Neutralization Reionization Research Articles

Ethylene bromonium and 1-bromoethyl cations and their neutral and anionic counterparts: a tandem mass spectrometry study of dissociations and gas phase redox reactions

The unimolecular chemistry of ethylene bromonium cation (cyclo-CH2CH2Br+, 1+) and 1-bromoethyl cation (CH3CH+Br, 2+) has been probed by metastable ion (MI) characteristics, collisionally activated dissociation (CAD) and neutral fragment reionization (NfR). These isomers undergo many common decompositions but can, nevertheless, be distinguished based on the structurally indicative >˙>CH3vs. CH2 losses. Neutralization–reionization (NR) experiments have further shown that the gas phase reduction of 1+ and 2+ leads to 2-bromoethyl (>˙>CH2CH2Br, 3) and 1-bromoethyl (CH3ĊHBr, 2) radicals, respectively, both of which are stable species. However, from the incipient C2H4Br– anions emerging upon charge reversal of 1+ and 2+, only CH3CH–Br (2–) is found to be a bound anion.

Mass spectrometric approaches to the reactivity of transient neutrals

During the past few years, Neutralisation–Reionisation Mass Spectrometry (NRMS) has developed from a method for the generation and structural characterisation of elusive and highly reactive neutral molecules to a useful tool for probing their chemical reactivity. Three major principles can be distinguished: (i) peak shape analysis, (ii) activation of the neutrals by collisions or light, and (iii) variation of the neutrals’ lifetimes. Several methodological approaches are discussed in conjunction with illustrating examples for the chemical reactivity of transient neutrals.

Novel β-Distonic Radical Cations [CnH2n+2S]•+ (n = 2, 3) Formed upon Decarbonylation of Ionized S-Alkyl Thioformates: A Mass Spectrometric and ab Initio Study

Decarbonylation is commonly observed in the 70 eV electron impact mass spectra of a series of S-alkyl thioformates, H−C(O)SR 1−7. Using a combination of collisional activation (CA) and neutralization−reionization (NR) mass spectra, it is shown that the spectra of the so-formed [M−CO]•+ ions differ from the isomeric alkane thiol ions (RSH•+), if R > CH3. These fragment ions are therefore distonic radical cations, and given the fact that the α-distonic •CH2−S+H2 ions are not present under our experimental conditions, it is proposed that they actually are β-distonic ions. Strong evidence for the distonic nature of the fragment ions derived from 2−7•+ has been obtained owing to the use of a new hybrid tandem mass spectrometer presenting a sector−quadrupole−sector configuration. In the case of ethyl derivatives (R = C2H5), both the distonic •CH2CH2S+H2 (2b) and conventional CH3CH2SH•+ (2a) ions are observed. The nonclassical structure 2b reacts indeed with nitric oxide (NO•) giving the production of H2S+−NO io...

A Study of the Gas‐Phase Reactivity of Neutral Alkoxy Radicals by Mass Spectrometry: α‐Cleavages and Barton‐type Hydrogen Migrations

AbstractThe reactivity of neutral alkoxy radicals in the absence of any interfering intermolecular interactions is investigated by means of the recently introduced method of neutral and ion decomposition difference (NIDD) spectra. These are obtained from quantitative analysis of the corresponding neutralization–reionization (NR) and charge reversal (CR) mass spectra. The following trends emerge: alkoxy radicals with short (C1C3) or branched alkyl chains give rise to α‐cleavage products, whereas longer‐chained alkoxy radicals undergo 1,5‐hydrogen migrations from carbon to oxygen, that is, Barton‐type chemistry. This facile rearrangement has been studied in detail for n‐pentoxy radicals by isotopic labeling experiments and computation at the Becke 3 LYP/6‐31 G* level of theory. Further, the NIDD spectra of 3‐methylpentoxy radicals permit for the first time the identification of the diastereoselectivity of the gas‐phase hydrogen migrations. The results from the NIDD method are compared to those from earlier studies in the condensed phase. This new mass spectrometric approach is suggested as a tool for the examination of intramolecular reactions of free alkoxy radicals which can usefully complement theoretical studies.

