Ion-molecule reactions observed for nitroaromatic compounds in negative ion fast atom bombardment mass spectrometry

Analyses of a series of nitroaromatic compounds using fast atom bombardment (FAB) mass spectrometry are discussed. An interesting ion-molecule reaction leading to [M + O ? H]^? ions is observed in the negative ion FAB spectra. Evidence from linked-scan and collision-induced dissociation spectra proved that [M + O ? H]^? ions are produced by the following reaction: M + NO₂^? → [M + NO₂]^? → [M + O ? H]^?. These experiments also showed that M^?˙ ions are produced in part by the exchange of an electron between M and NO₂^? species. All samples showed M^?˙, [M ? H]^? or both ions in their negative ion FAB spectra. Not all analytes studied showed either [M + H]⁺ and/or M⁺˙ in the positive ion FAB spectra. No M⁺˙ ions were observed for ions having ionization energies above ～9 eV.     

Complexes of Cu(II) and Co(II) with NN′-bis-8-quinolylethylenediamine

This article describes some complexes of Cu(II) and Co(II with NN′-bis-8-quinolylethylenediamine ligand (nn′). All the compounds are of stoichiometry [MX₂(nn′)] (M = Cu or Co; X = Cl^?, Br^?, I^?, NO^?₃ or SCN^?). The electronic spectra are consistent with distorted octahedral geometry around the ions, indicating the four coordination of the nn′ ligand. Magnetic susceptibility measurements down to 100 K show antiferromagnetic interactions in all the Cu(II) compounds demonstrating the existence of the ionic and bridging X group. Infrared spectra show the presence of ionic and bridging nitrate in the [M(NO₃)₂(nn′)] (M = Co or Cu) compounds and ionic and bridging NCS group in the [Cu(NCS)₂(nn′)] compound.     

Ion/molecule reactions of ammonia and methylamine with chloro- and nitrobenzene. A comparison of chemical ionization and ion cyclotron resonance experiments

It is shown by ion cyclotron resonance measurements that ion/molecule reactions, leading to substitution or reduction product ions from chloro- and nitrobenzene with the title amines, are those between the molecular ions [RNH₂]⁺ or [C₆H₅X]⁺˙ and their respective counterparts C₆H₅X or RNH₂. The protonated reagent gas ions [RNH₃]⁺ are not involved in these reactions. In the case of nitrobenzene, adduct ions [C₆H₅NO₂·RNH₃]⁺ do not decompose within the time scale of the measurements. The results obtained are compared with those found under chemical ionization conditions.     

Electron impact,chemical ionization and negative chemical ionization mass spectra,and mass-analysed ion kinetic energy spectrometry–collision-induced dissociation fragmentation pathways of some deuterated 2,4,6-trinitrotoluene (TNT) derivatives

Mass-analysed ion kinetic energy spectrometry (MIKES) with collision-induced dissociation (CID) has been used to study the fragmentation processes of a series of deuterated 2,4,6-trinitrotoluene (TNT) and deuterated 2,4,6-trinitrobenzylchloride (TNTCI) derivatives. Typical fragment ions observed in both groups were due to loss of OR′ (R′ = H or D) and NO. In TNT, additional fragment ibns are due to the loss of R₂′O and 3NO₂, whilst in TNTCI fragment ions are formed by the loss of OCI and R₂′OCI. The TNTCI derivatives did not produce molecular ions. In chemical ionization (Cl) of both groups. MH⁺ ions were observed, with [M – OR′]⁺ fragments in TNT and [M – OCI]⁺ fragments in TNTCI. In negative chemical ionization (NCI) TNT derivatives produced M^?′, [M–R′]^?, [M–OR′]^? and [M–NO]^? ions, while TNTCI derivatives produced [M–R]^?, [M–Cl]^? and [M – NO₂]^? fragment ions without a molecular ion.     

