RATE CONSTANTS AND TEMPERATURE DEPENDENCES FOR THE REACTIONS OF HYDROXYL RADICAL WITH SEVERAL HALOGENATED METHANES, ETHANES, AND PROPANES BY RELATIVE RATE MEASUREMENTS

Rate constants of 15 OH reactions with halogen-substituted alkanes, C1 to C3, were studied using a relative rate technique in the temperatur e range 283-403 K. Compounds studied were CHF2Cl (22), CHF2Br (22B), C H3F (41), CH2F2 (32), CHF3 (23), CHClFCCl2F (122a), CHCl2CF3 (123), CH ClFCF3 (124), CH3CF3 (143a), CH3CH2F (161), CF3CHFCF3 (227ea), CF3CH2C F3 (236fa), CF3CHFCHF2 (236ea), and CHF2CF2CH2F (245ca). Using CH4, CH 3CCl3, CF3CF2H, and C2H6 as primary reference standards (JPL 92-20 rat e constants), absolute rate constants are derived. Results are in good agreement with previous experimental results for six of the compounds studied, including CHF2Cl, CHF2Br, CH2F2, CH3CF3, CHFClCFCl2, and CF3 CHFCF3. For the remainder the relative rate constants are lower than t hose derived from experiments in which OH loss was used to measure the reaction rate. Comparisons of the derived Arrhenius A factors with pr evious literature transition-state calculations(2,3) show order of mag nitude agreement in most cases. However, the experimental A factors sh ow a much closer proportionality to the number of H atoms in the molec ule than is evident from the transition state calculations. For most o f the compounds studied, an A factor of (8 +/- 3)E-13 cm(3)/(molecule s) per C-H bond is observed. A new measurement of the ratio k(CH3CCl3) /k(CH4) is reported that is in good agreement with previous data.