molecular orbitals of ammonia
DESCRIPTION
Drawing and calculating molecular orbitals of ammonia.Disclaimer: Some of the pictures are taken from net.TRANSCRIPT
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.