Thionitrosyl cyanide (NCNS)

Thionitrosyl cyanide and its radical-cation have been generated for the first time by dissociative ionization of 1,2,5-thiadiazolo [3,4-c](1,2,5)-thiadiazole and characterized by collisional activation (CA) and neutralization-reionization (NR) mass spectrometry. Both neutral and ionized forms are stable species and do not undergo unimolecular rearrangement under the experiment conditions. The NCNS molecule is further characterized by ab initio molecular orbital calculations using various levels of theory up to QCISD(T)/6-311++G(3df,2p) based on MP2/6-311 ++G(d,p) geometries. The stability ordering of the four possible [C, N 2, S] isomers is (kcal mol −1): NCNS 1 (0) > NCSN 2 (14) > CNSN 4 (25) > CNNS 3 (34) > CN + NS (73). The classical energy barrier for the 1,2-migration of the cyano and thionitrosyl groups connecting 1 with 2 and 1 with 3 amount to 80 and 83 kcal mol −1, respectively, values being larger than the dissociation energy. The geometry and rotational constants of NCNS were predicted using calculations and empirical corrections based on available data on similar molecules; A = 51.986 ± 0.7 GHz, B = 3.521 ± 0.1 GHz, C = 3.298 ± 0.1 GHz. Vibrational wavenumbers of both NCNS and NCSN isomers were computed using QCISD/6-311G(d) wavefunctions. The protonation of thionitrosyl cyanide occurs preferentially at the cyano-nitrogen with a proton affinity PA (NCNS) = 178 ± 3 kcal mol −1. Several molecular properties of NCNS have also been computed. Mass spectrometric experiments confirm that the product of the reaction NSCl + AgCN → AgCl + [ C, N 2, S] is the isomeric thiazyl cyanide (NCSN).

Ion-molecule reaction of pyridine with CS3 radical cations: experimental evidence for the production of pyridineN-thioxide distonic ions

Chemical ionization of pyridine using carbon disulphide as the reagent gas leads to the formation of new distonic ions, pyridine N-thioxide radical cations, 2+•. The origin of these ions is unambiguously attributed to the reaction of CS3+• ions with neutral pyridine owing to the use of a new hybrid mass spectrometer combining sectors and an r.f.-only quadrupole collision cell floated at a voltage similar to the accelerating voltage of the ions. Collisional activation (CA) of appropriate reference ions (the molecular ions of isomeric mercaptopyridines, 4–6+•) demonstrates the actual structure of the ions, while neutralization–reionization experiments indicate the stability of the corresponding neutral dipole in the gas phase. Several reactions were performed in the quadrupole collision cell with molecules recognized as excellent trapping reagents of distonic ions: dimethyl disulphide, dimethyl diselenide and nitric oxide. The 2+• ions react with nitric oxide generating NOS+ ions; this reaction is not observed for the reference 4–6+• ions. Although the transfer of thiomethyl radicals or selenomethyl radicals is observed for all the radical cations, the resulting [2(4–6)+S(Se)]+ cations are clearly differentiated by CA. It is also shown that the transfer of S+• to perdeuterated pyridine is a specific reaction of the distonic 2+• ions. © 1997 John Wiley & Sons, Ltd.

Cross sections for electron-stimulated desorption of alkali-metal atoms from adlayers on oxidized tungsten

Earlier cross-section measurements of electron-stimulated desorption (ESD) of Li, Na, K, and Cs atoms from adlayers on oxidized tungsten are analyzed with respect to substrate temperature and degree of oxidation. It is conjectured that the ESD cross sections are determined by the ratio of the rates of neutral alkali-metal re-ionization on the surface and of relaxation of the O+ charge in the substrate. A comparison with experiment revealed the dependence of the re-ionization rate of an alkali-metal adatom on its size and mass, as well as the dependence of the O+ charge relaxation rate on substrate temperature and degree of oxidation. The relation of the charge relaxation time to the substrate band structure is discussed.

