Hi Ashutosh,
You can use
the Eyring-Polanyi equation to calculate k the unimolecular rate constant, given
Gibbs Free Energy of Activation delGdoubledagger. Google "Eyring equation
wiki" to see the equation written out.
To a good approximation at moderate temperatures, the free energy of
activation is not dependent on temperature. Reactions go faster at higher
temperatures because a higher proportion of molecules have attained a kinetic
energy greater or equal to the free energy of activation.
To estimate free energy of activation, you would need to optimize and
do frequency calculation for the starting compound and transition state, and
take the difference in free energies between these two. Locating the T.S. is
sometimes difficult: if you are using Gaussian, the Opt=TS, or OPT=QST2, or
Opt=QST3 methods are used.