Experimental and computational studies of hydroxyl radical kinetics at very low temperatures

• Main Author: Shannon, Robin Joseph
• Other Authors: Heard, D.
• Published: University of Leeds 2012
• Subjects: 541.39
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.566356

In this thesis low temperature rate coefficients have been measured for a number of reactions involving the OH radical using a pulsed Laval nozzle apparatus. All these reactions exhibit an energy barrier between reactants and products and in each case this barrier is preceded by either a hydrogen bonded complex ( OH + acetone, OH + DME, OH + methanol) or a weaker Van der Walls complex (OH + acetylene, OH + ammonia). The rate coefficients for these reactions are observed to increase by up to three orders of magnitude between 200 K and 63 K and complimentary Master equation calculations are able to reproduce the complicated temperature dependences that these rate coefficients exhibit. From these master equation calculations, the negative temperature dependencies of the measured rate coefficients are attributed to a mechanism involving the prereaction complex, in most cases including a contribution from quantum mechanical tunnelling. This tunnelling contribution is most especially important in the case of OH + methanol and in this case, hydrogen abstraction products through a 15 kJ mol-1 barrier are observed directly at 79 K on the same timescale as OH loss. The association between an OH radical and its co-reagent to form a weakly bound adduct, is further explored through performing the first proxy method experiments below 200 K. The proxy method is designed to give the high pressure limiting rate coefficient for two associating reactants A and B through measuring the rate coefficients for the A(v=i ) + B reaction. The reactions listed above are revisited and OH vibrational levels up to v = 3 are probed. From these measurements, lower limits for the high pressure limiting rate coefficients of these reactions are obtained at 80 K and the validity of the proxy method is explored in each case.