KINETICS OF THE REACTIONS OF CL ATOMS WITH C2D6 AND C2D5 AND THE DISPROPORTIONATION OF 2C(2)D(5) - THE DEUTERIUM-ISOTOPE EFFECT

The kinetics of the Cl atom reaction with perdeuterated ethane has been reinvestigated at 298 K using the complete mechanism of reactions occurring in the very low-pressure reactor (VLPR) experimental flow system. The following rate constants (cm(3)/(molecule s)) were measured for the following elementary steps: Cl + C2D6 -->(1) DCl + C2D5, k(1D) = (2.11 +/- 0.05) x 10(-11); Cl + C2D5 -->(2) DCl + C2D4, k(2D) (1.21 +/- 0.06) x 10(-11); and 2C(2)D(5) -->(3) C2D6 + C2D4, k(3D) (2.08 +/- 0.18) x 10(-12). A rough estimate for the reaction Cl + C2D4 -->(4) DCl + C2D3 gives k(4D) similar or equal to (9.4 +/- 2.0) x 10(-14), indicating that it is an unimportant side process in the system. C-2 and Cl mass balances are good to 98.5 +/- 1% without taking reaction 4 into account. Combining these rate constants with earlier measurements from the study of the Cl + C2H6 system, the kinetic deuterium isotope effect for the first step is obtained as k(1H)/k(1D) = 2.89 +/- 0.08. This is now in good agreement with literature values. It is about 43% of the theoretical maximum value of simple H/D abstraction, and this weak isotope effect is attributed to the isotope sensitive zero-point energy increase of a bent transition state geometry. Highly exothermic reactions 2 and 3 disclose no measurable deuterium isotope effects. Recently reported 25-fold higher values of k(2H) are shown to require abnormal collision diameters in excess of 11 Angstrom.