46303-AC6
Product Imaging of the Dissociation of Combustion Radicals
Objectives Studies of the photodissociation dynamics of hydrocarbon radicals provides a wealth of thermochemical, kinetic, and mechanistic data that is of use in modeling combustion chemistry. This project seeks to investigate the photodissociation properties of radical species using a photolytic or pyrolytic molecular beam source coupled with product imaging techniques. Progress The competition between rearrangement of the excited allyl radical via a 1,3 sigmatropic shift versus sequential 1,2 shifts has been observed and characterized using isotopic substitution, laser excitation, and molecular beam techniques. Both rearrangements produce a 1-propenyl radical that subsequently dissociates to methyl plus acetylene. The 1,3 shift and 1,2 shift mechanisms are equally probable for CH2CHCH2, whereas the 1,3 shift is favored by a factor of 1.6 in CH2CDCH2. The translational energy distributions for the methyl and acetylene products of these two mechanisms are substantially different. Both of these allyl dissociation channels are minor pathways compared to hydrogen atom loss.
Ab initio calculations have been performed, using the GAUSSIAN03 package, on the energies and vibrational frequencies of the stable and transition state structures in the reaction path. These structures were optimized at the B3LYP/ccpVDZ level, followed by single point electronic energy calculations at the QCISD.T./cc-pVTZ level and zero point energy corrections by a B3LYP/cc-pVDZ anharmonic frequency analysis. The energies, geometries, and vibrational frequencies obtained from the ab initio calculations were then used as input for RRKM calculations to investigate the microcanonical reaction rates. The rates calculated in this way allowed the branching ratios to be calculated by a full integration of the forward rate equations, including the H-loss reactions. Results from this calculation were in reasonable agreement with the measured branching ratios when the error of 2-3 kcal/mole in the barrier heights was taken into account.
Future work will compare the results of trajectory calculations on this system to the P(E) distributions that have been measured.
