Deprotonation of α-distonic ions. Proton affinities of the α-radicals

Audier, H. E.; Fossey, Jacques; Mourgues, P.; Leblanc, Danielle; Hammerum, Steen

doi:10.1016/s0168-1176(96)04397-2

Cited by 14 publications

(19 citation statements)

References 36 publications

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“…Calculated G2 gasphase proton affinities (PAs) of CH 3 X and • CH 2 X are listed in Table 6. Agreement with the values quoted in Lias et al 3 is quite variable, but there is considerable improvement when more recent experimental values are used, such as those of Szulejko and McMahon 5,44 and Audier and co-workers, 42,43 or those listed in a recent compendium by Hunter and Lias. 7 Comparison of the PAs of • CH 2 X radicals with the PAs of the corresponding CH 3 X molecules shows that for π-donor substituents, the proton affinities are 30-70 kJ mol -1 lower in the radicals than in the corresponding closed-shell molecules (Table 6).…”

Section: Resultsmentioning

confidence: 85%

“…5 e Using ∆fH°298( • CH2NH2) ) 150.6 kJ mol -1 (Griller and Lossing)34 and ∆fH°298( • CH2NH3 + ) e 841 kJ mol -1 (Lias et al). 3 f Audier et al42 g Mourgues et al43 h Using ∆fH°298( • CH2OH) ) -16.6 ( 0.9 kJ mol -1 (Ruscic and Berkowitz). 36 i Glukhovtsev et al44 j Calculated using ∆fH°298( • CH2FH + ) ) 965 ( 15 kJ mol -1 (J. L. Holmes, private communication, cited by Gauld et al…”

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confidence: 99%

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The Effects of Protonation on the Structure, Stability, and Thermochemistry of Carbon-Centered Organic Radicals

et al. 1997

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The effects of protonation on the geometries and stabilization energies of prototypical •CH2X radicals (X = NH2, OH, OCH3, PH2, SH, F, Cl, Br, CN, CHO, and NO2) have been studied with the use of ab initio molecular orbital calculations at the G2 level. The proton affinities at X of the •CH2X radicals and the analogous substituted methanes, CH3X, are compared and the corresponding heats of formation calculated. For π-donor substituents (X = NH2, OH, OCH3, PH2, SH, F, Cl and Br), protonation at X leads to considerable +C :XH character for both CH3XH+ and •CH2XH+, resulting in substantially lower heterolytic bond dissociation enthalpies and longer C−X bonds. Protonation also strengthens the C−H bonds in CH3XH+ and, in combination with the reduced interaction of the lone pair on X with the singly-occupied orbital at the radical center in the radicals, results in negative radical stabilization energies for •CH2XH+. For the π-acceptor substituents (CN, CHO, and NO2), protonation enhances hyperconjugative electron donation from the methyl group to the π* orbital of X, thereby resulting in C−X bonds in CH3XH+ and •CH2XH+ that are shorter than those in the unprotonated species. This also leads to weaker C−H bonds and, together with enhanced delocalization of the unpaired electron in the radical, leads to positive radical stabilization energies for •CH2XH+. The proton affinities of the radicals with π-donor substituents are 30−70 kJ mol-1 lower than those of their closed-shell counterparts. This may be attributed to the decreased availability of the lone-pair orbital(s) on X, resulting from interaction with the singly-occupied orbital at the radical center. The •CH2CHO and •CH2NO2 radicals have proton affinities that are similar to those of their closed-shell counterparts because protonation takes place in a plane almost orthogonal to the singly-occupied orbital on C, and so there is less effect in going from CH3X to •CH2X.

