Vibrational energy transfer probabilities of highly vibrationally excited 1,1,1-trifluoroethane

Abstract

We have also measured the 13C Isotropic shifts and line widths for the cobalt complexes of glycine and alanine with a 13C NMR spectrometer and obtained a pD dependence of the shift and line width similar to the case for histidine. (29) For Co2+ complexes, the experiment values of the electron spin relaxation time rs are in the range 10"12-10"13 s30 at room temperature while the rotational correlation time rr at the same temperature Is approximately 10-10-10-11 s31 for molecules of the size of a histidine-cobalt complex In solution. Therefore, under these conditions the relation 1/rs » 1/rr holds. Furthermore, we obtain the following relations In the case of very fast electron spin relaxation time: t, » tm"\ rc = ra = ts. Using these effective correlation times in combination with the relations, ^ ,2 » 1, =2 2 » 1, the Solomon-Bloembergen equations are reduced to32-34 the pressure variation of this ratio with model calculations, the average vibrational energy, (AE)¿own, removed from CH3CF3 per collision was assigned for each of the bath gases. For monatomic, diatomic, and triatomic gases, the (AE}down values ranged from 1 to 2 kcal moT1 and the distribution of transition probabilities has an exponential dependence on AE. For larger polyatomic bath gases, a Gaussian distribution, represented here by a stepladder model, provides a satisfactory description of the transition probability distribution and (AE}down increases to a maximum value of ~10 kcal moT1 for the most efficient bath gases. The present data, together with results already in the literature, provide a rather complete description for the vibrational deactivation of chemically activated CH3CF3*. Vibrational energy transfer from CH3CF3* occurs somewhat less readily than from many other chemically activated molecules, such as 1,2-dichloroethane, cyclopropane, or alkyl radicals.

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

“…31 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 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.…”

“…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.…”

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.…”