Low-Pressure and Nascent Yields of Thermalized Criegee Intermediate in Ozonolysis of Ethene.

The yields of thermalized formaldehyde oxide (CH2OO, the simplest Criegee intermediate) produced from ozonolysis of ethene at low pressures were measured indirectly using cavity ringdown spectroscopy (CRDS) and chemical titration with an excess amount of sulfur dioxide (SO2). The method of monitoring the consumption of SO2 as a scavenger allows better characterization of the CH2OO at low pressure and short residence time. The yield of thermalized CH2OO from ethene ozonolysis was found to decrease with decreasing pressure. The nascent yield of thermalized CH2OO was determined to be 20.1 ± 2.5% by extrapolation of the 7-19 Torr measurements to the zero-pressure limit. Kinetic models enable better evaluation and understanding of the different measurement methods of thermalized Criegee intermediates. The information on the low-pressure yields from this work serves as a benchmark for theoretical calculations and facilitates a better understanding of the alkene ozonolysis reaction mechanisms.