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5700362 
Dissertation 
Dissociation dynamics of radical intermediates in bimolecular reactions 
McCunn, LR 
2005 
Chicago, IL 
University of Chicago 
Doctoral Dissertation 
English 
This dissertation presents studies of radicals that are relevant to chemical addition reactions. In each experiment, the radical was formed by photolysis of a chlorinated precursor molecule in molecular beam scattering experiments. An advantage of this method is that photolysis of the precursor molecule produces the radical of interest in its ground electronic state with a known distribution of internal energy. This technique is applied to studies of the 2-buten-2-yl, methoxycarbonyl, propionyl and propargyl radicals. The 2-buten-2-yl radical is an intermediate in three addition reactions: H + 1,2-butadiene, CH3 + propyne and H + 2-butyne. Experimental data showed that the nascent 2-buten-2-yl radicals undergo both C-C and C-H bond fission. Detection of stable 2-buten-2-yl radicals determined that the lowest barrier to dissociation of the radical is 31 ± 2 kcal/mole, in agreement with previous calculations of the barrier for 2-buten-2-yl → CH3 + propyne. Analysis of CH3 and C3H 4 data also evidenced that 2-buten-2-yl radicals with high internal energy dissociated by a different mechanism than lower energy radicals, suggesting that the high internal-energy radicals dissociate from an excited state. The methoxycarbonyl radical, CH3OCO, is an intermediate on the potential energy surface of the CH3O + CO → CH3 + CO2 reaction. Our experimentally measured branching ratio and RRKM calculations showed that the CH3OCO → CH3 + CO2 product channel is heavily favored over CH3OCO → CH3O + CO. The data also evidenced a minor channel corresponding to dissociation of an excited state of the methoxycarbonyl radical. The propionyl radical, CH3CH2CO, can be formed in the addition of CO to CH3CH2. Detection of photofragments derived from stable propionyl radicals showed that the lowest barrier to dissociation of the radical is 16.3 ± 1.5 kcal/mole, a result in agreement with kinetic data and computational findings. The propargyl radical, HCCCH 2, is an intermediate in the C + C2H3 and CH + C2H2 reactions. Experimental results evidenced both C-H bond fission and H2 elimination channels for propargyl. Detection of stable propargyl radicals determined the lowest energy barrier to dissociation to be 71.5 kcal/mole.