Arrhenius parameters for the hydrogen abstraction from ammonia by CF₃ radicals

Abstract

The reaction of CF, radicals with NH, has been studied over a wide temperature range 298-673 K, using the photolysis and the thermal decomposition of CF,I as the free radical source.It was found that the reaction could not be explained in terms of a simple mechanismCF3 + CF3 CZFG in the whole temperature range because a marked pressure dependence on the rate of products formation and the presence of a dark reaction complicate the system a t low temperatures. Thus, Arrhenius parameters for reaction (1) have been calculated relative to the CF, recombination from data in the range 523-673 K where pure hydrogen transfer occurs. The rate constant expression is given by log k"/kt2 = (6.17 -t 0.28) -(57.8 2 2.9)/0 where kH/k:2 is in units of ~m~~/ m o l '~ sl' and 0 = 2.303 RT/kJ/mol.

Kinetics of the reactions of halogenated methyl radicals (CF3, CF2Cl, CFCl2, and CCl3) with molecular chlorine

“…This stabilizing or destabilizing effect would not be as important in CX3 -* H * -B transition states since the leaving group (B) in most of the H-atom transfer reactions which have been studied is generally not as electronegative as the C1 atom. However, among those few H-atom transfer reactions which have been investigated in which the leaving group is also very electronegative (e.g., the reactions with HC1, HBr, NH3, and H,S) a reversal of reactivity is observed: The activation energies for the CF3 reactions are higher than those for the CH3 reactions [6,7,29,30]. This anomalous behavior has been attributed to a repulsive force between the permanent dipole moment of the H-atom donor and the polar CF3 radical [6,71.…”