Functional group-selective ion-molecule reactions of ethylene glycol and its monomethyl and dimethyl ethers

The selective methylation and methylene substitution reactions of dimethyl ether ions with ethylene glycol, ethylene glycol monomethyl ether, and ethylene glycol dimethyl ether were investigated in a quadrupole ion trap mass spectrometer. Whereas the reactions of ethylene glycol and ethylene glycol monomethyl ether with the methoxymethylene cation 45⁺ gave only [M + 13]⁺ product ions, the reaction of ethylene glycol dimethyl ether with the same reagent ion yielded exclusively [M + 15]⁺ ions. The relative rates of formation of these products and those from competing reactions were examined and rationalized on the basis of structural and electronic considerations. The heats of formation for various relevant species were estimated by computational methods and showed that the reactions leading to the [M + 13]⁺ ions were more energetically favorable than those leading to the [M + 15]⁺ products for cases in which both reactions are possible. Finally, the collision-induced dissociation behavior of the [M + H]⁺, [M + 13]⁺, and [M + 15]⁺ ions indicated that the and [M + H]⁺ rons dissociated by analogous pathways and were thus structurally similar, whereas the [M + 13]⁺ ions possessed distinctly different structural characteristics.     

Phase Evolution of Cu?S System in Ethylene Glycol Solution: the Effect of Anion and PVP on the Transformation of Thiourea

The transformation mechanisms of thiourea in ethylene glycol solution was systematically investigated in this report, which shows the transformation process is influenced by the anion (NO^3?, Cl^?, Br^?) and polyvinylpyrrolidone (PVP). Thiourea (tu) isomerizes into ammonium thiocyanate when NO^3? is present, regardless of the existence of PVP. For Cl^?, thiourea coordinates with copper anion to form [Cu(tu)]Cl·1/2H₂O complex whether PVP is present. When it comes to Br^?, thiourea hydrolyzes in the cooperation of PVP or coordinates with copper anion to form [Cu(tu)Br]·1/2H₂O complex without PVP. The different transformation routes will lead to different phase evolution of the Cu? S system. This work may provide a new understanding of the transformation of thiourea in ethylene glycol solution. The optical properties of the as‐prepared copper sulfides exhibit signi?cant stoichiometry‐dependent features which may have potential applications in semiconductor photovoltaic devices.     

Study of gas‐phase reactions of NO2+ with aromatic compounds using proton transfer reaction time‐of‐flight mass spectrometry

The study of ion chemistry involving the NO₂⁺ is currently the focus of considerable fundamental interest and is relevant in diverse fields ranging from mechanistic organic chemistry to atmospheric chemistry. A very intense source of NO₂⁺ was generated by injecting the products from the dielectric barrier discharge of a nitrogen and oxygen mixture upstream into the drift tube of a proton transfer reaction time‐of‐flight mass spectrometry (PTR‐TOF‐MS) apparatus with H₃O⁺ as the reagent ion. The NO₂⁺ intensity is controllable and related to the dielectric barrier discharge operation conditions and ratio of oxygen to nitrogen. The purity of NO₂⁺ can reach more than 99% after optimization. Using NO₂⁺ as the chemical reagent ion, the gas‐phase reactions of NO₂⁺ with 11 aromatic compounds were studied by PTR‐TOF‐MS. The reaction rate coefficients for these reactions were measured, and the product ions and their formation mechanisms were analyzed. All the samples reacted with NO₂⁺ rapidly with reaction rate coefficients being close to the corresponding capture ones. In addition to electron transfer producing [M]⁺, oxygen ion transfer forming [MO]⁺, and 3‐body association forming [M·NO₂]⁺, a new product ion [M−C]⁺ was also formed owing to the loss of C═O from [MO]⁺.This work not only developed a new chemical reagent ion NO₂⁺ based on PTR‐MS but also provided significant interesting fundamental data on reactions involving aromatic compounds, which will probably broaden the applications of PTR‐MS to measure these compounds in the atmosphere in real time.     

Atmospheric pressure chemical ionization of explosives induced by soft X‐radiation in ion mobility spectrometry: mass spectrometric investigation of the ionization reactions of drift gasses,dopants and alkyl nitrates

