The rate constants for the reactions of CN radicals with methane, ethane, propane, cyclo-propane, isobutane, and neopentane have been measured over a temperature range from 275 to 455 K. Laser photolysis was used to produce the radicals and time delayed laser induced fluorescence was used to follow the radical concentration as a function of time. The temperature dependence of the observed rate constants could be fitted with a three-parameter Arrhenius plot. The activation energies that were observed were all small and in some cases they were negative. Time resolved ir emission was used to follow the formation of the HCN(0n2) and HCN(0n′1) product emission. The time dependence of the relative emission intensities, as well as computer modeling of the decay curves, suggest that vibrational population inversion occurs for all of the hydrocarbons studied except methane and cyclopropane. These observations are discussed in terms of the current theories for these type of reactions.
The contribution of the title reaction to the total reaction of CN + 0 2 has been determined, using time-resolved IR emission spectroscopy of the product CO(u"). This product channel is found to contribute up to 29% with 2% experimental variance to the total reaction. A contribution of this magnitude to the total reaction of CN + 0 2 by this product channel proceeding via a four-center transition state is energetically improbable. A b initio calculations have been performed on two-and four-center transition states which are presumably formed in the reaction between CN radicals and molecular oxygen. The results of these calculations confirm the existence of a very high barrier to the formation of a four-center transition state. Consequently, a new mechanism for the formation of CO in reaction 1 is proposed. This new mechanism consists of two sequential steps rather than two parallel steps to explain the formation of the products NCO, 0, and CO in the reaction CN + 0 2 .
Time-resolved I R emission spectroscopy has been used to monitor the fluorescence in the C-H stretch region of methyl radicals produced in the 193-nm photolysis of acetone. Spectra collected at 20-cm-l resolution in the u3 spectral region do not exhibit any structure. This indicates that the emission in this region is due to both the u3 fundamental of CH3 and combination bands of the radical which overlap each other. Modes other than ~~( 0 0 n O ) must contribute to the observed emission in the 3000-3350-cm-l region. Translationally hot methyl radicals are also found to undergo very fast T -V energy-transfer processes via collisions with various noble gases, resulting in enhanced infrared emission. The intensity of the enhanced emission is a factor of 4 or 5 times the emission intensity in the absence of the noble gases, suggesting that most of the radicals are formed in other vibrational states. The results are explained by assuming that the CH3 radical is initially produced in a broad range of vibrational states.
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