Subject: Rebirth of resonance...

From: "John R. Nash" <[email-masked]>
Date: Wed, 22 Jan 1997 09:44:56 -0600

[email-masked] asked about "resonance" of a different kind, but
in of the CCL replies:

[email-masked] said:
>Probably, the most close analogy among the resonance "theory" and quantum
>mechanics is represented by generalized valence bond (GVB) scheme. But
>the valence structures in GVB is not exactly what organic chemists mean
>by "resonance". We can say that a few valence structures in GVB correspond
>to one structure in resonance theory.
>
>You may look in Dewar books. I do not think computational and theoretical
>chemist care a lot about "resonance" nowdays.

Since "this kind" of resonance has been brought up, I thought I should
mention that many chemists still do care about it.  It is certainly a
useful teaching aid for undergraduate organic chemistry students, and
practicing lab and theoretical chemists often find resonance structures
useful "when one Lewis Structure just won't do" to describe a delocalized
system.  This is especially true in organometallics, where one structure
seldom tells the whole story, even for a simple complex.

As far as the theoreticians go, F. A. Weinhold's NBO program (most often
used along with G94, but available as a standalone program) can generate
weighted resonance structures (i.e. 40% contribution from A, 23%
contribution from B, etc.) for a delocalized system.  So Pauling's and
others' "approximate, chemical intuition" ideas on resonance can actually
be quantified into useful numbers these days.

See the NBO manual and Weinhold's papers for more details.

-john

-==-John R. Nash-==[email-masked]==-UW-Madison Chem. Dept-==-
                 --- "When in doubt, roll!" ---