A promising replacement for the radioactive sources commonly encountered in ion mobility spectrometers is a miniaturized, energy‐efficient photoionization source that produce the reactant ions via soft X‐radiation (2.8 keV). In order to successfully apply the photoionization source, it is imperative to know the spectrum of reactant ions and the subsequent ionization reactions leading to the detection of analytes. To that end, an ionization chamber based on the photoionization source that reproduces the ionization processes in the ion mobility spectrometer and facilitates efficient transfer of the product ions into a mass spectrometer was developed. Photoionization of pure gasses and gas mixtures containing air, N₂, CO₂ and N₂O and the dopant CH₂Cl₂ is discussed. The main product ions of photoionization are identified and compared with the spectrum of reactant ions formed by radioactive and corona discharge sources on the basis of literature data. The results suggest that photoionization by soft X‐radiation in the negative mode is more selective than the other sources. In air, adduct ions of O₂^– with H₂O and CO₂ were exclusively detected. Traces of CO₂ impact the formation of adduct ions of O₂^– and Cl^– (upon addition of dopant) and are capable of suppressing them almost completely at high CO₂ concentrations. Additionally, the ionization products of four alkyl nitrates (ethylene glycol dinitrate, nitroglycerin, erythritol tetranitrate and pentaerythritol tetranitrate) formed by atmospheric pressure chemical ionization induced by X‐ray photoionization in different gasses (air, N₂ and N₂O) and dopants (CH₂Cl₂, C₂H₅Br and CH₃I) are investigated. The experimental studies are complemented by density functional theory calculations of the most important adduct ions of the alkyl nitrates (M) used for their spectrometric identification. In addition to the adduct ions [M + NO₃]^– and [M + Cl]^–, adduct ions such as [M + N₂O₂]^–, [M + Br]^– and [M + I]^– were detected, and their gas‐phase structures and energetics are investigated by density functional theory calculations. Copyright © 2016 John Wiley & Sons, Ltd.     

Electron-impact studies. XCIX—negative ion mass spectrometry of the o-NO2?C6H4?X?C6H5 systems. Aryl hydrogen scrambling and proximity effects

We report the first example of aryl hydrogen scrambling occurring in a molecular anion prior to or accompanying fragmentation, i.e. for the reaction [M]^?· → [M ? H₂NO₂.]^? from o-NO₂? C₆H₄? X? C₆H₅ (X = O or S). Proximity effects occur in these spectra when X = CO, NH, O, or S, and certain of these have been substantiated by ²H and ¹⁸O labelling.     

Pseudoelementverbindungen. XI. [1] Untersuchungen zum Komplexbildungsverhalten von Cyanamidonitrat [NO2NCN]−

Pseudoelement Compounds. XI. [1] Investigations on the Coordination Behaviour of Cyanamidonitrate [NO₂NCN]^? With the ionic, potentially ambidentate ligand cyanamidonitrate complexes of the types [MX(PPh₃)₃], [MX(PPh₃)₂]₂ (M?Cu^I, Ag^I) and trans-[Pt(H)X(PPh₃)₂] (X^??[NO₂NCN]^?) are introduced. The new compounds are characterized by ¹H NMR, ³¹P NMR, and IR spectroscopy. The crystal structures of [Cu(NO₂NCN)(PPh₃)₂]₂ and [Ag(NO₂NCN)(PPh₃)₂]₂ are reported. In the complexes [MX(PPh₃)₃] and trans-[Pt(H)X(PPh₃)₂] cyanamidonitrate is unidentately coordinated through the nitrile group end-on. In the dimeric complexes [MX(PPh₃)₂]₂ the anion acts bidentately as a bridging ligand. Surprisingly, both coordinative bonds are formed through nitrogen atoms of the NCN group.     

C3i‐Symmetric Octanuclear Cadmium Cages: Double‐Anion‐Templated Synthesis,Formation Mechanism,and Properties

A series of C_3i‐symmetric bicapped trigonal antiprismatic Cd₈ cages [2X@Cd₈L₆(H₂O)₆] ? n Y ? solvents (X=Cl^?, Y=NO₃^?, n=2: MOCC‐4 ; X=Br^?, Y=NO₃^?, n=2: MOCC‐5 ; X=NO₃^?, Y=NO₃^?, n=2: MOCC‐6 ; X=NO₃^?, Y=BF₄^?, n=2: MOCC‐7 ; X=NO₃^?, Y=ClO₄^?, n=2: MOCC‐8 ; X=CO₃^2?, n=0: MOCC‐9 ), doubly anion templated by different anions, were solvothermally synthesized by means of a flexible ligand. Interestingly, the CO₃^2? template for MOCC‐9 was generated in situ by two‐step decomposition of DMF solvent. For other MOCCs, spherical or trigonal monovalent anions could also play the role of template in their formation. The template abilities of these anions in the formation of the cages were experimentally studied and are discussed for the first time. Anion exchange of MOCC‐8 was carried out and showed anion‐size selectivity. All of the cage‐like compounds emit strong luminescence at room temperature.     

