Subject: another dumb question about molecular mechanics

From: [email-masked] (Alexandre HOCQUET)
Date: Tue, 30 Apr 1996 18:37:00 +0200 (MET DST)

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]
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