Characteristic neutral loss of CH3CHO from Thr‐containing sodium‐associated peptides

A characteristic neutral loss of 44 Da is observed in the MS/MS spectra of Thr‐containing sodiated peptides. A combination of tandem mass spectrometry and quantum chemical calculations calculated at the B3LYP/6‐311G (d, p) level of ab initio theory is used to elucidate this fragmentation pathway. The high resolution mass spectrometry data indicate this neutral loss is acetaldehyde lost from the side chain of Thr rather than CO₂. The intensity of this neutral loss can be enhanced when Thr residue is far from the C‐terminus and when the C‐terminus is esterified as well. The mechanism of the acetaldehyde loss is proposed to adopt a McLafferty‐type rearrangement reaction, which involves a proton transfer from the hydroxyl of Thr side chain to its C‐terminal neighboring carbonyl oxygen inducing the cleavage of the C_a–C_β bond. This mechanism is further supported by examining the fragmentation of a [GT(tBu)G + Na]⁺ peptide derivative and by comparing the product ion spectra of [M + Na‐44]⁺ of [GTGA + Na]⁺ with [M + Na]⁺ of [GGGA + Na]⁺. A similar neutral loss of HCHO can also be detected in Ser‐containing peptides. Our computational results reveal that the most stable [GTG + Na]⁺ ion is present as a tridentate charge‐solvated structure and the dissociation leading to the 44 loss is dynamically and energetically favorable. Copyright © 2015 John Wiley & Sons, Ltd.     

Exploring the intrinsic polar [4 + 2+] cycloaddition reactivity of gaseous carbosulfonium and carboxonium ions

Gas‐phase reactions of model carbosulfonium ions (CH₃‐S⁺ = CH_2; CH₃CH₂‐S⁺ = CH₂ and Ph‐S⁺ = CH₂) and an O‐analogue carboxonium ion (CH₃‐O⁺ = CH₂) with acyclic (isoprene, 1,3‐butadiene, methyl vinyl ketone) and cyclic (1,3‐cyclohexadiene, thiophene, furan) conjugated dienes were systematically investigated by pentaquadrupole mass spectrometry. As corroborated by B3LYP/6‐311 G(d,p) calculations, the carbosulfonium ions first react at large extents with the dienes forming adducts via simple addition. The nascent adducts, depending on their stability and internal energy, react further via two competitive channels: (1) in reactions with acyclic dienes via cyclization that yields formally [4 + 2⁺] cycloadducts, or (2) in reactions with the cyclic dienes via dissociation by HSR loss that yields methylenation (net CH⁺ transfer) products. In great contrast to its S‐analogues, CH₃‐O⁺ = CH₂ (as well as C₂H₅‐O⁺ = CH₂ and Ph‐O⁺ = CH₂ in reactions with isoprene) forms little or no adduct and proton transfer is the dominant reaction channel. Isomerization to more acidic protonated aldehydes in the course of reaction seems to be the most plausible cause of the contrasting reactivity of carboxonium ions. The CH₂ = CH‐O⁺ = CH₂ ion forms an abundant [4 + 2⁺] cycloadduct with isoprene, but similar to the behavior of such α,β‐unsaturated carboxonium ions in solution, seems to occur across the C = C bond. Copyright © 2012 John Wiley & Sons, Ltd.     

Charge and substituent effects on the stability of porphyrin/G‐quadruplex adducts

