Ammonia

This species is postulated as an intermediate in some This species is postulated as an intermediate in some

reactions.

It is the simplest triatomic molecule.

In it 2 electrons hold the molecule together. This is an

example of a three center two electron bond.

Each H atom has only a 1s orbital, so to obtain MO’s

for the H3+ cation, we must make linear combinations

of the three 1s orbitals.

Since the terminal H atoms are symmetry related and

must be considered as a pair, we must make Symmetry

Adapted Linear Combinations (SALC’s) of their Adapted Linear Combinations (SALC’s) of their

orbitals to interact with the central atom:

1.) 2.)

This is the approach that we must use for all

polyatomic molecules.

0 nodes so most stabile.

1 node so less stabile than 1σ.

The 1s orbital of H(B) does not

have appropriate symmetry to

interact with the combination of

1s(A) - 1s(C).

2 nodes � the least stabile.

Correlation diagram / Walsh diagrams

odnosno:

odnosno:

odnosno:

The doubly-degenerate pair of MO’s have e symmetry.

Although they do not look alike, each orbital in the pair

has the same energy. Notice that the orbitals have roughly

the same symmetry as would px and py orbitals in the

middle of the H3 ring.

There are three SALCs, one with a1 symmetry and

two (considered together) with e symmetry.

s and pz have a1 symmetry and px and py (as a

pair) possess e symmetry. C3pair) possess e symmetry. C3

1s N orbital has the lowest

Combining MO of ammonia:

1s N orbital has the lowest

energy, so it will belong

mostly to N and will be 0a1

nonbonding MO.

2s N (a1) with a1 SALC gives

1a1 lowest energy bonding MO.

2pz N (a1) added with a1 SALC

gives 2a1 bonding MO (actually gives 2a1 bonding MO (actually

nonbonding due to low overlap.)

2pz N (a1) subtrackted with a1 SALC

gives 3a1 highest energy antibonding

MO.

A combination of N 2px and 2py orbitals and linear

combinations of 3× H1s orbitals have e symmetry

Two bonding e symetry

low energy orbitals

2x 1e

Two antibonding e symetry

high energy orbitals

2x 2e

MO diagram in Figure

– a1 symmetry orbitals

• Bondings, nonbonding, and antibonding MO

– e symmetry orbitals

• These are doubly degenerate

orbitals which means that there is

a pair at low energies and a pair at

high energies (the same energy).high energies (the same energy).

In NH3, the HOMO (Highly Occupied

Molecular Orbital) is a mostly nitrogen

based orbital that corresponds to the

Lone pair of electrons. This is why

Properties of ammonia:

Lone pair of electrons. This is why

ammonia acts as a Lewis base at

the N atom. The LUMO (Lowest

Unoccupied Molecular Orbital) is the

2e level that has more H character –

this shows why NH3 can also act as

a Lewis acid through the H atoms.