Sputtered ammonium dinitramide: Tandem mass spectrometry of a new ionic nitramine

The recently synthesized ammonium dinitramide (ADN) is an ionic compound containing the ammonium ion and a new oxide of nitrogen, the dinitramide anion (O₂N? N? NO₂^?). ADN has been investigated using high-energy xenon atoms to sputter ions directly from the surface of the neat crystalline solid. Tandem mass spectrometric techniques were used to study dissociation pathways and products of the sputtered ions. Among the sputtered ionic products were NH₄⁺, NO⁺, NO₂^?, N₂O₂^?, N₂O, N₃O₄^? and an unexpected high abundance of NO₃^?. Tandem mass spectra of the dinitramide anion reveal the uncommon situation where a product ion (NO₃^?) is formed in high relative abundance from metastable parent ions but is formed in very low relative abundance from collisionally activated parent ions. It is proposed that the nitrate anion is formed in the gas phase by a rate-determining isomerization of the dinitramide anion that proceeds through a four-centered transition state. The formation of the strong gas-phase acid, dinitraminic acid (HN₃O₄), the conjugate acid of the dinitramide anion, was observed to occur by dissociation of protonated ADN and by dissociation of ADN aggregate ions with the general formula [NH₄(N(NO₂)₂)_n] NH₄⁺, where n = 1–30.     

Mass spectral fragmentation pathways in RDX and HMX. A mass analyzed ion kinetic energy spectrometric/collisional induced dissociation study

A collisional induced dissociation study of 1,3,5-trinitro-1,3,5 triazacyclohexane (RDX) and 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX) was carried out using mass analyzed kinetic energy spectrometry. High resolution mass spectra and mass analyzed ion kinetic energy/collisional induced dissociation spectra of RDX and HMX were recorded in the electron impact, chemical ionization and negative ion chemical ionization modes. Fragmentation pathways of the compounds investigated were determined in all three modes of ionization. It was found that a major part of the fragment ions in RDX and HMX originate from formation of the aduct ions [M+NO]⁺ and [M+NO₂]⁺ in electron impact and chemical ionization, and from [M+NO]^? and [M+NO₂]^? in negative chemical ionization, followed by dissociation.     

Collision-Induced Dissociation Analysis of Negative Atmospheric Ion Adducts in Atmospheric Pressure Corona Discharge Ionization Mass Spectrometry

Collision-induced dissociation (CID) experiments were performed on atmospheric ion adducts [M + R]^– formed between various types of organic compounds M and atmospheric negative ions R^- [such as O₂ ^–, HCO₃ ^–, COO^–(COOH), NO₂ ^–, NO₃ ^–, and NO₃ ^–(HNO₃)] in negative-ion mode atmospheric pressure corona discharge ionization (APCDI) mass spectrometry. All of the [M + R]^– adducts were fragmented to form deprotonated analytes [M – H]^– and/or atmospheric ions R^–, whose intensities in the CID spectra were dependent on the proton affinities of the [M – H]^– and R^– fragments. Precursor ions [M + R]^– for which R^- have higher proton affinities than [M – H]^– formed [M – H]^– as the dominant product. Furthermore, the CID of the adducts with HCO₃ ^– and NO₃ ^-(HNO₃) led to other product ions such as [M + HO]^– and NO₃ ^–, respectively. The fragmentation behavior of [M + R]^– for each R^– observed was independent of analyte type (e.g., whether the analyte was aliphatic or aromatic, or possessed certain functional groups).      

The Structural Systematics of Protonation of Some Important Nitrogen‐base Ligands. I Some Univalent Anion Salts of Doubly Protonated 2, 2′:6′, 2″‐Terpyridyl

