Platonic Point Group
The platonic rotation group is defined by having multiple Cn
axes where n>2. These molecules are considered to have high symmetry
and are non-polar. This rotation group can be further divided into
3 subgroups: tetrahedral (T), octahedral (O) and icosahedral (I) depending
on whether n=3, n=4 or n=5 respectively.
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T - If more than one C3 is present, the molecule
will be tetrahedral. Molecules in this group will have a full point
group of either Th or Td.
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Th - These molecules will also have either an inversion
center (i) or a perpendicular (to the C3) mirror plane
(sh). These molecules do not
look like tetrahedra. Examples are relatively rare; one is [Fe(C5H5N)6]2+.
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Td - Most tetrahedral molecules will belong to this
point group. They will have a dihedral mirror plan (sd)
bisecting two C3's. A common example is methane
(CH4).
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O - This group is defined by the presence of multiple C4's.
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O - If there is no inversion center and no mirror planes, then
the full point group is O, and the molecule is chiral. This point
group is quite rare.
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Oh - If either an inversion center or a mirror plane
exist, then the full point group is Oh. These molecules
are achiral. An example is [PtCl6]2-.
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Ih - The icosahedral group is defined by the presence
of multiple C5's. There will in fact be 12C5's,
as well as 20C3's, 15C2's, 15s's
and an i.. An example is [B12H12]2-.