Isotope effects in hot atom abstraction reactions of hot hydrogen and hot deuterium atoms with ethane-1,1,1-d3

Abstract

Hot H and D (H* and D*) reactions with ethane were studied in Br/sub 2/ scavenged systems using HBr and DBr as photodissociation hot atom sources. Quantum yields for the abstraction reactions of H* + C/sub 2/D/sub 6/ and H* + CH/sub 3/CD/sub 3/ were determined at an initial hot atom energy E/sub i/ = 1.6 eV, and found to be in good agreement with previous yields determined by the integral reaction probability method. Abstraction isotope effects for H* and D* + CH/sub 3/CD/sub 3/ were determined over the range E/sub i/ = 1.1 to 2.9 eV, in 94 percent ethane systems. With H* + CH/sub 3/CD/sub 3/ the H/sub 2//HD product ratio increased from 2.4 +- 0.1 at 1.1 eV to 2.8 +- 0.1 at 2.9 eV, whereas with D* + CH/sub 3/CD/sub 3/ the HD/D/sub 2/ product ratio was approximately constant with change in E/sub i/, 2.0 +- 0.1 at 1.6 eV and 1.9 +- 0.1 at 2.0 and 2.9 eV. In mixtures with a large mole fraction of Br/sub 2/ to scavenge low energy atoms the isotope effects at E/sub i/ = 2.9 eV increased to 3.1 +- 0.1 for H* + CH/sub 3/CD/sub 3/ and 2.8more » +- 0.4 for D* + CH/sub 3/CD/sub 3/. These results, taken together with a previous determination of the abstraction isotope effect in the T* + CH/sub 4/--CD/sub 4/ system at 2.9 eV, give a consistent trend favoring H over D abstraction, with the H/D isotope effect increasing with decreasing hot atom mass. It is concluded that both the H/D preference and the hot atom mass effect can be understood in terms of inertial considerations and that a zero point energy effect may also contribute to the H/D preference.« less