Hydrogen Atom Addition to Hydrocarbon Guests in Radiolyzed Zeolites

Abstract

The formation of H adducts during radiolysis of zeolites containing olefinic and aromatic hydrocarbon guests was demonstrated to occur by H atom transfer from the zeolite to the adsorbed molecules. The H adducts and other paramagnetic radiolysis products (radical cations and H-loss radicals) were detected by EPR spectroscopy. The mechanism of H adduct formation was confirmed by, inter alia, H/D isotopic labeling and comparison of results in HZSM5 and H−Mordenite to results in their Na+-exchanged counterparts. The effects of charge transfer among radiolysis products and zeolite−adsorbate interactions on the relative yields of trapped species is discussed. It is proposed that molecules smaller that a critical size adsorb inhomogeneously in ZSM5, forming aggregates in which ion−molecule reactions occur. Interpretation of the experimental results was aided by quantum chemical calculations of the molecular properties of the intermediates and their reaction energetics. This included evaluation of electron-transfer exothermicities, activation energies for H addition, and the relative acidities of the radical cations.

Theoretical study of hydrogenation of thiouracils and their base pairs with adenine

“…Forming the two radicals, the systems will pass over transition states, found to lie 1.8 kcal/mol above the isolated reactants for 5H‐U and 2.9 kcal/mol for 6H‐U (including ZPVE corrections). The observed barriers are highly similar to those found for hydrogenation of unsaturated hydrocarbons (e.g., 2.3 kcal/mol in 1,2,‐dimethylcyclopentene; 1.9 kcal/mol in cyclohexene 20). In the transition states, the attacking hydrogen atom is positioned ca.…”

“…The energy gain upon formation of the C 5 ‐hydrogenated uracil (5H‐U, for short), relative to the isolated reactants, is 37.3 kcal/mol (31.6 kcal/mol after ZPVE), whereas for 6H‐U it is 35.2 kcal/mol (29.6 kcal/mol including ZPVE). The exothermicities for hydrogenation of uracil are hence highly similar to those of, e.g., 1,2‐dimethylcyclopentene (31.2 kcal/mol) or cyclohexene (33.6 kcal/mol) 20. Correcting for thermal energy (cf.…”

“…We begin with considering some of the key features of the hydrogenated uracil 12, 20. Uracil favors hydrogenation at its C 5 and C 6 carbons (see Fig.…”

“…We earlier explored hydrogenation reactions to (unsaturated) hydrocarbon guests in radiolyzed zeolites 20, which has provided valuable insight into the energetics and pathways for hydrogenation across double bonds in related systems. It was concluded that initial van der Waals complexes are highly difficult to localize when performing computations of these model systems in vacuo and that such initial complexes actually provide little information about the reactions occurring in real systems.…”

Muon Spin Spectroscopy of Transient Radicals in Zeolites

“…Radicals are mobile under catalytically relevant conditions and this leads to termination reactions and the loss of the EPR signal, which in practice limits EPR studies of radicals in zeolites to low temperatures [8,9]. The principle method for studying radicals is electron paramagnetic resonance (EPR).…”