Journal article

Laser-induced temperature jump measurements in the kinetics of association and dissociation of the nitrogen oxide (N2O3) + M = nitrogen dioxide + nitrous oxide + M system

Kinetic measurements have been made on the gas-phase reaction N2O4 + M .dblharw. 2NO2 + M by applying the temp.-jump relaxation method. Equil. mixts. contg. NO2, N2O4, a small quantity of an IR absorber, and an excess of a third-body bath gas, have been subjected to a short CO2 laser pulse. The induced temp. jump, of the order of 1 K, displaces the equil. towards NO2 formation. After the fast heating of the mixt., the time dependence of the NO2 and N2O4 concns. has been monitored at 420 and 250 nm, resp. For small perturbations, the relaxation to the new equil. concns. at the higher temp. follows first-order kinetics, and the thermal rate const. can be deduced from the measured relaxation rate const. Measurements have been made at 255 K at 0.3-200 bars He, which corresponds to the falloff range for this reaction. The high- and low-pressure limiting rate consts., krec,... = (7.0 +- 0.7) * 1011 cm3 mol-1 s-1 and krec,0/[He] = (2.1 +- 0.2) * 1014 cm3 mol-1 s-1, resp., extrapolated from the exptl. data at 255 K, were in agreement with those calcd. with a simplified statistic adiabatic channel model. The temp. dependence of the high- and low-pressure recombination rate const., detd. at 255-273 K, is given by the relations krec,... = (2.2 +- 0.2) * 106 * T(2.3+-0.2) cm3 mol-1 s-1 and krec,0/[He] = (7.5 +- 0.8) * 1035 * T(-9.0+-0.9) cm3 mol-1 s-1. The corresponding high- and low-pressure dissocn. rate consts. are kdiss,... = (2.8 +- 0.3) * 1013 * T(1.3+-0.2) exp{-(6790 +- 700)/T} s-1 and kdiss,0/[He] = (9.6 +- 0.9) * 1042T(-10.0+-1.0) * exp{-(6790 +- 700)/T} s-1, resp.
The kinetics of the N2O3 + M = NO + NO2 + M reaction was studied over a wide range of pressure by a laser-induced temp. jump relaxation method. Equil. mixts. at 225 K contg. nitrogen dioxide, dinitrogen trioxide, dinitrogen tetroxide, nitric oxide, a small quantity of SiF4 as an IR absorber, and an excess of Ar as a 3rd-body gas collider are rapidly heated by a short pulse of CO2 laser radiation. The induced temp. jump, at 0.5-3 K, displaces the equil. toward NO2 and NO formation. The rate of recombination reaction to the new equil. concns. at higher temp. is monitored by observing spectroscopically the time dependence of the N2O3 concn. in the UV. For small perturbations, simple mathematics relates the measured chem. relaxation time const. to the thermal rate consts. of the system. Exptl. results (obtained in the 0.5-200 bar pressure range of Ar corresponding to the 3rd-order and falloff region of the reaction) were fitted by falloff curves established accordingly to a method developed by Troe. High- and low-pressure limiting rate consts. for the NO + NO2 recombination at 227 K are extrapolated from the exptl. data. These values lie within a factor of 2 of those that may be predicted using a simplified statistic adiabatic channel (SAC) model. The temp. dependence of the recombination rate const., measured exptl. at 227-260 K is reported as well as the calcd. kinetics of dissocn. for N2O3.


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