Subject: Re-REPLY: CCL:1,3,5-cyclohexatriene

From: Valentine Ananikov <[email-masked]>
Date: Mon, 22 May 2000 09:56:49 +0400

Hello,

Not all of the possible 1,3,5-cyclohexatriene isomers could be
assumed as a resonance structures, neither the vibrational
extremes.

For example, cis,cis,trans-1,3,5-cyclohexatriene, so called
"Mobius benzene", does not seem to fit into the resonance
structures active space. It may be converted to classical
planar forms through the trans/cis isomerization, however
this implies some activation barrier.

The comments below concerning the resonance hybrid and real 
existence of a compound are qualitatively correct. Perhaps,
somebody may formulate the quantitative rules also?

best regards, Valentin.

> 
> 2000 mAY 21
> 
> Hello,
> 
> The answer to your question (below), phrased as a general problem, is
> not as quite simple as one may think. The conventional response would be
> that cyclohexatrine structures are resonance structures (canonical
> structures) contributing to the actual structure, a resonance hybrid,
> and so can't have any real existence: they are just bond-stretching
> vibrational extremes with benzene vibrating about a geometry of mean
> (average) hexagonal symmetry (think of a Morse curve). the energy of the
> purely hypothetical 1,3,5-cyclohexatriene has been evaluted in
> connection with its use as areference molecule in assigning a resonance
> energy to benzene (e.g. work by M. J. S. Dewar--treated in some org chem
> textbooks).
> 
>  So much for benzene, but for cyclobutadiene the alternating C-C/C=C
> structures are real: the potential energy surface has two minima
> separated by a transition state. Predicting the results for simple
> polyene cases requires, usually, only knowing about aromaticity and
> antiaromaticity and the Hueckel rule, but _generally_ distinguishing
> between resonance (as in benzene) and valence tautomerism (as in
> cyclobutadiene) requires actual calculation. The problem of "the thin
> line" (quoting Paquette) between the two was discussedby L. Paquette in
> J Am Chem Soc in a paper on homoaromaticity about 10-15 years ago.
> 
> E. Lewars
> ====
> 
> -------- Original Message --------
> Subject: CCL:1,3,5-cyclohexatriene
> Date: Thu, 11 May 2000 09:21:17 +0400
> From: Valentine Ananikov <[email-masked]>
> Organization: IOC
> To: CCL_post <[email-masked]>
> 
>    Dear List Members,
> 
> Does anybody have an information about predicted structure and
> stability for 1,3,5-cyclohexatriene?
> 
> I mean a six-membered ring with alternating single and double
> C-C bonds, but without aromatic pi-electrons delocalization due
> to non-planar conformation (i.e. non-planar benzene isomer).
> Would such structure exist in principle?
> 
> Thanks!  Valentin.
> 
> ====================================================================
>                                              ,         ,      ,   ,
> Valentine P. Ananikov                        |\\\\ ////|     /////|
> NMR Group                                    | \\\|/// |    ///// |
> ND Zelinsky Institute of Organic Chemistry   |  |~~~|  |   |~~~|  |
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====================================================================
                                             ,         ,      ,   ,
Valentine P. Ananikov                        |\\\\ ////|     /////|
NMR Group                                    | \\\|/// |    ///// |
ND Zelinsky Institute of Organic Chemistry   |  |~~~|  |   |~~~|  |
Leninsky Prospect 47                         |  |===|  |   |===|  |
Moscow  117913                               |  |   |  |   |   |  |
Russia                                       |  | A |  |   | Z |  |
                                              \ |   | /    |   | /
e-mail: [email-masked]                     \|===|/     |===|/
http://nmr.ioc.ac.ru/Staff/AnanikovVP/          '---'      '---'
  Fax +7 (095)1355328   Phone +7 (095)9383536
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