Fragmentation Of Radical Cations Research Articles

Coherence mapping to identify the intermediates of multi-channel dissociative ionization

Identifying the short-lived intermediates and reaction mechanisms of multi-channel radical cation fragmentation processes remains a current and important challenge to understanding and predicting mass spectra. We find that coherent oscillations in the femtosecond time-dependent yields of several product ions following ultrafast strong-field ionization represent spectroscopic signatures that elucidate their mechanism of formation and identify the intermediate(s) they originate from. Experiments on endo-dicyclopentadiene show that vibrational frequencies from various intermediates are mapped onto their resulting products. Aided by ab initio methods, we identify the vibrational modes of both the cleaved and intact molecular ion intermediates. These results confirm stepwise and concerted fragmentation pathways of the dicyclopentadiene ion. This study highlights the power of tracking the femtosecond dynamics of all product ions simultaneously and sheds further light onto one of the fundamental reaction mechanisms in mass spectrometry, the retro-Diels Alder reaction.

Tracking Molecular Fragmentation in Electron-Ionization Mass Spectrometry with Ultrafast Time Resolution.

ConspectusMass spectrometry is a powerful analytical method capable of identifying compounds given a minute amount of material. The fragmentation pattern that results following molecular activation serves as a fingerprint that can be matched to a database compound for identification. Over the past half century, studies have addressed and, in many cases, named the chemical reactions that lead to some of the principal fragment ions. Theories have been developed to predict the observed fragmentation patterns, many of which assume that energy redistributes prior to dissociation. However, the existence of rearrangements and nonergodic processes complicates the prediction of fragmentation patterns and the identification of compounds that have yet to be entered into a curated database. To date, very few studies have addressed the time-dependent nature of the fragmentation of radical cations and, in particular, processes occurring with picosecond or shorter time scales where one expects to find nonergodic reactions.This Account focuses on a novel approach that enables tracking of molecular fragmentation in electron-ionization mass spectrometry with ultrafast time resolution. The two challenges that have prevented the time-resolved studies following electron ionization are the random impact parameter and moment of ionization of each molecule. In addition, medium-sized molecules can produce fragmentation patterns with tens if not hundreds of product ions. Spectroscopically interrogating all of these ions as a function of time is another major challenge. We describe strong field disruptive probing, a method that ionizes molecules on a femtosecond time scale and allows us to track in time the formation of all fragment ions simultaneously.Molecular fragmentation following ionization can occur on a very wide range of time scales. Metastable ions can survive from nanoseconds to microseconds; reactions that depend on vibrational energy redistribution can take picoseconds to nanoseconds; and direct dissociation processes and some rearrangements can take place in femtoseconds to picoseconds. All of these processes depend on the dynamics that occur during attoseconds and femtoseconds following the ionization process. Following a discussion of these time scales, we provide three examples of fragmentations that have been studied with femtosecond time resolution. Each of these examples include unforeseen reaction dynamics that involve a nonergodic process, highlighting the importance of time resolution in mass spectrometry. Finally, we explore future challenges and unresolved questions in mass spectrometry and, more broadly, in the domain of electron-initiated chemical reactions.

Visible‐Light‐Mediated Regioselective Allylation, Benzylation, and Silylation of Methylene‐Malononitriles via Photoredox‐Induced Radical Cation Fragmentation

Visible‐light‐mediated regioselective allylation, benzylation, and silylation of methylene‐malononitriles using allylic/benzylic silane and disilane reagents have been developed, delivering the corresponding allylated, benzylated, and silylated products in moderate to excellent yields. These reactions proceed smoothly with exclusive regioselectivity by employing only an organo‐photoredox catalyst under very mild conditions. A photoredox‐induced radical cation fragmentation is proposed for the generation of the key allylic, benzylic, and silyl radicals.

N‐Methylimidazolidin‐4‐one organocatalysts: gas‐phase fragmentations of radical cations by experiment and theory

Electron ionisation mass spectra of N-methylimidazolidin-4-one organocatalysts were studied by experimental and theoretical means. The molecular ions mostly undergo alpha cleavages of exocyclic substituents that leave the five-membered ring intact. The type of substituent strongly dominates the appearance of the spectra. Fragmentation cascades are corroborated by metastable ion mass spectra. Quantum Chemistry Electron Ionisation Mass Spectra calculations correlate reasonably well with the experimental electron ionisation spectra and reveal mechanistic details of fragmentation pathways. The drawbacks and benefits of such calculations are discussed. Copyright © 2017 John Wiley & Sons, Ltd.

