CHM 501 Lecture

Are VBT and weak interactions adequate for description of the chemical and physical properties in simple molecules?
bond length
bond energy
magnetic properties
triple bond
77 K
110 pm
942 kJ/mol
double bond
81 K
121 pm
494 kJ/mol
moderately reactive
single bond
85 K
141 pm
155 kJ/mol
very reactive

VBT helps explain everything except the paramagnetism of oxygen - this requires a different model.

Molecular Orbital Theory

Linear Combination of Atomic Orbitals LCAO

Basic assumptions:

1) Orbitals in molecules look a lot like atomic orbitals

2) Perturbations are caused by wave interference (overlap) of atomic orbitals;

3) The new molecular orbitals fill with electrons according to the Aufbau and Pauli Principles

Constructive interference - lowers energy

Destructive interference - raises energy

Only orbital of the same irreducible representation can overlap with each other.

Increase of electron density in bonding orbitals between nuclei has two effects:

1) Screening of nuclear-nuclear repulsion by the extra electrons between nuclei

2) Electron-nuclear attraction in the direction that moves nuclei toward each other

Consider mixing orbitals in a linear diatomic molecule (Dh)

s orbitals: a1g

p orbitals:

pz a1u

(px, py) e1u

To distinguish various types of molecular orbitals

Constructing Molecular Orbital energy diagrams:

Basic principles

1. Choose atomic orbitals as basis set; #AOs initially = #MOs created

2. AOs of like size and energy overlap better with each other

3. Only orbitals of the same symmetry can overlap with each other

4. A larger overlap leads to a larger energy change

5. Fill electrons into orbitals following the Pauli and Aufbau principles


electron configuration 2

BO = bond order = ½(Nb-Na)


Na = number of electrons in antibonding orbitals;

Nb = number of electrons in bonding orbitals

BO = 1 (in agreement with VBT)