Chemical Bonding: The Molecular Orbital Approach
(outline, part 5)
Valence Bond Theory vs Molecular Orbital Theory
Valence Bond Theory:
Most appropriate for:
Molecular Orbital (MO) Theory:
Most appropriate for:
MO Bonding in H2
Classical Picture of H2
Wave Mechanical Picture of H2:
(H + H)
(H-H)
__________________ overlap
(H + H)
______________ overlap
Principle: The greater the number of nodal planes, the higher the energy of the MO. If nodal planes are between nuclei, the orbital is antibonding.
MO Diagram for H2:
Potential Energy Curve for H2:
Rules for forming MO's
1. Overlapping orbitals must have ______________________________________________________.
Good covalent bond.
Which atom is more electronegative? Bond will be polar covalent.
(not necessarily weaker, but less covalent)
2. Overlap must be ______________ to form a bonding MO.
(see chalkboard)
3. Bond energy is roughly proportional to ________________________________.
f < d < p < s (s is less diffuse)
d < p < s
4. MO's are filled with electrons using _________________________________________.
5. Number of MO's = Number of AO's from which they are formed.
MO's for Diatomic Molecules
Homonuclear
Let's look at F2:
MO diagram for F2 (from Hyperchem)
(Go to chalkboard, then Hyperchem)
MO diagrams for Li2 through F2
Heteronuclear
General Principle: When 3 AO's overlap with each other, 3 MO's will form: one bonding, one antibonding, and one approximately nonbonding.
MO Diagram for HF
Bond order is _______
Electron density is shifted toward________
Tools for more complex molecules:
1. Group Theory (deals with symmetry)
2. Molecular Modeling Software
Hyperchem
Spartan, PC Spartan, PC Spartan Plus, PC Spartan Pro
Titan
Gaussian
Cache
Alchemy
MO Diagram for NH3
Note that there are not three equivalent bonding pairs of electrons.
Plot the orbital at -13.71 eV.
Photoelectron Spectroscopy
PES for NH3
What does the peak at 11 eV represent?
What does the peak at ca. 16 eV represent?