We wish to communicate results from studies of the vibrational deactivation of chemically activated molecules [ 11. The present measurements are for CHaCFZ with a wide variety of bath gases which include inefficient rare gases as well as polyatomic gases ranging up to the efficiency of those investigated by Chang, Craig, and Setser [2]. The results consist of comparisons of data at high pressure, which give a measure of relative collisional deactivation efficiencies, and of extensive low-pressure data, which permit approximate assignment of a collisional transition probability model and the average energy ( A E ) lost per collision according to that model. For several bath gases experiments were done at 300°K and 195°K. The important general conclusions are (a) there is a wide spread in collisional efficiencies for various bath gases, and (b) CH 3CF 3 is particularly difficult to deactivate relative to 1,2-dichloroethane [3] and cyclopropane [4].The photolysis [5] of CF3N2CH3 generates CH3 and CF3 radicals from which chemically activated CHBCF3" molecules are formed, with an average energy of 102 kcal/rnole [Z]. The critical energy for H F elimination producing CHz=CF2 is -68 kcal/mole [6]. Chemically activated CH3CF3* may either undergo unimolecular reaction or collisional stabilization. The nonequilibrium rate constant for this process is defined by k, = wD/S, where D is the number of molecules decomposing per unit time, S is the number of molecules stabilized per unit time, and w is the collision frequency. The proportionality between w and pressure allows the rate constant to be initially recorded as k, = P(D/S); conversion to sec-' units was done in the usual manner. The experiments consisted of mapping the increase in k, with decreasing pressure by gas chromatographic measurement of D / S over a wide pressure range. For a particular bath gas the mole ratio of bath gas to CH 3N2CF 3 was constant and sufficiently high that collisions with CH3N2CF3 could be neglected. The experimental results for SFs, CFr, CH4, and N P are presented in Figure 1. The limiting high-pressure rate constants k,", the standard deviations, and the "best fit" curves were cjbtained by polynomial regression analysis. Table I presents a summary of the results.The competition between unimolecular reaction and cascade deactivation for the i t h energy increment can be represented by the master equation [1,2,4]. Computational solution of the master equation to obtain steady-state concentra- 473
Notes 1457 were connected with Sj 18/9 ball joints. An exit bubbler filled with Kel-F oil was connected to the last trap. The entire apparatus was situated in a well-ventilated walk-in hood behind a transparent safety shield.A gas stream consisting of a mixture of helium (550 cc/min) and fluorine (30 cc/min) was passed through the system. Periodically, the gas stream was diverted and passed through an infrared gas cell by means of a tee and stopcock in the line between the two traps at -78°. The infrared spectra showed weak bands due to F3NCN and CO2. The solution became yellowish and then dark orange as the run progressed. During the fluorination, the infrared spectra of the product stream indicated a steadily increasing amount of CO2 and a relatively constant F2NCN concentration. After 1 hr, the gas stream was stopped, the fluorinated solution emptied, and the flask recharged with fresh reagents. Fluorination was continued for another hour.The water bath surrounding the reaction flask was held between 9 and 12°b y adding ice.Isolation of the material retained in the two -196°t raps by vacuum line techniques yielded 14 mmoles of crude product. Infrared spectra of this product indicated that the composition was approximately 65% F2NCN, 30% CO2, and 5% other species. The molecular weight (gas density) of the mixture was 69. B. Colthup for interpretation of the infrared spectra, Dr. F. C. Schaefer for helpful discussions, and Mr. S. W. Grant for carrying out the early fluorination experiments.
The competition between unimolecular dehydrohalogenation and collisional deactivation of chemically activated 1,2-C2H4Cl2, <E>=88.3 kcal mol−1, and 1,2-C2D4Cl2, <E>=89.3 kcal mol−1, formed by combination of CH2Cl and CD2Cl radicals at 300°K has been investigated in CH3Cl(CD3Cl), CO, Ne, and He bath gases. For each gas, the kinetic isotope effect, ka (H) /ka (D), was measured over a wide pressure range. Within experimental error, all gases gave the same isotope effect at high pressure; ka∞(H)/ka∞(D=3.2± 0.2. At lower pressures the isotope effect markedly increased and for the lowest pressures the rate constant ratio was ∼8. Cascade vibration deactivation is shown to be the source of the pressure dependence of the kinetic isotope effect. The cascade deactivation of C2H4Cl2 and C2D4Cl2 by CH3Cl(CD3Cl) and CO was adequately described by a stepladder model with average energy losses per collision of 6 and 4 kcal mol−1. For Ne and He bath gases, an exponential model with average energy losses of 2.9 and 1.4 kcal mol−1 was satisfactory. There was no specific isotope effect on the collisional transition probabilities for C2H4Cl2 and C2D4Cl2.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.