Selective Quantitation of Organic Sulfur with the Precursor Scan Method of Neutralization-Reionization Mass Spectrometry

Quantitation of organosulfur compounds in a hydrocarbon matrix was studied by the precursor scan method of neutralization–reionization mass spectrometry. The CHS+ ion was used as a marker selectively to detect and quantify sulfur compounds dissolved in a synthetic hydrocarbon mixture. Precursor ion abundances showed a linear dependence on the organosulfur content in the 1–100% concentration range. Detection limits of 0.5% were achieved, which were limited by the instrument sensitivity. Isobaric interferences of 13C isotopomers from hydrocarbons were negligible. Small interferences from oxygen- and nitrogen-containing additives are discussed. Several neutralization gases were examined to minimize interferences and to maximize the ion abundances. © 1997 by John Wiley & Sons, Ltd.

First-wall cleaning and isotope control studies by ICRF conditioning in Tore Supra with a permanent magnetic field

The use of a permanent magnetic field in superconducting magnetic fusion devices impedes conditioning by glow discharges as actually applied in most pulsed machines. An alternative has been studied in Tore Supra for this purpose with a 3.8 T permanent field: a discharge produced by ion cyclotron range of frequency wave injection (ICRF). Helium ICRF discharge conditioning (ICRF-DC) has already been shown to desaturate the deuterium-loaded carbon first wall efficiently. In this paper, we describe how ICRF-DC can be applied to clean the wall or change its hydrogen isotopic ratio. This is achieved by pumping wall-desorbed molecules induced by particle bombardment from the ICRF plasma. The conditioning efficiency is optimized as a function of two input parameters: the gas pressure and the applied power. A plasma characterization is also given as a function of these parameters and a global model has been used to compare the experimental results and to interpret the main processes to which the neutral and charged species are subjected. The key to optimizing ICRF-DC is the wave ion heating to produce energetic particle bombardment of the first wall, while still achieving a low electron density, low-temperature plasma , to avoid reionization and redeposition of wall-desorbed neutrals before they can be evacuated by the torus pumping system.

Amide bond dissociation in protonated peptides. Structures of the N-terminal ionic and neutral fragments

Three-stage tandem mass spectrometry (MS 3) and neutral fragment reionization (N fR) are utilized to investigate the structures of the N-terminal ionic (b i ) and neutral backbone fragments, respectively, produced from break-up of the amide bond in protonated peptides that have been collisionally activated. The b-type ion from [M+H] + of C 6H 5CO-GF, which is produced by loss of the C-terminal phenylalanine, has the structure of protonated 2-phenyl-5-oxazolone. Conversely, the neutral fragment accompanying the y-type ion (protonated phenylalanine) is 2-phenyl-5-oxazolone. The b 2 ions arising from [M+H] + of underivatized tripeptides are also found to be protonated oxazolones. On the other hand, the neutral fragments released from the N-terminus of the tripeptides upon formation of y 1 are shown to be diketopiperazines and not oxazolones. The combined MS 3 and N fR data help propose dissociation mechanisms that account for the observed structures of ionic and neutral backbone fragments.

Letter: False recovery signals in neutralization–reionization mass spectra

Letter: False recovery signals in neutralization–reionization mass spectra

Characterization of neutral fragments in tandem mass spectrometry: a unique route to mechanistic and structural information.

The neutral species eliminated upon fragmentation of fast-moving mass-selected ions can be directly identified by collisional ionization and detection in neutral fragment reionization (Nf R) mass spectra. Establishment of the identity of neutral fragments yields valuable insight into the decomposition mechanism of a precursor ion, as demonstrated for fullerene and alkali metal iodide cluster ions as well as metal ion adducts of amino acids. In addition, neutral fragment reionization also provides structural information that may not be available from the complementary ionic fragments alone; this is illustrated in the differentiation of isomeric mononucleotides. The parameters influencing the appearance of Nf R spectra are discussed and the scope and general applicability of the method are briefly evaluated.

Differentiation of N-from C-protonated aniline by neutralization-reionization.

Amino- and ring-protonated aniline are distinguished in the gas phase by neutralization-reionization mass spectrometry. This method takes advantage of the dramatically different stabilities and reactivities of the neutralized forms of N- and C-protonated aniline, to ascertain thereby the specific protonation site(s). Fast atom bombardment ionization of aniline is found to yield primarily the anilinium cation (N-protonated tautomer). In contrast, chemical ionization with a variety of reagent gases is shown to generate mixtures in which the ring-protonated species predominates.

