Subject: Re: Second order TS's crossroads

From: "Stefan Fau" <[email-masked]>
Date: Tue, 3 Oct 2000 15:16:30 -0400

Hi Valentine,

to 1)

I think that 2nd order TSs are of little importance, chemically.
If you have a 2nd order TS with one imaginary mode connecting
your reactants and products, you can optimize along the other
imaginary mode and get a 1st order TS that should still connect
your reactants and products. (I don't know of any exceptions to
that, but maybe there are a few.) Due to the optimization, the energy
of this 1st order TS is lower, and therefore more reacting molecules
should pass along here (assuming that the effects of nuclear motion
are small enough).

Classically, every point (geometry) on the potential energy
hypersurface which is below the molecules energy is accessible
by that molecule. So if you supply your molecule with lots of energy
lots of geometries are possible. If you give just enough energy to
keep
a very slow reaction going virtually all product molecules will have
passed the 1st order TS.

to 2)

A 2nd order TS is not generally connected to four 1st order TSs (two
for each imaginary mode). I once had a 2nd order TS connected to
two mirror-symmetric 1st order TSs and two minima. The first order
TSs connected the same minima. I think that this scheme might be
common with 2nd order TSs of higher symmetry.

The example is published in
S. Fau, G. Frenking, Theochem - J. Mol. Struct. 1995, 338, 117-130.
I think the 2nd order TS was a C2v structure of B2CH4, the 1st order
TS was non-planar Cs and the minimum was planar Cs.

Stefan
______________________________________________________________________
Dr. Stefan Fau                    |      [email-masked]
Quantum Theory Project     |     (352) 392-6714
University of Florida
Gainesville, FL 32611-8435