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 a new academic year! Learn how to see differently…..

x Symmetry may be defined as a feature of an object which is invariant to transformation There are 5 types of symmetry elements. 1. Mirror plane of reflection,  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, C n 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, S n where n = order of rotation Something NEW!!! C n followed by  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 C 1 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 property 2) demonstrates associativity 3) 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 C 3 axis NOTE: that only symmetry operations form groups, not symmetry elements.

Now, observe what the C 3 operation does: C3C3 C32C32

A useful way to check the 4 group properties is to make a “multiplication” table: C3C3 C32C32

Now, observe what happens when two symmetry elements exist together: Start with an object that has only a C 3 axis

Now, observe what happens when two symmetry elements exist together: Now add one mirror plane,  11 2 2

Now, observe what happens when two symmetry elements exist together: C3C3 1

Here’s the thing: Do the set of operations, {C 3 C 3 2  1 } still form a group? How can you make that decision? C3C3 11 11

This is the problem, right? How to get from A to C in ONE step! What is needed? C3C3 11 11 ACB

What is needed? Another mirror plane! C3C3 11 1 22

And if there’s a 2 nd mirror, there must be …. 33 1 22

Today in Inorganic…. 1. How to Assign Point Groups “the flowchart” 2. Classes of Point Groups 3. Inhuman Transformations 4. Symmetry and Chirality Previously in Inorganic Chemistry ….. 1. Symmetry elements and operations 2. Properties of Groups 3. Symmetry Groups, i.e., Point Groups And as always, Learning how to see differently…..

Does the set of operations {E, C 3 C 3 2  1  2  3 } form a group? 33 1 2 C3C3 C32C32

The set of symmetry operations that forms a Group is call a Point Group—it describes 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, C 3 C 3 2  1  2  3 } is the operations of the C 3v point group.

The Types of point groups If an object has no symmetry (only the identity E) it belongs to group C 1 Axial Point groups or C n class C n = E + n C n ( n operations) C nh = E + n C n +  h (2n operations) C nv = E + n C n + n  v ( 2n operations) Dihedral Point Groups or Dn class D n = C n + nC2 (  ) D nd = C n + nC2 (  ) + n  d D nh = C n + nC2 (  ) +  h Sn groups: S 1 = C s S 2 = C i S 3 = C 3h S 4, S 6 forms a group S 5 = C 5h

Linear Groups or cylindrical class C∞v and D∞h = C∞ + infinite  v = D∞ + infinite  h Cubic groups or the Platonic solids.. T: 4C 3 and 3C 2, mutually perpendicular T d (tetrahedral group): T + 3S 4 axes + 6  v O: 3C 4 and 4C 3, many C 2 O h (octahedral group): O + i + 3  h + 6  v Icosahedral group: I h : 6C 5, 10C 3, 15C 2, i, 15  v

See any repeating relationship among the Cubic groups ? T: 4C 3 and 3C 2, mutually perpendicular T d (tetrahedral group): T + 3S 4 axes + 6  v O: 3C 4 and 4C 3, many C 2 O h (octahedral group): O + i + 3  h + 6  v Icosahedral group: I h : 6C 5, 10C 3, 15C 2, i, 15  v

See any repeating relationship among the Cubic groups ? T: 4C 3 and 3C 2, mutually perpendicular T d (tetrahedral group): T + 3S 4 axes + 6  v O: 3C 4 and 4C 3, many C 2 O h : 3C 4 and 4C 3, many C 2 + i + 3  h + 6  v Icosahedral group: I h : 6C 5, 10C 3, 15C 2, i, 15  v How is the point symmetry of a cube related to an octahedron? …. Let’s see! How is the symmetry of an octahedron related to a tetrahedron?

What’s the difference between:  v and  h  h is perpendicular to major rotation axis, C n vv  v is parallel to major rotation axis, C n hh

5 types of symmetry operations. Which one(s) can you do?? Rotation Reflection Inversion Improper rotation Identity

Today in Inorganic…. 1.Symmetry and Chirality 2.Introducing: Character Tables 3. Symmetry and Vibrational Spectroscopy Previously in Inorganic Chemistry ….. 1. How to Assign Point Groups “the flowchart” 2. Classes of Point Groups 3. Inhuman Transformations Still learning how to see differently…..

First, some housekeeping 1.What sections of Chapter 4 are we covering? (in Housecroft) In Chapter 4: first part, through p.104 (not pp ) and Point Group (or Symmetry Group) Assignments: checking in 3. 1 st introspection due Friday Sept. 16 and Problems set #2 due next Tuesday.

Chirality What is it?? How do you look for it? Is this molecule chiral?It’s mirror image…

Chirality: dissymmetric vs. asymmetric

Chirality: Dissymmetric: having a non-superimposible mirror image (dissymmetric = chiral) vs. Asymmetric: without any symmetry (has C 1 point symmetry)

Chirality as defined through Symmetry: A Dissymmetric molecule has no S n axis. Is this contradictory to what you learned in Organic Chemistry? NO because: a S 1 axis = mirror plane a S 2 axis = inversion center

Chirality as defined through Symmetry: A Dissymmetric molecule has no S n axis. These molecules: do not have mirror symmetry do not have an inversion BUT they are not chiral because they have a S 4 axis.