Probing gas phase catalysis by atomic metal cations with flow tube mass spectrometry.

The evolution and applications of flow tube mass spectrometry in the study of catalysis promoted by atomic metal ions are tracked from the pioneering days in Boulder, Colorado, to the construction and application of the ICP/SIFT/QqQ and ESI/qQ/SIFT/QqQ instruments at York University and the VISTA-SIFT instrument at the Air Force Research Laboratory. The physical separation of various sources of atomic metal ions from the flow tube in the latter instruments facilitates the spatial resolution of redox reactions and allows the separate measurement of the kinetics of both legs of a two-step catalytic cycle, while also allowing a view of the catalytic cycle in progress downstream in the reaction region of the flow tube. We focus on measurements on O-atom transfer and bond activation catalysis as first identified in Boulder and emphasize fundamental aspects such as the thermodynamic window of opportunity for catalysis, catalytic efficiency, and computed energy landscapes for atomic metal cation catalysis. Gas-phase applications include: the catalytic oxidation of CO to CO2 , of H2 to H2 O, and of C2 H4 to CH3 CHO all with N2 O as the source of oxygen; the catalytic oxidation of CH4 to CH3 OH with O3 ; the catalytic oxidation of C6 H6 with O2 . We also address the environmentally important catalytic reduction of NO2 and NO to N2 with CO and H2 by catalytic coupling of two-step catalytic cycles in a multistep cycle. Overall, the power of atomic metal cations in catalysis, and the use of flow tube mass spectrometry in revealing this power, is clearly demonstrated.

Relativistic Effects in the Ligation of Atomic Coinage Metal Cations with O2 and C6H6: Anomalous Formation of Relativistic Mono- and Bis-adducts with Au.

The interaction of the atomic coinage metal cations Cu+, Ag+, and Au+ with O2, a weak ligand, and C6H6, a strong ligand, was investigated with measurements of rate coefficients of ligation and quantum-chemical computations of ligation energies with an eye on relativistic effects going down the periodic table. Strong "third row enhancements" were observed for both the rate coefficients of ligation and ligation energies with the O2 ligand and for the formation of both the mono- and bis-adducts of M+ and the monoadduct of M+(C6H6). The computations revealed that the third-row enhancement in the ligation energy is attributable to a relativistic increase in the ligation energy. This means that rate coefficient measurements down the periodic table for the ligation of coinage metal cations with O2 provide a probe of the relativistic effect in ligation reactions, as expected from the known dependence of the rate coefficient of ligation on the ligation energy. The much stronger benzene ligand was observed to ligate the atomic coinage metal cations with nearly 100% efficiency so that there is no, or only slightly, visible third-row enhancement despite the strong relativistic effect in the binding energy that is revealed by the calculations. Relativistic effects contribute substantially to the extraordinary stability against deligation of all the observed mono- and bis-adducts of Au+ relative to Ag+, truly a "third-row enhancement".

Toward ICP-SIFT mass spectrometry and atomic cation ligation as a probe of relativistic effects-A personal journey.

The ICP-SIFT mass spectrometer at York University, a derivative of flowing afterglow (FA) and selected-ion flow tube (SIFT) mass spectrometers, has provided a powerful technique to measure the chemistry and kinetics of atomic cation-molecule reactions. Here, I focus on periodic trends in the kinetics of ligation reactions of atomic ions with small molecules. I examine trends in ammonia ligation kinetics across the first two rows of the atomic transition metal cations and their correlation with ligand bond enthalpies and ligand field stabilization energies. Also explored are trends down Groups 1 and 2 in the kinetics of noncovalent electrostatic ligand bonding and the tendency for s electron solvation of the atomic alkaline-earth cations with ammonia. Finally, I briefly review trends observed with 12 different ligands in the ligation rate down the periodic table with Group 9-12 transition atomic metal cations. These trends provide a compelling probe for the presence of relativistic effects that influence the strengths of the metal-ion ligand bonds that are formed. There is a clear third-row rate enhancement with Ir+ , Pt+ , Au+ , and Hg+ , the extent of which depends on the nature of the ligand. This large set of kinetic data provides an unprecedented broad perspective of relativistic effects in ligand bonding. With CS2 as a ligand, the third-row relativistic effect is apparent in the formation of both the first and the second ligand bond with the Groups 10 and 11 atomic cations as predicted by our quantum chemical calculations of ligation energies.

