today in inorganic…. symmetry elements and operations properties of groups symmetry groups, i.e.,...
Post on 19-Dec-2015
224 views
TRANSCRIPT
Today in Inorganic….
Symmetry elements and operations
Properties of Groups
Symmetry Groups, i.e., Point Groups
Classes of Point Groups
How to Assign Point Groups
Previously:
Welcome to 2011!
x
Symmetry may be defined as a feature of an object which is invariant to transformation
Symmetry elements are geometrical items about which symmetry transformations—or symmetry operations—occur.
There are 5 types of symmetry elements.1. Mirror plane of reflection, s
z
y
Symmetry may be defined as a feature of an object which is invariant to transformation
There are 5 types of symmetry elements.2. Inversion center, i
z
y
x
Symmetry may be defined as a feature of an object which is invariant to transformation
There are 5 types of symmetry elements.3. Proper Rotation axis, Cn
where n = order of rotation
z
y
x
Symmetry may be defined as a feature of an object which is invariant to transformation
There are 5 types of symmetry elements.
y
4. Improper Rotation axis, Sn
where n = order of rotationSomething NEW!!! Cn followed by s
z
Symmetry may be defined as a feature of an object which is invariant to transformation
There are 5 types of symmetry elements.5. Identity, E, same as a C1 axis
z
y
x
When all the Symmetry of an item are taken together, magical things happen.
The set of symmetry operations NOT elements)in an object can form a Group
A “group” is a mathematical construct that has four criteria (‘properties”)
A Group is a set of things that:1) has closure property2) demonstrates
associativity3) possesses an
identity 4) possesses an
inversion for each operation
Let’s see how this works with symmetry operations.
Start with an object that has a C3 axis.
1
23
NOTE: that only symmetry operations form groups, not symmetry elements.
A useful way to check the 4 group properties is to make a “multiplication” table:
1
23
3
12
2
31
C3 C32
Now, observe what happens when two symmetry elements exist together:
Start with an object that has only a C3 axis.
1
23
Now, observe what happens when two symmetry elements exist together:
Now add one mirror plane, s1.
1
3
s1
2
Here’s the thing:
Do the set of operations, {C3 C32 s1}
still form a group?
1
23
3
12
3
21
How can you make that decision?
C3 s1
s1
This is the problem, right?How to get from A to C in ONE step!
1
23
3
12
3
21
What is needed?
C3 s1
s1
A CB
3
12
3
21
Does the set of operations {E, C3 C32 s1 s2
s3}form a group?
s3
s1
1
23
s2
1
23
3
12
2
31
C3 C32
The set of symmetry operations that forms aGroup is call a Point Group—it describers completely the symmetry of an object around a point.
Point Group symmetry assignments for any object can most easily be assigned by following a flowchart.
The set {E, C3 C32 s1 s2 s3} is the
operations of the C3v point group.
The Types of point groups
If an object has no symmetry (only the identity E) it belongs to group C1
Axial Point groups or Cn class Cn = E + n Cn ( n operations)Cnh= E + n Cn + sh (2n operations)Cnv = E + n Cn + n sv ( 2n operations)
Dihedral Point Groups or Dn class Dn = Cn + nC2 (^)
Dnd = Cn + nC2 (^) + n sd
Dnh = Cn + nC2 (^) + sh
Sn groups:
S1 = Cs
S2 = Ci
S3 = C3h
S4 , S6 forms a groupS5 = C5h
Linear Groups or cylindrical class
C∞v and D∞h= C∞ + infinite sv= D∞ + infinite
sh
Cubic groups or the Platonic solids..
T: 4C3 and 3C2, mutually perpendicularTd (tetrahedral group): T + 3S4 axes + 6 s
O: 4C3, mutually perpendicular, and 3C2 +
6C2Oh (octahedral group): O + i + 3 sh + 6 sdIcosahedral group:Ih : 6C5, 10C3, 15C2, i, 15 s
What’s the difference between: sv and sh
1
23
3
12
3
21
sh is perpendicular to major rotation axis, Cn
sv
sv is parallel to major rotation axis, Cn
sh
5 types of symmetry operations.
Which one(s) can you do??
RotationReflectionInversionImproper rotationIdentity