Are VBT and weak interactions adequate for description of the chemical and physical properties in simple molecules?

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 (D_∞h)

s orbitals: a_1g

p orbitals:

p_z a_1u

(p_x, p_y) e_1u

To distinguish various types of molecular orbitals

No node perpendicular to the atom-atom axis : bonding

One node perpendicular to the atom-atom axis : antibonding

No nodes parallel to the atom-atom axis : sigma

1 node parallel to the atom-atom axis : pi

2 nodes parallel to the atom-atom axis : delta

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

H₂

electron configuration σ²

BO = bond order = ½(N_b-N_a)

where

N_a = number of electrons in antibonding orbitals;

N_b = number of electrons in bonding orbitals

BO = 1 (in agreement with VBT)

First row diatomics: Li₂ to Ne₂

Note that 1σ* and 2σ are close in energy and of the same symmetry (σ) thus they can overlap (called a configuration interaction); overlap causes an energy change - an avoided crossing that changes the order of the orbitals:

Neither B₂ nor C₂ has σ bond, only π bonds so the bond energies are quite low

Heteronuclear diatomics:

CO as an example (very important molecule in inorganic chemistry; it has the strongest homonuclear bond energy and is a very good Lewis base towards low valent metals)

basis set is still 2s, 2p but now the orbital energies are different

Hybridization to form sp hybrids gives required lone pairs and then form molecular orbitals

σ²1nb²π⁴2nb²

BO = 3

valence orbital is a lone pair on C so the basic end is C