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

[chan()at()curl.gkcl.yorku.ca (Wai-To-Chan)]

      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