Subject: Re: CCL:keto/enolate TS structure in Solvation model

From: Andreas Klamt <klamt{at}cosmologic.de>
Date: Thu, 04 Dec 2003 08:13:13 +0100

Dear Laurence, dear Yong,

although I have not too much experience in searching TS, but based on 
all my COSMO experience -  I strongly disagree with the opinion that it 
makes any sense to use simple spherical or ellipsoidal cavity Onsager 
models for TS search in solution. The solvation energies strongly depend 
on the size, orientation and position of the cavity relative to the 
molecule, that these kinds of cavities anyway only make sense of quite 
spherical or ellipsoidal molecules. For all others you could as well use 
a random number generator, with the only difference that the sign of the 
dielectric energy correction will always be right.  This is so much more 
true for TS, which tend to be very "non-spherical", and the degree of 
deviation from a spherical shape changes strongly along the reaction path.

For TS search in continuum solvation models everything else than a good 
molecular shaped cavity is useless. I am not absolutely convinced that 
the way of smoothing between atomic spheres with auxiliary spheres as 
done in PCM (GEPOL) is optimal, because each of the auxiliary spheres 
has the wrong curvature (convex), while in the interpolation region 
(crevices) we need a concave surface. In our more advanced COSMO 
implementations in TURBOMOLE, GAMESS (not yet officially released), and 
MOPAC98/2000/2002 we have a better interpolation for those regions, 
which become of special importance during reactions.

Another point of importance in the application of dielectric continuum 
solvation methods to TS is the choice of radii for the cavity 
construction. United atom definitions and atom-type based definitions 
are quite useless, because the atoms loose their clear bond partners and 
atom types during a reaction. In our COSMO and COSMO-RS 
parameterizations we only use element specific radii, which should keep 
valid during a reaction. This set of radii is also available in 
Gaussion03 now.

The fact that DFT might not be optimal for TS at all is a different 
issue. Use of more refined schemes like COSMO-RS (COSMOtherm program) 
for corrections beyond the dielectric level (including hydrogen bonding) 
may be required as well.
So I agree, that TS search in solution is still a very demanding area of 
research, and no turn key solutions exist here. But going back to the 
Onsager level is the wrong direction!

Andreas

Laurence Cuffe wrote:

>On 1 Dec 2003, at 14:46, yonyang wrote:
>
>  
>
>>Dear CCL users:
>>I am playing around with a keto/enolate formation reaction catalyzed by
>>gernal acid and base(methyl amine/ammonium ion). I would like to locate
>>the TS structure in organic solvent such as THF or cyclohexane based on
>>the TS structure previously located in gas phase. Despite semi-empirical
>>method AM1 in spartan was able to locate the initial strucuture in
>>organic
>>solvation, but further optimization in Gaussian was unsuccesful and
>>failed
>>many times.
>>    
>>
>Dear Yong 
>you may find our paper:
>"Experimental and theoretical studies of rotational barriers in aceto, 
>n-methylaceto and n-phenylacetohydroxamic acid"
>David A. Brown, Laurence P, Cuffe, Geraldine M. Fitzpatrick, Noel 
>J. fitzpatrick, William K. Glass and Kara M. Herlihy
>in  the collection of Czechoslovak Chemical Communications
> 2001, 66(1), 99-108
>of interest.  I think there's an abstract on the webpage:
>http://cccc.uochb.cas.cz/Vol/66/No01/20010099.html
>Finding solvated Transistion states is not easy and when we did 
>find them we did not find the barrier heights reliable using DFT and 
>PCM, although this may have changed with the new version of 
>Gaussian.  If your molecule has a dipole moment you might 
>consider using the Onsager method,  Its old fashioned, but also 
>computationaly cheap and it seemed to improve over gas phase 
>results for our systems. It may also provide a better starting point 
>for more complex calculations. 
>Hope this helps
>All the best
>Laurence Cuffe
>
>
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-- 
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Dr. Andreas Klamt
COSMOlogic GmbH&CoKG
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51381 Leverkusen, Germany

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Fax:  +49-2171-73168-9
e-mail: klamt{at}cosmologic.de
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