Yields of elementary reactions initiated when HI and DI are activated by 254 nm radiation in 3-methylpentane-h14 and 3MP-d14 at 20–300 °K; H atom trapping

Abstract

This paper reports work designed to give a more quantitative understanding of the reactions of hot and thermal hydrogen atoms with hydrocarbons at 298, 77, and 30°K and to explain the absence of trapped H atoms during photolysis of HI in 3-methylpentane-h14 (3MP-h14) glass at 30°K. Quantum yields of H2, HD, and D2 from the photolysis of HI and DI in 3MP-h14 have been determined for samples photolyzed in the liquid state at 298°K and the glassy state at 77 and 30°K. The yields of trapped free radicals and of alkyl iodides and iodine have been determined on samples photolyzed at 77°K. Trapped hydrogen atoms have been produced by photolysis of HI in 3-methylheptane-d18 and methylcyclohexane-d14 glasses, and their production in 3MP-d14 and nonproduction in 3MP-h14 confirmed. Photolysis of HI in 3MP-d14 at 30°K yields H2 as well as HD, indicating that thermalized H atoms are produced, but photolysis of DI in 3MP-h14 at 30°K yeilds no D2. This indicates that all the D atoms produced in 3MP-h14 abstract H from C–H bonds and implies that the reason D and H atoms from the photolysis of DI and HI in 3MP-h14 are not trapped is because the abstraction reaction consumes all of the atoms before they can be trapped. Information on isotope effects in both the liquid and solid phase has been obtained. The data further indicate that: (1) the quantum yield for stabilization of HI (DI) without reaction after having absorbed a photon is about 0.13 at 298°K and 0.5 at 77°K, in either 3MP-h14 or 3MP-d14; (2) the yield of HD from HI in 3MP-d14 or DI in 3MP-h14 at 77 °K is much greater than the yield of trapped radicals, implying that C6D13I(C6H13I) + HD must be formed by a concerted reaction; (3) at 298 °K the reaction D+3MP-h14 competes significantly for thermalized D atoms with the atom scavenging reaction D+DI→D2+I at DI concentrations less than ∼100 mM.