Subject: CCL: Conformational preference of N,N'-dimethyl urea

From: "David Maxwell" <[email-masked]>
Date: Tue, 09 Dec 1997 12:43 -0600

Dear CCL Members,

I am running into difficulties with determining the conformational preference
for N,N'-dimethyl urea.  Based on the force field available in Insight II, I can
get
a preference for either cis-trans or trans-trans.  See table below.

Force Field*                tt              ct           Delta E (tt-ct)
========================================================================
cff91                    -74.673         -78.743            +4.070
cvff                     -18.809         -17.001            -1.808
amber                    -27.845         -27.866            +0.021
esff                     -40.441         -42.713            +2.272

* All were calculated in InsightII V 95.0 with discover_3.  No cutoffs were
used.

I am fully aware of the problems associated with force fields and
parameterization in general, but this seems way too much of a difference for
such a simple system.  As an added point, I ran this calculation with DGAUSS
method in Unichem and found almost no energy difference for the two
conformational forms.  For this ab initio calculation, I utilized the DZVP
basis set with LDA approximation.  Is there any consensus as to the
conformational preference for this system?  I am most interested in experimental
or high level ab initio support.  Of course, I haven't even brought up the issue
of solvation!

Thanks in advance for you help.

David Maxwell
Senior Scientist
Texas Biotechnology Corp.
** Please send all comments to:  [email-masked]

Background:

I ran quench dynamics on some larger ureas (CFF91 FF) and found a large
preference for cis-trans.  I wanted to justify my results, so I turned to a
simpler system.  On the basis of the N,N'-dimethyl urea results, I now have
little confidence in my results for the larger urea.