Reionization Mass Spectrometry Research Articles

The unimolecular chemistry of protonated and deprotonated 2,2-dinitroethene-1,1-diamine (FOX-7) studied by tandem mass spectrometry and computational chemistry.

2,2-Dinitroethene-1,1-diamine (FOX-7) was studied by means of electrospray ionization (ESI) and chemical ionization (CI) mass spectrometry in both positive and negative ion mode. Detailed mechanisms of unimolecular fragmentations of protonated and deprotonated FOX-7 were investigated using high- and low- energy collision-induced dissociation (CID) mass spectrometry, neutral fragment reionization mass spectrometry and quantum chemistry calculations. In deprotonated FOX-7, elimination of the carbodiimide molecule was identified as the energetically most favored fragmentation channel, closely resembling the base hydrolysis of FOX-7. The dinitromethanide ion is formed during this fragmentation as revealed by comparison with CID mass spectra of an isobaric ion prepared by the ESI of authentic sodium dinitromethanide. The proton affinity of FOX-7 was estimated as 855 kJ mo(-1) by high-accuracy quantum chemistry calculations. This value corresponds to protonation at the C-2 position, though the oxygen-protonated tautomer was found to be nearly isoenergetic in the gas phase. In acetonitrile, the nitro group-protonated FOX-7 was found to be significantly less stable then its C-2 tautomer. These theoretical findings are clearly reflected in differences in fragmentations of ESI- and CI-generated [M+H(]+) ions. Interestingly, the consecutive losses of OH∙ and NO2∙ radicals instead of a whole HNO3 molecule were found to account for the most abundant fragment ion in the positive ESI CID mass spectra. In the CI-generated [M+H](+) and [M+D](+) ions, substantial internal energy effects upon the CID were observed.

The Elusive Ethenediselone, Se=C=C=Se

The neutral ethenediselone, Se=C=C=Se, has been characterised by neutralisation–reionisation mass spectrometry, which implies a minimum lifetime of the order of microseconds. Tetraselenafulvalene 1 and tetramethyltetraselenafulvalene 2 were used as precursor molecules. Flash vacuum thermolysis (FVT) of these compounds with isolation of the products in Ar matrices permitted the identification of ethyne, 2-butyne, CSe2, and selenoketene, H2C=C=Se, but at best traces of Se=C=C=Se survived the FVT/matrix isolation experiment. Multiconfigurational calculations indicate that Se=C=C=Se is a ground state triplet molecule with a very small singlet-triplet gap.

Ligand effects on single-electron transfer of isolated iron atoms in the gaseous complexes [(OC)mFe(OH2)n]+ (m, n = 0–2, m + n = 1, 2)

The properties of neutral and monocationic complexes of iron with one and two water or carbonyl ligands (i.e. FeCO0/+, Fe(OH2)0/+, (OC)Fe(OH2)0/+, and (H2O)Fe(OH2)0/+) are studied by means of wave-function based ab initio theory. Not surprisingly, the addition of ligands to the metal center drastically changes the energetics of the cationic species, whereas the ligation of neutral iron has much smaller effects and the interaction is even repulsive for some neutral quintet states. As a consequence, the associated adiabatic and vertical transitions between the manifold of the neutral and monocationic species experience substantial changes with differences exceeding 1eV in the ionization energies. The results are used to explain the results of earlier studies of (OC)Fe(OH2)0/+ and (H2O)Fe(OH2)0/+ by means of neutralization–reionization mass spectrometry.

Gas-Phase Ion Chemistry and Organic Chemistry: The Story of a Hybrid Six-Sector Mass Spectrometer—The “AutoSpec 6F”

The AutoSpec 6F mass spectrometer is a large, floor standing instrument comprising a pair of commercial EBE geometry (AutoSpec) mass spectrometers coupled in series to provide an hybrid EBE-EBE configuration, (E and B being respectively electrostatic and magnetic sectors.) It was designed in close collaboration between Professor R. Flammang and VG Analytical in Manchester, UK. It was equipped with five collision cells and allowed the recording of high energy CID (collision induced dissociation), MIKES (mass analyzed ion kinetic energy spectrometry) and NRMS (neutralization re-ionization mass spectrometry) data as well as consecutive MSn analyses. The field-free regions between sectors allowed the study of unimolecular decomposition products from long-lived metastable ions. The mass spectrometer became even more versatile when an RF-only quadrupole collision cell was installed between the second and the third electric sector. This allowed the study of associative ion/molecule reactions in the low kinetic energy regime. Bimolecular chemical reactions were performed inside the quadrupole cell when a neutral reagent was introduced and the reaction products were analyzed by high energy CID in the downstream sectors. This paper tells the history and summarizes the capabilities of this versatile instrument.

