Study of systems relevant to combustion and atmospheric chemistry
Two approaches have been used in this investigation. The first involved the employment of the technique of molecular modulation kinetic (MMK) spectroscopy in the study of reaction kinetics of transient species. Previous research at Aberdeen had established a systematic error in both the kinetic (kobs) and spectroscopic (σλ) parameters determined. Therefore it was intended to investigate the source of this error through the study of the kinetics of the association of the trichloromethyl radical with oxygen, producing the trichloromethyl peroxy (CCl3O2) radical. However, over a period of thirty-three months, no sensible results were obtained, as a result of severe instrumental component problems and the lack of in-house technical expertise. The project ultimately had to be abandoned. Since the kinetic investigation and the attempted rectification of the many problems encountered with the MMK spectrometer constituted a major portion of the allocated research time, they are reported in the final chapter of this thesis. In the second approach, two computational procedures, applying the Rice-Ramsperger-Kassel-Marcus (RRKM) theory of unimolecular reactions, have been used to evaluate kinetic parameters for the decomposition of two distinct chemical systems, namely tert-butoxy radical and peracetic acid. The programmes used are the Rabinovitch-RRKM programme and the programme UNIMOL (Gilbert et al), the latter of which is the more sophisticated. Fall-off data for the decomposition of tert-butoxy radical have been modelled over the temperature range 402-443 K relative to the experimental Arrhenius parameters, evaluated from both thermal and photolysis rate data obtained in previous investigations. The reaction was found to be pressure dependent over the range 1-1500 Torr. The refined high-pressure limit rate parameters obtained in this work are found to be in reasonable agreement with related rate constants reported from different laboratories. A value of the decomposition rate constant is extrapolated for atmospheric conditions. The efficiencies of energy transfer for three inert bath-gases, namely sulphur hexafluoride, carbon tetrafluoride and nitrogen, at 402 K, are examined using the weak collision model. RRKM modelling predicts very little difference in the efficiencies of these bath-gases - an unexpected result.