Week 4 content (M-1)

Question 1

Basic Rule #1 of MO Theory:

Answer 1

Rule #1: The interaction of n AOs leads to the formation of n MOs. If n= 2, one MO is bonding and one antibonding. The bonding orbital is more stable than the lower-energy AO. The antibonding orbital is less stable than the higher-energy AO. The bonding orbital is stabilized less than the
antibonding orbital is destabilized.

Question 2

Basic Rule #2 of MO Theory

Answer 2

Rule #2: If the AOs are degenerate, their interaction is proportional to their overlap integral, S.

-Greater the degree of overlap, stronger bonding/antibonding

Question 3

Basic Rule #3 of MO Theory:

Answer 3

ule #3: Orbitals must have the same symmetry (same irreducible
representation) to have non-zero overlap.

-if S NOT (=) 0, bonding and antibonding is result.

-if S (=) 0, with all other orbitals in molecule, 100% non-bonding orbital

Question 4

Antisymmetry (def’n)

Answer 4

An orbital is antisymmetric if there is an inversion center, but the resulting
inversion has the opposite sign of the original orbital:

Question 5

The extent of overlap depends on…

Answer 5

the internuclear seperation, the natuire of the orbitals invon=lved (s, p, or d), their relative orientation, and angle

Question 6

Non-bonding overlap:

Answer 6

If the orbitals are combined in such a way that a lobe overlaps equally
with two lobes of opposite shading, there is not net change in orbital
energy. This is a non-bonding interaction, and no molecular orbital is
formed.

SAB = 0

Question 7

Basic Rule #4 of MO Theory:

Answer 7

Rule #4: If the AOs are non-degenerate, their interaction is proportional
to S2/DE, where DE is the energy separation between the AOs. In this
case, the bonding orbital is mostly localized on the atom with the deeper lying AO, usually the more electronegative atom. The antibonding orbital is mostly localized on the atom with the higher AO.

Question 8

Pi Bonds:

Answer 8

The designation  is made if the bonding MO has a
nodal plane in the internuclear axis (i.e. a C=C
bond).

Question 9

Sigma Bonds:

Answer 9

When the molecular orbitals are constructed from
the atomic orbitals, the designation  is made if the
resulting MOs are symmetric about the internuclear
axis (i.e. the H-H bond in H2).
* Another way of looking at this is to pretend that the
molecule is being viewed end on (looking down the
bond) and if the sign of the wavefunction is the
same when a circle is traced around the bond, then
the MO is a sigma orbital.

Question 10

Bonding/Antibonding

Answer 10

• Depending how the orbitals are mixed with respect to shading, a
  bonding or antibonding orbital will result:
• Note that the bonding orbital has electron density in the plane
  perpendicular to the bond.
• The antibonding orbital has a node in the plane perpendicular to the
  bond through its midpoint. Antibonding orbitals are always labelled
  with an asterisk.
• The bonding orbital becomes more stable in energy, while the antibonding
  orbital becomes less stable in energy.

Question 11

Overlap Criteria: Antibonding:

Answer 11

• If the orbitals are combined in such a way that their signs (shading)
  are opposite when they overlap, cancellation of the lobes occur,
  and the energy of the resulting antibonding orbitals is higher than
  the free atomic orbitals.
• SAB < 0

Question 12

Overlap Criteria for Bonding:

Answer 12

• If the orbitals are combined in such a way that their signs (shading)
  are the same where they overlap, there is increase of electron
  density in the lobe between the atomic centres. The energy of the
  resulting molecular orbital is lower than the free atomic orbitals.
• SAB > 0

Question 13

Molecular Orbital Theory: For the AOs to interact and form MOs…

Answer 13

• The AOs must have the same symmetry
• The AOs must have similar energy
• AOs must have spatial overlap

Question 14

Molecular Orbital Theory:

Answer 14

One approach to understanding the electronic structure of molecules is
called Molecular Orbital Theory.
* Symmetry will allow us to treat more complex molecules by helping us
to determine which AOs combine to make MOs….more advanced
symmetry next year!
* MO theory assumes that the valence electrons of the atoms within
a molecule become the valence electrons of the entire molecule.
* Molecular orbitals are constructed by taking linear combinations of
the valence orbitals of atoms within the molecule. For example,
consider H2