define its point group. The name of the point group indicates the main symmetry elements: for example: D4h indicates a C4 axis and σh) Try the program at http://molwave.com “3DSymm” to see the symmetry operation in 3Dhttp://molwave.com