Differentiation of N from CProtonated Aniline by NeutralizationReionization

Amino- and ring-protonated aniline are distinguished in the gas phase by neutralization–reionization mass spectrometry. This method takes advantage of the dramatically different stabilities and reactivities of the neutralized forms of N- and C-protonated aniline, to ascertain thereby the specific protonation site(s). Fast atom bombardment ionization of aniline is found to yield primarily the anilinium cation (N-protonated tautomer). In contrast, chemical ionization with a variety of reagent gases is shown to generate mixtures in which the ring-protonated species predominates.

The Long Elusive Acepentalene—Experimental and Theoretical Evidence for its Existence

Neutralization–reionization mass spectrometry (NRMS) proved the existence of the highly strained acepentalene 2 generated from the precursor 1. The experimental data agree well with the results of density functional theory computations, which predict that the Cs singlet 2 is about 3.9 kcal mol−1 more stable than the C3v triplet 2.

A neutralization-reionization mass spectrometric study of alkyl hydroperoxide cation radicals and four distinguishable [C,H 3,O 2] + isomers

The cation radicals of simple alkyl hydroperoxides ROOH + (R = CH 3, C 2H 5, iso-C 3H 7, tert-C 4H 9) have been studied by various mass spectrometric techniques, and neutralization-reionization (NR) methods in particular. Collisional activation (CA) of the beam of fast neutrals before reionization (NCR) and collision experiments with the mass-selected survivor ions (NR/CA) demonstrate that the peroxide OO bond remains intact in ionized methyl hydroperoxide. In the series of hydroperoxides, the abundances of the survivor ions decrease with α-substitution, which can be traced back to increasing contributions of [ R + OOH ] ion/dipole complexes to the parent ion beam. For the elucidation of ion structures, +NR − experiments are shown to be particularly helpful; in addition, they also provide insight into the bonding situations in hydroperoxides. Unimolecular loss of H' from metastable ions (MI) of CH 3OOH + leads to hydroperoxy methyl cations, CH 2OOH +, which are characterized by their MI/CA mass spectra. For comparison, four distinguishable [C,H 3,O 2] + isomers (CH 2OOH +, CH 3OO +, HC(OH) 2 , and H 3O +·CO) have been generated and examined by MI, CA, and NR experiments. With the exception of the proton-bound species H 3O +·CO, the corresponding neutral [C,H 3,O 2] . radicals exist as well and do not interconvert into each other. In addition, HOCH 2O . radicals can be probed by −NR + experiments with HOCH 2O − anions. Unimolecular and collision-induced decomposition of CH 2OOH + gives rise inter alia to a composite [C,H 2,O] + peak, which consists of a narrow gaussian and a broad, dish-topped component. The narrow component vanishes in the +NR + experiment.

Intramolecular 1,2-alkyl shifts in unsymmetric dialkoxycarbenes studied by very low vapour pressure (VLVP) pyroiysis – mass spectrometry

Methoxy-(2,2,2-trifluoroethoxy)carbene radical cations, CH3O-C-OCH2CF3•+, 1•+, are cleanly generated by the dissociative electron ionization (EI) of 2-methoxy-5,5-dimethyl-2-(2,2,2-trifluoroethoxy)-Δ3-1,3,4-oxadiazoline I. Neutralization–reionization (NR) mass spectrometry of the neutral carbene 1, generated by one-electron reduction of 1•+, shows no recovery ion signal and thus 1 is not a viable species within the μs time scale of the experiment. Very low vapour pressure (VLVP) pyrolysis – mass spectrometry of I in conjunction with (multiple) collision experiments shows that 1 completely isomerizes, via a 1,2-trifluoroethyl shift, into methyl 3,3,3-trifluoropropionate, CF3CH2C(=O)OCH3, 1a. This technique was also used to study the related dialkoxycarbenes C2H5O-C-OCH2CF3, 2, CH3O-C-OC2H5, 3, and CH3O-C-OCH(CH3)2, 4, generated from the corresponding 2,2-dialkoxy-5,5-dimethyl-Δ3-1,3,4-oxadiazolines. The pyrolytically generated carbene 2 behaves analogously to 1 and completely isomerizes to ethyl 3,3,3-trifluoropropionate, 2a. The neutral carbenes 3 and 4 undergo only a partial isomerization via 1,2-alkyl shifts in which the ethyl and isopropyl groups show a slightly greater migratory aptitude, respectively, than the methyl group. The differences in migratory aptitude are explained in terms of a transition state model similar to that of a 1,2-H shift in carbenes, with development of negative charge in the migrating group. The greater migratory aptitude of CF3CH2, as compared to CH3 and CH3CH2, is attributed to the stabilization of negative charge in the transition state by strongly electron-withdrawing β-fluorines whereas the differences in migratory aptitude between the alkyl groups in 3 and 4 are largely due to the greater polarizability of isopropyl and ethyl groups, as compared to the methyl group. Key words: dialkoxycarbenes, pyrolysis, tandem mass spectrometry.

