Reactions of first-row transition metal ions with propargyl alcohol in the gas phase

The gas-phase reactions with propargyl alcohol (PPA) of all the singly charged ions of the first-row transition metals, generated by laser ablation in an external ion source, were studied by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICRMS.). The reactivities of the metal ions change irregularly across the periodic table, and the reactivity of each ion is a function of its electronic configuration and corresponding metal-oxygen (M-O) bond energies. The 10 metal ions were classified into three categories according to their reactivities: Sc(+), Ti(+) and V(+) are the most reactive ions which react with PPA to give many kinds of oxygen-rich products due to stronger M-O bonds; Fe(+), Co(+) and Ni(+) are less reactive; Cr(+), Mn(+), Cu(+) and Zn(+) are the most unreactive ions, due to the half and completely occupied valence electronic configurations. The order of reactivity is Ti(+) > V(+) > Sc(+) > Co(+) > Fe(+) approximately Ni(+) > Zn(+) > Cr(+) approximately Mn(+) approximately Cu(+).     

Dipole effects on cation-pi interactions: absolute bond dissociation energies of complexes of alkali metal cations to N-methylaniline and N,N-dimethylaniline

Threshold collision-induced dissociation of M (+)( nMA) x with Xe is studied using guided ion beam mass spectrometry, where nMA = N-methylaniline and N, N-dimethylaniline and x = 1 and 2. M (+) includes the following alkali metal cations: Li (+), Na (+), K (+), Rb (+), and Cs (+). In all cases, the primary dissociation pathway corresponds to the endothermic loss of an intact nMA ligand. The primary cross section thresholds are interpreted to yield 0 and 298 K bond dissociation energies (BDEs) for ( nMA) x-1 M (+)-( nMA) after accounting for the effects of multiple ion-neutral collisions, the internal and kinetic energy distributions of the reactants, and the dissociation lifetimes. Density functional theory calculations at the B3LYP/6-31G* level of theory are used to determine the structures of these complexes, which are also used in single-point calculations at the MP2(full)/6-311+G(2d,2p) level to determine theoretical BDEs. The results of these studies are compared to previous studies of the analogous M (+)(aniline) x complexes to examine the effects of methylation of the amino group on the binding interactions. Comparisons are also made to a wide variety of cation-pi complexes previously studied to elucidate the contributions that ion-dipole, ion-induced-dipole, and ion-quadrupole interactions make to the overall binding.     

Tautomerization in the formation and collision-induced dissociation of alkali metal cation-cytosine complexes

Noncovalent interactions between alkali metal cations and the various low-energy tautomeric forms of cytosine are investigated both experimentally and theoretically. Threshold collision-induced dissociation (CID) of M(+)(cytosine) complexes with Xe is studied using guided ion beam tandem mass spectrometry, where M(+) = Li(+), Na(+), and K(+). In all cases, the only dissociation pathway observed corresponds to endothermic loss of the intact cytosine molecule. The cross-section thresholds are interpreted to yield 0 and 298 K bond dissociation energies (BDEs) for the M(+)(cytosine) complexes after accounting for the effects of multiple ion-neutral collisions, the kinetic and internal energy distributions of the reactants, and dissociation lifetimes. Ab initio calculations are performed at the MP2(full)/6-31G* level of theory to determine the structures of the neutral cytosine tautomers, the M(+)(cytosine) complexes, and the TSs for unimolecular tautomerization. The molecular parameters derived from these structures are employed for the calculation of the unimolecular rates for tautomerization and the thermochemical analysis of the experimental data. Theoretical BDEs of the various M(+)(cytosine) complexes and the energy barriers for the unimolecular tautomerization of these complexes are determined at MP2(full)/6-311+G(2d,2p) level of theory using the MP2(full)/6-31G* optimized geometries. In addition, BDEs for the Li(+)(cytosine) complexes are also determined at the G3 level of theory. Based upon the tautomeric mixture generated upon thermal vaporization of cytosine, calculated M(+)-cytosine BDEs and barriers to tautomerization for the low-energy tautomeric forms of M(+)(cytosine), and measured thresholds for CID of M(+)(cytosine) complexes, we conclude that tautomerization occurs during both complex formation and CID.     

