No abstract

H20 + 02 reaction. However, it is unlikely that a hydrogen-bonded intermediate can produce a negative temperature dependence for the atom + H02 reactions in Figure 10 as atoms possess no permanent dipole moment. Alternatively, it does not appear necessary to invoke a long-range attraction for these atom + H02 reactions since only the Cl + H02 -HC1 + 02 reaction exhibits a negative temperature dependence, and indeed only a negligible one within experimental error.31 More likely, the explanation for the somewhat large A factors for the X + H02 -<• HX + 02 series lies in the Hammond postulate, which in essence states that the transition state of a direct reaction increasingly resembles reactants as the exothermicity of the reaction increases.32 The X + H02 -»-HX + 02 reactions, with unusually large exothermicities (40 kcal mol"1 greater than the X + H202 counterparts), would thus be expected to have somewhat large A factors as the transition states would occur early along the reaction coordinate.The following picture then emerges for the reactions of H02 with radicals. Formation of the (XOOH)* complex is a rapid process, occurring at or near the gas kinetic rate. At high pressure, this intermediate can undergo collisional stabilization, but at low pressure the lifetime of the excited Complex is long enough for the excess energy to induce bond breakage of either the H-O bond (reformation of reactants) or the 0-0 bond (formation of XO and OH products). Much slower is the direct attack on the hydrogen atom to form the HX and 02 products; however, this

Absolute rate coefficients for the reactions of OH with C2H6 (kl), C2H3D3 (k2), and C2D6 ( k 3 ) were measured with the laser photolysis/laser-induced fluorescence technique. Kinetic data were obtained at 600 torr of helium pressure over the temperature range 292.5-705 K. The measurements were fit to modified Arrhenius expressions of the form ki(T) = Ail'" exp(-Ei/Rr). Kinetic isotope effects were characterized by two simple relations: k2(T) = 0.5[k1(T) + k 3 ( T ) ] , and k l ( T)/k3(T) = (1.01 A 0.06) exp(907 f 52 cal mol-'/RT), where the quoted uncertainties represent *2u estimates of the total experimental error. Nonempirical electronic structure calculations were performed on the reactants and transition-state structures. They are consistent with and support the experimental findings. (27) Baulch, D. L.; Craven, R. J. B.; Din, M.; Drysdale, D. D.; Grant, S.; Richardson, D. J.; Walker, A.; Watling, G.

range 290-903 K. For reaction of OH at the secondary site in propane, kHs/kOs varies from 2.6 to 1.5 over a comparable temperature range. As discussed above, for reaction of OH at the tertiary site in isobutane, &htAdt varied from 1.9 to near (or below) the classical limit of 1.4 as the temperature increased from 300 to 750 K. Theoretical attempts to reproduce this trend in k-n/ky, magnitudes are in progress.

Absolute rate coeficients for the reactions of the hydroxyl radical with dimethyl ether i k , ) and diethyl ether ( k 2 ) were measured over the temperature range 295-442 K. The rate coefficient data, in the units cm3 molecule s-l, were fitted to the Arrhenius equationsk,(T) = (1.04 2 0.10) x 10-"exp[-(739 ? 67 calmol-')/RTl andk,iT) = (9.13 2 0.35) x lo-" exp[+(228 2 27 kcal m~l -~) / R T l , respectively, in which the stated error limits are 20. values. Our results are compared with those of previous studies of hydrogen-atom abstraction from saturated hydrocarbons by OH. Correlations between measured reaction-rate coefficients and C -H bond-dissociation energies are discussed.