Subject: Correct representation of carbanions in HyperChem MM+

From: "Klaus-D. Warzecha" <[email-masked]>
Date: Wed, 25 Nov 1998 16:27:29 +0100

Dear all,
concerning the proper handling of tertiary alkyl carbanions in force field
calculations (HyperChem MM+), I would like to ask for some advice. Taking
methyl cyclohexane as a model, removal of the hydrogen from the tertiary
carbon changes the atom type of the latter from C4 (sp3) to CR (radical,
sp2). Adding a charge of -1 to this centre then gives a first representation
of the carbanion.

This simple model however fails to reproduce the expected pyramidal geometry
of the carbanionic centre.

Which modifications would necessary to obtain a more accurate representation
(still in HyperChem's MM+):
1. Changing the type of the negatively charged carbon from CR to C4 or,
2. Addition of a lone pair, or
3. A combination of both ?

Would this refined approach allow to calculate the relative energies of two
conformers emerging from the "umbrella effect" of a carbanion ? (The rate
for "flipping the carbanion" certainly is  much higher than for
chair-chair-interconversion of a cyclohexane and thus for any given ring
conformer, two "anion conformers" should be in equilibrium.) It's rather
meaningless in the the case of the methyl cyclohexane anion but might be
interesting for higher substituted cyclohexanes. In that cases, these
energies might be taken to explain possible stereoselectivity in the
protonation of the respective species.

I'm looking forward to your hints and comments. If you feel that the problem
shouldn't be handled at all using a force field model, don't hesitate to
suggest alternative approaches. However please notice that I'm not a
theoretical chemist and thus would prefer the most simple model, rather than
the most complex but more accutate one.

Regards,
Klaus-D. Warzecha