Unimolecular decomposition rate theory at the high pressure limit and its application to the dissociations of methanes and ethanes.
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A rate theory for the thermal decomposition by a single bond fission at the high pressure limit was derived from the conceptions that the equilibrium between the active molecule and the activated complex is adiabatic and that the zero point energy difference between the molecule and the complex (Δ<I>E</I><SUB>z</SUB><SUP>\neweq</SUP>) participates in the entropy term of the rate constant (<I>k</I><SUB>d</SUB><SUP>∞</SUP>) and is not related to the enthalpy term. It was also proposed a method calculating <I>k</I><SUB>d</SUB><SUP>∞</SUP> by replacing the vibrational state sum with the entropy term. The values of <I>k</I><SUB>d</SUB><SUP>∞</SUP> for methanes and ethanes were calculated using the present theory and the Gorin activated complex model. From good correspondences between the experimental and calculated values, the values of Δ<I>H</I><SUB>f,298</SUB>°(CH<SUB>3</SUB>(g))=143.2, <I>D</I><SUB>0</SUB>(CH<SUB>3</SUB>–CH<SUB>3</SUB>)=393.0, and <I>D</I><SUB>0</SUB>(CH<SUB>3</SUB>–H)=449.1 kJ mol<SUP>−1</SUP> were obtained for the standard heat of formation and the chemical bond dissociation energy. The apparent activation energy for <I>k</I><SUB>d</SUB><SUP>∞</SUP> decreases drastically by increasing of the reaction temperature. This fact means that the estimation of thermodynamic values from the apparent activation energy is not adequate. It was also found that the vibrational temperature in the complex is considerably higher than the reaction temperature by the contribution of Δ<I>E</I><SUB>z</SUB><SUP>\neweq</SUP> mainly. This result denotes that the chemical equilibrium constant cannot be evaluated from the ratio of <I>k</I><SUB>d</SUB><SUP>∞</SUP>⁄<I>k</I><SUB>c</SUB><SUP>∞</SUP>, where <I>k</I><SUB>c</SUB><SUP>∞</SUP> means the high pressure limiting rate constant of the radical-radical recombination reaction.
- 公益社団法人 日本化学会の論文
公益社団法人 日本化学会 | 論文
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