A number of salts of 2,2′:6′,2″ ‐terpyridyl (‘tpy’) with univalent anions (halides : X = Cl, Br, I; oxyanions of increasing basicity: ClO₄, NO₃, ‘tfa’ = trifluoroacetate, ‘tca’ = trichloroacetate), variously solvated, have been structurally characterized by single crystal X‐ray studies. In all cases the tpy moieties are found to be doubly protonated [tpyH₂]²⁺, the hydrogen atoms being associated with the nitrogen atoms of the peripheral rings, these together with the central nitrogen atom being directed towards a common focus, in most cases ‘chelating’ one of the counter‐ion components in diverse ways. Thus the chloride and bromide compounds are isomorphous [(tpyH₂)X]⁺X⁻·H₂O arrays; a second dihydrate phase is also described for the chloride, the two forms having the unchelated anion and water molecules engaged in hydrogen‐bonded networks essentially independent of [(tpyH₂)X]⁺. The iodide is anhydrous, and of a different structural type, the anions, presumably too large for chelation, lying out of plane to either side, and linking different cations into a one‐dimensional polymer; in the perchlorate, the unsolvated aggregate is now discrete [(tpyH₂)X₂], a pair of perchlorate ions disposed to either side of the tpy plane, lying each with one oxygen atom interacting with both of the two protonating hydrogen atoms. In the anhydrous X = NO₃, tfa, tca arrays, the lattices are solvated by the parent acids; one oxygen atom of each anion is chelated by the [tpyH₂]²⁺ as in the chlorides, the other anion, with the acid, forming an independent ‘acid salt’ counterion [XHX]⁻ in each case, retaining the additional protonic hydrogen rather than further protonating the central ring, all being of the form [(tpyH₂)X]X·HX = [(tpyH₂)X][X(HX)].     

Direct analysis in real time mass spectrometry (DART‐MS) of highly non‐polar low molecular weight polyisobutylenes

Low molecular weight polyisobutylenes (PIB) with chlorine, olefin and succinic acid end‐groups were studied using direct analysis in real time mass spectrometry (DART‐MS). To facilitate the adduct ion formation under DART conditions, NH₄Cl as an auxiliary reagent was deposited onto the PIB surface. It was found that chlorinated adduct ions of olefin and chlorine telechelic PIBs, i.e. [M + Cl]^? up to m/z 1100, and the deprotonated polyisobutylene succinic acid [M? H]^? were formed as observed in the negative ion mode. In the positive ion mode formation of [M + NH₄]⁺, adduct ions were detected. In the tandem mass (MS/MS) spectra of [M + Cl]^?, product ions were absent, suggesting a simple dissociation of the precursor [M + Cl]^? into a Cl^? ion and a neutral M without fragmentation of the PIB backbones. However, structurally important product ions were produced from the corresponding [M + NH₄]⁺ ions, allowing us to obtain valuable information on the arm‐length distributions of the PIBs containing aromatic initiator moiety. In addition, a model was developed to interpret the oligomer distributions and the number average molecular weights observed in DART‐MS for PIBs and other polymers of low molecular weight. Copyright © 2015 John Wiley & Sons, Ltd.     

Towards a Molecular Understanding of Cation–Anion Interactions—Probing the Electronic Structure of Imidazolium Ionic Liquids by NMR Spectroscopy,X‐ray Photoelectron Spectroscopy and Theoretical Calculations

Ten [C₈C₁Im]⁺ (1‐methyl‐3‐octylimidazolium)‐based ionic liquids with anions Cl^?, Br^?, I^?, [NO₃]^?, [BF₄]^?, [TfO]^?, [PF₆]^?, [Tf₂N]^?, [Pf₂N]^?, and [FAP]^? (TfO=trifluoromethylsulfonate, Tf₂N=bis(trifluoromethylsulfonyl)imide, Pf₂N=bis(pentafluoroethylsulfonyl)imide, FAP=tris(pentafluoroethyl)trifluorophosphate) and two [C₈C₁C₁Im]⁺ (1,2‐dimethyl‐3‐octylimidazolium)‐based ionic liquids with anions Br^? and [Tf₂N]^? were investigated by using X‐ray photoelectron spectroscopy (XPS), NMR spectroscopy and theoretical calculations. While ¹H NMR spectroscopy is found to probe very specifically the strongest hydrogen‐bond interaction between the hydrogen attached to the C² position and the anion, a comparative XPS study provides first direct experimental evidence for cation–anion charge‐transfer phenomena in ionic liquids as a function of the ionic liquid’s anion. These charge‐transfer effects are found to be surprisingly similar for [C₈C₁Im]⁺ and [C₈C₁C₁Im]⁺ salts of the same anion, which in combination with theoretical calculations leads to the conclusion that hydrogen bonding and charge transfer occur independently from each other, but are both more pronounced for small and more strongly coordinating anions, and are greatly reduced in the case of large and weakly coordinating anions.     