Infrared spectroscopy of Cu+(H2O)(n) and Ag+(H2O)(n): coordination and solvation of noble-metal ions

M(+)(H(2)O)(n) and M(+)(H(2)O)(n)Ar ions (M=Cu and Ag) are studied for exploring coordination and solvation structures of noble-metal ions. These species are produced in a laser-vaporization cluster source and probed with infrared (IR) photodissociation spectroscopy in the OH-stretch region using a triple quadrupole mass spectrometer. Density functional theory calculations are also carried out for analyzing the experimental IR spectra. Partially resolved rotational structure observed in the spectrum of Ag(+)(H(2)O)(1) x Ar indicates that the complex is quasilinear in an Ar-Ag(+)-O configuration with the H atoms symmetrically displaced off axis. The spectra of the Ar-tagged M(+)(H(2)O)(2) are consistent with twofold coordination with a linear O-M(+)-O arrangement for these ions, which is stabilized by the s-d hybridization in M(+). Hydrogen bonding between H(2)O molecules is absent in Ag(+)(H(2)O)(3) x Ar but detected in Cu(+)(H(2)O)(3) x Ar through characteristic changes in the position and intensity of the OH-stretch transitions. The third H(2)O attaches directly to Ag(+) in a tricoordinated form, while it occupies a hydrogen-bonding site in the second shell of the dicoordinated Cu(+). The preference of the tricoordination is attributable to the inefficient 5s-4d hybridization in Ag(+), in contrast to the extensive 4s-3d hybridization in Cu(+) which retains the dicoordination. This is most likely because the s-d energy gap of Ag(+) is much larger than that of Cu(+). The fourth H(2)O occupies the second shells of the tricoordinated Ag(+) and the dicoordinated Cu(+), as extensive hydrogen bonding is observed in M(+)(H(2)O)(4) x Ar. Interestingly, the Ag(+)(H(2)O)(4) x Ar ions adopt not only the tricoordinated form but also the dicoordinated forms, which are absent in Ag(+)(H(2)O)(3) x Ar but revived at n=4. Size dependent variations in the spectra of Cu(+)(H(2)O)(n) for n=5-7 provide evidence for the completion of the second shell at n=6, where the dicoordinated Cu(+)(H(2)O)(2) subunit is surrounded by four H(2)O molecules. The gas-phase coordination number of Cu(+) is 2 and the resulting linearly coordinated structure acts as the core of further solvation processes.     

Cation-pi interactions: structures and energetics of complexation of Na+ and K+ with the aromatic amino acids, phenylalanine, tyrosine, and tryptophan

Threshold collision-induced dissociation of M(+)(AAA) with Xe is studied using guided ion beam tandem mass spectrometry. M(+) include the alkali metal ions Na(+) and K(+). The three aromatic amino acids are examined, AAA = phenylalanine, tyrosine, or tryptophan. In all cases, endothermic loss of the intact aromatic amino acid is the dominant reaction pathway. The threshold regions of the cross sections are interpreted to extract 0 and 298 K bond dissociation energies for the M(+)-AAA complexes after accounting for the effects of multiple ion-neutral collisions, internal energy of the reactant ions, and dissociation lifetimes. Density functional theory calculations at the B3LYP/6-31G level of theory are used to determine the structures of the neutral aromatic amino acids and their complexes to Na(+) and K(+) and to provide molecular constants required for the thermochemical analysis of the experimental data. Theoretical bond dissociation energies are determined from single-point energy calculations at the B3LYP/6-311++G(3df,3pd) level using the B3LYP/6-31G geometries. Good agreement between theory and experiment is found for all systems. The present results are compared to earlier studies of these systems performed via kinetic and equilibrium methods. The present results are also compared to the analogous Na(+) and K(+) complexes to glycine, benzene, phenol, and indole to elucidate the relative contributions that each of the functional components of these aromatic amino acids make to the overall binding in these complexes.     

Noncovalent interactions of Zn+ with N-donor ligands (pyridine, 4,4'-dipyridyl, 2,2'-dipyridyl, and 1,10-phenanthroline): collision-induced dissociation and theoretical studies

The binding interactions in complexes of Zn(+) with nitrogen donor ligands, (N-L) = pyridine (x = 1-4), 4,4'-dipyridyl (x = 1-3), 2,2'-dipyridyl (x = 1-2), and 1,10-phenanthroline (x = 1-2), are examined in detail. The bond dissociation energies (BDEs) for loss of an intact ligand from the Zn(+)(N-L)(x) complexes are reported. Experimental BDEs are obtained from thermochemical analyses of the threshold regions of the collision-induced dissociation cross sections of Zn(+)(N-L)(x) complexes. Density functional theory calculations at the B3LYP/6-31G* level of theory are performed to determine stable structures of these species and to provide molecular parameters needed for the thermochemical analysis of experimental data. Relative stabilities of the various conformations of these N-donor ligands and their complexes to Zn(+) as well as theoretical BDEs are determined from single point energy calculations at the B3LYP/6-311+G(2d,2p) and M06/6-311+G(2d,2p) levels of theory using the B3LYP/6-31G* optimized geometries. The experimental BDEs for the Zn(+)(N-L)(x) complexes are in reasonably good agreement with values derived from density functional theory calculations. BDEs derived from M06 calculations provide better agreement with the measured values than those based on B3LYP calculations. Trends in the sequential BDEs are explained in terms of sp polarization of Zn(+) and repulsive ligand-ligand interactions. Comparisons are made to the analogous Cu(+)(N-L)(x) and Ni(+)(N-L)(x) complexes previously studied.     

