Week 4 Content Flashcards
What does the MO theory assume?
It assumes that valence electrons of atom within a molecule become the valence electrons of the entire molecule
What are molecular orbitals constructed by?
They are constructed by taking linear combinations of
the valence orbitals of atoms within the molecule. For example,
consider H2
:
How can symmetry help in complex molecules?
They help by allowing us to treat more complex molecules through assisting in the determination of which AOs combine to make MOs
For the AOs to interact and form MOs…
(three requirements)
- The AOs must have the same symmetry
- The AOs must have similar energy
- AOs must have spatial overlap
Bonding criteria:
Orbitals are combined in so that their signs (shading)
are the same where they overlap. Increase of electron
density in the lobe between the atomic centres. energy of the
resulting molecular orbital is lower than the free atomic orbitals.
SAB > 0
Overlap Criteria: Antibonding
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
Sigma Bonds Overview:
Molecular orbitals are constructed from
the atomic orbitals, the designation sigma is made if the
resulting MOs are symmetric about the internuclear
axis
Pi Bonds Overview:
The designation pi is made if the bonding MO has a
nodal plane in the internuclear axis
Rule 1 of MO theory:
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.
Rule 2 of MO theory:
If the AOs are degenerate, their interaction is proportional to
their overlap integral, S.
Rule 3 of MO theory:
Orbitals must have the same symmetry (same irreducible
representation) to have non-zero overlap
Rule 4 of MO theory:
: 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.
Antisymmetry: Definition
Occurs when there is an inversion center, but the resulting
inversion has the opposite sign of the original orbital
Overlap Criteria for Non-Bonding:
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
What does the extent of the overlap depend on?
The internuclear seperation, the nature of the orbitals involved (s,p, or d) and their relative orientation
