The neutralization of organic cations

Holmes, John L.

doi:10.1002/mas.1280080604

Cited by 268 publications

(90 citation statements)

References 106 publications

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“…Examples of both exothermic (*E \ 0) and endothermic (*E [ 0) electron transfer are well documented. 25 The reason why endothermic electron transfer works at high collision energies is that a small fraction of the non-conserved (center-of-mass) kinetic energy of the ion can be converted to the internal energy of the system and thus make up for the negative energy balance. This feature is of immense practical importance, because stable molecules can be used as electron donors for ions of very low…”

Section: Methodsmentioning

confidence: 99%

“…Even-electron anions are also useful precursors for the formation of radicals.38,39 Collisional electron detachment is typically accomplished with molecular oxygen as target gas. 25,38 The second important feature of collisional electron transfer is that it occurs on an extremely short timescale. Interaction between the unoccupied orbitals in the fast ion that accept the electron and the Ðlled orbitals in the donor molecule fades away exponentially with distance.…”

Section: Methodsmentioning

confidence: 99%

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Modeling nucleobase radicals in the mass spectrometer

Tureček¹

1998

J. Mass Spectrom.

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Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Modeling nucleobase radicals in the mass spectrometer

Tureček¹

1998

J. Mass Spectrom.

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“…NRMS relies on the generation of gas-phase ions of well-defined structures that are separated by mass, accelerated to a high velocity (typically 100,000 -200,000 ms Ϫ1 at kinetic energies in the 5-10 keV range), and discharged by a glancing collision with a gaseous neutral target [11]. For neutralization of cations, collisional electron-transfer from a thermal neutral donor, which is usually a polarizable molecule such as CH 3 SSCH 3 , (CH 3 ) 3 N, N,N-dimethylaniline, etc., occurs on a time scale of a few femtoseconds to form the transient neutral species [12].…”

mentioning

confidence: 99%

Simple b ions have cyclic oxazolone structures. A neutralization-reionization mass spectrometric and computational study of oxazolone radicals

Chen

Turecček

2005

J. Am. Soc. Mass Spectrom.

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The 2-methyloxazol-5-on-2-yl radical (3) and its deuterium labeled analogs were generated in the gas-phase by femtosecond electron-transfer and studied by neutralization-reionization mass spectrometry and quantum chemical calculations. Radical 3 undergoes fast dissociation by ring opening and elimination of CO and CH 3 CO. Loss of hydrogen is less abundant and involves hydrogen atoms from both the ring and side-chain positions. The experimental results are corroborated by the analysis of the potential energy surface of the ground electronic state in 3 using density functional, perturbational, and coupled-cluster theories up to CCSD(T) and extrapolated to the 6-311 ϩϩ G(3df,2p) basis set. RRKM calculations of radical dissociations gave branching ratios for loss of CO and H that were k CO /k H Ͼ 10 over an 80 -300 kJ mol U pon low-energy collisions, gas-phase peptide ions fragment by peptide bond cleavages that result in the formation of ions containing Nterminal residues (b-series ions) and C-terminal residues (y-ion series) [1]. Based on both ion dissociations and computational studies of ion structure and energetics, it is generally accepted that y ions correspond to truncated protonated peptides [2]. In contrast, there has been some recent disagreement regarding b ions for which as many as four different structures have been proposed, e.g., (1) open chain acylium; (2) protonated oxazolone; (3) protonated diketopiperazine; and (4) immonium ions formed from oxazolone structures [3]. Quantum chemical calculations at various levels of theory (Hartree-Fock, Møller-Plesset, and density functional theory) prefer protonated oxazolone structures for b-ions that are usually the most stable or the only stable isomers [4,5]. The formation of protonated oxazolone structure for b-ions is also consistent with the mechanism of peptide ion dissociations that was elucidated by detailed ab initio calculations on model diand tripeptides [6], as discussed in detail in a recent authoritative review [3]. The fragmentation mechanism (Scheme 1) involves proton migration to the amide nitrogen followed by a nucleophilic attack at the amide carbonyl by the neighboring carbonyl oxygen from the N-terminal site. This forms an intermediate with an N-protonated oxazolone ring that can eliminate a molecule of the neutral C-terminal-truncated peptide to form the b-ion [4]. Alternative structures and mechanisms for b-ion formation have been proposed for peptides having polar groups in side chains of amino acid residues, His, Glu, Asn, Lys, and Arg, if these were at N-terminal sites of the peptide bond to be cleaved [7].Experimental support for oxazolone b-ion structures was provided by CAD spectra of model systems. For example, Yalcin et al. showed that the CAD spectrum of the b ion from protonated C 6 H 5 CO-Gly-Gly-OH matched that of protonated 2-phenyloxazol-5-one [4]. The reported low-energy CAD spectra showed only two fragment ions due to loss of CO and formation of C 6 H 5 CO ϩ [4]. Evidence for oxazolone-like neutral counterparts of y-typ...

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“…The cation is discharged by a glancing collision with CH 3 SSCH 3 as an electron donor. Because of the short duration of the collision, which is estimated at 7.5 fs from the collision kinematics, the electron transfer is considered to be a vertical process, so that the nascent radical is formed with the geometry of the precursor cation [26]. Unimolecular dissociations of transient radicals are observed on the time scale of 4.4 s, and the products are analyzed by mass spectrometry following non-selective ionization by collisions with O 2 .…”

mentioning

confidence: 99%

Modeling deoxyribose radicals by neutralization-reionization mass spectrometry. Part 2. Preparation, dissociations, and energetics of 3-hydroxyoxolan-3-yl radical and cation

Shetty

Sadı́lek

Chen

et al. 2004

J. Am. Soc. Mass Spectrom.

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The title radical (1) is generated in the gas-phase by collisional neutralization of carbonylprotonated oxolan-3-one. A 1.5% fraction of 1 does not dissociate and is detected following reionization as survivor ions. The major dissociation of 1 (ϳ56%) occurs as loss of the hydroxyl H atom forming oxolan-3-one (2). The competing ring cleavages by O™C-2 and C-4™C-5 bond dissociations combined account for ϳ42% of dissociation and result in the formation of formaldehyde and 2-hydroxyallyl radical. Additional ring-cleavage dissociations of 1 resulting in the formation of C 2 H 3 O and C 2 H 4 O cannot be explained as occurring competitively on the doublet ground (X) electronic state of 1, but are energetically accessible from the A and higher electronic states accessed by vertical electron transfer. Exothermic protonation of 2 also produces 3-oxo-(1H)-oxolanium cation (3 ؉ ) which upon collisional neutralization gives hypervalent 3-oxo-(1H)-oxolanium radical (3). The latter dissociates spontaneously by ring opening and expulsion of hydroxy radical. Experiment and calculations suggest that carbohydrate radicals incorporating the 3-hydroxyoxolan-3-yl motif will prefer ring-cleavage dissociations at low internal energies or upon photoexcitation by absorbing light at ϳ590 and

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The neutralization of organic cations

Cited by 268 publications

References 106 publications

Modeling nucleobase radicals in the mass spectrometer

Modeling nucleobase radicals in the mass spectrometer

Simple b ions have cyclic oxazolone structures. A neutralization-reionization mass spectrometric and computational study of oxazolone radicals

Modeling deoxyribose radicals by neutralization-reionization mass spectrometry. Part 2. Preparation, dissociations, and energetics of 3-hydroxyoxolan-3-yl radical and cation

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