The adduct ions of two tetramolecular G‐quadruplexes formed from the d(TGGGGT) and d(TTGGGGGT) single strands with a group of cationic porphyrins, with different charges and substituents, and one neutral porphyrin, were investigated by ESI‐MS and ESI‐MS/MS in the negative ion mode. Formation of [Q + nNH₄⁺+P^p+‐(z + n + p)H⁺]^z‐ adduct ions (where Q = quadruplex, n = number of quartets minus 1, P = porphyrin and p⁺ =0,1,2,3,4) indicates that the porphyrins are bound outside the quadruplexes providing an additional stabilization to those structures. The fragmentation pathways of the [Q + nNH₄⁺+P^p+‐(z + n + p)H⁺]^z‐ adduct ions depend on the number of positive charges (p⁺) of the porphyrins and on the overall complex charge (z^‐), but do not show a significant dependence on the type of the substituent groups in the porphyrins. Formation of the ‘unfilled’ ions [Q + P^p+‐(z + p)H⁺]^z‐ predominates for porphyrins with a higher number of positive charges. Strand separation with the formation of [T + P^p+‐(z‐2 + p)H⁺]^(z‐2)‐ and (SS‐2H⁺)^2‐ ions, where T = [d(TG₄T)]₃ and [d(T₂G₅T)]₃ and SS = d(TG₄T) and d(T₂G₅T) is only observed for the complexes with a higher overall negative charge. Porphyrin loss with the formation of [Q + nNH₄⁺‐(z + n)H⁺]^z‐ ions occurs predominantly for the neutral and monocharged porphyrins. The predominant formation of the ‘unfilled’ ions, [Q + P^p+‐(z + n)H⁺]^z‐, for porphyrins with a higher number of charges shows that these porphyrins can prevent strand separation and preserve, at least partially, the quadruplex structure. Copyright © 2012 John Wiley & Sons, Ltd.     

Gas‐phase reaction mechanism of Pd+ with CH3CHO: A density functional theoretical study

Details on the reactions of: (1) Pd⁺ + CH₃CHO → PdCO⁺ + CH₄ and (2) Pd⁺ + CH₃CHO → PdH + CH₃CO⁺ in the gas phase were investigated using density functional theory (B3LYP), in conjunction with the LANL2DZ+6‐311+G(d) basis set. Three encounter complexes were located on the potential energy surfaces and the calculations indicated that both the C? C and aldehyde C? H bond activation of acetaldehyde could lead to the dominant demethanation reaction. The charge transfer process for PdH abstraction was caused by an intramolecular PdH rearrangement of the newly found η¹‐aldehyde attached complex. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Mass Spectrometric Observation of Doubly Charged Alkaline‐Earth Argon Ions

Doubly charged diatomic ions MAr²⁺ where M=Mg, Ca, Sr or Ba have been observed by mass spectrometry with an inductively coupled plasma ion source. Abundance ratios are quite high, 0.1 % for MgAr²⁺, 0.4 % for CaAr²⁺, 0.2 % for SrAr²⁺ and 0.1 % for BaAr²⁺ relative to the corresponding doubly charged atomic ions M²⁺. It is assumed that these molecular ions are formed through reactions of the doubly charged metal ions with neutral argon atoms within the ion source. Bond dissociation energies (D₀) were calculated and agree well with previously published values. The abundance ratios MAr⁺/M⁺ and MAr²⁺/M²⁺ generally follow the predicted bond dissociation energies with the exception of MgAr²⁺. Mg²⁺ should form the strongest bond with Ar [D₀ (MgAr²⁺)=124 to 130 kJ mol^?1] but its relative abundance is similar to that of the weakest bound BaAr²⁺ (D₀=34 to 42 kJ mol^?1). The relative abundances of the various MAr²⁺ ions are higher than those expected from an argon plasma at T=6000 K, indicating that collisions during ion extraction reduce the abundance of the MAr²⁺ ions relative to the composition in the source. The corresponding singly charged MAr⁺ ions are also observed but occur at about three orders of magnitude lower intensity than MAr²⁺.     