Hydrogenated Gold Clusters from Helium Nanodroplets: Cluster Ionization and Affinities for Protons and Hydrogen Molecules.

We report the mass spectrometric detection of hydrogenated gold clusters ionized by electron transfer and proton transfer. The cations appear after the pickup of hydrogen molecules and gold atoms by helium nanodroplets (HNDs) near zero K and subsequent exposure to electron impact. We focus on the size distributions of the gold cluster cations and their hydrogen content, the electron energy dependence of the ion yield, patterns of hydrogenated gold cluster cation stability, and the presence of "magic" clusters. Ab initio molecular orbital calculations were performed to provide insight into ionization energies and proton affinities of gold clusters as well as into molecular hydrogen affinities of the ionized and protonated gold cluster cations.

Ligation Kinetics as a Probe for Non-Covalent Electrostatic Bonding and Electron Solvation of Alkali and Alkaline Earth Cations with Ammonia.

Mono-ligation kinetics were measured for ammonia reacting with atomic cations in the first two groups of the periodic table (K+, Rb+, Cs+ and Ca+, Sr+, Ba+). Also, mono-ligation energies were computed using density functional theory (DFT) in an attempt to assess the role of non-covalent electrostatic interactions in these chemical reactions. The measurements were performed at room temperature in helium bath gas at 0.35 Torr using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Rate coefficients are reported for ammonia addition, the only reaction channel that was observed with all these cations. A systematic decrease in the rate of addition of NH3 was observed for both group 1 and 2 cations going down the periodic table. The computational studies predict a decrease in the adduct binding energy and an increase in the bond separation going down groups 1 and 2 of the periodic table and provide some insight into the role of the extra selectron in the group 2 radical cations in ligand bonding. A correlation is seen between the efficiency of ligation and the binding energy of the adduct ion and attributed to the lifetime of the intermediate encounter complex against back dissociation which is dependent on its well depth. Higher-order additions of ammonia were also observed. Remarkable differences in the extent and kinetics were seen between the group 1 and 2 cations, and these were attributed to the occurrence of ammonia solvation of the extra s electron in the higher-order adducts of the alkaline earth cations.

Ligation kinetics as a probe for gas-phase ligand field effects: Ligation of atomic transition metal cations with ammonia at room temperature.

The kinetics of ammonia ligation to atomic first and second row transition metal cations were measured in an attempt to assess the role of ligand field effects in gas-phase ion-molecule reaction kinetics. Measurements were performed at 295 ± 2 K in helium bath gas at 0.35 Torr using an inductively coupled plasma/selected-ion flow tube tandem mass spectrometer. The atomic cations were produced at ca. 5500 K in an inductively coupled plasma source and were allowed to decay radiatively and to thermalize by collisions with argon and helium atoms prior to reaction. A strong correlation was observed across the periodic table between the measured rate coefficients for ammonia ligation and measured/calculated bond dissociation energies. A similar strong correlation is seen with the ligand field stabilization energy. So ligand field stabilization energies should provide a useful predictor of relative rates of ligation of atomic metal ions.

Uptake and accommodation of water clusters by adamantane clusters in helium droplets: interplay between magic number clusters.

