Reionization Mass Spectrometry Research Articles

Pyrazine diradicals, carbenes, ylides, and distonic ions probed by theory and experiment

The [C 4,H 4,N 2] potential energy hypersurface relating to pyrazine and its hydrogen shift isomers has been investigated computationally using hybrid Hartree–Fock/density functional theory and through a variety of tandem mass spectrometry experiments (metastable ion, collision-induced dissociation, and neutralization reionization mass spectrometry). In addition to the conventional pyrazine structure 1, its α-ylide 2, β-ylide 3, and the 1,4-diradical 4 were generated and characterized through neutralization reionization mass spectrometry experiments. Also, the corresponding radical cations 1 ·+ – 4 ·+ were accessible by dissociative electron ionization of the appropriate pyrazine esters. Quantum chemical calculations at the B3LYP/TZVP level of theory reveal that all these species correspond to minima that are separated by significant barriers thus preventing facile isomerization. As an additional, albeit high lying isomer, the 1,3-diradical 5 was computationally identified. In the case of the radical cations the energy differences between the various isomers are much smaller than for the corresponding neutrals; however, pyrazine represents in both cases the most stable species.

Metastable states of dimethylsulfonium radical, (CH 3) 2SH ·: : A neutralization–reionization mass spectrometric and ab initio computational study

Variable-time neutralization–reionization mass spectrometry was used to generate and detect the hypervalent radical dimethylsulfonium, (CH 3) 2SH ·( 1H · ), and its isotopomers (CH 3) 2SD ·( 1HD · ), (CD 3) 2SH ·( 1DH · ), and (CD 3) 2SD ·( 1DD · ). The successful detection of 1H · –1DD · was achieved in spite of severe interferences due to isobaric overlaps by the 13C, 33S, and 34S isotope satellites of residual dimethylsulfide cation radicals. Radicals 1H · –1DD · dissociated by cleavages of the S–(H,D) and S–C bonds in a ∼3:1 branching ratio. Ab initio calculations with large basis sets failed to find a local energy minimum for the ( X) 2 A′ ground electronic state of 1H · , which dissociated without barrier to (CH 3) 2S and H ·. Several excited states of 1H · were found by configuration interaction singles CIS/6-311++G(3 df,2 p) calculations that were within 2 eV of the repulsive ground state potential energy surface. The microsecond metastability and dissociations of 1H · –1DD · were ascribed to the properties of excited electronic states.

Cu(NH 3) 1–2 complexes: formation and characterization in the gas phase by neutralization–reionization mass spectrometry

The Cu(0) complexes Cu(NH 3) and Cu(NH 3) 2 are formed in the gas phase by neutralization of the corresponding monocations. The mass spectra obtained after reionization indicate that both complexes survive intact for ≥0.3 μs and, thus, are stable species residing in potential energy minima. A fraction of the Cu(NH 3) and Cu(NH 3) 2 molecules generated in the neutralization step undergo dissociation to Cu(0)+NH 3 and Cu(NH 3)+NH 3, respectively, before reionization.

Pyridine N-selenide radical cations: synthesis, characterization, neutralization, and ion–molecule reactions in the gas phase

Cyanogen N-selenide radical cations, NCCNSe ·+, react efficiently with neutral pyridine producing the elusive pyridine N-selenide radical cations. The structure of these new distonic ions has been established by high energy collisional activation (CA), neutralization–reionization mass spectrometry, and associative ion–molecule reactions with nitric oxide, methyl isocyanide, pyridine-d 5, dimethyl disulfide, and dimethyl diselenide. These experiments have been performed in a new hybrid mass spectrometer having the sectors–quadrupole–sectors configuration.

Glycine radicals in the gas phase †‡

2-Glycyl radical (1), 2-glycyl methyl ester (2), and 2-glycyl N-methylamide radicals (3) were generated in the gas phase by collisional neutralization of the corresponding cations and studied by neutralization–reionization (NR) mass spectrometry and ab initio and density functional theory calculations. Stable fractions were found for 1–3 that did not dissociate on the 4 µs timescale of the experiment. The major unimolecular dissociations observed were cleavages of the CO–X bonds (X = OH, OCH3, and NHCH3) to form aminoketene, and cleavages of the C–CO bond to form aminocarbene, which gave rise to prominent fragments in the NR mass spectra. In addition, hydrogen rearrangements occurred in 1–3 and an unusual methyl migration took place in 3. G2(MP2) bond dissociation energies are reported for 1 and 1+. Combined B3LYP and PMP2/6-311+G(2df,p) calculations were used to provide bond dissociation energies for 2, 2+, 3 and 3+.

