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 o f the rate constants for reaction 1 (CH3 + O2 + M --> CH3O2 + M) and r eaction 3 (CH3 + NO + M --> CH3NO + M) relative to reaction 2 (CH3 + C l2 --> CH3Cl + Cl). The pressure dependence of the rate constant of re action 3 at 297 K can be represented in the Troe equation by the param eters (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. Th e values of k3 are identical to those observed in recent absolute rate measurements at 296 K and 27-600 Torr, verifying that the rate consta nt chosen for reaction 2, k2 = 3.95 X 10(-12) exp(-530/RT), is both pr essure 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 Tor r 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(c ent) = 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 diluen ts the relative third-body efficiencies are 0.56:1.0:1.52, respectivel y, assuming that F(cent) is independent of diluent. The high-pressure limit in SF6 is identical to that in N2. Rate constant ratios were als o 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.