Molecular view of charge exchange in ion–C60 collisions

We present a theoretical study of charge transfer in H⁺+C₆₀ and He²⁺+C₆₀ collisions using an extension of the molecular time‐dependent method of ion–atom collisions. Energy‐correlation diagrams have been evaluated for the corresponding (C₆₀–H)⁺ and (C₆₀–He)²⁺ quasi‐molecules. Single and double charge‐transfer cross sections in C₆₀+He²⁺ collisions are reported for the first time. The results show that double charge‐transfer cross sections are only one order of magnitude smaller than single charge‐transfer cross sections. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Attachment of Chloride Anion to Sugars: Mechanistic Investigation and Discovery of a New Dopant for Efficient Sugar Ionization/Detection in Mass Spectrometers

A new method for efficient ionization of sugars in the negative‐ion mode of electrospray mass spectrometry is presented. Instead of using strongly hydrophobic dopants such as dichloromethane or chloroform, efficient ionization of sugars has been achieved by using aqueous HCl solution for the first time. This methodology makes it possible to use hydrophilic dopants, which are more appropriate for chromatographic separation techniques with efficient sugar ionization and detection in mass spectrometry. The interaction between chloride anions and monosaccharides (glucose and galactose) was studied by DFT in the gas phase and by implementing the polarizable continuum model (PCM) for calculations in solution at the high B3LYP/6‐31+G(d,p)//B3LYP/6‐311+G(2d,p) level of theory. In all optimized geometries of identified [M+Cl]^? anions, a non‐covalent interaction exists. Differences were revealed between monodentate and bidentate complex anions, with the latter having noticeably higher binding energies. The calculated affinity of glucose and galactose toward the chloride anion in the gas phase and their chloride anion binding energies in solution are in excellent agreement with glucose and galactose [M+Cl]^? experimental intensity profiles that are represented as a function of the chloride ion concentration. Density functional calculations of gas‐phase affinities toward chloride anion were also performed for the studied disaccharides sucrose and gentiobiose. All calculations are in excellent agreement with the experimental data. An example is introduced wherein HCl was used to effectively ionize sugars and form chlorinated adduct anions to detect sugars and glycosylated metabolites (anthocyanins) in real biological systems (Vitis vinifera grape extracts and wines), whereas they would not have been easily detectable under standard infusion electrospray mass spectrometry conditions as deprotonated species.     

Fingerprinting stereoisomers by survivor-ion mass spectrometry

Survivor-ion mass spectrometry is used to distinguish stereoisomeric cis- and trans-4-methylcyclohexanol. The method involves producing ions by electron impact ionization and submitting them without mass selection to collisional neutralization and reionization, followed by selective monitoring of non-dissociating ions. The differences in the electron impact mass spectra of the stereoisomers, due to the different fragment ion elemental compositions and structures, are highlighted by collisional neutralization with Xe, NO and CH₃SSCH₃, followed by reionization with oxygen. The differences in the survivor-ion spectra are due to different neutralization efficiencies of the isobaric and isomeric ions produced by electron impact ionization, different stabilities of the intermediate neutral species, different reionization efficiencies and reionized ion stabilities. Neutralization-reionization spectra of the C₇H₁₂^+., C₆H₉⁺, C₃H₆O^+. and C₃H₅O⁺ ions from stereoisomeric 4-methylcyclohexanols are also reported.     

