Subject: Re: CCL:G:SUMMARY OF RATE/ACTIVATION ENERGY

From: [email-masked] (Wai-To-Chan)
Date: Wed, 13 Jan 1999 19:56:33 -0500 (EST)

Having read Dr Topper's information about POLYRATE
I realize I should take back my remarks about the program
being inappropriate for isomerisation reaction rate calculation.

   In writing my previous message I had in mind that 
it is not always neccessary to apply unimolecular rate theory 
rigorously using a package as sophiscated as POLYRATE. 
One can estimate rate constants using a calculator merely by 
substitution of thermodynamic quantities readily obtained from
ab initio MO calculations into the Arrhenius rate 
equation, K = A * exp(-E/RT). Evaluation of the rate constant 
this way is not strictly valid as E is the high-pressure- 
-activation-energy which is not equivalent to the barrier height
obtained from ab initio calculation. Nonetheless the ab initio
barrier height should approximate E reasonably well and I would 
not recommend one going through the trouble of calculating it 
using RRKM method. My opinion is that applying RRKM method
safely requires a better than casual understanding 
of the background theory which happened to be a difficult 
learning experience to me. However I should have presented 
my thoughts on using POLYRATE, which I never used, more carefully. 

   Finally I should point out I mistakenly referred to 
Q the "partition function" from the Arrhenius rate equation as 
the "entropy" in my early comments. My apology for the blunder.

Wai-To Chan
Research Associate
Department of Chemistry 
York University
Toronto, Canada