Vibrational energy transfer probabilities of highly vibrationally excited fluoroethane and 1,2-difluoroethane molecules

Abstract

The collisional loss of vibrational energy from chemically activated CH3CH2F and CH2FCH2F formed with average energies of 91 and 92.5 kcal mol'1, respectively, has been studied at 300 K with four bath gases, SFe, C02, N2, and He. These chemically activated molecules were formed by combination of CH3 with CH2F and of CH2F with CH2F. The data cover an extensive range of pressure and permit the assignment of the mean energy transfer per collision and the form of the transition probability distribution. For He the (AEd) values were 1.0 kcal mol"1 with an exponential distribution for both C2H6F and C2H4F2. The ( ) values for C2H6F or C2H4F2 were virtually the same and ranged from 2.0 to 5.0 kcal mol"1 for N2, C02, and SFe; these transition probability distributions were of the Gaussian type (represented here by a stepladder model). The results for CH3CH2F and CH2FCH2F are compared to previous findings for CH3CF3 and CH2C1CH2C1 from this laboratory. The deactivation efficiency for SF6 is similar for all four molecules. However, the deactivation of CH3CF3 by N2 and C02 is less efficient than for for the other three molecules. The He deactivation efficiencies for the fluoroethanes are all similar, but substantially smaller than for C2H4C12.

Isomerisation of CF2ClCH2Cl and CFCl2CH2F by Interchange of Cl and F Atoms with Analysis of the Unimolecular Reactions of Both Molecules

“…stants for HF elimination from the two molecules seem to follow the respective reaction path degeneracy. The rate constants measured in SF 6 were reduced by a factor of 0.8 to account for collisional cascade [31][32][33] before comparison to the calculated rate constants in Table 2.…”

“…The difference between the two sets of rate constants is partly a consequence of the reversal of the reaction path degeneracies. Based on the extensive studies of CF 3 CH 3 in various bath gases, [31,32] a factor of 0.8 should be used to convert the observed rate constant in SF 6 to the value for unit deactivation efficiency and these scaled rate constants are used in Table 2 to assign the threshold energies.…”

“…The bath gases belong to the efficient category of collision partners with vibrationally excited haloethanes, and 25-33 kJ mol À1 of energy, on average, should be removed from CF 2 ClCH 2 Cl* and CFCl 2 CH 2 F* per collision. [31][32][33] The rate constants measured at high pressure, that is for D/S 1.5, can be associated with the rate constant averaged over the energy distribution, < k E > . Since the energy distribution of the CF 2 ClCH 2 Cl* and CFCl 2 CH 2 F* molecules formed by recombination at room temperature is narrow and < k E > can be closely approximated by the rate constant at the average energy, k <E> .…”

“…The latter requires the collision cross sections between CF 2 ClCH 2 Cl* and the bath gas molecules, which are given in a footnote of Table 1. The principal bath gas, CH 2 ClI, belongs to the efficient bath gas category based on comparison to studies of energy transfer [31][32][33] in similar systems, and the measured rate constants should be no more than 1.2 times larger than the unit deactivation limit. The uncertainty in the collision cross sections are as large as the factors that would be selected to account for estimates of cascade deactivation, and we have assigned the measured rate constants as unit deactivation rate constants.…”

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.…”