A DFT study of hydride transfers to the carbonyl oxygen of DDQ

Hydride‐transfer reactions between benzylic substrates and 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ) were investigated by DFT (density functional theory) calculations. The lowest unoccupied molecular orbital of DDQ has the largest extension on two carbonyl oxygens, which comes from two‐step mixing of antisymmetric orbitals of fragment π MOs. Transition‐state (TS) geometries and activation energies of reactions of four benzylic substrates R²? CH₂‐para‐C₆H₄? R¹ (R¹, R² = H and/or OCH₃) with DDQ were calculated. M06‐2X/6‐311(+*)G* was found to be a practical computational method, giving energies and geometries similar to those of M06‐2X/6‐311++G(3df,2pd) and wB97xD/6‐311++G(3df,2pd). For toluene (R¹ = R² = H), an initiation‐propagation model was suggested, and the calculated kinetic isotope effect k(H)/k(D) = 5.0 with the tunnel correction at the propagating step is in good agreement with the experimental value 5.2. A reaction of para‐MeO? C₆H₄? CH₂(OMe) + DDQ + (H₂O)₁₄ → para‐MeO? C₆H₄? C(?O)H + HOMe + DDQH₂ + (H₂O)₁₃ was investigated by M06‐2X/6‐311(+*)G*. Four elementary processes were found and the hydride transfer (TS1) is the rate‐determining step. The hydride transfer was promoted by association with the water cluster. The size of the water cluster, (H₂O)_n, at TS1 was examined. Three models of n = 14, 20, and 26 were found to give similar activation energies. Metal‐free neutral hydride transfers from activated benzylic substrates to DDQ were proposed to be ready processes both kinetically and thermodynamically. © 2015 Wiley Periodicals, Inc.     

Positive chemical ionization triple‐quadrupole mass spectrometry and ab initio computational studies of the multi‐pathway fragmentation of phthalates

We report the first positive chemical ionization (PCI) fragmentation mechanisms of phthalates using triple‐quadrupole mass spectrometry and ab initio computational studies using density functional theories (DFT). Methane PCI spectra showed abundant [M + H]⁺, together with [M + C₂H₅]⁺ and [M + C₃H₅]⁺. Fragmentation of [M + H]⁺, [M + C₂H₅]⁺ and [M + C₃H₅]⁺ involved characteristic ions at m/z 149, 177 and 189, assigned as protonated phthalic anhydride and an adduct of phthalic anhydride with C₂H₅⁺ and C₃H₅⁺, respectively. Fragmentation of these ions provided more structural information from the PCI spectra. A multi‐pathway fragmentation was proposed for these ions leading to the protonated phthalic anhydride. DFT methods were used to calculate relative free energies and to determine structures of intermediate ions for these pathways. The first step of the fragmentation of [M + C₂H₅]⁺ and [M + C₃H₅]⁺ is the elimination of [R? H] from an ester group. The second ester group undergoes either a McLafferty rearrangement route or a neutral loss elimination of ROH. DFT calculations (B3LYP, B3PW91 and BPW91) using 6‐311G(d,p) basis sets showed that McLafferty rearrangement of dibutyl, di(‐n‐octyl) and di(2‐ethyl‐n‐hexyl) phthalates is an energetically more favorable pathway than loss of an alcohol moiety. Prominent ions in these pathways were confirmed with deuterium labeled phthalates. Copyright © 2010 John Wiley & Sons, Ltd.     

Electron‐induced dissociation of doubly protonated betaine clusters: controlling fragmentation chemistry through electron energy

The [M₂₁+2H]²⁺ cluster of the zwitterion betaine, M = (CH₃)₃NCH₂CO₂, formed via electrospray ionisation (ESI), has been allowed to interact with electrons with energies ranging from >0 to 50 eV in a Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometer. The types of gas‐phase electron‐induced dissociation (EID) reactions observed are dependent on the energy of the electrons. In the low‐energy region up to 10 eV, electrons are mainly captured, forming the charge‐reduced species, {[M₂₁+2H]^{+ .} }*, in an excited state, which stabilises via the ejection of an H atom and one or more neutral betaines. In the higher energy region, above 12 eV, a Coulomb explosion of the multiply charged clusters is observed in highly asymmetric fission with singly charged fragments carrying away more than 70% of the parent mass. Neutral betaine evaporation is also observed in this energy region. In addition, a series of singly charged fragments appears which arise from C? X bond cleavage reactions, including decarboxylation and CH₃ group transfer. These latter reactions may arise from access of electronic excited states of the precursor ions. Copyright © 2009 John Wiley & Sons, Ltd.     

