Subject: Summary: CCL:modeling of phosphonates...

From: "Victor M. Rosas Garcia" <[email-masked]>
Date: Mon, 1 Jul 1996 10:18:39 -0400

Hi everybody,

Some time ago I posted the following question:

> 	Is there anyone out there modeling phosphonates?  I'd like to discuss
> some experiences (both good and bad) I've had.  Comments about the
> appropriateness of any method (molecular mechanics, semiempirical and ab
> initio) are welcome. I'm particularly interested in modeling including
solvent
> effects.

I got several responses that were most useful.  Thanx to:

Michael Colvin
Christopher Cramer
Beverly Bendiksen
Rainer Koch
and
Steve Cabaniss

To make a long story short:
DO NOT use semiempirical methods for modeling phosphorus(V) species.  They'are
bad.  The problem I ran into was sort of a catch-22 situation: AM1 was way off
in the P-C bond length (1.63 angs vs 1.77 exp) while PM3 was pretty close (1.73
vs 1.77), but PM3 went awry when the solvent was included, probably because of
the quality of the charges produced by PM3 (that's my best guess).  I also got
some references:

Denmark, S. E.; Cramer, C. J. "The Theoretical Structures of Neutral,
Anionic and Lithiated P-Allylphosphonic Diamide" J. Org. Chem. 1990, 55,
1806.

Cramer, C. J.; Denmark, S. E.; Miller, P. C.; Dorow, R. L.; Swiss, K. A.;
Wilson, S. R. "Structure and Dynamics of Phosphorus(V)-Stabilized Carbanions:
A Comparison of Theoretical, Crystallographic, and Solution Structures"
J. Am. Chem. Soc. 1994, 116, 2437.

Koch, R.; Anders, E.; "Ab Initio and PM3-MO calculations of Lithiophosphonates"
J. Org. Chem. 1995, 60, 5861-5866

Interesting thing:  after reading a few papers, I found that the P-C bond
length is very sensitive to the substituents on the P.  These are the P-C bond
lengths for a few P species:
Phosphines:          1.81 angs
Phosphonates:        1.77 angs
Phosphonic diamides: 1.71 angs.

Most molecular mechanics force fields have P-C parameters derived from
phosphines (probably based on Allinger's original publications).  I haven't
found any force fields that included phosphonates in the parametrization
(although I have not looked very hard on this).

So, once more, participating in the CCL provided a valuable learning
experience.
Thank you all.

Victor.

-- 
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Victor M. Rosas Garcia                   * "How can we contrive to be 
[email-masked]                *  at once astonished at the  
Virginia Tech doesn't necessarily share  *  world and yet at home in it?"
the opinions you just read.	         *  G. K. Chesterton
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