Determination of rates of hydrogen atom reactions with alkenes at 298 K by a double modulation technique

Abstract

Hydrogen atoms and HgH radicals are the two precursors of the HNO(1A′′) chemiluminescence in the mercury photosensitized reaction of gaseous mixtures of H2, NO, and Hg and are responsible, respectively, for its slow and fast decaying component. The total intensity of the chemiluminescence is decreased when an olefin is added to the gaseous mixture and, with an excess of the olefin added, 85% of the luminescence is eliminated. This fraction corresponds to the slow component of the chemiluminescence, due to the reaction H + NO + M → HNO* + M, while the residual 15% is due to the reaction HgH + NO → HNO* + Hg. The magnitudes of the decrease of chemiluminescence intensity and of the increase of the rate of decay of its slow component at a given concentration of an added olefin provide information on the rate of H atom reaction with the olefin.Relative values of the rate constants of H atom reactions with ethylene, propene, 1-butene, cis-2-butene, trans-2-butene, isobutene, and 1,3-butadiene have been determined in the present work from the observed dependence of the luminescence intensity on olefin concentration. At the same time, the absolute values of these rate constants have been determined from the relation between the decay rate of the slower component of the chemiluminescence and olefin concentration. The relative and the absolute values of the rate constants are compared with each other, to check their mutual consistency, and with the available relative and absolute values in the literature.