Kinetics of radical reactions of tropospheric importance
The object of the research described in this thesis is to provide, by means of laboratory experiments, data required to understand the atmospheric chemistry of the nitrate radical, from both a mechanistic and kinetic point of view. The low pressure discharge-flow kinetic technique coupled detection of the NO3 radical by optical absorption was used to measure the temperature-dependent rate coefficients for the reaction of NO3 with 1-butene, 1-chloro-l-butene, 2-chloro-2-butene, 3-chloro-l-butene, 1-chloro-2-butene, 2-chloro-2-butene, 1-chloromethylpropene, 3-chloromethylpropene, 3-bromo-1-butene, 4-bromo-l-butene and 2-bromo-2-butene. The atmospheric implications for the reaction of NO3 with these compounds are discussed. In order to understand the patterns of reactivity towards NO3 exhibited by these compounds, a number of approaches were adopted. First, a non-quantitative approach employing the simple ideas of inductive and mesomeric effects. Secondly the observed reactivity of NO3 towards these compounds was discussed, quantitatively, in terms of the relative energies of the interacting orbitals. This quantitative analysis required an extensive set of molecular orbital calculations were undertaken at various levels of sophistication. A good correlation was found, for compounds not containing vinylic halogen atoms, between -E(HOMO) and the measured rate constant; the data were used also to calculate "group-reactivity factors". In an extension to this work a new empirical correlation is presented that takes account of the contribution, in terms of the atomic orbital coefficients, of vinylic halogen atoms to the observed rate constant. To provide support for these calculations, work was undertaken using photoelectron spectroscopy to characterise experimentally the individual molecular orbitals. A correlation between the inverse of the molecular polarisability and the activation energy of the reaction is also described. Complementary work was undertaken in a unconventional flash-photolysis system to look at the kinetics of the reaction NO3 + NO3 andlongrightarrow; 2 NO2 + O2 over a pressure range of 2 to 100 Torr in helium. The measured rate coefficient was found to be pressure independent and to have a value of (2.2±1.0) x 10-16 cm3 molecule-1 s-1. The experimental work on the nitrate radical was extended to look at the laser-induced fluorescence (LIF) spectrum and at the quenching of the excited state of the nitrate radical.