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.

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.

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.

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.

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.

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.

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".

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.

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.

UV-induced bond modifications in thymine and thymine dideoxynucleotide: structural elucidation of isomers by differential mobility mass spectrometry.

Differential mobility spectrometry has been applied to reveal the occurrence of isomerization of thymine nucleobase and of thymine dideoxynucleotide d(5'-TT-3') due to bond redisposition induced by UV irradiation at 254 nm of frozen aqueous solutions of these molecules. Collision-induced dissociation (CID) spectra of electrosprayed photoproducts of the thymine solution suggest the presence of two isomers (the so-called cyclobutane and 6,4-photoproducts) in addition to the proton-bound thymine dimer, and these were separated using differential mobility spectrometry/mass spectrometry (DMS/MS) techniques with water as the modifier. Similar experiments with d(5'-TT-3') revealed the formation of a new isomer of deprotonated thymine dideoxynucleotide upon UV irradiation that was easily distinguished using DMS/MS with isopropanol as the modifier. The results reinforce the usefulness of DMS/MS in isomer separation.

Buckminsterfullerene cations: new dimensions in gas-phase ion chemistry.

The author provides a brief overview, and shares the extraordinary excitement, of the years of unprecedented discoveries in ion chemistry that followed the first production of fullerene powder in 1990. Various charge states of the buckminsterfullerene C60n+ cation became available by conventional electron-impact ionization of the vapor of this powder and so for mass-spectrometric measurements of ion reactivity. The emphasis here will be on fullerene-ion research performed in the author's own laboratory at York University using electron ionization flow-tube mass spectrometry techniques.

Sequential penning ionization: harvesting energy with ions.

We report the observation of the ejection of electrons caused by collisions of excited atoms with ions, rather than neutrals, leading to the production of doubly charged ions. Doping superfluid He droplets with methyl iodide and exposing them to electrons enhances the formation of doubly charged iodine atoms at the threshold for the production of two metastable He atoms. These observations point toward a novel ionization process where doubly charged ions are produced by sequential Penning ionization. In some cases, depending on the neutral target, the process also leads to a subsequent Coulomb explosion of the dopant.

Conversion of methane to methanol: nickel, palladium, and platinum (d9) cations as catalysts for the oxidation of methane by ozone at room temperature.

The room-temperature chemical kinetics has been measured for the catalytic activity of Group 10 atomic cations in the oxidation of methane to methanol by ozone. Ni(+) is observed to be the most efficient catalyst. The complete catalytic cycle with Ni(+) is interpreted with a computed potential energy landscape and, in principle, has an infinite turnover number for the oxidation of methane, without poisoning side reactions. The somewhat lower catalytic activity of Pd(+) is reported for the first time and also explored with DFT calculations. Pt(+) is seen to be ineffective as a catalyst because of the observed failure of PtO(+) to convert methane to methanol.

Peptide quantitation with methyl iodide isotopic tags and mass spectrometry.

A novel method is presented for the quantitation of peptides based on their methylation by in vacuo chemical reaction with methyl iodide. Samples of two small peptides, hexaglycine and pentaalanine, were labeled with CH(3)I and CD(3)I, representing the "unknown" and "standard" respectively, and then subjected to a series of tests using mass spectrometry to ascertain the suitability of the isotopic labels for peptide quantitation. The experiments show methyl iodide to be a very quantitative label, exhibiting a linear relationship in concentration over the dynamic range of the mass spectrometer used in the analysis (up to 4 orders of magnitude) both as pure samples and in a complex mixture of peptides. The tendency of trimethylated peptides to preferentially form a(2) fragment ions in MS(2) produces a significant increase in sensitivity, especially when the mass spectrometer is used in the MRM mode. Tests were also performed to verify the stability of the label against H/D exchange and its suitability for long-term storage, showing little degradation while in solution and during subsequent chemical processing.

Nitrogen dioxide reactions with atomic lanthanide cations and their monoxides: gas-phase kinetics at room temperature.

Results of experimental investigations are reported for the gas-phase kinetics of chemical reactions between nitrogen dioxide (NO(2)) and 14 different atomic cations of the lanthanide series, Ln(+) (Ln = La-Lu, excluding Pm), and their monoxides, LnO(+). 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 lanthanide cations were produced at ca. 5500 K in an ICP source and allowed to decay radiatively and to thermalize by collisions with Ar and He atoms prior to reaction with NO(2). The atomic ions were observed to react rapidly with NO(2) with large rate coefficients, k > 2 x 10(-10) cm(3) molecule(-1) s(-1), and almost exclusively by oxygen-atom abstraction to produce lanthanide-oxide LnO(+) cations. In contrast to results of previous studies with many other molecules, the reaction efficiency exhibits essentially no dependence upon the energy required to promote an electron to achieve a d(1)s(1) excited electronic configuration, in which two non-f electrons are available to Ln(+) for chemical bonding. Apparently the radical character of NO(2) (X (2)A(1)) leads to the efficient formation of LnO(+) by the end-on abstraction of an oxygen atom by Ln(+). In the reactions with La(+), Ce(+), Pr(+) and Gd(+) an additional minor channel (less than 2%) leads to the formation of NO(+). The LnO(+) product ions participate in various secondary and higher order reactions with NO(2) resulting in the formation of ions of the type LnO(x)(NO)(y)(NO(2))(z)(+) with x = 1-2, y = 0-2, and z = 0-2, as well as the ions NO(+) and NO(2)(+).

Gas-phase reactions of atomic lanthanide cations with ammonia: room-temperature kinetics and periodicity in reactivity.

