Resonance and equilibrium

Luis P. Candeias (
Sun, 04 Aug 1996 14:06:54 -0500

Dear All,

while some of you are probably enjoying a summer break, others are
slaving in front of the computer and trying to solve the assessed

I would like to make myself unpopular again by picking on a matter of
detail in the (excellent) Oliver Smart's section of molecular forces. It
has to do with resonance and equilibrium.

I note that it became common practice to denote the resonance of the
Kekule structures of benzene by putting two arrows (equilibrium sign)
between them, instead of the double headed arrow (resonance sign). This
unimportant detail contributes to a fuzzy image of the very important
distinction between resonance and equilibrium.

In an equilibrium, two forms of a system exist and can be interconverted
(with or without exchange of energy) at variable rates. Both forms have
real existence and can usually be characterized. An example relevant to
protein structure would be the rotational isomers of a peptide bond.

A resonance, on the contrary, is a conceptual way of representing a
system that exists in a defined state that is a sort of hybrid of the
contributing (mesomeric) structures. The Kekule forms of benzene have no
real existence: they are a Lewis valence formalism of describing a
molecule that is better described in terms of molecular orbitals. (By
the way, the Dewar benzene can be isolated, but that's another story).

Here is the clearest metaphor of resonance I've ever read (I'm afraid I
forget the source): benzene is a hybrid of the resonance forms in the
same way a rhinoceros is a hybrid between a dragon and a unicorn. The
latter two don't exist but a rhinoceros does.


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