Subject: imaginary frequencies in cycloaddition's TS

From: Wai-To Chan <chan*at*curl.gkcl.yorku.ca>
Date: Tue, 18 Nov 2003 02:49:47 -0500 (EST)

In the case of the reaction of ozone + ethene/propene 
which I've studied the structural difference between the 
TSs for the concerted cycloaddition and the stepwise pathway is
so obvious that not even a visualization of their imaginary vibrations
are required. 

	It could be easier if you differentiate 
the two TS structures in terms of their activation entropies. 
The looser structure of the asyncronous-path TS implicates 
a higher entropy than the syncronous TS which is expected to have 
a more compact ring-like structure for cycloaddition. 
The magnitude of the Arrhenius factor
of the asyncronus pathway is likely to be significantly greater 
than that of the syncronous pathway. In fact the A-factor is one 
important quantity for the experimentalists to deduce qualitatively 
the structure of a TS. 

  In case this is of interest this paper 

"Mechanisms for the ozonolysis of ethene and propene:
 Reliability of quantum chemical predictions",
J. Chem. Phys., 118, 1688-1701 (2003)

reports a comparison between theoretically calculated
A-factors for the concerted and stepwise pathway 
of the ozone+alkene reactions and experimental values.
Both calculated A-factors and imaginary frequencies for the 
asyncronous TSs are greater. 

Wai-To Chan

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Hi,
This could be a naive question, but I'd like to ask if is it possible to
state if a cycloaddition reaction proceeds by a syncronous or asyncronous
mechanism by visual inspection of the TS' imaginary frequency.
Any suggestion or litt. ref. will be appreciated.

Thanks a lot

Alessandro
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