Oxidation of Aryl Diphenylmethyl Sulfides Promoted by a Nonheme Iron(IV)-Oxo Complex: Evidence for an Electron Transfer-Oxygen Transfer Mechanism.

The oxidation of a series of aryl diphenylmethyl sulfides (4-X-C6H4SCH(C6H5)2, where X = OCH3(1), X = CH3 (2), X = H (3), and X = CF3(4)) promoted by the nonheme iron(IV)-oxo complex [(N4Py)Fe(IV)═O](2+) occurs by an electron transfer-oxygen transfer (ET-OT) mechanism as supported by the observation of products (diphenylmethanol, benzophenone, and diaryl disulfides) deriving from α-C-S and α-C-H fragmentation of radical cations 1(+•)-4(+•), formed besides the S-oxidation products (aryl diphenylmethyl sulfoxides). The fragmentation/S-oxidation product ratios regularly increase through a decrease in the electron-donating power of the aryl substituents, that is, by increasing the fragmentation rate constants of the radical cations as indicated by a laser flash photolysis (LFP) study of the photochemical oxidation of 1-4 carried out in the presence of N-methoxyphenanthridinium hexafluorophosphate (MeOP(+)PF6(-)).

Improvement of chlorophyll identification in foodstuffs by MALDI ToF/ToF mass spectrometry using 1,5-diaminonaphthalene electron transfer secondary reaction matrix

Chlorophylls (Chls) are important pigments responsible for the characteristic green color of chloroplasts in algae and plants. In this study, 1,5-diaminonaphthalene (DAN) was introduced as an electron transfer secondary reaction matrix for the identification of intact chlorophylls and their derivatives, by matrix-assisted laser desorption ionization (MALDI) mass spectrometry (MS). DAN was proved to drastically outperform conventional matrices such as α-cyano-4-hydroxycinnnamic acid, dithranol, antracene, and even terthiophene, since loss of the metal ion and fragmentation of the phytol-ester linkage are negligible. Absence of significant fragmentation of radical cations of Chls a and b at m/z 892.529 and 906.513, respectively, makes MALDI MS capable of following natural degradation of intact porphyrin-based pigments whose initial steps are just represented by demetalation and dephytylation. Chl by-products, such as pyropheophytins, have been identified in dried tea leaves showing the potential of MALDI MS to follow chlorophyll biotransformation occurring in processed foodstuffs. Finally, preliminary results show the potential of MALDI MS to detect illegal vegetable oil re-greening practices.

ChemInform Abstract: Radical Cation Reactions Associated with the Thiocarbonyl Group.

Abstract The radical cation reactions of different thiocarbonyl compounds were examined. Compounds 7, 8, 11, 14 and 18 underwent a radical cation fragmentation in a photoinduced electron transfer (PET) reaction, in the presence of tris(4-bromophenyl)aminium hexachloroantimonate. The examined thiocarbonyl compounds underwent a radical cation fragmentation in a photoinduced electron transfer (PET) reaction, in the presence of tris(4-bromophenyl)aminium hexachloroantimonate.

ChemInform Abstract: Photochemical Alkylation of Ketene Dithioacetal S,S-Dioxides. An Example of Captodative Olefin Functionalization.

Radical alkylation of some ketene dithioacetal S,S-dioxides failed through the tin hydride promoted chain process but was successfully performed through stoichiometric photochemical initiation, either by electron transfer or hydrogen abstraction. In the first case, alkyl radicals were produced from tetralkylstannanes (t-Bu-, i-Pr-, n-Bu-SnR(3)) via radical cation fragmentation, while in the second case these were produced from alkanes (cyclohexane, adamantane) by benzophenone triplet. When bulky radicals (t-Bu, adamantyl) were involved, the addition occurred with complete diastereoselectivity.

Dissociation of the N–C α Bond and Competitive Formation of the [z n – H] •+ and [c n + 2H] + Product Ions in Radical Peptide Ions Containing Tyrosine and Tryptophan: The Influence of Proton Affinities on Product Formation

Dissociations at the N-C(alpha) bond of tryptophan and tyrosine residues are the prevalent pathways in the fragmentations of radical cations of tripeptides that contain such as residues. This process involves a proton transfer from the beta-carbon of the tryptophan or tyrosine residue to the carbonyl oxygen of the amide group, followed by cleavage of the N-C(alpha) bond, generating low-lying proton-bound dimers that dissociate to give each an ionic and a neutral product. Formation of the [z(n) - H](*+) or [c(n) + 2H](+) ion is a competition between the two incipient fragments for the proton in a dissociating proton-bound dimer.