We report an experimental study of water clusters as guests in interactions with clusters of adamantane (Ad) as hosts that occur in doped helium droplets at extremely low temperatures. Separate experiments with pure water as dopant showed ready formation of a distribution of water clusters (H2O)mH+ that peaks at m = 11 and extends beyond m = 100 with local maxima at m = 4, 11, 21, 28 and 30 with (H2O)21H+ being the most anomalous and showing the greatest stability with respect to clusters immediately adjacent in water content. When adamantane is also added as a dopant, extensive hydration is seen in the formation of water/adamantane clusters, (H2O)mAdn+; magic number clusters (H2O)21Adn+ are seen for all the adamantane clusters. Other magic numbers for water clusters attached to adamantane, (H2O)mAdn+, are as for pristine protonated water, with m = 28 and m = 30. The icosahedral shell closure of pure adamantane at n = 13 and 19 appears to be preserved with (H2O)21 replacing one adamantane. (H2O)21Ad12+ and (H2O)21Ad18+ stand out in intensity and demonstrate the interplay of magic number water clusters with magic number adamantane clusters, observed perhaps for the first time in gas-phase cluster chemistry. There was no clear evidence for the formation of clathrate hydrates in which adamantane is trapped within structured water.

Fullerene ion chemistry: a journey of discovery and achievement.

An account is provided of the extraordinary features of buckminster fullerene cations and their chemistry that we discovered in our Ion Chemistry Laboratory at York University (Canada) during a 'golden' period of research in the early 1990s, just after C60 powder became available. We identified new chemical ways of C60 ionization and tracked novel chemistry of C60 (n+) as a function of charge state (n=1-3) with some 50 different reagent molecules. We found that multiple charges enhance reaction rates and diversify reaction products and mechanisms. Strong electrostatic interactions with reagent molecules were seen to reduce barriers to carbon surface bonding and charge-separation reactions, while intramolecular Coulomb repulsion appeared to localize charge on the surface or the substituent and so influence higher order chemistry, including 'spindle', 'star', 'fuzzy ball', 'ball-and-chain' and dimer ion formation. We introduced the notion of 'apparent' gas-phase acidity with measurements of proton-transfer reactions of multiply charged fullerene cations. We also explored the attachment of atomic metal cations to C60 and their subsequent reactions. All these findings were applied to the possible chemistry of fullerene cations in the interstellar medium with a focus on multiply charged fullerene ion formation and the intervention of fullerene cations in fullerene derivatization and molecular synthesis, with a view to their possible future detection.This article is part of the themed issue 'Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene'.

Building Carbon Bridges on and between Fullerenes in Helium Nanodroplets.

We report the observation of sequential encounters of fullerenes with C atoms in an extremely cold environment. Experiments were performed with helium droplets at 0.37 K doped with C60 molecules and C atoms derived from a novel, pure source of C atoms. Very high-resolution mass spectra revealed the formation of carbenes of the type C60(C:)n with n up to 6. Bridge-type bonding of the C adatoms to form the known dumbbell C60═C═C60 also was observed. Density functional theory calculations were performed that elucidated the carbene character of the C60(C:)n species and their structures. Mass spectra taken in the presence of water impurities and in separate experiments with added H2 also revealed the formation of the adducts C60C(n)(H2O)n and C60C(n)(H2)n probably by H-OH and H-H bond insertion, respectively, and nonreactivity for the dumbell. So C adatoms that form carbenes C60(C:)n can endow pristine C60 with a higher chemical reactivity.

Experimental evidence for the influence of charge on the adsorption capacity of carbon dioxide on charged fullerenes.

We show, both experimentally and theoretically, that the adsorption of CO2 is sensitive to charge on a capturing model carbonaceous surface. In the experiment we doped superfluid helium droplets with C60 and CO2 and exposed them to ionising free electrons. Both positively and negatively charged C60(CO2)n(+/-) cluster ion distributions are observed using a high-resolution mass spectrometer and they show remarkable and reproducible anomalies in intensities that are strongly dependent on the charge. The highest adsorption capacity is seen with C60(+). Complementary density functional theory calculations and molecular dynamics simulations provided insight into the nature of the interaction of charged C60 with CO2 as well as trends in the packing of C60(+) and C60(-). The quadrupole moment of CO2 itself was found to be decisive in determining the charge dependence of the observed adsorption features. Our findings are expected to be applied for the adsorption of CO2 on charged surfaces in general.

