Rate constants for the reaction, H+D2→HD+D, over the temperature range, 724-2061 K, by the flash photolysis shock tube technique An ultraclean 2 in. Ld. single pulse shock tube coupled to an atomic resonance absorption system was constructed in order to determine hydrogen atom concentration by Lyman-a absorption. The tube was baked to 300·C and pumped down to _10-7 Torr. Ultrapure argon could be shock heated to -2500 K with no spurious H atom absorption. The system was constructed in order to study the kinetics of chemical reactions which are strongly catalyzed by H atoms, under the conditions where no such atoms are present. Specifically, the role of H atoms in the H2 + D2---+2HD exchange reaction was studied. Mixtures of hydrogen and deuterium diluted in argon were shock heated to 1375-1760 K; samples were then taken from the tube and analyzed mass spectrometrically for the ratio [HD]/[D2J. 1400 K was the highest temperature at which no spurious H atom absorption was observed in a shocked mixture of 1% H2-1 % D 2 . At 1400 K, under the conditions of no absorption, no, or s; I %HD conversion was obtained. At higher temperatures Lyman-a absorption was detected and more HD conversion was observed. A comparison between these results and results obtained previously in conventional systems suggests that the high HD conversion observed in the past was caused by hydrogen atoms generated from impurities. The existence of a molecular mechanism in the H2-D2 exchange reaction is highly doubtful.2742

The thermal decomposition of perfluorocyc1obutane was studied behind reflected shocks in a single-pulse shock tube over the temperature range 1040 o -1200oK. Mixtures of approximately 1% to 1.5% of perfluorocyclobutane in argon were used. The reaction products were analyzed by vapor-phase chromatography. The first-order rate constants obtained at high temperatures are slightly higher than the values deduced from an Arrhenius extrapolation of the low-temperature data; however, the activation energy is the same. The rate constant is: kuni"'=2.1X10 16 exp[ -74.3X10 3 /RT]'

The ignition of COS + 0 2 mixtures diluted in argon was studied behind reflected shocks in a single-pulse shock tube over the temperature range of 1100-1 700°K. Ignition delay times and the distribution of reaction products before and after ignition were determined experimentally. From a total of 63 tests run at varying initial conditions, the following correlation for the induction times was derived:where Using a reaction scheme of 14 steps, the following values were obtained by a computer modeling of the induction times: = f0.22, 8 2 = -1.55, and E = 17.3 kcal/mole. The calculations showed that the reaction COS + S -+ CO f SZ caused the inhibiting effect of the COS. The reaction COS -+ 0 -+ COZ + S has a very strong accelerating effect, whereas the parallel channel COS + 0 ---f CO + SO shows the opposite effect. It was also shown that the reaction 0 f S2 -+ SO + 0 is very slow and does not contribute to the overall oxidation reaction. It is suggested that the rate constant given to the four-center reaction COS + SO --f COz -+ Sz, that is, 10" cm3/mole.sec at 300°K is incorrect. This constant is not much higher than lo8 cm3/mole .set at 1300°K.= +0.30, 0 2 = -1.12, and E = 16.9 kcal/mole.

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