Subject: semiempirical methods for hypervalent sulfur

From: [email-masked] (Daniel Glossman)
Date: Thu, 13 Jan 94 14:07:17 ARG

Dear netters:

Perhaps these are two FAQs but:

1) Is there any semiempirical method that can deal successfully with the 
optimization of geometries of organic molecules containing hypervalent
sulfur atoms? 

2) A bit more theoretical question: If a semiempirical method, i.e., MNDO
is not reliable in the calculation of heats of formation of organic 
molecules containing hypervalent sulfur atoms, this means that the optimized
geometries with the same method are also not reliable? Is there any 
relationship or general rule to be used as a guide in these situations?  

I do not want to start a discussion (perhaps it has been already discussed
in the list), so please answer directly to me:  [email-masked] 
or [email-masked]. I will summarize the answers for the list
if there exists enough interest.

Thanks in advance

				Daniel Glossman

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Dr. Daniel Glossman                    Phone: (54) 1 - 553 3797
Programa QUINOR - CONICET                     (54) 21 - 259485
Facultad de Ciencias Exactas           FAX: (54) 1 - 856 5963
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Republica Argentina
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From: "DR. DOUGLAS A. SMITH, UNIVERSITY OF TOLEDO" <[email-masked]>
Date: Thu, 13 Jan 1994 13:20:45 -0500 (EST)

Dan:
	Please excuse me for answering to the list, but I think that I have
seen too many questions and not enough discussions recently.  I have been
looking for a good thread to expand and discuss (I am not qualified to
get into the MM of TM discussion) and you offer a good opportunity.

	A while back I published a paper inthe Journal of Fluorine Chemistry
on "Semiempirical studies on the structure and bonding of fluorosulfuranes
and aminofluorosulfuranes"  (1990, 50, 427-432). At that time, we knew that
the MNDO parameters for sulfur were not particularly great and the AM1
sulfur parameters had not yet been published.  We therefore used PM3.  The
bottom line is that the optimized structures of SF4 and (Me)2NSF3, did not 
agree well with the experimental geometries, particularly because there was 
no real difference between the axial and equitorial positions on sulfur.  
This was in agreement with ab initio calculations by Hay (JACS 1977, 99, 
1003) and Reed and Schleyer (JACS 1990, 112, 1434) whose results indicated 
the need for 3d orbitals in the calculations.  Ionization potentials at the
PM3 level were inconsistent with experiment.

	The calculated structure of [(Me)2N]2SF2 was in much better agreement
with experiment, and the trend upon adding amino groups was for bond angles
to become more accurate than bond lengths.  This may be a simple steric
result rather than an electronic one.

	I have not seen a test of SAM1 on these types of compounds.  If
the parameters for CHNFS are all available, it might be worth my going back
and doing a small follow up project (Andy Holder, are you listening?).

	If anyone wants reprints of the J. Fluorine Chem. paper, send me
your mailing address.  I know that journal is not always readily available.

Doug

Douglas A. Smith
Assistant Professor
Department of Chemistry
 and member,
Center for Drug Design and Development
The University of Toledo
Toledo, OH  43606-3390

voice    419-537-2116
fax      419-537-4033
email    [email-masked]