Subject: HOCQUET's cyclohexadiene - good pedagogical example

From: [email-masked] (Jim Gano)
Date: Fri, 03 May 1996 09:52:14 -0600

We had occasion to look at the 1,3-cyclohexadiene problem with AM1
mentioned by Alexandre HOCQUET.  It is really interesting and a fine
pedagocgical example for students.  HOCQUET's conclusion that AM1 gives the
wrong conformation, the planar conformation, is not true.  However, this is
a commonly observed result.  The calculation EASILY FINDS THE WRONG
CONFORMATION.  The wrong conformation is not an energy minimum but rather a
maximum at the transition state.  That is what makes it so interesting.
This is nicely evident by calculating and viewing the IR spectrum, using
for example the CAChe system. The IR spectrum of the planar structure
contains an absorption at a NEGATIVE frequency. Inspection of the nature of
the vibration indicates the distortion necessary to find the proper
nonplanar conformation which is lower in energy.

>Dear CCLers and Hyperchemers,
>
>In their molecular mechanics principles review (J. Chem. Ed. , 59, 269-274,
>1982) Boyd and Lipkowitz present the conformations of cyclohexa-1,4-diene as
>an example of supremacy of molecular mechanics upon hand held molecular
>models : the dreiding model predicts boat geometry whereas molecular
>mechanics calculate it planar.
>I do believe that this example is demonstrative, presenting molecular
>mechanics to students as a sophisticated hand held model.
>Unfortunately, as i tried to minimize cyclohexa-1,4-diene with the MM+ force
>field provided by Hyperchem, the results show that the minimum (9.10
>kcal.mol-1) corresponds to a torsion angle between 1,2,3 and 4 carbons of
>26°, in a boat conformation. The constrained planar structure corresponds to
>9.73 kcal.mol-1 and goes back to the former conformation when constrain is
>removed.
>Besides, An AM1 semiempirical calculation does find the minimum to be planar.
>So my questions are :
>Does the MM+ forcefield, or the parameters provided in Hyperchem, lack the
>accuracy needed to perform this calculation ?
>Does any references about molecular mechanics calculations of this molecule
>exist ?
>Does anybody know the references about calculations, Xray and NMR results
>that Boyd and Lipkowitz cite with no references (or, to be honnest, the
>reference cited was "in press" : Lipkowitz et al. in J. Org. Chem.) ?
>Am i doing something wrong ?
>
>Thanks in advance for any answers (if somebody is interested, i will
>summarize the answers)
>
>Alexandre Hocquet
>
>Laboratoire de Chimie Analytique
>ESPCI
>10, Rue Vauquelin
>75231 Paris Cedex 05
>France
>
>tel : 33 1 40794420
>fax : 33 1 40794425
>email : [email-masked]
>ou [email-masked]
>ou [email-masked]
>
>
>
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James E. Gano, Director
Instrumentation Center in Arts and Sciences
Bowman-Oddy Laboratories
University of Toledo
Toledo, Ohio 43606

Instrumentation Center :  http://www.icenter.utoledo.edu
Department of Chemistry:  http://www.chem.utoledo.edu

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