Substitution reactions of gaseous ions in a three‐dimensional quadrupole ion trap

Substitution reactions between gaseous ions and neutral substrate molecules are of ongoing high interest. To investigate these processes in a qualitative and quantitative manner, we have constructed a device, with which a defined amount of a volatile substrate can be mixed with a defined amount of helium gas and added into a three‐dimensional quadrupole ion trap. From the known inner volume of the device, the known ratio n_substrate:n_He of the mixture, and the determined absolute partial pressure of helium in the ion trap, we can derive the partial pressure of the substrate in the ion trap and, thus, convert the directly observable pseudo–first‐order rate constants of the substitution reactions into absolute bimolecular rate constants. We have tested the device by investigating a series of S_N2 reactions of Br ^? and CF₃CH₂O ^? anions as well as ligand exchange reactions of ligated Na⁺ cations. As the obtained results suggest, the described device makes it possible to determine the bimolecular rate constants of substitution reactions as well as other ion‐molecule reactions with satisfactory accuracy and reliability.     

HPLC‐DAD and HPLC‐ESI‐MS separation,determination and identification of the spin‐labeled diastereoisomers of podophyllotoxin

Spin‐labeled nitroxide derivatives of podophyllotoxin had better antitumor activity and less toxicity than that of the parent compounds. However, the 2‐H configurations of these spin‐labeled derivatives cannot be determined by nuclear magnetic resonance (NMR) methods. In the present paper, a high‐performance liquid chromatography‐diode array detection (HPLC‐DAD) and a high‐performance liquid chromatography‐electrospray ionization tandem mass spectrometry (HPLC‐ESI/MS/MS) method were developed and validated for the separation, identification of four pairs of diastereoisomers of spin‐labeled derivatives of podophyllotoxin at C‐2 position. In the HPLC‐ESI/MS spectra, each pair of diastereoisomers of the spin‐labeled derivatives in the mixture was directly confirmed and identified by [M+H]⁺ ions and ion ratios of relative abundance of [M‐ROH+H]⁺ (ion 397) to [M+H]⁺. When the [M‐ROH+H]⁺ ions (at m/z 397) were selected as the precursor ions to perform the MS/MS product ion scan. The product ions at m/z 313, 282, and 229 were the common diagnostic ions. The ion ratios of relative abundance of the [M‐ROH+H]⁺ (ion 397) to [M+H]⁺, [A+H]⁺ (ion 313) to [M‐ROH+H]⁺, [A+H‐OCH₃]⁺ (ion 282) to [M‐ROH+H]⁺ and [M‐ROH‐ArH+H]⁺ (ion 229) to [M‐ROH+H]⁺ of each pair of diastereoisomers of the derivatives specifically exhibited a stereochemical effect. Thus, by using identical chromatographic conditions, the combination of DAD and MS/MS data permitted the separation and identification of the four pairs of diastereoisomers of spin‐labeled derivatives of podophyllotoxin at C‐2 in the mixture.     

Estimation of peptide N–Cα bond cleavage efficiency during MALDI‐ISD using a cyclic peptide

Matrix‐assisted laser desorption/ionization in‐source decay (MALDI‐ISD) induces N–C_α bond cleavage via hydrogen transfer from the matrix to the peptide backbone, which produces a c′/z? fragment pair. Subsequently, the z? generates z′ and [z + matrix] fragments via further radical reactions because of the low stability of the z?. In the present study, we investigated MALDI‐ISD of a cyclic peptide. The N–C_α bond cleavage in the cyclic peptide by MALDI‐ISD produced the hydrogen‐abundant peptide radical [M + 2H]⁺? with a radical site on the α‐carbon atom, which then reacted with the matrix to give [M + 3H]⁺ and [M + H + matrix]⁺. For 1,5‐diaminonaphthalene (1,5‐DAN) adducts with z fragments, post‐source decay of [M + H + 1,5‐DAN]⁺ generated from the cyclic peptide showed predominant loss of an amino acid with 1,5‐DAN. Additionally, MALDI‐ISD with Fourier transform‐ion cyclotron resonance mass spectrometry allowed for the detection of both [M + 3H]⁺ and [M + H]⁺ with two ¹³C atoms. These results strongly suggested that [M + 3H]⁺ and [M + H + 1,5‐DAN]⁺ were formed by N–C_α bond cleavage with further radical reactions. As a consequence, the cleavage efficiency of the N–C_α bond during MALDI‐ISD could be estimated by the ratio of the intensity of [M + H]⁺ and [M + 3H]⁺ in the Fourier transform‐ion cyclotron resonance spectrum. Because the reduction efficiency of a matrix for the cyclic peptide cyclo(Arg‐Gly‐Asp‐D‐Phe‐Val) was correlated to its tendency to cleave the N–C_α bond in linear peptides, the present method could allow the evaluation of the efficiency of N–C_α bond cleavage for MALDI matrix development. Copyright © 2016 John Wiley & Sons, Ltd.     

