Collisional deactivation probabilities for chemically activated 1,1,1‐trifluoroethane

Abstract

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

Unimolecular Rate Constant and Threshold Energy for the HF Elimination from Chemically Activated CF3CHFCF3

“…For cyclo-CF 2 CF 2 CH 2 * colliding with CF 2 CF 2 the average energy removed per collision was 9 kcal/mol . The more exhaustive study of CF 3 CH 3 * with numerous collision partners showed that for molecules similar to our bath gas the average energy removed per collision would be 6−10 kcal/mol.…”

Unimolecular Rate Constants for the HF and HCl Elimination Reactions from Chemically Activated CF2ClSH

“…Our reaction mixture is 67% SF 6 , 20% (CF 2 Cl) 2 C=O, and 13% H 2 S. Previous work suggests that for collisions involving SF 6 and (CF 2 Cl) 2 C=O with a variety of collision partners, a stepladder (SL) model with 25–42 kJ/mol removed per collision should be used for the collisional transition probability. It appears that little is known about the average energy transferred for collisions between hydrogen sulfide and either haloethanes or halomethanes, but since H 2 S comprises only 13% of our mixture we will assume that the SL model with about 32 kJ/mol (Δ E = 8 kcal/mol) removed per collision is an acceptable representation for our system.…”

CF3CH3 → HF + CF2CH2: A Non-RRKM Reaction?

“…12,13 We also simulate some of the many experiments in the literature on TFE thermal activation [14][15][16][17] and chemical activation. 18 As discussed below, we find that the KKSST data at the two highest pressures are not consistent with either RRKM theory ("standard" model) or the theoretical modification to include slow IVR ("slow-IVR" model). [7][8][9][10][11] We show that it is possible to fit the KKSST data by empirically modifying k(E) and that the same model ("truncated" model) will also fit the chemical activation data of Marcoux and Setser 19 when no constraints are placed on empirical parameters for energy transfer.…”

“…In the present paper, we address this question by determining to what extent the KKSST data are consistent with a full RRKM/master equation treatment, with the IVR theory described by Leitner and Wolynes, − with a strictly empirical non-RRKM adjustment of k ( E ), and with other extant data on trifluroroethane(s). Our approach is to simulate the KKSST shock experiments with the MultiWell master equation code. , We also simulate some of the many experiments in the literature on TFE thermal activation − and chemical activation …”