Reactions of (14) atomic lanthanide cations (excluding Pm(+)) with ammonia have been surveyed in the gas phase by using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer to measure rate coefficients and product distributions in He at 0.35 +/- 0.01 Torr and 295 +/- 2 K. Primary reaction channels were observed corresponding to H(2) elimination with formation of the protonated lanthanum nitride and NH(3) addition. H(2) elimination was seen only in the reactions with La(+), Ce(+), Gd(+), and Tb(+) and occurs with these ions exclusively. NH(3) addition was seen with Pr(+), Nd(+), Sm(+), Eu(+), Dy(+), Ho(+), Er(+), Tm(+), Yb(+), and Lu(+). Higher-order sequential addition of up to five NH(3) molecules was observed with the Ln(+)(NH(3)) and LnNH(+) ions. The reaction efficiency of the primary reactions is seen to decrease as the energy required to promote an electron to make two non-f electrons available for bonding increases. The periodic trend in reaction efficiency along the lanthanide series matches quite closely the periodic trend in the electron-promotion energy required to achieve a d(1)s(1) or d(2) excited electronic configuration in the lanthanide cation. With La(+), Ce(+), Gd(+), and Tb(+), the electrostatic attraction between the atomic lanthanide cation and ammonia is sufficiently strong to provide enough energy to achieve electron promotion and to overcome any barriers to subsequent N-H bond insertion and H(2) loss, but this is not the case with the other lanthanide cations with which collisional stabilization of the intermediate adduct ion, with or without insertion of Ln(+), predominates.

Mass-spectrometric studies of the interactions of selected metalloantibiotics and drugs with deprotonated hexadeoxynucleotide GCATGC.

ESI tandem mass spectrometry is employed in a detailed study of the interactions of a hexameric duplex d(5'GCATGC) with three types of ligated first-row transition metal dications M(2+): metallated bleomycins, singly, doubly, and triply ligated metallophenanthrolines and [M(triethylenetetramine)](2+). The singly, doubly, and triply metallated species were found to dissociate by noncovalent separation into two strands with metal ions attached either to one or to both. Relative gas-phase stabilities of the double-stranded oligodeoxynucleotide (ODN)-M(2+) complexes were found to follow the order Mn(II) > Fe(II) > Co(II) > Ni(II) > Zn(II) > Cu(II). Overall, the presence of metal dications is found to increase the gas-phase stability of the duplex against noncovalent dissociation with the exception of one and three copper dications. An analysis of the dissociation pathways and relative gas-phase stabilities of the species that were investigated provided a basis for the assessment of the possible binding modes between duplex oligonucleotides and metallocomplexes.

Gas-phase reactions of sulfur hexafluoride with transition metal and main group atomic cations: room-temperature kinetics and periodicities in reactivity.

Gas-phase reactions of SF(6) were investigated with 46 different atomic metal and main group cations at room temperature using an Inductively-Coupled Plasma/Selected-Ion Flow Tube (ICP/SIFT) tandem mass spectrometer. The atomic ions were produced at about 5500 K in the ICP source and allowed to decay radiatively and to thermalize by collisions with argon and helium atoms prior to reaction downstream in a flow tube in helium buffer gas at 0.35 +/- 0.01 Torr and 295 +/- 2 K. Rate coefficients and product distributions were measured for the reactions of fourth-row atomic ions from K(+) to Se(+), of fifth-row atomic ions from Rb(+) to Te(+) (excluding Tc(+)), and of sixth-row atomic ions from Cs(+) to Bi(+). The early transition metal ions react with SF(6) very efficiently (k/k(c) = 0.56-0.96) to produce MF(m)(+) (m = 1-4) and SF(n)(+) (n = 1-4) ions, whereas the late transition metal ions react much less efficiently (k/k(c) < 0.2) to form M(+)(SF(6)) adduct ions. Reactions of SF(6) with Ca(+), Sr(+), Ba(+), Ge(+), and As(+) proceed efficiently (k/k(c) = 0.35-0.85) through various channels, while all other main group metal ions are inert toward sulfur hexafluoride. Primary and secondary adduct formation was observed to exhibit equilibrium kinetics, and the standard free energy change for SF(6) addition is found to correlate with the efficiency of addition according to log[k/k(c)] = -8.7 + 7.8 log[-DeltaG(o)/(kcal mol(-1))]. Several MF(m)(+) ions were observed to react further with SF(6) to produce MF(m+k)(+), SF(n)(+), and MF(m)(+)(SF(6)) as secondary products.

Reactions of atomic cations with methane: gas phase room-temperature kinetics and periodicities in reactivity.

Reactions of methane have been measured with 59 atomic metal cations 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. The atomic cations were produced at approximately 5500 K in an ICP source and allowed to decay radiatively and to thermalize by collisions with argon and helium atoms prior to reaction. Rate coefficients and product distributions are reported for the reactions of fourth-row atomic cations from K(+) to Se(+), of fifth-row atomic cations from Rb(+) to Te(+) (excluding Tc(+)), of sixth-row atomic cations from Cs(+) to Bi(+), and of the lanthanide cations from La(+) to Lu(+) (excluding Pm(+)). Two primary reaction channels were observed: C-H bond insertion with elimination of H(2), and CH(4) addition. The bimolecular H(2) elimination was observed in the reactions of CH(4) with As(+), Nb(+), and some sixth-row metal cations, i.e., Ta(+), W(+), Os(+), Ir(+), Pt(+); secondary and higher-order H(2) elimination was observed exclusively for Ta(+), W(+), and Ir(+) ions. All other transition-metal cations except Mn(+) and Re(+) were observed to react with CH(4) exclusively by addition, and up to two methane molecules were observed to add sequentially to most transition-metal ions. CH(4) addition was also observed for Ge(+), Se(+), La(+), Ce(+), and Gd(+) ions, while the other main-group and lanthanide cations did not react measurably with methane.