Multi-component ion modifiers and arcing suppressants to enhance differential mobility spectrometry for separation of peptides and drug molecules.

The optimization of ion/molecule chemistry in a differential mobility spectrometer (DMS) is shown to result in improved peak capacity, separation, and sensitivity. We have experimented with a modifier composed of multiple components, where each component accomplishes a specific task on mixtures of peptides and small drug molecules. Use of a higher proton affinity modifier (hexanol) provides increased peak capacity and separation. Analyte ion/modifier proton transfer is suppressed by adding a large excess of low proton affinity modifier (water or methanol), significantly increasing signal intensity and sensitivity for low proton affinity analytes. Finally, addition of an electrical arcing suppressant (chloroform) allows the device to operate reliably at higher separation fields, improving peak capacity and separation. We demonstrate a 20% increase in the device peak capacity without any loss of sensitivity and estimate that further optimization of the modifier composition can increase this to 50%. Use of 3-, 4-, or even 5-component modifiers offers the opportunity for the user to fine-tune the modifier performance to maximize the device performance, something not possible with a single component modifier.

Locating Pb2+ and Zn2+ in zinc finger-like peptides using mass spectrometry.

The binding preferences of Pb(2+)and Zn(2+) in doubly charged complexes with zinc finger-like 12-residue peptides (Pep), [Mn(Pep-2(n-1)H)](2+) have been explored using tandem mass spectrometry. The peptides were synthesized strategically by blocking the N-terminus with an acetyl group and with four cysteine and/or histidine residues in positions 2, 5, 8, and 11, arranged in different motifs: CCHH, CHCH, and CCCC. The MS(2) spectra of the Pb(2+) and Zn(2+) complexes show multiple losses of water and a single methane loss and these provide a sensitive method for locating the metal dication and so elucidating its coordination. The elimination of a methane molecule indicated the position of the metal at the Cys2 residue. Whereas lead was observed to preferentially bind to cysteine residues, zinc was found to primarily bind to histidine residues and secondarily to cysteine residues. Preferential binding of lead to cysteine is preserved in the complexes with more than one Pb(2+). Key to the mechanism of the loss of water and methane is the metal dication withdrawing electrons from the proximal amidic nitrogen. This acidic nitrogen loses its hydrogen to an amidic oxygen situated four atoms away leading to formation of a five-member ring and the elimination of water.

Role of (NO)2 dimer in reactions of Fe+ with NO and NO2 studied by ICP-SIFT mass spectrometry.

In a recent publication by J. J. Melko et al. (J. Phys. Chem. A2012, 116, 11500-11508) on the reactions of Fe(+) cations with NO and NO2, these authors made a number of assertions regarding the work previously published in our laboratory. Melko et al. assert that our previously reported data was erroneously analyzed, resulting in our misreporting of the Fe(+) + NO2 reaction branching ratio for NO(+). Also, they proposed that this alleged misreporting made it likely for the second-order chemistry observed in our Fe(+) + NO experiments to be a product of an impurity of NO2 in our NO reagent and, furthermore, that our reported rate coefficient for the effective second-order chemistry was unreasonably high on the basis of their model calculations. Despite extensive private communications in which we presented detailed data supporting our original data analysis to Melko et al., these authors proceeded to publish their critique without any reference to this data. Here, we present the data communicated by us to Melko et al. and show that our result reported earlier for the Fe(+) + NO2 reaction branching ratio to form NO(+) is accurate and, furthermore, that there is no evidence for a sufficient NO2 impurity in any of our NO experiments. We suggest that the discrepancy in the results observed by us and Melko et al. may be attributed to a reaction with the dimer (NO)2. This possibility was dismissed in our earlier work as the dimer concentration under the flow tube conditions was calculated to be below 10(-5)% of the monomer, but the new results of J. J. Melko et al. raise the dimer reaction as a real possibility. Finally, J. J. Melko et al. appear to have misunderstood the mechanism of the second-order NO chemistry that we had proposed.