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Section: Resultsmentioning

confidence: 85%

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The Effects of Protonation on the Structure, Stability, and Thermochemistry of Carbon-Centered Organic Radicals

et al. 1997

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“…However, it is not the case of the ␣-distonic ion • CH 2 OH 2 + . This ion is strongly acidic [23,27] and, in the presence of most of the neutral molecules, the only reaction observed is protonation of the neutral. This latter reaction is not observed with alkanes whose proton affinities are too low.…”

Section: H • Abstraction By An α-Distonic Ionmentioning

confidence: 99%

Hydrogen radical abstraction by small ionized molecules, distonic ions and ionized carbenes in the gas phase

Nedev

Rest²,

Mourgues³

et al. 2004

International Journal of Mass Spectrometry

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“…8,13,14 For example, ions containing a proton charge, such as • CH 2 OH 2 + , have been shown to dissociate via • H rearrangement to the radical site and, also, to react via proton transfer with appropriately basic molecules. 15,16 In distonic ions carrying well-stabilized or inert charge centers, on the other hand, radical reactivity prevails; 14,[17][18][19][20][21][22][23][24] such behavior was demonstrated for distonic ions with, inter alia, tertiary sulfonium, 14 quaternary pyridinium [18][19][20][21][22][23] or metal ion charges. 24 In the latter cases, the charge provides a convenient means to study the gas-phase chemistry of radicals by mass spectrometry.…”

Section: Introductionmentioning

confidence: 99%

Li⁺-Coordinated Polyglycol Radicals: Ion—Molecule Reactions with Alkenes and Ethers

Polce

Modarelli

Wesdemiotis

2004

Eur J Mass Spectrom (Chichester)

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The Li + complexes of the radicals HOCH 2 CH 2 O • ("oxy" radical), HOCH 2 CH 2 OCH 2 • ("methyl" radical) and HOCH 2 CH 2 OCH 2 CH 2 • ("ethyl" radical) are produced in the gas phase by fast atom bombardment of poly(ethylene glycol) 200 and the ion-molecule reactions of these novel distonic ions, [R • + Li] + , with 1-butene, dimethyl ether and methyl vinyl ether are examined in the hexapole collision cell of a sector/quadrupole tandem mass spectrometer. The products arising from these reactions depend on the reagent used and the type of radical site present in [R • + Li] +. With 1-butene, predominantly radical-site addition-eliminations take place, initiated by addition of the unpaired electron of [R • + Li] + to the olefinic double bond; fragmentations, induced by the unpaired electron of the adduct, follow to create stable closed-shell products. Dimethyl ether attaches to the charge site of [R • + Li] + (i.e. at the metal ion) and the adduct reacts further by elimination of (mainly) small radicals from either the original [R • + Li] + segment or the added reagent. Methyl vinyl ether, which blends the structural features of alkenes and ethers, gives rise to a combination of such radicaland charge-site addition-fragmentation processes. The dissociations promoted by the unpaired electron involve β-scissions and • H rearrangements, which are the typical reactions of neutral radicals. All lithiated radicals undergo addition to double bonds. A significant fraction of the oxy radical also reacts via hydrogen atom abstraction from the added reagent. The charge site (metal ion) does not directly participate in the radical-site reactions observed, but facilitates specific channels by transiently coordinating to dissociating ligands, so that inter-ligand hydrogen atom rearrangements between these ligands can occur. This type of metal ion involvement in radical reactivity could be particularly important in biological milieus where metal ions abound.

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Deprotonation of α-distonic ions. Proton affinities of the α-radicals

Cited by 14 publications

References 36 publications

The Effects of Protonation on the Structure, Stability, and Thermochemistry of Carbon-Centered Organic Radicals

The Effects of Protonation on the Structure, Stability, and Thermochemistry of Carbon-Centered Organic Radicals

Hydrogen radical abstraction by small ionized molecules, distonic ions and ionized carbenes in the gas phase

Li⁺-Coordinated Polyglycol Radicals: Ion—Molecule Reactions with Alkenes and Ethers

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Deprotonation of α-distonic ions. Proton affinities of the α-radicals

Cited by 14 publications

References 36 publications

The Effects of Protonation on the Structure, Stability, and Thermochemistry of Carbon-Centered Organic Radicals

The Effects of Protonation on the Structure, Stability, and Thermochemistry of Carbon-Centered Organic Radicals

Hydrogen radical abstraction by small ionized molecules, distonic ions and ionized carbenes in the gas phase

Li+-Coordinated Polyglycol Radicals: Ion—Molecule Reactions with Alkenes and Ethers

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Li⁺-Coordinated Polyglycol Radicals: Ion—Molecule Reactions with Alkenes and Ethers