Direct Observation of Tetrahydrofuranyl and Tetrahydropyranyl Peroxy Radicals via Cavity Ring-Down Spectroscopy

Influence of the Ether Functional Group on Ketohydroperoxide Formation in Cyclic Hydrocarbons: Tetrahydropyran and Cyclohexane

Photolytically initiated oxidation experiments were conducted on cyclohexane and tetrahydropyran using multiplexed photoionization mass spectrometry to assess the impact of the ether functional group in the latter species on reaction mechanisms relevant to autoignition. Pseudo-first-order conditions, with [O₂]₀:[R^•]₀ > 2000, were used to ensure that R^• + O₂ → products were the dominant reactions. Quasi-continuous, tunable vacuum ultraviolet light from a synchrotron was employed over the range 8.0-11.0 eV to measure photoionization spectra of the products at two pressures (10 and 1520 Torr) and three temperatures (500, 600, and 700 K). Photoionization spectra of ketohydroperoxides were measured in both species and were qualitatively identical, within the limit of experimental noise, to those of analogous species formed in n-butane oxidation. However, differences were noted in the temperature dependence of ketohydroperoxide formation between the two species. Whereas the yield from cyclohexane is evident up to 700 K, ketohydroperoxides in tetrahydropyran were not detected above 650 K. The difference indicates that reaction mechanisms change due to the ether group, likely affecting the requisite ^•QOOH + O₂ addition step. Branching fractions of nine species from tetrahydropyran were quantified with the objective of determining the role of ring-opening reactions in diminishing ketohydroperoxide. The results indicate that products formed from unimolecular decomposition of R^• and ^•QOOH radicals via concerted C-C and C-O β-scission are pronounced in tetrahydropyran and are insignificant in cyclohexane oxidation. The main conclusion drawn is that, under the conditions herein, ring-opening pathways reduce the already low steady-state concentration of ^•QOOH, which in the case of tetrahydropyran prevents ^•QOOH + O₂ reactions necessary for ketohydroperoxide formation. Carbon balance calculations reveal that products from ring opening of both R^• and ^•QOOH, at 700 K, account for >70% at 10 Torr and >55% at 1520 Torr. Three pathways are confirmed to contribute to the depletion of ^•QOOH in tetrahydropyran including (i) γ-^•QOOH → pentanedial + ^•OH, (ii) γ-^•QOOH → vinyl formate + ethene + ^•OH, and (iii) γ-^•QOOH → 3-butenal + formaldehyde + ^•OH. Analogous mechanisms in cyclohexane oxidation leading to similar intermediates are compared and, on the basis of mass spectral results, confirm that no such ring-opening reactions occur. The implication from the comparison to cyclohexane is that the ether group in tetrahydropyran increases the propensity for ring-opening reactions and inhibits the formation of ketohydroperoxide isomers that precede chain-branching. On the contrary, the absence of such reactions in cyclohexane enables ketohydroperoxide formation up to 700 K and perhaps higher temperature.

Room-Temperature Cavity Ring-Down Spectroscopy of Methylallyl Peroxy Radicals

We report room-temperature cavity ring-down (CRD) spectra of the à ← X̃ electronic transition of 1-, 2-, and 3-methylallyl peroxy (MAOO^•) radicals produced by 193 nm photolysis of methyl-substituted allyl chlorides in the presence of O₂. Vibronic structure of experimentally observed spectra was simulated using calculated relative populations of MAOO^• conformers, their electronic transition frequencies and oscillator strengths, as well as their vibrational frequencies and Franck-Condon factors of the à ← X̃ electronic transition. The reaction intermediate for the production of 1- and 3-MAOO^• radicals, CH₃CHCHCH₂, is a resonance-stabilized free radical. CRD spectra of 1- and 3-MAOO^• radicals obtained using different precursors suggest that allylic rearrangement between the two resonance structures (CH₃CH=CHCH₂^• and CH₃CH^•CH=CH₂) is significantly faster than oxygen addition. Branching ratio between terminal and nonterminal oxygen addition was predicted to be 52:48 on the basis of calculated spin densities, which agrees qualitatively with the experimental CRD spectra of 1- and 3-MAOO^• radicals.