Radical Addition to Cation Pool . Reverse Process of Radical Cation Fragmentation

not Available.

Radical Addition to “Cation Pool”. Reverse Process of Radical Cation Fragmentation.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Radical Addition to “Cation Pool”. Reverse Process of Radical Cation Fragmentation

The reaction of an N-acyliminium ion with an alkyl iodide and hexabutyldistannane took place to give the alkylation product. A mechanism involving the addition of an alkyl radical to an N-acyliminium ion to produce the corresponding radical cation has been suggested.

Photochemical alkylation of ketene dithioacetal S,S-dioxides. An example of captodative olefin functionalization

Radical alkylation of some ketene dithioacetal S,S-dioxides failed through the tin hydride promoted chain process but was successfully performed through stoichiometric photochemical initiation, either by electron transfer or hydrogen abstraction. In the first case, alkyl radicals were produced from tetralkylstannanes (t-Bu-, i-Pr-, n-Bu-SnR(3)) via radical cation fragmentation, while in the second case these were produced from alkanes (cyclohexane, adamantane) by benzophenone triplet. When bulky radicals (t-Bu, adamantyl) were involved, the addition occurred with complete diastereoselectivity.

New synthetic methods via radical cation fragmentation

Photoinduced single electron transfer followed by radical cation cleavage is a selective method for generating radicals and cations from unconventional substrates under unparalleled mild conditions. The versatility of the method and its synthetic significance are demonstrated through the discussion of selected examples involving alkyl radicals as the key species, such as nucleophilic aromatic substitution, addition to unsatured esters, nitriles and ketones, and reduction, as well as of an example of addition to cations.

Heptene radical cations: how well is the double bond localized?

The fragmentation of radical cations of multiply labeled 1-, 2-, and 3-heptenes is studied with different mass spectrometric techniques, allowing, together with published data, conclusions to be drawn about the time dependence of the fragmentations. No skeletal isomerization can be observed, but the position of the double bond seems to be shifted rather rapidly, making the ion in the time average rather symmetric. This can be explained either by an equilibrium of different positions (where the end positions have less weight) or some unknown intermediate structure(s). The important loss of neutral fragments from within the ions at longer time scales may speak in favor of the latter proposal. On the other hand, reactions such as allyl scission or McLafferty-type fragmentations take place before the double bond is displaced. The additional energy gained by shifting the double bond from a terminal position may explain the differences observed between 1- and 2 or 3-heptenes even at longer time scales. In particular, it remains unexplained that even after a few milliseconds the non-fragmented molecular ions in their ground state seem to retain some information about the initial localization of the double bond.

Radical cation reactions associated with the thiocarbonyl group

The radical cation reactions of different thiocarbonyl compounds were examined. Compounds 7, 8, 11, 14 and 18 underwent a radical cation fragmentation in a photoinduced electron transfer (PET) reaction, in the presence of tris(4-bromophenyl)aminium hexachloroantimonate. The examined thiocarbonyl compounds underwent a radical cation fragmentation in a photoinduced electron transfer (PET) reaction, in the presence of tris(4-bromophenyl)aminium hexachloroantimonate.

Side-chain oxidation of some phenyl-substituted alcohols in aqueous solution: Formation and fragmentation of radical-cations

The reactions of the hydroxyl radical (at low pH) and SO-4 have been employed to oxidize a number of phenyl-substituted alcohols in aqueous solution; ESR spectroscopy has been employed to study directly the radicals formed by fragmentation of first-formed radical-cations. Examples of deprotonation (to give benzyl radicals), Cα-Cβ bond scission (e.g. PhCH2 CH2 OH to PhCH2.) and longer-range fragmentation (e.g. PhCH2 CH2 CMe2 OH to PhCH2 CH2.) are described and discussed: evidence is obtained for intramolecular nucleophilic attack as a route for overall electron-transfer from side-chain to aromatic ring. Solvation of the proton and of oxygen-conjugated carbonium ions is thought, at least in part, to account for differences between fragmentations induced in the gaseous and aqueous phases.