Subject: Summary Question on resonance structures

From: sergiusz kwasniewski <[email-masked]>
Date: Mon, 14 Apr 2003 15:26:38 +0200 (MET DST)

Hi,

I would like to thank the people who responded on my question.

What I learn from the reactions is that the "resonance" concept is
irrelevant in solid state systems 
like the infinite all-trans-poly-acetyleen molecule. The two distinct
electronic structures lay 
at the same energy, and are connected through a transition state. This bond
alternation picture 
(symmetry breaking) arises due to correlation energy, and is imposed by the
Jahn-Teller effect
(or the one-dimensional Peierls distortion).

Is this correct ?

Thanks
Serge

Hello, Serge.
I believe "resonance" concept is irrelevant in systems with strong
electron-lattice coupling, like Peierls transitions or Jahn-Teller
phenomena.  It is a pure electronic concept and should not include
symmetry-breaking nuclear displacements.  Don't you think so?

In order to treat electronic and nuclear motions at the same time,
you should go to vibronic state, a linear combination of products
of electronic and nuclear wavefunctions.

Motohiro NAKANO    <[email-masked]>
----------------------------------------------------

I think you are talking about the *acyclic* molecule H2C=CH-CH=CH-CH=CH-....  I
know that 1,3-butadiene is well-described by just *one* resonance structure, and
1,3,5-hexatriene by one resonance structure too. I would not be surprised if
this
is not true for polyacetylene too, although Zi realize that the pi-MO levels are
squeezed together (causing near-degeneracy?) for a big, polymeric C=C-C type
molecule.

E. Lewars

----------------------------------------------------

The original question was:

> As you know, poly-acetylene has a doubly degenerate ground state structure,
> because of the Peierls distortion. The skelet structures can be seen in terms
> of bond alternations as struct1 = (=-=-=)n and struct2 = (-=-=-)n. They both
> consist of a linear combination of partial wavefunctions describing
> wave1 = (=-=-=)n and wave2 = (-=-=-)n (in VB theory).
>
> If I'm not mistaken wave1 and wave2 are resonance (canonics ?) structures and
> should therefore be written as wave1 <--> wave2. On the other hand, struct1
> and struct2 are different molecular structures and they should therefore be
> written as struct1 <==> (i.e. as a chemical equilibrium). I believe these
> structures are called bondisomers.
>
> Each of these structures can thus be given by A*wave1+B*wave2.
>
> I would like your opinion on this and would like to get advise on the accurate
> nomenclature of structs and waves.

___________________________________________________

	Sergiusz Kwasniewski
	LUC SBG/TS
	Universitaire Campus Gebouw D
	3590 Diepenbeek
	BELGIUM

	tel(direct): 032 (0)11/268315
	fax	   : 032 (0)11/268301
	email      : [email-masked]
	http://www.luc.ac.be/TheoChem
___________________________________________________