Characterization of New Cumulenes C2NX2 (X = O or S): Tandem Mass Spectrometry and ab Initio Studies

New cumulenic ions, SNCCO+, SCNCO+, SNCCS+, and SCNCS+, have been generated by dissociative ionization and characterized by tandem mass spectrometry techniques. The stabilities of the corresponding radicals were evaluated by neutralization-reionization (NR) experiments and supported by ab initio molecular orbital calculations at the G2(MP2,SVP) level. The neutral radicals, SNCCS., SCNCS., and SNCCO., are found by experiment and theory to be observable species in the gas phase, while SCNCO. is not accessible under NR conditions. All the cations are potential energy minima, and their calculated fragmentation energies are in reasonable accord with experiment.

Generation of New Nitrile N-Sulfides (NCCNS, R2NCNS, H3CSCNS, and ClCNS) as Ions and Neutrals in the Gas Phase: Tandem Mass Spectrometry, Flash Vacuum Pyrolysis, and ab Initio MO Study

1,2,5-Thiadiazoles 1−5 bearing CN, CONH2, SCH3, and Cl substituents have been investigated under electron impact conditions. Collisional activation (CA) and neutralization−reionization (NR) mass spectrometries have allowed the identification of new nitrile N-sulfides (NCCNS, H2NCNS, H3CSCNS, and ClCNS) as ions or neutrals in the gas phase of a mass spectrometer. These results were supported by chemical ionization experiments of nitriles using carbon disulfide as the reagent gas, flash vacuum pyrolysis experiments, and ab initio molecular orbital calculations at the G2(MP2,SVP) level. The neutral nitrile N-sulfides are found by experiment and theory to be observable species in the gas phase. All the cations are potential energy minima, and their calculated fragmentation energies are in accord with experimental observations.

Combined quantum chemical and mass spectrometric study of [Si,C,H3,O]+isomers

The potential energy surface of [Si,C,H3,O]+ has been explored by means of ab initio MO calculations at the G2 level of theory as well as by mass spectrometric techniques. The silicon–methoxide cation H3COSi+ has been identified as the global minimum (ΔfH= 151 kcal mol–1), followed by the silaacetyl cations H3SiCO+(ΔfH= 172 kcal mol–1) and H3CSiO+(ΔfH= 180 kcal mor–1). A number of other intermediates, the transition structures associated with the mutual interconversion reactions and the energetics of possible fragmentation channels are explored computationally. It turns out that the energy demands for the unimolecular isomerizations of most of the isomers are substantially lower than those of fragmentations. Consequently, an identification of the different isomers based solely on mass spectrometric information is made difficult by intramolecular rearrangements which precede the structure-indicative fragmentation reactions, even though H3SiCO+ is clearly distinguishable from H3CSiO+ and H3COSi+. However, when the experimental findings are combined with the computationally predicted [Si,C,H3,O]+ potential-energy surface and with thermochemical information, this difficulty can be overcome, and a coherent picture of this complex system emerges. In addition, the existence of the neutral radicals H3COSi˙, H3CSiO˙, as well as H3SiCO˙ and/or H3SiOC˙ is explored by means of neutralization–reionization mass spectrometry.