Sent to CCL by: "Ernst Horkel" [ehorkel : ioc.tuwien.ac.at]
Dear all,
I have a (at least to me) strange problem when I try to describe a keto-enol
tautomerism. The substance under inverstigation is a beta-keto carboxylic acid
amide derivative; there is one possible keto-tautomer (having the ketone and the
amide a C=O bond). Between the two possible enol-tautomers (one having the O-H
on carbon C3 and the other having the O-H on the amide C1), there should be a
transition state; I thik I found it, being planar in geometry with the hydrogen
oszillating between the two oxygen atoms. A frequency run returned one single
imaginary frequency with v=-1086cm-1. So this seems to make sence to me, right?
But now the problem: in order to get the both tautomers connected via this TS, I
followed the imag.freq. in both directions (manual displacement in gauss view,
both in positive and negative direction) and optimized those structures. Results
are chemically correct tautomers, frequency runs showed no imaginary frequency.
BUT: in one of the tautomers, the gi!
bbs free energy is HIGHER than in the TS! How is this possible? Does that make
sence (I don't think so). Please help me and leave comments on this, as I'm
really stucking at this point for a while.
some background:
*planned investigation: realative stability of two tautomers; calculation of the
activation energy of the tautomerization.
*used software: Gaussian 09, GaussView 5.0 on a Linux system
*level of theory: DFT, M06-2X, 6-311G(d,p)++, solvent H2O, PCM theory
*input (representative): "opt=tight freq 6-311++g(d,p) scrf=(solvent=water)
geom=connectivity int=ultrafine m062x"
*for calculation of "delta G" I used the calculated values in the
".log" file printed in the line (e.g.): "Sum of electronic and
thermal Free Energies= -670.274904"
So please can anyone help (guide) me out of this. Any helpful ideas or comments
are really welcome.
Thank You all in advance,
best reagrds
Ernst>