A NEW APPROACH TO FREE-RADICAL KINETICS - RADIALLY AND AXIALLY RESOLVED HIGH-PRESSURE DISCHARGE FLOW WITH RESULTS FOR OH+(C2H6,C3H8,N-C4H10,N-C5H12)-] PRODUCTS AT 297K

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 fact 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 orders of magnitude, eliminates heterogeneous interference, provides marked improvement in the precision of pressure and temperature scans, and simplifies data analysis. The performance of the new flow system was evaluated by studying the pressure-independent reactions of the OH radical with C2H6, C3H8, n-C4H10, and n-C5H12 at 297 ± 2 K. The experimentally determined rate constants, with uncertainties at the 95% confidence level, are (2.38 ± 0.16) × 10-13, (1.21 ± 0.10) × 10-12, (2.25 ± 0.10) × 10-12, and (4.21 ± 0.18) × 10-12 cm3 molecule-1 s-1, respectively. The rate constant for the first reaction was found to be pressure independent from 7.3 to 381 Torr of N2 buffer gas. The other reactions were studied at a single pressure, 51 ± 1 Torr, of N2. Rate constants within the series of reactions of OH with straight-chain alkanes are found to correlate strongly with the ionization potential of the alkane, and it is argued that this reactivity trend imparts more information about the character of the transition state for simple H atom abstraction reactions than does the more frequently used exothermicity/bond energy trend. © 1990 American Chemical Society.