Subject: Second order TS's crossroads

From: Valentine Ananikov <[email-masked]>
Date: Thu, 05 Oct 2000 11:38:02 +0400

Dear CCL'ers,

There are two questions concerning second order TS's, which I
found being rather contradictory in the literature. So:

1) in some articles one may read something like that: "... we
found that reaction proceeds through the second order transition
state ... and gives desired products ..."

while the other authors state: "... second order saddle points
are of no interest for chemical reactions ..."

Where is the truth? May a chemical transformation proceed through
a second order transition state?

2) Next,

If there is a second order TS, does it mean that there are
also two independent first order TS's representing each of the 
imaginary vectors of the former? Is the statement always correct?

Any suggestions and references are very welcome.

best regards,
Valentine.

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From: TREVOR D POWER <[email-masked]>
Date: Tue, 3 Oct 2000 12:00:11 -0500 (CDT)

Valentine,
	I actually ran into this head first when I was doing modeling of
organolithium reactions about a year ago. Many theoreticians will tell you
that a second-order transition state is a theoretical entity and cannot
be called a transition state nor a molecule anyway.  If fact the only
correct designation would probably be a second-order saddle point.  When
one is looking for a transition state that describes a particular reaction
and they instead find such a maximum, this usually means that the reaction
they are trying to model is actually not concerted and may go through two
transition states and 1 intermediate.  I would think that one needs to go
to extreme measures to find a second-order saddle point; such as symmetry
or geometry restrictions (a partial optimization --- which does not make 
a lot of sense).  
	I would expect that those papers describing a reaction to go
through second-order saddle points are in journals not set up to
rigorously review computational manuscripts (like J. Org. Chem. for 
example).  You would do well to find papers on this subject in J. Phys.
Chem or J. Chem. Phys.  Alan Isaacson, Miami University, wrote a few
papers (none are reviews though) that began my literature search on this
topic.
	I would agree with the "other authors" as stated below.  The
principle of least motion is violated in a second-order saddle point.  

Hope my posting does not spark too many fires out there.

Regards,
Trevor D. Power
Department of Chemistry
University of North Texas
NT Station, Box 305070
Denton, Tx 76201-5070
[email-masked]

On Thu, 5 Oct 2000, Valentine Ananikov wrote:

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From: George Vacek <[email-masked]>
Date: Tue, 03 Oct 2000 14:55:55 -0700

TREVOR D POWER wrote:

>         Many theoreticians will tell you
> to extreme measures to find a second-order saddle point;

A well stated response Trevor.  A couple papers where theoreticians concede that
experimentalists may have seen evidence of a second order saddle point are:

``The Anomalous Behavior of the Zeeman Anticrossing Spectra of \~A
$^1A_u$ Acetylene: Theoretical Considerations,'' G. Vacek,
C. D. Sherrill, Y. Yamaguchi and H. F. Schaefer, {\it J. Chem. Phys.}
{\bf 104}, 1774-8 (1996).

``The \~A $^1A_u$ State and the $T_2$ Potential Surface of Acetylene:
Implications for Triplet Perturbations in the Flourescence Spectra of
the \~A State,'' C. D. Sherrill, G. Vacek, Y. Yamaguchi,
H. F. Schaefer, J. F. Stanton and J. Gauss, {\it J. Chem. Phys.} {\bf
104}, 8507-15 (1996).

Regards,
George Vacek
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