A NEW DARK SOURCE OF THE GASEOUS HYDROXYL RADICAL FOR RELATIVE RATE MEASUREMENTS
Journal of Physical Chemistry, 97 (6):
1172--1177 (January 1993)Abstract
A new dark source of OH for relative rate studies is described. This technique, which is simple to apply and yields results of high precision, is applicable to organics which react very slowly with O3. This method involves measuring the relative rates of decay of pairs of simple alkanes (ethane, propane, n-butane, isobutane, and n-hexane) or toluene in the presence of relatively high concentrations of O3/O2 (4-36 Torr O3) at 298 K and 1 atm total pressure in the dark. The conclusion that OH is the reactant is reached by comparison of the measured relative rates of decay of the organics to literature values for the OH reactions and to experiments reported here in which known photolytic sources of OH, specifically O3-H2O or CH3ONO-NO-air, were used. The same technique was applied to measure the temperature dependence of the relative rate constants for the ethane/propane reactions from 298 to 373 K. The value of (E1 - E2)/R, where E1 is the activation energy for the OH-ethane reaction and E2 that for the OH-propane reaction, was found to be (353 +/- 36) K (+/- 2sigma), compared to a literature value2,12 of (380 +/- 180) K based on the absolute rate constants. Similarly, our ratio of the preexponential factors, A1/A2, was (0.82 +/- 0.09), compared to 0.80 from recent literature evaluations.2,12 Thus this dark reaction provides a new means of measuring relative rate constants of OH reactions for those organics which do not themselves react with O3 at a significant rate. From the absolute rates of decay of the organics, the average concentration of OH generated in these dark reactions was found to be comparable at 298 K to that from the photolysis of CH3ONO-NO-air mixtures, which is approximately 2 orders of magnitude less than during photolysis of the O3-H2O mixtures at 254 nm. In the dark system, the average OH concentration increases with the pressure of O3 and with the temperature. Potential mechanisms of formation of OH in this system are discussed. This technique could prove to be particularly useful for studying the kinetics of compounds which themselves photolyze and hence for which application of photolytic sources of OH is not appropriate.