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Announcements
n ne ass gnments First online assessment on cha 2 and 3
started, and will be until Wednesday Jan 30 at11.59 pm
You have one hour to answer 10 quiz
questions picked randomly in a larger set ofques ons
Next two classes taught by Laura Pavelka
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Office hours on Thursday cancelled.
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4. S mmetr and
group point
Inor anic chemistr : Main rou
CHEM 281
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r. u rey . oores
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CHEM 201-281: II. Symmetry elements and operation of symmetry
. Inversion (i): Each point ends up in an opposite position
respective to the centre of symmetry (which must be center
of the molecule)E.g.: benzene vs methane
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CHEM 201-281: II. Symmetry elements and operation of symmetry
. Rotation-reflection operation (Sn)= improper rotation
It is a com osite transformation: Sn o Cn
Sn is a rotation about an Cn axis followed by a reflectionthrough a perpendicular plane
. .
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and then another. The second operation is done first, then comes the
first. It is like a multiplication.
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CHEM 201-281: III. Point groups: group of symmetry of operations
.Def:A group is a mathematical object. It contains some operations of
s mmetr and the com osition and obe s 4 rules described below
The composition of two elements of the group is also an element ofthe group
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Application: multiplication table
o E C2 v v
E
C2
E C2 v v
C2 E v v
4 Associative ro ert : A o B o C = A o B o C
vv v
v v C2 E
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III.3 S mmetr rou s nomenclature
CHEM 201-281: III. Point groups: group of symmetry of operations
The collection of the symmetry operations of a molecule(+ o) = symmetry group of the molecule. These groupsare given names.
E.g.:
Water molecule C2v C2, v, v, E associated with o
Starwars Sk walker
Springfield Simpson
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III.3 S mmetr rou s nomenclature
CHEM 201-281: III. Point groups: group of symmetry of operations
C1: Group containing E only (no symmetry)
C : E + 1 reflection
C : E + inversion center
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III.3 S mmetr rou s nomenclature
CHEM 201-281: III. Point groups: group of symmetry of operations
Cn (n>1): E + (n-1) rotations about an axis Cn
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o e: a mo ecu e as one ax s n, en n, n n- are ro a ons a a
belong to the group. Thus we say that the group possesses (n-1) Cn
rotations. E.g.: C3 and C32 C3 group => The group has 2C3
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III.3) Symmetry groups nomenclature
CHEM 201-281: III. Point groups: group of symmetry of operations
Cnv (n>1): E + (n-1) rotations Cn + n vertical reflections
C (n>1): E + (n-1) rotations C + 1 horizontal reflection
Cv: E + rotations of any angle about an axis C+
infinite number of vertical planes
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III.3 S mmetr rou s nomenclature
CHEM 201-281: III. Point groups: group of symmetry of operations
Dn (n>1): E + (n-1) rotations Cn (principal axis) + nrotations C2 (the C2 axes are perpendicular to Cn
Dnh (n>1): E + (n-1) rotations Cn (principal axis) + n C2
inversion when n is even Sn when n3
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III.3 S mmetr rou s nomenclature
CHEM 201-281: III. Point groups: group of symmetry of operations
Dnd (n>1): E + 2 improper rotations S2n + (n-1) Cnrotations + n C2 rotations + n vertical reflections
nvers on w en n s uneven
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III.3 S mmetr rou s nomenclature
CHEM 201-281: III. Point groups: group of symmetry of operations
Dh: E + rotations of any angle + number of vertical
planes + horizontal plane + i + number of C2 rotations +
Reminder: Cv:
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III.3 S mmetr rou s nomenclature
CHEM 201-281: III. Point groups: group of symmetry of operations
Sn (n even): E + 2 improper rotations Sn, 2 Cn/2 + iif n/2 is uneven
See Molwave software
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III.3 S mmetr rou s nomenclature
CHEM 201-281: III. Point groups: group of symmetry of operations
Oh (octahedron): E + 4C3 + 6C2 + 3C4 + i + 3S4 +4S6 + 3 h + 6 d
http://www.mathsisfun.com/geometry/octahedron-model.html
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III.3 S mmetr rou s nomenclature
CHEM 201-281: III. Point groups: group of symmetry of operations
Ih (Icosahedron): E + 6S5 + 6C5 + 10 C3 + 15 C2 +i + 6 S10 + 10 S6 + 15
htt ://www.mathsisfun.com/ eometr /icosahedron-model.html
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III.3 S mmetr rou s nomenclature
CHEM 201-281: III. Point groups: group of symmetry of operations
Point group assignment tree You can find it on myCourses
To assign the point group of a molecule, we
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cons er so e y epos on o e a oms an nothe lone pairs, the orbitals
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