Atmospheric pressure chemical ionization mass spectrometry of pyridine and isoprene: potential breath exposure and disease biomarkers.

Volatile organic compounds (VOCs) in exhaled human breath can serve as potential disease-specific and exposure biomarkers and therefore can reveal information about a subject's health and environment. Pyridine, a VOC marker for exposure to tobacco smoke, and isoprene, a liver disease biomarker, were studied using atmospheric pressure chemical ionization mass spectrometry (APCI-MS). While both molecules could be detected in low-ppb levels, interactions of the ionized analytes with their neutral forms and ambient air led to unusual ion/molecule chemistry. The result was a highly dynamic system and a nonlinear response to changes in analyte concentration. Increased presence of ambient water was found to greatly enhance the detection limit of pyridine and only slightly decrease that of isoprene. APCI-MS is shown to be a promising analytical tool in breath analysis with good detection limits, but its application requires a better understanding of the ion/molecule chemistry that may affect VOC quantification from a chemically complex system such as human breath.

Nitrogen dioxide reactions with 46 atomic main-group and transition metal cations in the gas phase: room temperature kinetics and periodicities in reactivity.

Experimental results are reported for the gas-phase room-temperature kinetics of chemical reactions between nitrogen dioxide (NO(2)) and 46 atomic main-group and transition metal cations (M(+)). Measurements were taken with an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer in helium buffer gas at a pressure of 0.35 ± 0.01 Torr and at 295 ± 2 K. The atomic cations were produced at ca. 5500 K in an ICP source and allowed to decay radiatively and to thermalize to room temperature by collisions with Ar and He atoms prior to reaction with NO(2). Measured apparent bimolecular rate coefficients and primary reaction product distributions are reported for all 46 atomic metal cations and these provide an overview of trends across and down the periodic table. Three main types of reactions were observed: O-atom transfer to form either MO(+) or NO(+), electron transfer to form NO(2)(+), and addition to form MNO(2)(+). Bimolecular O-atom transfer was observed to predominate. Correlations are presented between reaction efficiency and the O-atom affinity of the metal cation and between the prevalence of NO(+) product formation and the electron recombination energy of the product metal oxide cation. Some second-order reactions are evident with metal cations that react inefficiently. Most interesting of these is the formation of the MNO(+) cation with Rh(+) and Pd(+). The higher-order chemistry with NO(2) is very diverse and includes the formation of numerous NO(2) ion clusters and a number of tri- and tetraoxide metal cations. Group 2 metal dioxide cations (CaO(2)(+), SrO(2)(+), BaO(2)(+)) exhibit a unique reaction with NO(2) to form MO(NO)(+) ions perhaps by NO transfer from NO(2) concurrent with O(2) formation by recombination of a NO(2) and an oxide oxygen.

Trimethylation and Differential Mobility Spectroscopy in Quantitative Peptide Analysis: Increasing Selectivity and Sensitivity through Ion/Molecule Chemistry.

Isotopic labeling of peptides by trimethylation creates a charged quaternary amine group on the peptide that provides clear differentiation from unlabeled protonated peptides and protonated or sodiated chemical background. Differential mobility spectrometry, with its use of a chemical modifier, allows otherwise undesirable ion/molecule reactions in the mobility cell to increase selectivity and sensitivity of quantitative peptide analysis. A high proton affinity modifier selectively removes protonated and sodiated interference and background ions by proton and sodium transfer, while leaving the trimethylated ions with their quaternary amine groups unchanged.

Lead(II)-catalyzed oxidation of guanine in solution studied with electrospray ionization mass spectrometry.

