APPLICATION OF SQMFF VIBRATIONAL CALCULATIONS TO TRANSITION-STATES - DFT AND AB-INITIO STUDY OF THE KINETICS OF METHYL AZIDE AND ETHYL AZIDE THERMOLYSIS

DFT including nonlocal corrections and ab initio calculations at MP2 a nd MP4 levels of theory have been performed in order to provide inform ation concerning the mechanism of the rate limiting step of the therma l decomposition of methyl azide and ethyl azide. The chemically intere sting points of the ground-state potential energy surface have been fu lly optimized, and a detailed normal-mode analysis for the reagents an d the transition states is presented. The well-established scaled quan tum mechanical force field method has been used to obtain reliable vib rational frequencies for these molecular structures. The force fields of transition states have been modified by using the scale factors com puted for the force fields of the azides in their ground state. Finall y, the activation energies and the Arrhenius preexponential factors fo r the rate constant have been computed according to transition state t heory. The best values for the activation energies are provided by B3- LYP/6-311+G*. For the preexponential factor, the agreement with exper iment seems to be independent of the level of theory used.