Thermal decompositions of CH3I and CD3I

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 68.2x10-6 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 CD3I, respectively. (author)