CCL: calculation of Arrhenius kinetic parameters
- From: "Daria Khvostichenko"
- Subject: CCL: calculation of Arrhenius kinetic parameters
- Date: Wed, 21 May 2008 13:30:06 -0400
Sent to CCL by: "Daria Khvostichenko" [dkhvosti_+_uiuc.edu]
I am trying to estimate Arrhenuis kinetic parameters for a unimolecular reaction
from QM calculations. According to Christopher Cramer's "Essentials of
Computational Chemistry", the rate constant can be calculated as follows
(Eq. 15.28, 1st Ed.):
k_rate = kT/h * exp[-dHa / RT] * exp[ dSa / R]
where dHa and dSa are the standard enthalpy and the standard entropy of
activation, respectively. However, the way it is split into A and Ea of the
Arrhenius equation k_rate = A * exp[-Ea / RT] is as follows (Eq. 15.31 and
Ea = dHa + RT
and A = kT/h * exp[1 + dSa/R]
whereas a seemingly more intuitive partition would be:
Ea = dHa, A = kT/h * exp[dSa/R].
Both ways of partitioning give the same equation for k_rate from the rate theory
because the "extra" terms in -(dHa+RT)/RT and (1+dSa/R) cancel out.
I apologize if the question is too trivial, but could somebody explain why these
extra terms, RT for dHa and R for S, must be added taken into account in
calculation of Arrhenius kinetic parameters?
Thank you in advance,