Neutralization-reionization mass spectrometry applied to organometallic and coordination chemistry (update: 1994-1998)

This review presents the results from the last 5 years on neutralization–reionization mass spectrometry (NRMS) studies of metal-containing ions. The results for silicon- and phosphorus-containing species are also discussed. The major aim of NRMS experiments is to generate (in the gas phase) neutrals that cannot be easily produced and/or detected in other experimental conditions. A variety of elusive complexes of coordinatively unsaturated and saturated metal(s), including important reaction intermediates, have been produced by collisional neutralization of their positively or negatively charged counterparts in the mass spectrometer. The detection of these species helps in understanding the mechanisms of reactions in the gas and condensed phases (interstellar chemistry, catalytic transformations, reactions on surfaces, etc.). The reviewed period of time has been characterized by the development of new experimental techniques. These techniques allow studying stability, electronic structure, and reactivity of selected molecules and radicals. The review provides a complete list of metal-, Si-, and P-containing ions that have been studied by using NRMS and related methods prior to 1999. © 1999 John Wiley & Sons, Inc., Mass Spec Rev 18: 87–118, 1999

Modeling nucleobase radicals in the mass spectrometer

Neutralization–reionization mass spectrometry, complemented by ab initio and density-functional theory calculations, provides a powerful tool for the investigation of polyatomic radicals relevant to transient intermediates in the process of radiation or chemically induced DNA damage. Precursor ions for analogues of DNA radicals are prepared by gas-phase protonation of nucleobases or their heterocyclic models. The proton affinity of the gas-phase acid is tuned to the topical proton affinity in the substrate molecule to achieve selective protonation. Femtosecond electron transfer imprints the structure of the precursor ion on to that of the radical. Deuterium labeling, collisional activation and variable-time measurements on a microsecond time-scale are used to study unimolecular dissociations of transient heterocyclic radicals and distinguish them from ion dissociations. Ab initio and density-functional theory calculations provide energies which are not available from neutralization–reionization experiments. Topical proton affinities, Franck-Condon energies pertinent to vertical electron transfer, radical stabilities and isomerization barriers are used to elucidate the structures and dissociations of isomeric heterocyclic radicals. © 1998 John Wiley & Sons, Ltd.

Hypervalent pyrrolidinium radicals by neutralization—reionization mass spectrometry: Metastability and radical leaving group abilities

Neutralization-reionization mass spectrometry was used to generate hypervalent radicals pyrrolidinium (1H·), N-methylpyrrolidinium (2H·), N-ethylpyrrolidinium (3H·), N-phenylpyrrolidinium (4H·), N,N-dimethylpyrrolidinium (5·), N-methyl-N-ethylpyrrolidinium (6·), and their deuterium-labeled derivatives and to study their dissociations in the gas phase. Isotopomers of pyrrolidinium and N-phenylpyrrolidinium showed small fractions of stable radicals of microsecond lifetimes that were detected following collisional reionization. The leaving group abilities in radical dissociations were established as H· » C2H5· ≈ C6H5·> CH3·. The hydrogen atom was the best leaving group in secondary and tertiary pyrrolidinium radicals 1H·–4H·, whereas losses of ethyl, phenyl, and ring openings by N-C bond cleavages were less facile. Methyl was the worst leaving group among those studied. Ring cleavages dominated the dissociations of quaternary pyrrolidinium radicals 5· and 6·, whereas losses of alkyl substituents were less efficient. The electronic properties of hypervalent ammonium radicals are discussed to rationalize the experimental leaving group abilities of hydrogen atom, alkyl, and phenyl radicals.

The gas-phase R nX–NO + (X=O, N, S) cations: nitroso onium cations versus ion–molecule complexes

Collisional activation (CA) and ion–molecule reaction experiments making use of a hybrid tandem mass spectrometer provide evidence for the dilute gas-phase existence of the nitroso sulfonium cation (H 2SNO +). Ab initio molecular orbital calculations using the coupled-cluster method, CCSD(T), with both 6-31G(d,p) and 6-311++G(3df,2p) basis sets confirm that, while H 2SNO + is relatively stable with respect to fragmentation, it can best be regarded as a strong molecular complex between NO + and SH 2 rather than an onium cation. Other nitroso onium cations have also been generated by ionization of mixtures incorporating NO · and water, methanol, ammonia and alkane thiols and characterized by collisional activation and by neutralization–reionization (NR) mass spectrometry.

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.

Sulfur Oxyacids and Radicals in the Gas Phase. A Variable-Time Neutralization−Photoexcitation−Reionization Mass Spectrometric and Ab Initio/RRKM Study

