Subject: Re: simple way to measure delocalization

From: [email-masked] (Douglas A. Smith Ph.D.)
Date: Wed, 19 Feb 1997 08:33:05 -0500

This has been a very good discussion to date, so I thought I would add my $0.02.

I am not sure in which subdiscipline of chemistry the term "delocalized"
originated, but I first heard it as an organic student (then, much more
often and with better understanding, as a Professor of Organic Chemistry).
It seems we have to worry here about the difference between "de jure" and
"de facto" definitions.

De jure.  I agree with the definitions below, put forth by others.  And, for
this group of CCL readers, this is probably a very valid set of definitions.
Certainly, MOs exist over the entire molecule (not all MOs over all atoms,
except perhaps at the basis set limit).  And perhaps quantum chemists are
more adept at thinking about fuzzy clouds of electrons than organic chemists.

De facto.  Even with organic chemistry being taught today (I hope!) based on
molecular orbitals, most organic chemists write canonical formulas based on
the Lewis formalism, that is, lines representing electron pairs.  Thus,
organic chemistry defines a localized picture of MOs, or still subscribes to
an atomic orbital view of the world.  Occasionally, we organic chemists will
deign to write a circle in a hexagon to indicate "delocalized" electrons in
a pheny ring.

The answer seems to be that, if you are speaking organic chemistry to an
organic chemist (or a biochemist or similar type) then the normal state of
affairs is localized (i.e. Lewis structure) electrons.  Delocalization then
means that you cannot write a simple and obviously correct single Lewis
structure that accurately represents the "bonding" in the molecule.

So, as always, the meaning of words is context- and audience-dependent.

Doug

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Thanks.

>> 
>>    1. In quantum chemistry, electrons are delocalized means
>> they occupy the molecular orbitals which are physically
>> distributed over a large region.
>> 
>
>Molecular orbitals are always delocalized, if species has a symmetry.
>Methane is a good example. Hardly, someone will call it as a "delocalized",
>it is rather used as an example of a "localized" system. And electrons
>are always delocalized by exchange interactions.
>
>>    2. In organic chemistry, it seems to me the word 'local'
>> means the electron density distribution is concentrated in
>> a small region.
>> 

> 
>>     Could any expert tell me how should I use the word
>> 'delocalized' correctly?
>> 
>
>
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