Subject: [email-masked]

From: <[email-masked]>
Date: Fri, 14 Jan 1994 17:22:59 -0600 (CST)

Hello Netters.

Daniel Glossman asked some interesting questions about hypervalent 
sulfur species computed by semiempirical methods.  It's an excellent 
question in that none of the "mainline" (i.e. Dewar-style) methods up 
til the recent advent of SAM1 has included an explicit treatment of 
d-orbitals.  This precluded the old standard sp3d2 hybridization 
pattern that we all know from Cootn and Wilkinson.  It was shown how-
ever that successful results could be obtained within this limitation
in a method using Gaussian corrections to the core-core replusion
term in the approximation (PM3 : J. J. P. Stewart, J. Comput. Chem.  
10, 221 (1989);  AM1 : M. J. S. Dewar,  Y.-C. Yuan, Inorg. Chem.  29, 
3881 (1990)).  MNDO's performance was substantially poorer than AM1 
or PM3, especially for hypervalent systems.

Which brings us to an intertesting question:  Is it really sp3d2 hybrid-
ization or are the d-orbitlas used as some sort of extra flexibility
(polarization?)?  This has been bandied about for quite some time.  A rel-
atively definitive article (A. E. Reed,  P. v. R. Schleyer, J. Am. Chem. 
Soc. 112, 1434 (1990)) seems to suggest that its not d-orbitals, but d-
functions.  I tend to come down in this camp myself.  Anyway, whether
or not its "orbital" or "function" it seesm that computationally they 
are needed for a proper description of these systems.

Next:  How do sp only semiempirical methods treat these systems?  Answer:
Badly and non-systematically.  It is my belief that the hypervalent 
systems that AM1 or PM3 might get correct are fortuitous.  Basically, I
would not trust these results for anything important.  To really
get these systems, we need d-functions/orbitals in the description.
TA DA:  SAM1 to the rescue.  Michael Dewar has been working over a 
period of some 8 years I know of on a way to bring d-orbitals into semi-
empirical theory.  This was accomplished with the new SAM1 method
(M. J. S. Dewar, C. Jie,  G. Yu, Tetrahedron  23, 5003 (1993);
A. J. Holder, R. D. Dennington,  C. Jie, Tetrahedron  in press, 
(1994)).  Here in Kansas City we have finished up the sulfur param-
eterization with SAM1 and the results for hypervalent systems are much
better.  (All of this will be published shortly.  This is not meant
to be a stall, as we are still working on some of this.  SAM1 with sulfur
will be available in AMPAC 5.0 from Semichem to be released in March
at the ACS meeting in San Diego.)  There is some selected data below
to illustrate the point.

SF4
---
                       SAM1       Exptl

Heat of Formation    -179.5      -182.4
SF(1)                  1.53        1.54
SF(2)                  1.68        1.65
FSF(1)                128.8       129.2
FSF(2)                 96.7        93.4
Ioniz. Pot.           12.55       12.03

General Results for Sulfur with SAM1
------------------------------------
(The following are unsigned average errors.)

                                  SAM1      AM1         PM3
Heat of Formation (76 molecs)     6.45      7.09        9.33
Dipole Moments (45 molecs)        0.40      0.49        0.59
Ioniz. Pot. (67 molecs)           0.39      0.33        0.39

The results for P are even more dramatic, with the average errors in
heats of formation being HALVED over PM3 and AM1!

As well as hypervalent heavy main group elements, we are working on 
transition metals and hope to have iron and copper available in the
5.0 release as well.  Hope this wasn't too boring.  And YES, Doug,
I'm always listening.  You'll get 5.0 when its ready and you can repeat
the study you mentioned with SAM1.

Cheerio,  Andy Holder

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
                              DR. ANDREW HOLDER
             Assistant Professor of Computational/Organic Chemistry
Department of Chemistry              ||  BITNET Addr:   AHOLDER@UMKCVAX1
University of Missouri - Kansas City ||  Internet Addr: [email-masked]
Spencer Chemistry, Room 315          ||  Phone Number:  (816) 235-2293
Kansas City, Missouri 64110          ||  FAX Number:    (816) 235-5502
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