A neutralization-reionization and reactivity mass spectrometry study of the generation of neutral hydroxymethylene

Neutral hydroxymethylene HCOH is an important intermediate in several chemical reactions; however, it is difficult to observe due to its high reactivity. In this work, neutral hydroxymethylene and formaldehyde were generated by charge exchange neutralization of their respective ionic counterparts and then were reionized and detected as positive-ion recovery signals in neutralization-reionization mass spectrometry in a magnetic sector instrument of BEE geometry. The reionized species were characterized by their subsequent collision-induced dissociation mass spectra. The transient hydroxymethylene neutral was observed to isomerize to formaldehyde with an experimental time span exceeding 13.9 µs. The vertical neutralization energy of the HCOH(+•) ion has also been assayed using charge transfer reactions between the fast ions and stationary target gases of differing ionization energy. The measured values match the result of ab initio calculations at the QCISD/6-311 + G(d,p) and CCSD(T)/6-311 + + G(3df,2p) levels of theory. Neutral hydroxymethylene was also produced by proton transfer from CH(2) OH(+) to a strong base such as pyridine, confirmed by appropriate isotopic labeling. There is a kinetic isotope effect (KIE) for H(+) versus D(+) transfer from the C atom of the hydroxymethyl cation of ∼3, consistent with a primary KIE of a nearly thermoneutral reaction.

The covalently bound dimer ion HC[dbnd]N[sbnd]C[dbnd]NH[rad]+ and its neutral counterpart

Model chemistry calculations (CBS-QB3 and CBS-APNO methods) and tandem mass spectrometry based experiments indicate that dissociative ionization of 2-methoxy-s-triazine (consecutive losses of CH2O and HCN) yields the elusive covalently bound [H,C,N] dimer ion HCNCNH+, a species of interest in astrochemistry. Neutralization–Reionization Mass Spectrometry (NRMS) experiments indicate that its neutral counterpart, HCNCNH, is a kinetically stable molecule in the rarefied gas-phase.

ChemInform Abstract: 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

The covalently bound HC [tbnd]N dimer ions HC [dbnd]N−N [dbnd]CH [rad]+ and HC [dbnd]N−C( [dbnd]N)H [rad]+ are stable species in the gas-phase, but the neutral counterparts are not

Tandem mass spectrometry based experiments and computational chemistry (CBS-QB3/APNO methods) indicate that ionized s-tetrazine eliminates N 2 to yield the elusive linear HC N dimer ion HC N–N CH + ( 1). The ion is stable in the rarefied gas-phase but, as it is generated with considerable internal energy, it readily interconverts with HC N–C( N)H + ( 2), via a 1,2-HCN shift. This more stable HCN dimer ion was generated independently by loss of HCN from ionized s-triazine. Whereas the HN C dimer ion HN C C NH + has a (kinetically) stable neutral counterpart, theory and experiment (neutralization–reionization mass spectrometry) agree that the HC N dimer ions 1 and 2 do not.

Surface neutralization/surface reionization (Ping–Pong) mass spectrometry

Experiments are reported that utilize surface neutralization of hyperthermal organic ions to obtain hyperthermal neutral species that are reionized on oxygen-activated rhenium surface and detected by mass spectrometry. A special Ping–Pong mass spectrometer was designed and coupled to a double-focusing mass spectrometer to allow energy and angle-resolved measurements. Neutralization of pyridine and benzene ions on gold, copper, and aluminum surfaces generated substantial ion currents into the collecting plate. The ion currents obtained on gold surfaces were found to depend on the recombination energy of the ion projectile. Hyperthermal neutrals coming off the first surface were found to mainly originate from hydrocarbon adsorbates.

Electron Super-Rich Radicals in the Gas Phase. A Neutralization−Reionization Mass Spectrometric and ab Initio/RRKM Study of Diaminohydroxymethyl and Triaminomethyl Radicals

