Hi all,
I have a conversion problem with rate constant calculations. My start point is the isoprene molecule with a chlorine atom bond to C1. I want to obtain the rate constant of:
isoprene + Cl --> isoprene-Cl
For that, I have performed different calculations with Gaussian as follows:
- Geometry optimization: of isoprene + Cl system, and isoprene-Cl using B3LYP/6-31G(d,p).
- Frequency calcs: same methodology and basis set.
- QST2: for finding the TS.
- CCSD(T): using B3LYP/6-311G(d,p).
After all, I did the thermochemical study with all of these obtained values. Using the equation:
k = (kb*T/h)*(Qts/Qadduct)*e(-( ΔE)/kb*T)
, I have tried to calculate "k" value. My problem implies that:
- I do not understand clearly what ΔE means. Is it referred to ZPE energy included in .log of Gaussian? Is it the difference between any other values?
- Result units are "s-1", how could I convert it to "cm3*molecule-1s-1", which are the units presented in all papers?
- If I transform the equation into this:
k=(Kb*T/h)*e(-ΔG/RT)
, the obtained values of k are very high. Maybe am I taking erroneous values from .log file?
Thank you so much for your attention and time!!
Hi Francisco Javier Hermoso,
The ΔE in your first equation means a diference in potential energy (I'm pretty sure ZPE is already accounted for in the vibrational partition function that shows up in the pre-exponential factor). I know it is confusing, that's why I prefer the second equation, in terms of ΔG, better.
Be aware that most computational chemistry packages will calculate things in the gas phase only. This means you'll end up with reaction rate constants in atm-1 s-1 (since you have two reactants; the molecularity of the reaction is two), not in s-1. It is impossible to have a two-reactant reaction end up with a rate constant in s-1 units (units wouldn't otherwise match in, e.g., d[P]/dt = k [A] [B]).
For unit conversion, I recommend this single-page gem from 1969 will very likely help you: Article Tables of conversion factors for reaction rate constants
All the best,
Felipe
Hi Felipe Silveira de Souza Schneider ,
Thank you so much for your answer! Sometimes we don't see something obvious until someone helps us. I said that it was an unimolecular reaction because Cl in gas phase is in excess, so "k" can be considered as unimolecular constant in this case.
All the best and thanks for your time and attention,
Francisco Javier
Hi Francisco Javier Hermoso,
I see! Nevermind my digression on the units then :)
Best,
Felipe
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