Decomposition of vibrationally excited 1,1,1-trideuterio-2,2-difluoroethane

Abstract

Evidence is presented that the vibrationally "hot" molecule CF2HCD3*, formed by the combination of CF2H and CD3 radicals, may decompose by three competing unimolecular processes to form CFH=CD2 + DF, CFD=CD2 + HF, and CF2=CD2 + HD. The second process, a three-center a,a. elimination of HF from the same carbon atom, has not previously been observed. The evidence for the third process, HD elimination, is more tentative. An estimation of the Arrhenius parameters for the decompositions using classical Rice-Ramsperger-Kassel (RRK) theory is made, and the effect of collisional quenching of the "hot" molecules is discussed in terms of the critical energies for the eliminations. A similar experiment with CFH2CD3* showed no a,a elimination of 18, 122 (1964).

The unimolecular decomposition of vibrationally excited 1‐deuterio‐1,1,2‐trifluoroethane

“…There is excellent agreement between their data and our previous data [2] on this "hot" molecule, the a,a-to +elimination ratio varying from about 0.2 at roorfi temperature to about 0.5 at around 250°C. This marked increase in the ratio with temperature is not readily explained [22].7 RRKM > EoarB for the energy barriers for the two processes.…”

“…In the absence of an isotope effect for the a,@ process the critical energy separation is 1 kcal/mole larger. Perhaps fortuitously, the original RRK calculation also gave Ef.O = 64 kcal/mole [2]. However, the RRK calculated value of Ep.8 = 53 kcal/mole [2] is certainly much too low…”

“…The pressure effect originally reported [2] at 250°C for some of the data with c-CsFlz as an added quencher is presumably in error due to some unrealized experimental factor. Table I.…”

“…All of these were not obtained under all conditions, nor, in many runs, was an analysis made for them. The CFzDCFHz is the stabilized compound formed in reaction (2). Below 2 torr it was a minor product, and became undetectable at very low pressures and higher temperatures.…”

“…To gain further insight into these reaction possibilities we have investigated the decomposition of the "hot" molecule CFZDCFH 2 *, formed by radical combination The decomposition of the "hot" molecule CDCltCH $21 * has recently been described, and a series of reactions analogous to those given above were reported [3]. The present study was initiated prior to the communication of Kim and Setser [3], and represents a further part of the continuing investigation of the a,a process in these laboratories [ 1, 2,8]. The possibility of a halogen shift was neglected by Kim and Setser, and also by us in reaction (5).…”