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.