CHM 501 Lecture



Are VBT and weak interactions adequate for description of the chemical and physical properties in simple molecules?
molecule
bond
bp
bond length
bond energy
reactivity
magnetic properties
N2
triple bond
77 K
110 pm
942 kJ/mol
unreactive
diamagnetic
O2
double bond
90 K
121 pm
494 kJ/mol
moderately reactive
paramagnetic
F2
single bond
85 K
141 pm
155 kJ/mol
very reactive
diamagnetic

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

H2


electron configuration 2

BO = bond order = ½(Nb-Na)

where

Na = number of electrons in antibonding orbitals;

Nb = number of electrons in bonding orbitals

BO = 1 (in agreement with VBT)

First row diatomics: Li2 to Ne2


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

X2
configuration
Bond Order
predicted spin
observed spin
Li2 12
1
0
0
Be2 121*2
0
0
B2 121*222
1
2
2
C2 121*24
2
0
0
N2 121*2224
3
0
0
O2 121*2224*2
2
2
2
F2 121*2224*4
1
0
0
Ne2 121*2224*42*2
0
0

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

Better:



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


21nb242nb2

BO = 3

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