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91.
The negative temperature dependence, pressure dependence, and isotope effects of the self-reaction of HO2 are modeled, using RRKM theory, by assuming that the reaction proceeds via a cyclic, hydrogen-bonded intermediate. The negative temperature dependence is due to a tight transition state, with a negative threshold energy relative to reactants, for decomposition of the intermediate to products. A symmetric structure for this transition state reproduces the observed isotope effect. The weak pressure dependence for DO2 self-reaction is due to the approach to the high-pressure limit. Addition of a polar collision partner, such as ammonia or water vapor, enhances the rate by forming an adduct that reacts to produce deexcited intermediate. A detailed model is presented to fit the data for these effects. Large ammonia concentrations should make it possible to reach the high-pressure limit of the self-reaction of HO2. 相似文献
92.
93.
Products of radical combination from the free-radical buffer system \documentclass{article}\usepackage{amssymb}\pagestyle{empty}\begin{document}$${{\rm R}^{\rm .} + {\rm R}^{\rm '} {\rm I}\mathop {\leftrightharpoons}\limits^{{\rm K}_{{\rm RR}}}{\rm RI} + {\rm R}^{'}}$$\end{document}. have been analyzed for the two cases, R = Me, R′ = iPr and R = Et, R′ = iPr. Results are consistent with the previously examined system where R = Me, R′ = Et, and give a value of kP for iPr· combination of 108.6±1.1 M?1 sec?1. 相似文献
94.
A search for γ-rays preceding isomeric fission in the reaction238U(α, 2n)240mPu yielded negative results. Upper limits are given for the number of unconverted photons per isomer formed. 相似文献
95.
An analysis of the thermochemistry of the kinetic parameters of the elementary reactions involved in the pyrolysis of pentachloroethane has resolved several disputed, unclarified, or inconsistent aspects of the reaction mechanism. The resulting mechanisms for the inhibited and uninhibited pyrolysis account for all reported experimental findings. On the basis of this interpretation, first experimentally based values have been derived for the following: DH0(CCl3–CHCl2) = 79.0 ± 1.0 kcal/mol, ΔH(CHCl2) = 25.7 ± 1.0 kcal/mol, and E1 = 59.7 ± 1.0 kcal/mol C2HCl5 . 相似文献
96.
Oku N Krishnamoorthy R Benson AG Ferguson RL Lipton MA Phillips LR Gustafson KR McMahon JB 《The Journal of organic chemistry》2005,70(17):6842-6847
The absolute stereochemistry of the three unresolved structural components in neamphamide A (1) was determined to be (R)-beta-methoxy-L-tyrosine, (2R,3R,4S)-4-amino-7-guanidino-2,3-dihydroxyheptanoic acid, and (2R,3R,4R)-3-hydroxy-2,4,6-trimethylheptanoic acid. Stereochemical assignments were made by chemical degradation of 1, derivatization of the resulting products, and then spectroscopic and chromatographic comparison of the derivatives with synthetically prepared standards. Using the same analytical protocol developed for 1, the beta-methoxytyrosine residue in papuamide B (2) was found to be (R)-beta-methoxy-D-tyrosine. This represents a rare example of divergent stereochemistry in an unusual amino acid residue that is present in two closely related classes of peptides. 相似文献
97.
Equilibrium constants for the reaction CH3COCH2CH3 + I2 ? CH3COCHICH3 + HI have been computed to fit the kinetics of the reaction of iodine atoms with methyl ethyl ketone. From a calculated value of S(CH3COCHICH3) = 93.9 ± 1.0 gibbs/mole and the experimental equilibrium constants, ΔH(CH3COCHICH3) is found to be ?38.2 ± 0.6 kcal/mole. The Δ(ΔH) value on substitution of a hydrogen atom by an iodine atom in the title compound is compared with that for isopropyl iodide. The relative instability of 2-iodo-3-butanone (3.4 kcal/mole) is presented as further evidence for intramolecular coulombic interaction between partial charges in polar molecules. The unimolecular decomposition of 2-iodo-3-butanone to methyl vinyl ketone and hydrogen iodide was also measured in the same system. This reaction is relatively slow compared to the formation of the above equilibrium. Rate constants for the reaction over the temperature range 281°–355°C fit the Arrhenius equation: where θ = 2.303RT kcal/mole. The stability of both the ground and transition states is discussed in comparing this activation energy with that reported for the unimolecular elimination of hydrogen iodide from other secondary iodides. The kinetics of the reaction of hydrogen iodide with methyl vinyl ketone were also measured. The addition of HI to the double bond is not rate controlling, but it may be shown that the rate of formation of 1-iodo-3-butanone is more rapid than that for 2-iodo-3-butanone. Both four- and six-center transition complexes and iodine atom-catalyzed addition are discussed in analyzing the relative rates. 相似文献
98.
99.
The decomposition rate constant of i-PrI under conditions of very low-pressure pyrolysis (VLPP) is completely consistent with the well-known high-pressure Arrhenius parameters and the RRK(M) theory. The decomposition of n-PrI under the same conditions proceeds via two pathways, the anti-Markownikoff dehydroiodination and C? I bond scission. The data, analyzed by taking into account the mutual interaction of the two pathways, is completely consistent with the known Arrhenius parameters for the bond scission step and, when combined with a reasonable A-factor, yields an activation energy for HI elimination which is as predicted for these semi-ion pair transition states. 相似文献
100.
The very low pressure reactor (VLPR) technique has been used to measure the bimolecular rate constant of the title reaction at 300 K. The rate constant is given by log k1 (1/mol s) = (11.6 ± 0.4) ? (5.9 ± 0.6)/θ the equilibrium constant has also been measured at the same temperature and is given by K1 = (5.6 ± 1) × 10?3 and hence log k?1 (1/mol s) = 9.5 ± 0.1. The results show that the reaction Br + t? C4H9 → HBr + i? C4H8 is unimportant under the present experimental conditions. Assigning the entropy of t-butyl radical to be 74 ± 2 eu which is in the possible range, the value of K1 gives ΔH (t-butyl) = 9.1 ± 0.6 kcal/mol?1. This yields for the bond dissociation, DH° (t-butyl-H) = 93.4 ± 0.6 kcal/mol. Both of these values are found to be in good agreement with recent VLPP studies. 相似文献