Direct Measurements and Kinetic Studies of Reaction Intermediates in the Ozonolysis of Alkenes Using Cavity Ring-Down Spectroscopy
As our understanding of the different physical and chemical aspects of tropospheric processes increases, complex reaction mechanisms are developed and tested in local, regional, and global atmospheric models. These complex mechanisms give rise to nonlinear dynamical processes that depend not only on the additive effects of the reactions that comprise them. In addition to studying kinetics of elementary reactions and unimolecular processes, it is necessary to study mechanisms as a whole. This work utilizes a flow reactor and cavity ring-down spectroscopy (CRDS) to study the mechanism of ozonolysis of various alkenes in real time. The use of a flow reactor as a cavity for CRDS measurements allows the simulation of concentration profiles of analytes at different reactor segments by modelling the plug-flow behaviour as a series of continuously-stirred tank reactors (CSTRs). Experimental measurements are used to validate ozonolysis mechanisms used for these simulations. Informed by kinetic modelling, direct measurements of formaldehyde oxide (CH2OO) produced in situ from ozonolysis of ethene and direct measurements of vinoxy radicals (∙CH2CHO) from ozonolysis of 2-butenes are carried out under various reaction conditions. New insights on the mechanisms of ozonolysis of ethene and 2-butenes are obtained by comparing measurements of these reaction intermediates and formaldehyde with simulations from mechanisms containing current kinetic information of elementary reactions, pointing out to the importance of existing and new reaction pathways. Yields of the fraction of stabilized carbonyl oxides produced from ozonolysis of several alkenes are also measured at low pressures, and nascent yields are determined by extrapolation to the zero-pressure limit, providing benchmarks for theoretical and master equation calculations.