THE KINETIC ISOTOPE EFFECT FOR C-H BOND ACTIVATION ON CU(110) - THE EFFECTS OF TUNNELING

The kinetic isotope effect (KIE; k(H)/k(D)) in the decomposition of formate on Cu(110) is 2.9 at 460 K (4.4 at 300 K). The absolute value of the pre-exponential factor and the values of E(D) - E(H) and A(D)/A(H) indicate that rate-limiting C-H(D) bond scission occurs without any appreciable effect of tunnelling. This kinetic isotope effect is reflected by a 15 K increase in the peak temperature observed for the CO2 product for the D versus the H isotope in the temperature programmed reaction spectrum; this temperature shift appears to be typical of first-order, rate-limiting C-H(D) bond activation on metal surfaces. The kinetic isotope effect for beta- and gamma-C-H(D) bond scission for a variety of intermediates adsorbed on various metal surfaces show temperature shifts of this magnitude. These results thus suggest that tunnelling does not usually contribute significantly to the rate of C-H bond activation of adsorbed intermediates on metals.