Subject: RE: A question on heat of formation

From: "Shobe, Dave" <dshobe{at}sud-chemieinc.com>
Date: Thu, 29 May 2003 08:44:17 -0400

Most people do the following.  They find the lowest-energy conformations of
the molecules involved, and use the energy at those geometries to calculate
delta-H.  The computational chemist should of course use any prudent,
reasonable means to ensure that these are indeed the *global* conformational
minima.

However...

I think to really "do it right" one has to calculate all the low-lying
conformations and determine the relative statistical-mechanical populations
of each conformation, and from that the average enthalpy per molecule (or
per mole).  

Usually there are enough other sources of error that the correct procedure
has little advantage over the first approach, at least for enthalpy.  But
the corresponding "correct" procedure is often done for entropy, where
population of low-lying conformational isomers can make a significant
difference in the outcome.

--David Shobe
Süd-Chemie Inc.
phone (502) 634-7409
fax     (502) 634-7724
email  dshobe{at}sud-chemieinc.com

Don't bother flaming me: I'm behind a firewall.

-----Original Message-----
From: Renxiao Wang [mailto:renxiao{at}med.umich.edu]
Sent: Wednesday, May 28, 2003 8:19 PM
To: chemistry{at}ccl.net
Subject: CCL:A question on heat of formation

Hi, CCLers!

Should the heat of formations of two enantiomers of a chiral compound be
identical? I think the answer is yes. Please point out some references
for this. 

A disturbing fact is that, no matter you use QM or MM to calculate the
heat of formation for a given molecule, the number is
conformation-depentdent. I guess this is because of the steric energy
part is conformation-dependent. So, if I really want to compare the
stability of two molecules using heat of formation, how may I calculate
it right?

Best regards,

Renxiao Wang
renxiao{at}med.umich.edu

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From: Paul.Mathias|at|aspentech.com
Date: Thu, 29 May 2003 13:40:13 -0400

The NIST-JANAF tables are primarily based upon experimental data.
Computational predictions are sratting to be used, but the contribution is
-- yet -- very small.
 
Paul Mathias

-----Original Message-----
From: Shobe, Dave [mailto:dshobe|at|sud-chemieinc.com]
Sent: Thursday, May 29, 2003 12:02 PM
To: 'Guosheng Wu'; 'Renxiao Wang'; 'CCL'
Subject: CCL:A question on heat of formation

---Guosheng Wu wrote: 
> It seems to me, dH has not much to do with propabilities of all
conformations, 
> which is essentially entropy contribution. 

There is a small effect on enthalpy, for temperatures above zero Kelvin.
Most people ignore it, since there are more important sources of error.  But
I think the NIST/JANAF tables do take this effect into account.  It may
affect dH/dT more than it affects dH itself.

--David Shobe 
Süd-Chemie Inc. 
phone (502) 634-7409 
fax     (502) 634-7724 
email  dshobe|at|sud-chemieinc.com 

-----Original Message----- 
From: Guosheng Wu [ mailto:wu_guosheng2002|at|yahoo.com
<mailto:wu_guosheng2002|at|yahoo.com> ] 
Sent: Thursday, May 29, 2003 11:20 AM 
To: Shobe, Dave; 'Renxiao Wang' 
Subject: Re: CCL:A question on heat of formation 

Were you talking about free energy(dG), instead of heat of formation (dH)? 

It seems to me, dH has not much to do with propabilities of all
conformations, 
which is essentially entropy contribution. 

BTW, John Pople and others had a few papers on Gn theory to get very high
quality 
heat of formation for a small molecule, by ab initio plus scaling
strategy... but 
obviously not interested to most of the Med. Chem. community. 

Guosheng 

> email  dshobe|at|sud-chemieinc.com 
> 
> From: Renxiao Wang [ mailto:renxiao|at|med.umich.edu
<mailto:renxiao|at|med.umich.edu> ] 
> To: chemistry|at|ccl.net 
> renxiao|at|med.umich.edu 
> Send your subscription/unsubscription requests to:
CHEMISTRY-REQUEST|at|ccl.net 
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