Reactions of methyl fluoride with atomic transition-metal and main-group cations: gas-phase room-temperature kinetics and periodicities in reactivity

Reactions of CH(3)F have been surveyed systematically at room temperature with 46 different atomic cations using an inductively coupled plasma/selected-ion flow tube tandem mass spectrometer. Rate coefficients and product distributions were measured for the reactions of fourth-period atomic ions from K(+) to Se(+), of fifth-period atomic ions from Rb(+) to Te(+) (excluding Tc(+)), and of sixth-period atomic ions from Cs(+) to Bi(+). Primary reaction channels were observed corresponding to F atom transfer, CH(3)F addition, HF elimination, and H(2) elimination. The early-transition-metal cations exhibit a much more active chemistry than the late-transition-metal cations, and there are periodic features in the chemical activity and reaction efficiency that maximize with Ti(+), As(+), Y(+), Hf(+), and Pt(+). F atom transfer appears to be thermodynamically controlled, although a periodic variation in efficiency is observed within the early-transition-metal cations which maximizes with Ti(+), Y(+), and Hf(+). Addition of CH(3)F was observed exclusively (>99%) with the late-fourth-period cations from Mn(+) to Ga(+), the fifth-period cations from Ru(+) to Te(+), and the sixth-period cations from Hg(+) to Bi(+) as well as Re(+). Periodic trends are observed in the effective bimolecular rate coefficient for CH(3)F addition, and these are consistent with expected trends in the electrostatic binding energies of the adduct ions and measured trends in the standard free energy of addition. HF elimination is the major reaction channel with As(+), while dehydrogenation dominates the reactions of W(+), Os(+), Ir(+), and Pt(+). Sequential F atom transfer is observed with the early-transition-metal cations, with the number of F atoms transferred increasing across the periodic table from two to four, maximizing at four for the group 5 cations Nb(+)(d(4)) and Ta(+)(d(3)s(1)), and stopping at two with V(+)(d(4)). Sequential CH(3)F addition was observed with many atomic cations and all of the metal mono- and multifluoride cations that were formed.     

Effect of metal ions (Li+, Na+, K+, Mg2+, Ca2+, Ni2+, Cu2+, and Zn2+) and water coordination on the structure and properties of L-arginine and zwitterionic L-arginine

Interactions between metal ions and amino acids are common both in solution and in the gas phase. The effect of metal ions and water on the structure of L-arginine is examined. The effects of metal ions (Li(+), Na(+), K(+), Mg(2+), Ca(2+), Ni(2+), Cu(2+), and Zn(2+)) and water on structures of Arg x M(H2O)m , m = 0, 1 complexes have been determined theoretically by employing the density functional theories (DFT) and using extended basis sets. Of the three stable complexes investigated, the relative stability of the gas-phase complexes computed with DFT methods (with the exception of K(+) systems) suggests metallic complexes of the neutral L-arginine to be the most stable species. The calculations of monohydrated systems show that even one water molecule has a profound effect on the relative stability of individual complexes. Proton dissociation enthalpies and Gibbs energies of arginine in the presence of the metal cations Li(+), Na(+), K(+), Mg(2+), Ca(2+), Ni(2+), Cu(2+), and Zn(2+) were also computed. Its gas-phase acidity considerably increases upon chelation. Of the Lewis acids investigated, the strongest affinity to arginine is exhibited by the Cu(2+) cation. The computed Gibbs energies DeltaG(o) are negative, span a rather broad energy interval (from -150 to -1500 kJ/mol), and are appreciably lowered upon hydration.     

Influence of thioketo substitution on the properties of uracil and its noncovalent interactions with alkali metal ions: threshold collision-induced dissociation and theoretical studies

Experimental and theoretical studies are carried out to determine the influence of thioketo substitution on the properties of uracil and its noncovalent interactions with alkali metal ions. Bond dissociation energies of alkali metal ion-thiouracil complexes, M(+)(SU), are determined using threshold collision-induced dissociation techniques in a guided ion beam mass spectrometer, where M(+) = Li(+), Na(+), and K(+) and SU = 2-thiouracil, 4-thiouracil, 2,4-dithiouracil, 5-methyl-2-thiouracil, and 6-methyl-2-thiouracil. Ab initio electronic structure calculations are performed to determine the structures and theoretical bond dissociation energies of these complexes and provide molecular constants necessary for thermodynamic analysis of the experimental data. Theoretical calculations are also performed to examine the influence of thioketo substitution on the acidities, proton affinities, and A::SU Watson-Crick base pairing energies. In general, thioketo substitution leads to an increase in both the proton affinity and the acidity of uracil. 2-Thio substitution generally results in an increase in the alkali metal ion binding affinities but has almost no affect on the stability of the A::SU base pair. In contrast, 4-thio substitution results in a decrease in the alkali metal ion binding affinities and a significant decrease in the stability of the A::SU base pair. In addition, alkali metal ion binding is expected to lead to an increase in the stability of both single-stranded and double-stranded nucleic acids by reducing the charge on the nucleic acid in a zwitterion effect as well as through additional noncovalent interactions between the alkali metal ion and the nucleobases.     

