A new approach to free-radical kinetics: radially and axially resolved high-pressure discharge flow with results for hydroxyl + (ethane, propane, n-butane, n-pentane) .fwdarw. products at 297 K

Abstract

Development of a new approach to experimental gas-phase free-radical kinetics is reported. The system is unique in that experimentally determined axial/radial velocity and free-radical concentration profiles are used to simultaneously quantify flux divergence (arising from mixing/diffusional processes) and chemical loss for any volume element in the flow tube. This eliminates the plug flow approximation that has historically limited fast-flow techniques to the pressure regime below 10 Torr. Motivation for the development stems first from the need for studies of radical--molecule reactivity focusing on two classes of reactions-those involving multiple transition states with weakly bound intermediates and those traversing loose transition states--and second from the fast that radical-radical kinetics at elevated pressures and low temperatures have emerged as a critical component in our understanding of ozone depletion in the earth's polar stratosphere. The approach reported here extends the dynamic range in pressure and thereby the stabilizing collision frequency for intermediates by 2 order of magnitude, eliminates heterogeneous interference, provides marked improvement in the precision of pressure and temperature scans, and simplifies data analysis.