Hydroxysulfinyl radical (1), hydrogensulfonyl radical (2), and dihydroxysulfane (6) were generated in the gas phase by collisional reduction of the corresponding cations and studied by the variable-time and photoexcitation methods of neutralization−reionization mass spectrometry and by ab initio and RRKM calculations. Radicals 1 and 2 were thermodynamically and kinetically stable. Two rotamers of 1, syn-1 and anti-1, were found computationally to be local energy minima. The computations suggested a complex potential energy surface for dissociations of 1. The minimum-energy reaction path was the rate-determining isomerization to 2 followed by fast loss of H• to form SO2. Direct H• loss from 1 was kinetically disfavored. Cleavage of the S−OH bond in 1 was highly endothermic and became kinetically significant at excitations >325 kJ mol-1. In contrast to ab initio/RRKM predictions, 1 formed by vertical reduction of hydroxysulfinyl cation (1+) dissociated mainly to OH• and SO, whereas loss of H• was less significant. Both dissociations showed microsecond kinetics as established by variable-time measurements. Photoexcitation of nondissociating 1 opened the H-loss channel, whereas collisional excitation did not change the branching ratio for the H• and OH• loss channels. The experimental results pointed to the formation of a large fraction of metastable and dissociative excited electronic states of 1 upon vertical electron transfer. Radical 2 was cogenerated with 1 by vertical reduction of a mixture of 1+ and 2+ produced by highly exothermic protonation of SO2 with H3+. Pronounced loss of H• from 2 occurred following collisional neutralization in accordance with RRKM predictions. Dihydroxysulfane (6) was stable following collisional neutralization of the cation-radical 6•+. The G2(MP2) potential energy surface predicted the isomerization to hydrogensulfinic acid (7) followed by loss of water to be the lowest-energy dissociation of 6. RRKM calculations showed the 6 → 7 isomerization to be the rate-determining step. Cation-radical 6•+ also eliminated water through unimolecular isomerization to a stable nonclassical isomer, OS•+···H2O (9). The thermochemistry of the neutral and ionic systems is discussed. The important role of excited electronic states in the formation of radicals by vertical electron transfer is emphasized.

Generation and characterization of ionic and neutral cyanoisothiocyanate [NCNCS]0 by neutralization – reionization mass spectrometry

Generation and characterization of ionic and neutral cyanoisothiocyanate [NCNCS]0 by neutralization – reionization mass spectrometry

Generation and characterization of ionic and neutral cyanoisothiocyanate [NCNCS]+·/0 by neutralization - reionization mass spectrometry

Generation and characterization of ionic and neutral cyanoisothiocyanate [NCNCS]+·/0 by neutralization - reionization mass spectrometry

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.

Metastable States of Dimethylammonium, (CH3)2NH2•

Hypervalent dimethylammonium radical, (CH3)2NH2•, and its deuterium-labeled isotopomers (CH3)2ND2•, (CH3)2NHD•, (CD3)2NH2•, and (CD3)2ND2• were generated as transient species by collisional neutralization of their cations in the gas phase and studied by neutralization−reionization mass spectrometry, laser photoexcitation, and ab initio theory. (CH3)2ND2•, (CH3)2NHD•, and (CD3)2NH2• gave fractions of metastable species of ≥3.3 μs lifetimes, whereas (CH3)2NH2• and (CD3)2ND2• dissociated completely on the same time scale. Metastable (CD3)2NH2• and (CH3)2ND2• were photoexcited but not photoionized with the combined 488 and 514.5 nm lines from an Ar-ion laser. Ab initio calculations with effective PMP4(SDTQ)/6-311++G(3df,2p) identified the (X̃)2A1 ground state of vertical ionization energy, IEv = 3.70 eV. RRKM calculations on the ab initio potential energy surface of the (X̃)2A1 state predicted predominant N−H and N−D bond dissociations but did not allow for competitive loss of CH3 or CD3. The four lowest excited states of (CH3)2NH2•, (Ã)2B1, (B̃)2A1, (C̃)2B2, and (D̃)2A1, were characterized by CIS/6-311++G(3df,2p) calculations, and their vertical ionization energies were calculated as 2.86, 2.57, 2.48, and 1.82 eV, respectively. The excited states were calculated to be strongly bound with respect to N−H bond dissociations. The N−C bond dissociations were interpreted by potential energy surface crossing of the B̃ and à states and transitions via conical intersection to the dissociative ground state.

Protonation Sites in Pyrimidine and Pyrimidinamines in the Gas Phase

Protonation sites in gaseous pyrimidine (1), 2-pyrimidinamine (2), and 4-pyrimidinamine (3) were elucidated by neutralization−reionization mass spectrometry and MP2/6-311G(2d,p) ab initio calculations. Pyrimidine was protonated with NH4+ and t-C4H9+ exclusively at the nitrogen atoms whose proton affinity (PA) was calculated at 879 kJ mol-1 in excellent agreement with the experimental datum. The C-2 and C-5 positions in 1 had PA = 573 and 645 kJ mol-1, respectively. 2 was protonated with t-C4H9+ at N-1 and/or N-3 and the amine group in a statistical 2:1 ratio, consistent with the calculated PA = 909 and 864 kJ mol-1, respectively. 3 was protonated at N-1, N-3, and the amine group, which had PA = 943, 905, and 835 kJ mol-1, respectively. The C-5 positions in 2 and 3 were less basic, PA = 807 and 781 kJ mol-1, respectively, and were not attacked by the gas-phase acids used. The thermodynamic proton affinities of gaseous 1, 2, and 3 followed the order of their basicities in solution. The substituent electroni...

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.

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.