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Radical Reactivity in the Condensed Phase: Intermolecular versus Intramolecular Reactions of Alkoxy Radicals.

ABSTRACT: Condensed-phase alkoxy (RO) radicals can undergo unimolecular (e.g., intramolecular H atom abstraction) reactions as well as bimolecular (intermolecular H atom abstraction) reactions, though the competition between these two channels is not well constrained. Here, we examine this branching by generating RO radicals from the photolysis of a large alkyl nitrite (C20H41ONO) in hexanes and nebulizing the mixture into an aerosol mass spectrometer for analysis. Product ions associated with unimolecular (isomerization) reactions were observed to increase upon photolysis. However, no formation of the C20 alcohol (C20H41OH, the expected product from RO + RH reactions) was observed, suggesting that bimolecular reactions are at most a minor channel for this condensed-phase system (involving saturated hydrocarbons). This result, combined with previous studies of liquid-phase RO radicals carried out at higher concentrations, suggests that when 1,5-H atom abstraction reactions are facile (i.e., in which a 1,5-H atom shift from a secondary or tertiary carbon can occur), this channel will dominate over bimolecular reactions.

SUBMITTER: Carrasquillo AJ

PROVIDER: S-EPMC5348301 | biostudies-literature | 2015 Jun

REPOSITORIES: biostudies-literature

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Radical Reactivity in the Condensed Phase: Intermolecular versus Intramolecular Reactions of Alkoxy Radicals.

Carrasquillo Anthony J AJ Daumit Kelly E KE Kroll Jesse H JH

The journal of physical chemistry letters 20150610 12

Condensed-phase alkoxy (RO) radicals can undergo unimolecular (e.g., intramolecular H atom abstraction) reactions as well as bimolecular (intermolecular H atom abstraction) reactions, though the competition between these two channels is not well constrained. Here, we examine this branching by generating RO radicals from the photolysis of a large alkyl nitrite (C20H41ONO) in hexanes and nebulizing the mixture into an aerosol mass spectrometer for analysis. Product ions associated with unimolecula ...[more]

PMID: 26266621

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Project description:The intramolecular gas-phase reactivity of four oxoiron(IV) complexes supported by tetradentate N(4) ligands (L) has been studied by means of tandem mass spectrometry measurements in which the gas-phase ions [Fe(IV)(O)(L)(OTf)](+) (OTf = trifluoromethanesulfonate) and [Fe(IV) (O)(L)](2+) were isolated and then allowed to fragment by collision-induced decay (CID). CID fragmentation of cations derived from oxoiron(IV) complexes of 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (tmc) and N,N'-bis(2-pyridylmethyl)-1,5-diazacyclooctane (L(8)Py(2)) afforded the same predominant products irrespective of whether they were hexacoordinate or pentacoordinate. These products resulted from the loss of water by dehydrogenation of ethylene or propylene linkers on the tetradentate ligand. In contrast, CID fragmentation of ions derived from oxoiron(IV) complexes of linear tetradentate ligands N,N'-bis(2-pyridylmethyl)-1,2-diaminoethane (bpmen) and N,N'-bis(2-pyridylmethyl)-1,3-diaminopropane (bpmpn) showed predominant oxidative N-dealkylation for the hexacoordinate [Fe(IV)(O)(L)(OTf)](+) cations and predominant dehydrogenation of the diaminoethane/propane backbone for the pentacoordinate [Fe(IV)(O)(L)](2+) cations. DFT calculations on [Fe(IV)(O)(bpmen)] ions showed that the experimentally observed preference for oxidative N-dealkylation versus dehydrogenation of the diaminoethane linker for the hexa- and pentacoordinate ions, respectively, is dictated by the proximity of the target C-H bond to the oxoiron(IV) moiety and the reactive spin state. Therefore, there must be a difference in ligand topology between the two ions. More importantly, despite the constraints on the geometries of the TS that prohibit the usual upright ? trajectory and prevent optimal ?(CH)-?*(z2) overlap, all the reactions still proceed preferentially on the quintet (S = 2) state surface, which increases the number of exchange interactions in the d block of iron and leads thereby to exchange enhanced reactivity (EER). As such, EER is responsible for the dominance of the S = 2 reactions for both hexa- and pentacoordinate complexes.

Dataset Information

Radical Reactivity in the Condensed Phase: Intermolecular versus Intramolecular Reactions of Alkoxy Radicals.

Publications

Radical Reactivity in the Condensed Phase: Intermolecular versus Intramolecular Reactions of Alkoxy Radicals.

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