Thermal Decompositions of CH3I and CD3I. Experimental and Theoretical Approaches to the Specific Rates at the High Pressure Limit

The rate constants on the thermal decompositions of CH3I and CD3I at the high pressure limit, kd∞, were evaluated from the decomposition rates, measured as functions of the reaction time and the pressure of methyl iodide. The values of kd∞(CH3I) at 606.8, 620.2, 633.5, and 649.5 K were 2.92, 8.22, 22.0, and 68.2×10−6 s−1, respectively. The values of kd∞(CD3I)/kd∞(CH3I) were 0.290 at 620.2 K, 0.285 at 633.5 K, and 0.279 at 649.5 K. Meanwhile, the calculations for kd∞ were carried out using a rate theory derived from the assumption that the zero point energy and the vibrational energy are concerned in the entropy term of kd∞. From good correspondence between the experimental and calculated values, the values of D0 (chemical bond dissociation energy)=243.7, ΔE0 (energy change at 0 K)=230.6, and ΔH298° (enthalpy change at 298 K)=236.3 kJ mol−1 were obtained for the CH3–I bond dissociation. On the decomposition of equimolar mixture of CH3I and CD3I at 633.5 K and 10.67 kPa of the total pressure, it was found that the initial rate ratio of R^°(CH4+CH3D)/R°(CD4+CD3H) is 1.94 and the excitations to the active molecules of CH3I and CD3I in the mixture are less effective than those in the separated systems of CH3I and CD3, respectively.