Mechanism of atmospheric pressure chemical ionization of morphine,codeine, and thebaine in corona discharge–ion mobility spectrometry: Protonation,ammonium attachment,and carbocation formation

Atmospheric pressure chemical ionizations (APCIs) of morphine, codeine, and thebaine were studied in a corona discharge ion source using ion mobility spectrometry (IMS) at temperature range of 100°C–200°C. Density functional theory (DFT) at the B3LYP/6‐311++G(d,p) and M062X/6‐311++G(d,p) levels of theory were used to interpret the experimental data. It was found that in the presence of H₃O⁺ as reactant ion (RI), ionization of morphine and codeine proceeds via both the protonation and carbocation formation, whereas thebaine participates only in protonation. Carbocation formation (fragmentation) was diminished with decrease in the temperature. At lower temperatures, proton‐bound dimers of the compounds were also formed. Ammonia was used as a dopant to produce NH₄⁺ as an alternative RI. In the presence of NH₄⁺, proton transfer from ammonium ion to morphine, codeine, and thebaine was the dominant mechanism of ionization. However, small amount of ammonium attachment was also observed. The theoretical calculations showed that nitrogen atom of the molecules is the most favorable proton acceptor site while the oxygen atoms participate in ammonium attachment. Furthermore, formation of the carbocations is because of the water elimination from the protonated forms of morphine and codeine.     

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.     

A quantum-chemical study of the interaction of the HeT<Superscript>+</Superscript> ion with cycloalkanes and their oxidation products

The RHF/6-311G*(3d), RHF/6-311++G**(3df, 3p) and MP2/6-311G*(3d) ab initio methods were used to calculate the equilibrium structure of the products of the ion-molecular reaction of tritium ion transfer from HeT⁺ to cyclopentane and cyclohexane. Similar reactions with cyclopentanol and cyclopentanone were calculated at the RHF/6-311G*(3d) level. The interaction of HeT⁺ with cycloalkanes was found to produce onium ions with cyclic structures, in which the tritium atom held neighboring methylene groups together. With the alcohol and ketone, not only cyclic but also stabler linear cations could be formed, and the addition of the tritium ion directly to the oxygen atom was possible. The suggestion was made that the chain of tritium ion transfer reactions was the mechanism of the accumulation of tritium by hydrocarbon oxidation products when T₂ was dissolved in mineral oils.     

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.     

Ion-molecule chemistry of C+2: Evidence for the production of excited state dicarbon cation

Ion-molecule reactions of C⁺₂ with several neutral partners have been studied using Fourier transform mass spectrometry. The reactant ion was formed by electron impact of various neutral precursors and the internal energy content of the ion estimated using charge transfer/energy-bracketing reactions. These reactions indicate the production of a long-lived excited state ion when C⁺₂ is formed from C₂N₂ and the production of a mixture of states when formed from small hydrocarbon molecules. The observed reactions and energetics are consistent with the calculated electronic structure of this ion.