Molecular Orbitals

1
Molecular Orbitals

• Be able to distinguish between the 3 structure
  and bonding models we have studied so far Lewis
  Model (8.10-8.12), VSEPR model (8.13) and
  Localized Bonding and Hybrid Orbital Model (9.1).
  The Molecular Orbital Model (9.2-9.4) is the
  fourth model we will study.
• Understand what diamagnetism and paramagnetism
  are (p 432).
• Be able to draw the sigma molecular orbitals and
  the energy level diagram for the H2 molecule (Fig
  8.26). Understand why one of them is a bonding
  orbital and the other one is an antibonding
  orbital. Also understand why the antibonding
  orbital is higher in energy than the bonding
  orbital. Be able to specify the electron
  configurations for H2, H2- and He2 as in Figs
  9.28-9.30). Be able to compute the bond order
  using the formula on p 429 and be able to explain
  why He2 is not a stable molecule while H2 is.
• Be able to draw the sigma molecular orbitals and
  the energy level diagrams appropriate for the
  second row diatomics figs 9.32-9.36.
  Understand the difference between Fig 9.36 and
  9.38. Be able to apply the aufbau principle to
  the second row diatomics, as in fig 9.39 and
  exercises 9.6-9.8.
• Be able to explain how the paramagnetism of the
  oxygen molecule is accounted for by the MO model
  and also be able to explain why it is not
  accoutned for by any of the localized models.

2
Models (so far)

• Lewis Model(8.10-8.12)
• VSEPR Model (8.13)
• Hybrid Orbital and Localized Electron Model
  (Section 9.1)
• Molecular Orbital Model (9.2-9.3)

3
Why Do We Need Another Model?

• 1) The first 3 models are localized bonding
  models. They view chemical bonding as a sharing
  of electrons between two atoms. Even the hybrid
  orbital model is constructing atomic orbitals
  about a central atom.
• Clearly there are problems with localization
  one big one is the dicey issue of resonance
  structures which show that electrons can be
  shared by more than two atoms and can be spread
  out over a molecule this sharing of electrons
  among several bonds is called delocalization.
• 2) Another problem is the magnetism
  (paramagnetism) of molecular oxygen (Fig 9.40).
  Magnetism means that molecular oxygen has
  unpaired electron spins. None of the other 3
  models can account for this.
• 3) The MO Model attempts to fix these two
  problems and come up with a more comprehensive
  theory.

4
Problem is...

• The MO model is most satisfactory only for
  homonuclear diatomic molecules.
• You can generalize it, but that's beyond the
  scope of this course.
• Nevertheless, we will look at it now just to see
  what it is like.

5
Overview of the MO Model

• Remember how we treated atoms? We used the
  hydrogenic orbitals as the energy states for
  electrons in all atoms. We then built up
  electron configurations by adding electrons to
  those energy levels using the aufbau principle
  we added the electrons to the orbitals following
  the Pauli Principle, Hund's Rule and the orbital
  filling order.
• We are going to do a totally analogous thing now
  we are going to construct molecular orbitals out
  of hydrogenic atomic orbitals, and then build up
  electron configurations by adding electrons to
  those energy levels using the aufbau principle
  we will add electrons to the orbitals following
  the Paului Principle, Hund's Rule and the orbital
  filling order.

6
The Hydrogen (H2) Molecule

• Here are the two molecular orbitals you can make
  out of the hydrogen 1s orbital one is called a
  bonding sigma orbital and the other is called an
  antibonding sigma orbital.
• Notice that (unlike hybrid orbitals, where we are
  combining orbitals on one single atom) here we
  are combining orbitals on two different atoms.
• The antibonding orbital has a node between the
  two nuclei.

7
We Also Get an Energy Level Diagram...

• We know that the energy of the system is lowered
  by the bond.
• Energy conservation says that there must also be
  a state that is higher by the same amount of
  energy. We suppose that it is the sigma orbital
  that is higher in energy.

8
Let's Put It All Together Now...(Fig 9.26)
9
AufBau Principle

• We can now describe H2, H2- and He2 using the
  aufbau principle.
• Here is H2

10
Here is H2- (left, fig 9.29) and He2 (right, fig
9.30)
11
Bond Order (BO)

• New definition of bond order (p 429)
• Bond order (½) (number of bonding electrons
  number antibonding electrons)
• e.g. H2 BO 1 He2 BO0

12
We Can Construct a Similar Model for All
Homonuclear Diatomic Molecules for Second Row
Elements(Fig 9.32)
13
The P Orbitals We'll Use (Fig 9.33)
14
The MO's We'll Make from the P Orbitals (Fig 9.34
and 9.35)
15
AufBau Principle for Second Row Diatomics (Fig
9.39)
16
Summary of MO Theory

• The BO's match well with the Lewis Model
• Oxygen is paramagnetic!!!
• Delocalization makes a lot more sense.