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Molecular symmetry
Uni of Notts, fundamentals of inorganic & organic chemistry, first year
| Term | Definition |
|---|---|
| Symmetry operation | An action which transforms a molecule, creating an apparently unchanged molecule which could be perfectly superimposed on the original |
| Rotation/n-fold axis (Cn) | Rotation around an axis, n = number of rotations in 360* that the molecule will remain superimposable: C2 = 180* or 1/2, H2O C3 = 120* or 1/3, BH3 C4 = 90* or 1/4, XeOF4 C6 = 60* or 1/6, benzene |
| Reflection/mirror plane (σ) | A plane through which the molecule, if reflected, will still be symmetrical. The highest order rotation will have a plane passed through it & is defined as vertical (σv) & there can be other planes after such as horizontal (σh) & diagonal (σd) |
| Inversion/centre of inversion (i) | An operation through which if every atom in the molecule were to have a negative coordinate will be superimposable. If each atom moved in a straight line through the centre, it would arrive at a similar atom. Happens in staggered conformations |
| Rotation-reflection/axis of improper rotation (Sn) | First the molecule is rotated around σv (its highest order rotation) & then it's reflected in the perpendicular σh axis it will be superimposable. n = rotations in 360* |
| Identity/whole of space (E) | A baseline mathematical function of every single molecule that states if we do nothing to it (or rotate it by 360*) it is superimposable. A molecule with only E is classified as C1 |
| Point group | A set of all the symmetry operations around a fixed point (usually centre of mass) can undergo & still be superimposable |
| Why molecules with i can't be polar | Dipoles form due to the asymmetrical distribution of charge across a molecule & an inversion centre mirrors every group to the other side so they'd all have equal charge & no poles form |
| Why molecules with any symmetry operation other than E (& occasionally Cn) can't be chiral | The whole point of chiral molecules is that they're non-superimposable & symmetry operations make molecules superimposable |
| Tetrahedral point group (Td) justification | If there are many potential axes with n>2 then they would have to be perfect AB4 models with C3 rotation down each bond. This cubic group doesn't have a centre of inversion |
| Octahedral point group (Oh) justification | If there are many potential axes with n>2 then they would have to be perfect AB6 models with C4 rotation down each bond |
| Icosahedral point group (Ih) justification | The only way it wouldn't be cubic with multiple N>2 angles is if it's a rare pentagonal planar (icosahedral) molecule with C5 |
| What "Select C with the highest n; then is nC2 _|_ Cn?" on the point group flow chart means | You select your highest order rotation group to set a σv then it's asking if the σh is C2 |
| What to do with non AB6 octahedrons | If it's an AB4C2 with a symmetrical structure, there may still be a C2 which allows it to pass through the largest box on the point group table since they're perpendicular |
Created by:
Beech47
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