Diaminohydroxymethyl (1) and triaminomethyl (2) radicals were generated by femtosecond collisional electron transfer to their corresponding cations (1+ and 2+, respectively) and characterized by neutralization-reionization mass spectrometry and ab initio/RRKM calculations at correlated levels of theory up to CCSD(T)/aug-cc-pVTZ. Ion 1+ was generated by gas-phase protonation of urea which was predicted to occur preferentially at the carbonyl oxygen with the 298 K proton affinity that was calculated as PA = 875 kJ mol-1. Upon formation, radical 1 gains vibrational excitation through Franck-Condon effects and rapidly dissociates by loss of a hydrogen atom, so that no survivor ions are observed after reionization. Two conformers of 1, syn-1 and anti-1, were found computationally as local energy minima that interconverted rapidly by inversion at one of the amine groups with a <7 kJ mol-1 barrier. The lowest energy dissociation of radical 1 was loss of the hydroxyl hydrogen atom from anti-1 with ETS = 65 kJ mol-1. The other dissociation pathways of 1 were a hydroxyl hydrogen migration to an amine group followed by dissociation to H2N-C=O* and NH3. Ion 2+ was generated by protonation of gas-phase guanidine with a PA = 985 kJ mol-1. Electron transfer to 2+ was accompanied by large Franck-Condon effects that caused complete dissociation of radical 2 by loss of an H atom on the experimental time scale of 4 mus. Radicals 1 and 2 were calculated to have extremely low ionization energies, 4.75 and 4.29 eV, respectively, which belong to the lowest among organic molecules and bracket the ionization energy of atomic potassium (4.34 eV). The stabilities of amino group containing methyl radicals, *CH2NH2, *CH(NH2)2, and 2, were calculated from isodesmic hydrogen atom exchange with methane. The pi-donating NH2 groups were found to increase the stability of the substituted methyl radicals, but the stabilities did not correlate with the radical ionization energies.

Cytosine neutral molecules and cation–radicals in the gas-phase: Structures, energetics, ion chemistry, and neutralization–reionization mass spectrometry

Gas-phase cytosine molecules and cation–radicals represent a complex system of several nearly isoenergetic tautomers within each group. Computational methods differ in ordering the relative enthalpies of neutral cytosine tautomers. At our highest level of theory, CCSD(T)/aug-cc-pVTZ calculations find an enol form, anti-2-hydroxy-4-aminopyrimidine ( 2), to be the most stable neutral tautomer in the gas-phase, followed by its rotamer, syn-2-hydroxy-4-aminopyrimidine ( 3), the canonical oxo-form, 4-amino-1,2-dihydropyrimidin-2( 1H)-one ( 1), imino-forms, 2-oxo-4-iminodihydro( 1H, 3H)pyrimidine ( 4 and 5), and another oxo-form, 4-amino-dihydropyrimidin-2( 3H)-one ( 6). Other tautomers, such as anti-anti, syn-syn and syn-anti-2-hydroxy-4-iminodihydro( 3H, 4H)pyrimidines ( 7– 9), are less stable. The adiabatic ionization energies of the major cytosine tautomers have been calculated to be 8.71, 8.64, 8.62, 8.58, 8.64, and 8.31 eV for 1, 2, 3, 4, 5, and 6, respectively. Cytosine cation–radicals show very close relative energies that increase in the order of 6 + (most stable) < 2 + ≈ 3 + < 4 + ≈ 7 + ≈ 1 + < 5 +. In addition, distonic ions having radical centers at C-5 ( 10 +) and C-6 ( 11 + are found as low-energy isomers of 1 +– 7 +. Metastable cytosine cation–radicals undergo ring-cleavage dissociations by eliminations of CO (major) and HN C O (minor). The energetics of these and other higher-energy dissociations, including the pertinent transition states, have been established by high-level ab initio and density functional theory calculations and plausible mechanisms have been proposed. Collisional neutralization of cytosine cation–radicals with trimethylamine and dimethyldisulfide as electron donors forms stable molecules that are detected as cation–radicals following collisional reionization. The dissociations observed upon neutralization–reionization mainly include ring-cleavages followed by loss of N C O, HN C O, and formation of C 2H 3N, C 2H 2N, and CO neutral fragments that are assigned to ion dissociations following reionization.

Can the CH 2BH 2 and CH 3BH radicals, cations and anions be observed by experiment?

The structure, thermochemistry, isomerization and dissociation of the CH 2BH 2 and CH 3BH radicals, cations and anions have been explored with a wide range of ab initio levels of computational theory and their formation has been probed by tandem mass spectrometry and RRKM kinetic theory. Calculations predict that CH 2 BH 2 ⌉ + is not an equilibrium structure on the potential energy surface. The CH 3 BH ⌉ + was characterized by its collision-induced dissociation mass spectrum and neutralization–reionization (NR) mass spectrometry was used to generate the CH 3 BH ⌉ • radical. While both CH 2 BH 2 ⌉ • and CH 3 BH ⌉ • are stable with respect to isomerization and dissociation, a relatively low barrier to the 1,2-hydrogen shift means that pure CH 3 BH ⌉ • can only be made cold, and so the above NR experiment likely produces a mixture of neutrals, of which only CH 3 BH ⌉ • can be reionized. Vertical electron attachment to CH 3 BH ⌉ • is endothermic which means the CH 3 BH ⌉ - anion is unlikely to be made from the cation or radical. In addition, the anion lies in a shallow well of only 48 kJ mol −1 and will undergo fast isomerization to CH 2 BH 2 ⌉ - below the dissociation limit.

