Resonance, Formal Charge,
etc.
Some molecules can't quite
completely satify all of the requirements that Lewis thought all molecules wanted to have.
Consider a very interesting and important allotrope of oxygen, ozone. This molecule
is known to be bent but not an equiliateral triangle. It is also known that it is
symmetric with repect to the O-O bonds. (It does in fact have a two-fold symmetry axis,
and two perpendicular symmetry planes, i.e. C2V symmetry) The simplest
Lewis theory cannot predict this without a 'fix' called resonance.
I have also shown the 'Formal Charge' on these structures, where this is defined as
the number of valence electrons minus the number of lone pair electrons minus one half
of the bonding electrons. Usually formal charges are not found in Lewis structures,
but some molecules require them. Formal charges do not always occur in molecules
with resonance structures, as is the most famous case of benzene. The resonance
stabilization of benzene and its analogs is sometimes called 'aromatic' stabilization.
What is the state of hybridization of the carbon atoms in benzene?
Sometimes the formal charges can help one decide between the relative importance of
non-equivalent resonance structures. The best structure has the fewest formal charges
and has the negative charge on the highest electronegativity atom, as can be seen in
the cyanate anion
What is the 'best' structure for the cyanate anion, above? The worst?
The nitrate anion is symmetric, so either an incomplete octet, or three equivalent
resonance structures can predict this. Resonance is the preferred explanation.
What are the formal charges on the structures above?
Molecular Orbitals and DiOxygen
We have no difficulty writing a perfectly reasonable Lewis structure for
O2 that has all of the electrons paired, a double bond and complete octets:
But, as we saw in our demo, oxygen molecules are paramagnetic, which means that
this structure cannot be correct! This means we have to think a little more carefully
about the orbitals that are involved in chemical bonding.
The orbitals that electrons are in when they exist in molecules are actually
different from even the most 'hybridized' or perturbed atomic orbitals.
How molecular orbitals for
stems from how waves interefer with one another and the concept of Phase. Adding
orbitals 'in phase' or 'out of phase' can make two different shapes, usually one that
is 'bonding' and one that is 'antibonding'. Antibonding orbitals have a new node
between the atoms and perpendicular to the bond axis.
Molecular orbitals arising from p orbitals can have two types of nodes: A node
containing the bond axis (makes a pi bond) or a node perpendicular the the bond axis
(making and antibonding orbital). Orbitals that have no nodes containing the
bond axis are called sigma orbitals (Greek for s). Orbitals that have one nodes containing the
bond axis are called pi orbitals (Greek for p). Orbitals that have two nodes containing the
bond axis are called delta orbitals (Greek for d), etc.
The energy level diagram for the lowest-lying molecular orbitals of a homonuclear
diatomic molecule look like this:
Including the p orbitals complicates matters considerably.
Molecular 'aufbau' can now predict the correct nature of DiOxygen (and DiBoron)!
Careful examination of electron-electron repulsion in these systems shows an
inversion of the MO energies between N2 and O2
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PJ Brucat // University of Florida