PRESSURE-DEPENDENCE OF THE RATE CONSTANTS FOR THE REACTIONS CH3+O2 AND CH3+NO FROM 3 TO 10(4) TORR

A relative rate technique is used to measure the pressure dependence of the rate constants for reaction 1 (CH3 + O2 + M --> CH3O2 + M) and reaction 3 (CH3 + NO + M --> CH3NO + M) relative to reaction 2 (CH3 + Cl2 --> CH3Cl + Cl). The pressure dependence of the rate constant of reaction 3 at 297 K can be represented in the Troe equation by the parameters (k3)0 = (3.5 +/- 0.4) X 10(-30) cm6 Molecule-2 s-1, (k3)infinity = (1.68 +/- 0. 1) X 10(-11) cm3 molecule-1 s-1,and F(cent) = 0.46. The values of k3 are identical to those observed in recent absolute rate measurements at 296 K and 27-600 Torr, verifying that the rate constant chosen for reaction 2, k2 = 3.95 X 10(-12) exp(-530/RT), is both pressure independent and correct at 296 K. This value of k2 was used to determine absolute values of k1 from the ratio k1/k2 in N2 diluent for pressures between 3 and 11 000 Torr at 297 K, between 20 and 1500 Torr at 370 K, and between 40 and 1500 Torr at 264 K. All data in N2 can be fitted using the following parameters in the Troe equation: (k1)0 = (7.56 +/- 1.1) X 10(-31) (T/300)-3.64+/-1.0 cm6 molecule-2 s-1; (k1). = (1.31 +/- 0.1) X 10(-12) (T/300)1.2 +/- 0.8 CM3 molecule-1 s-1; F(cent) = 0.48 (264 K), 0.46 (297 K), 0.42 (370 K). Error limits include statistical error and the uncertainty in k2. In He, N2, and SF6 diluents the relative third-body efficiencies are 0.56:1.0:1.52, respectively, assuming that F(cent) is independent of diluent. The high-pressure limit in SF6 is identical to that in N2. Rate constant ratios were also measured at 297 K for CD3 + O2 + M --> CD3O2 + M (1D) between 5 and 6000 Torr. Assuming that k2D = k2, the limiting rate constants using F(cent) = 0.46 are (k1D)0 = (11.8 +/- 1.6) X 10(-31) CM6 molecule-2 s-1 and (k1D)infinity = (1.38 +/- 0.08) x 10(-12) cm3 molecule-1 s-1.