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

From: "Olga Dmitrenko" <odmitr<<at>>UDel.Edu>
Date: Tue, 02 Dec 2003 10:47:19 -0500

Dear Yong,

First of all, I would not recommend to use  AM1 for this kind of system. It
gives wrong energetics and wrong TS  geometry. Maybe as a starting point...
I suspect your problem with using ab initio or DFT on that TS is not because
of including solvent effect. You may have a look at our paper
" C-H....Carboxylate Oxygen Hydrogen Bonding in Substrate Activation by the
 Acyl-CoA Dehydrogenases: Synergy between the H-bonds, R.D.Bach,, C.Thorpe,
 O.Dmitrenko, J.Phys.Chem.B. 2002, 106, 4325-4335.
where we discuss the problems of TS location in for the reaction (weak
base+thioester)
CH3CO0- + CH3C=OSCH3 --> CH3COOH  + CH2C=OSCH3-
Note: at HF level, TS can be found, but it does not exist if the electron
correlated method is used.
But, when one adds the H-donors to the >C=O of thioester, reaction becomes
less endothermic, and
late TS can be located. It's kind of PAs matching - one should make PA of
base and PA of acid closer.
AM1 and HF are not ok, when studying the reactions of bond
breaking/formation and, in particular,
in charged systems.
Best regards,
Olga

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.(Gaussian program complain that there is no negative engien
vector
while desired one)I am not sure if the initial structure located in
spartan was
not good enough or there was some trap in refinement in Gaussian.Can
anybody
with experiences of this kind of reaction in solvation give me any
suggestions?
Your answer will be highly appreciated! Have a good one.

The keyword I am using in SCRF caculation

#HF/6-31G(d) OPT=(Tight,TS) SCRF=(PCM,Read,Solvent=THF) Freq Test

I also attached the PDB file of the TS in gas phase (located in
HF/6-31G(d))
and the TS in hexadacane and water.( located in AM1 ) if you are
interested.

Cheers

Yong. L. Y.