Energy transfer in thermal methyl isocyanide isomerization. Dilution effects at low pressure.

Abstract

The effect of three inert gases, helium, ethane, and pentene-1, on the rate of activation and reaction of methyl isocyanide in the low-pressure region at 280.5°has been examined. A study of the variation of the relative collision efficiency ß0(D) of these gases, as compared with the parent substrate, has been made as a function of dilution over the range D = -m. The predicted decrease of ß (0) with increasing dilution is observed. The variation is qualitatively correct in magnitude and increases as expected on going from the quasi-strong collider, pentene-1, to the weaker collider, helium. The measurements can in principle provide information about the nature of the collisional transition probability model(s) involved and the values of relative collision cross sections of inert molecules to substrate molecules. The present measurements are not sufficiently precise to permit full exploitation of this aspect but can be taken to suggest that a stepladder model fits best for pentene-1 and ethane, and an exponential model for helium. The average value of the energy decrement per collision experienced by the isocyanide molecule is of the order of 1.0, >5.0, and >6.0 kcal mole-1 for helium, ethane, and pentene-1, respectively.(1) (a) This work was supported by the National Science Foundation. (b) Abstracted from a Ph.D. thesis to be presented by Y. N. Lin which should be consulted for further details.(2) (a) H. S.

Branching Ratio and Pressure Dependent Rate Constants of Multichannel Unimolecular Decomposition of Gas-Phase α-HMX: An Ab Initio Dynamics Study

“…Both canonical variational transition state theory , (CVT) and microcanonical variational transition state theory , (μVT) were employed to calculate the high-pressure limit thermal rate constants for both NO 2 fission and HONO elimination reactions. The multichannel and pressure-dependent rate constants were calculated by using the master equation (ME) method. − Quantum tunneling effects were included in the rate constant calculations of the HONO elimination reaction by using the small curvature tunneling , (SCT) method in the CVT framework and Miller's one-dimensional Eckart tunneling model 32 in the μVT framework. There is considerable literature on the CVT, μVT, and SCT methods, − thus we will not discuss them here.…”

“…Thus, it is important to provide rate constants as well as the branching ratio of these two channels as functions of both the temperature and pressure. The multichannel dynamics and pressure-dependent rate constants can be studied using the well-developed master equation (ME) method. − This method has been employed in many recently theoretical studies ,− on the pressure dependent rate constants. The ME method requires the microcanonical rate constants, k ( E ), for each channel.…”

Energy transfer in unimolecular reactions at high temperatures

“…For example, the value of 0, for helium and argon in cyclopropane isomerization a t 765% given firmed by experiment [14], although dilutions of greater than 100 correspond very closely to infinite dilution. Consequently, most shock-tube experiments have been carried out at effectively infinite dilution.…”

“…The various modrls give similar results, and it appears that for several systems the least oficirnt collisions (those with thr noble gases) each involve a transfer of energy cquivalent to about 4 kJ/mol, while collisions with polyatomic species transfer at least 20 kJ/mol. These results are entirely in agreement with those obtained from chemical activation experiments [ 131. For the methyl isocyanide-helium system, the variation of P with dilution has also been studied [14], and the prcdicted [lS] decrease was observed. Increasing temperature also lowered /3 from 0.29 a t 210°C to 0.21 a t 320°C [15]; this is also predicted by the theory [la, 161. The theory of collisional energy transfer has also been examined in detail by Troc [17], using a versatile thrce-parameter model of energy transfer.…”