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Will stick to isolated, finite molecules (not crystals).

Will stick to isolated, finite molecules (not crystals). SYMMETRY OPERATION. Carry out some operation on a molecule (or other object) - e.g. rotation. If final configuration is INDISTINGUISHABLE from the initial one - then the operation is a SYMMETRY OPERATION for that object.

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Will stick to isolated, finite molecules (not crystals).

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  1. Will stick to isolated, finite molecules (not crystals). SYMMETRY OPERATION Carry out some operation on a molecule (or other object) - e.g. rotation. If final configuration is INDISTINGUISHABLE from the initial one - then the operation is a SYMMETRY OPERATION for that object. N.B. “Indistinguishable” does not necessarily mean “identical”.

  2. e.g. for a square piece of card, rotate by 90º as shown below: Labels show final configuration is NOT identical to original. Further 90º rotations give other indistinguishable configurations - until after 4 (360º) the result is identical.

  3. SYMMETRY OPERATIONS Motions of molecule (rotations, reflections, inversions etc. - see below) which convert molecule into configuration indistinguishable from original. SYMMETRY ELEMENTS

  4. C3: Isle of Man Coat of Arms

  5. Where is the Isle of Man?

  6. C3 Picture by MC Escher

  7. When m = n we have a special case, which introduces a new type of symmetry operation.....

  8. MORE ROTATION AXES

  9. Some additional points about symmetry axes:

  10. Reflection

  11. Mirror Plane

  12. Greek letter ‘sigma’

  13. C2 σv σv’

  14. Best described in terms of cartesian axes:

  15. Centre of inversion

  16. Centre of inversion?

  17. Position of P given by x,y,z

  18. C2H6

  19. Summary of symmetry elements and operations:

  20. A collection of symmetry operations all of which pass through a single point A point group for a molecule is a quantitative measure of the symmetry of that molecule

  21. Assignment of molecules to point groups Is there a plane of symmetry? Step 1: Is there an axis of symmetry? N Y Molecule in point group Cs Y N Is there a horizontal plane of symmetry? Step 2: Are there C2 axes perpendicular to Cn? Is there a centre of symmetry? Y Molecule in point group Cj Y N Molecule in point group Cnh N N Y No symmetry except E: point group C1 Are there n vertical planes of symmetry? Step 3: There are nC2's perpendicular to Cn Is there a horizontal plane of symmetry? Y Y Molecule belongs to point group Dnh Molecule in point group Cnv N Are there n vertical planes of symmetry? Y Molecule in point group Dnd

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