Four‐Coordinate Copper Halonitrosyl {CuNO}10 Complexes

While copper nitrosyl complexes are implicated in numerous biological systems, isolable examples remain limited. In this report, we show that [Cl₃CuNO]^?, with a {CuNO}¹⁰ electron configuration, can be generated by nitrite reduction at a copper(I) dichloride anion or by nitric oxide addition to a copper(II) trichloride precursor. The bromide analogue, [Br₃CuNO]^? was synthesized analogously, and both copper halonitrosyl complexes were characterized by X‐ray diffraction and a variety of spectroscopic methods. Experimental data and multireference (CASSCF/NEVPT2) calculations provide strong evidence for a Cu^II–NO^. ground state. Both [Cl₃CuNO]^? and [Br₃CuNO]^? release and recapture NO^. reversibly, and exhibit nitrosative reactivities toward a wide range of biological nucleophiles, such as amines, alcohols, and thiols.     

Mass spectra of sulfinamide derivatives and identification of their thermal decomposition products

The mass spectra of 30 sulfinamide derivatives (RSONHR', R' alkyl or p-XC₆H₄) are reported. Most of the spectra had peaks attributable to thermal decomposition products. For some compounds these were identified by pyrolysis under similar conditions to be: RSO₂NHR', RSO₂SR, RSSR and NH₂R' (in all kinds of sulfinyl amides); RSNHR' (in the case of arylsulfinyl arylamides); RSO₂C₆H₄NH₂, RSOC₆H₄NH₂ and RSC₆H₄NH₂ (in the case of arylsulfinyl arylamides of the type of X = H) The mass spectra of the three thermally stable compounds showed that there are several kinds of common fragment ions. The mass spectra of the thermally labile compounds had two groups of ions; (i) characteristic fragment ions of the intact molecules and (ii) the molecular ions of the thermal decomposition products. It was concluded that the sulfinamides give the following ions after electron impact: [M]⁺, [M ? R]⁺, [M ? R + H]⁺, [M ? SO]⁺, [RS]⁺, [NHR']⁺, [NHR' + H]⁺, [RSO]⁺, [RSO + H]⁺, [R]⁺, [R + H]⁺, [R']⁺ and [M ? OH]⁺, and that the thermal decomposition products give the following ions: [RSO₂SR]⁺, [RSSR]⁺, [M ? O]⁺, [M + O]⁺ and [RSOC₆H₄NH₂]⁺.     

Mobile Proton Triggered Radical Fragmentation of Nitroarginine Containing Peptides

Protonated nitroarginine, [R^NO2 + H]⁺, which contains the nitroguanidine ‘explosophore,’ undergoes homolytic N – N nitro-imine bond cleavage to expel NO₂ ^? and form a radical cation of arginine in high yield (100 % relative abundance) upon low-energy collision-induced dissociation (CID). Other ionization states of nitroarginine, including [R^NO2 - H]^–, and a fixed-charge derivative of nitroarginine do not expel NO₂ ^? (<1 %), but instead dissociate via heterolytic bond cleavage with abundant losses of small molecules (N₂O and H₂N₂O₂) from the nitroguanidine group. The effects of proton mobility on the CID reactions of nitroarginine containing peptides was investigated for peptide derivatives of leucine enkephalin, including XYGGFLR^NO2, X = D, G, K, and R, by examining the different protonation states: [M – H]^–; [M + H]⁺; and [M + 2H]²⁺. For [M + H]⁺ containing the less basic N-terminal residues (X = D, G) and all [M + 2H]²⁺, mobile proton fragmentation reactions that result in peptide sequence ions dominate. In contrast, for peptides containing the basic N-terminal residues (R and K), the CID spectra of both the [M – H]^– and [M + H]⁺ are dominated by the losses of small even-electron neutrals from the nitroarginine side-chain. The fraction of nitroguanidine directed fragmentation of the nitroarginine side chain that results in bond homolysis to form [XYGGFLR]^+? by expulsion of NO₂ ^? increases by more than 10 times as the protonation state changes from [M – H]^– (<10 %) to [M + 2H]²⁺ (ca. 90 %) and by about four times as the acidity of the [M + H]⁺ N-terminal residue increases from R (19.0 %) to D (76.5 %). These results indicate that protonated peptides containing nitroarginine can undergo non-canonical mobile proton triggered radical fragmentation.