The oxidation of guanine was investigated in water/methanol solution both in the absence and in the presence of Pb(II) with a variable temperature reactor coupled to a tandem mass spectrometer that allowed signature ions of solution reagents and products to be monitored by electrospray ionization (ESI). Two different oxidizing agents were employed, one strong (peroxymonosulfuric acid) and one weaker (hydrogen peroxide). Peroxymonosulfuric acid was observed to oxidize guanine rapidly at room temperature, k(app) > 10(-2) s(-1), whether in the absence or in the presence of Pb(II), to produce spiroiminohydantoin. Guanine did not show measurable oxidation by hydrogen peroxide in the absence of Pb(II) at concentrations of H(2)O(2) up to 1 M at temperatures up to 333 K (k(app) < 3 × 10(-8) s(-1) at 298 K), but in the presence of Pb(II), it was observed to produce both 5-carboxamido-5-formamido-2-iminohydantoin (2-Ih) and imidazolone (Iz) in a ratio of 2.3 ± 0.1 with a total rate enhancement of more than 4 × 10(3). The activation energy was measured to be 82 ± 11 kJ mol(-1) and is more than 120 kJ mol(-1) lower than that for the uncatalyzed oxidation with hydrogen peroxide measured to be at least 208 ± 26 kJ mol(-1). An activation energy of 113 ± 9 kJ mol(-1) has been reported by Bruskov et al. (Nucleic Acids Res.2002, 30, 1354) for the heat-induced oxidation by hydrogen peroxide of guanine embedded as guanosine in DNA which leads to the production of 8-oxo-7,8-dihydro-guanine (8-oxo-Gua). The atomic lead dication lowers the activation energy by activating the hydrogen peroxide oxidant, possibly by O-O bond activation, and by directing the oxidation, possibly through coordination to the functional groups adjacent to the carbon C5: the C6 carbonyl group and the N7 nitrogen. The coupling of tandem mass spectrometry (MS(2)) with a simple variable temperature reactor by ESI proved to be very effective for measuring reaction kinetics and activation energies in solution. Signature ions of both reagents and products, as well as the catalyst, could be identified, and the data were acquired in real time. The technique should be suitable for exploring other chemical and biochemical reactions that occur on similar time scales (minutes to hours).

Identification of a higher-order organozincate intermediate involved in Negishi cross-coupling reactions by mass spectrometry and NMR spectroscopy.

Negishi cross-coupling reactions were analyzed in solution by mass spectrometry and NMR spectroscopy to identify both the effect of LiBr as an additive as well as the purpose of 3-dimethyl-2-imidazolidinone (DMI) as a co-solvent. The results suggest that the main role of DMI is to facilitate a higher order bromozincate formation during the addition of LiBr.

Differential mobility spectrometry of isomeric protonated dipeptides: modifier and field effects on ion mobility and stability.

The ability to resolve isomeric protonated dipeptides was investigated with the new technique of differential ion mobility mass spectrometry that uses "modifier" molecules to enhance differential mobility. Two pairs of protonated peptides [glycine-alanine (GlyAla) and alanine-glycine (AlaGly), glycine-serine (GlySer) and serine-glycine (SerGly)] and eight different modifiers (water, 2-propanol, 1,5-hexadiene, 2-chloropropane, chlorobenzene, dichloromethane, acetonitrile, and cyclohexane) were used in the initial study. Separation of the protonated peptides was found to be dependent on the mass and proton affinity of the modifier and combinations of functionalities present in the modifier and the analyte ion. Six of the eight modifiers (water, 2-propanol, chlorobenzene, cyclohexane, dichloromethane, and acetonitrile) were able to separate the protonated isomeric peptide pairs, and generally, modifiers with electron-rich groups performed the best. In the presence of some modifiers, a reduction of ion current was observed under the highest field conditions (>115 Td). Dopant-catalyzed isomerization, likely by proton-transport catalysis, and field-induced fragmentation may have contributed to these losses. Two high vapor pressure modifiers, 1,5-hexadiene and 2-chloropropane, significantly influenced ion formation leading to the formation of stable cluster populations that could be observed in the mass spectrometer. Although not a major concern, both fragmentation and influence of modifier evaporation warrant further studies in order to fully understand and possibly eliminate them.