Differentiation of ionised benzimidazole from its isomeric alpha-distonic ion by collision-induced dissociation and neutralisation-reionisation mass spectrometry.

Ionised benzimidazole and its isomeric alpha-distonic ion (or ionised ylid) have been examined by recording their metastable ion, collision-induced dissociation and neutralisation-reionisation mass spectra. These tautomers may be distinguished by careful consideration of key features of the collision-induced dissociation spectra, with or without prior neutralisation and reionisation. Formation of doubly-charged ions by charge stripping occurs preferentially when the alpha-distonic ion is subjected to collision. This alpha-distonic ion survives neutralisation and reionisation, thus establishing that the corresponding ylid is stable on the microsecond time frame. The effects of benzannulation on the ease of differentiation of classical and distonic radical cations derived from biologically important heterocycles are considered.

The Generation of Neutral Derivatives of Low-Valence Iron, RFe(I), by Netralization—Reionization Mass Spectrometry

Neutralization-reionization mass spectrometry is applied for the generation of low-valence iron derivatives. Neutralization of Rfe(+) (R=H, F, Cl, Br, I, CN, OH, NH(2), acac, C(6)H(5)) ions resulted in the corresponding neutrals having lifetimes of at least 5 micros. Atom connectivities in RFe ions and neutrals were elucidated by a variety of tandem mass spectrometry techniques. The present study provides the first experimental evidence of the intrinsic stability of neutral IFe, NCFe, acacFe and C(6)H(5)Fe.

Generation of the Elusive meta‐Benzoquinone in the Gas Phase

Ready for matrix isolation? The existence of meta-benzoquinone has been established unambiguously by a combination of neutralization–reionization mass spectrometry and computational studies (see picture). The results suggest that this elusive member of the family of organic biradicals should be accessible in matrix-isolation experiments.

Experimental Detection of Theoretically Predicted N2CO

The missing link in the 28-electron family of open-chain N4, N2CO, and C2O2 molecules has been detected in the gas phase as an isolated, intact N2CO species, whose lifetime exceeds 0.8 μs. The new metastable molecule has been prepared by neutralization–reionization mass spectrometry of labeled N2CO+ ions, which were generated in various N2/CO ionized mixtures (see picture).

Electron-Rich Radicals by Neutralization—Reionization Mass Spectrometry. Generation, Dissociations and Energetics of the Hydrogen Atom Adduct to Acetamide

Protonated acetamide exists as two planar conformers, the more stable anti-form (anti-1(+)) and the syn-form (syn-1(+)), DeltaG(degree) (298) (anti-->syn) = 10.8 kJ mol(-1). Collisional neutralization of 1(+) produces 1-hydroxy-1-amino-1-ethyl radicals (anti-1 and syn-1) which in part survive for 3.7 micros. The major dissociation of 1 is loss of the hydroxyl hydrogen atom (approximately 95%) which is accompanied by loss of one of the methyl hydrogen atoms (approximately 3%) and loss of the methyl group (approximately 2%). The most favorable dissociation of the OH bond is calculated to be only 34 kJ mol(1) endothermic but requires 88 kJ mol(-1) in the transition state. Other dissociations of 1, e.g., loss of one of the amide hydrogens, methyl hydrogens, and loss of ammonia are calculated to proceed through higher- energy transition states and are not kinetically competitive if proceeding from the ground doublet electronic state of 1. The unimolecular dissociation of 1 following collisional electron transfer is promoted by large Franck-Condon effects that result in 8090 kJ mol(-1) vibrational excitation in the radicals. Radicals 1 are calculated to exoergically abstract hydrogen atoms from acetamide in water, but not in the gas phase. The different reactivity is due to solvent effects that favor the products, (.)CH(2)CONH(2) and CH(3)CH(OH)NH(2), over the reactants.

Transient Intermediates of Chemical Reactions by Neutralization—Reionization Mass Spectrometry

AbstractFor Abstract see ChemInform Abstract in Full Text.

Gas‐Phase Chemistry and Mass Spectrometry of Ge‐, Sn‐, and Pb‐Containing Compounds

AbstractFor Abstract see ChemInform Abstract in Full Text.

Experimental detection of the H2NO3 radical.

It's in the air. The authors detected by neutralization–reionization mass spectrometry the nitrogen hydroxide oxide 3, which is relevant to atmospheric chemistry and a key intermediate in the NO32− reduction, as a gaseous radical species with a lifetime of 1 μs. The charged precursor utilized was the high-energy isomer 2 from the protonation of HNO3, whereas the more stable isomer 1 dissociates upon neutralization.