CCL: calculation of Arrhenius kinetic parameters

["Daria Khvostichenko" <dkhvosti*uiuc.edu>]

 Sent to CCL by: "Daria  Khvostichenko" [dkhvosti_+_uiuc.edu]
 Dear CCLers,
 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
 15.32):
 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,
 Daria