Metal cation dependence of interactions with amino acids: bond energies of Cs+ to Gly, Pro, Ser, Thr, and Cys

The interactions of cesium cations with five amino acids (AA) including glycine (Gly), proline (Pro), serine (Ser), threonine (Thr), and cysteine (Cys) are examined in detail. Experimentally, the bond dissociation energies (BDEs) are determined using threshold collision-induced dissociation of the Cs(+)(AA) complexes with xenon in a guided ion beam tandem mass spectrometer. Analyses of the energy-dependent cross sections include consideration of unimolecular decay rates, internal energy of the reactant ions, and multiple ion-neutral collisions. Bond dissociation energies (0 K) of 93.3 ± 2.5, 107.9 ± 4.6, 102.3 ± 4.1, 105.4 ± 4.3, and 96.8 ± 4.2 kJ/mol are determined for complexes of Cs(+) with Gly, Pro, Ser, Thr, and Cys, respectively. Quantum chemical calculations are conducted at the B3LYP, B3P86, MP2(full), and M06 levels of theory with geometries and zero-point energies calculated at the B3LYP level using both HW*/6-311+G(2d,2p) and def2-TZVPPD basis sets. Results obtained using the former basis sets are systematically low compared to the experimental bond energies, whereas the latter basis sets show good agreement. For Cs(+)(Gly), theory predicts the ground-state conformer has the cesium cation binding to the carbonyl group of the carboxylic acid. For Cs(+)(Pro), the secondary nitrogen accepts the carboxylic acid hydrogen to form the zwitterionic structure, and the metal cation binds to both oxygens. Cs(+)(Ser), Cs(+)(Thr), and Cs(+)(Cys) are found to have tridentate binding at the MP2(full) level, whereas the density functional approaches slightly prefer bidentate binding of Cs(+) at the carboxylic acid moiety. Comparison of these results to those for the smaller alkali cations provides insight into the trends in binding affinities and structures associated with metal cation variations.     

Multiphoton processes of CO at 230 nm

High resolution kinetic energy release spectra were obtained for C(+) and O(+) from CO multiphoton ionization followed by dissociation of CO(+). The excitation was through the CO (B (1)Sigma(+)) state via resonant two-photon excitation around 230 nm. A total of 5 and 6 photons are found to contribute to the production of carbon and oxygen cations. DC slice and Megapixel ion imaging techniques were used to acquire high quality images. Major features in both O(+) and C(+) spectra are assigned to the dissociation of some specific vibrational levels of CO(+)(X (2)Sigma(+)). The angular distributions of C(+) and O(+) are very distinct and those of various features of C(+) are also different. A dramatic change of the angular distribution of C(+) from dissociation of CO(+)(X (2)Sigma(+), nu(+) = 1) is attributed to an accidental one-photon resonance between CO(+)(X (2)Sigma(+), nu(+) = 1) and CO(+)(B (2)Sigma(+), nu(+) = 0) and explained well by a theoretical model. Both kinetic energy release and angular distributions were used to reveal the underlying dynamics.     

Hydration energies in the gas phase of select (MX)mM+ ions, where M+ = Na+, K+, Rb+, Cs+, NH4+ and X- = F-, Cl-, Br-, I-, NO2-, NO3-. Observed magic numbers of (MX)mM+ ions and their possible significance

The sequential hydration energies and entropies with up to four water molecules were obtained for MXM(+) = NaFNa(+), NaClNa(+), NaBrNa(+), NaINa(+), NaNO(2)Na(+), NaNO(3)Na(+), KFK(+), KBrK(+), KIK(+), RbIRb(+), CsICs(+), NH(4)BrNH(4)(+), and NH(4)INH(4)(+) from the hydration equilibria in the gas phase with a reaction chamber attached to a mass spectrometer. The MXM(+) ions as well as (MX)(m)M(+) and higher charged ions such as (MX)(m)M(2)(2+) were obtained with electrospray. The observed trends of the hydration energies of MXM(+) with changing positive ion M(+) or the negative ion X(-) could be rationalized on the basis of simple electrostatics. The most important contribution to the (MXM-OH(2))(+) bond is the interaction of the permanent and induced dipole of water with the positive charge of the nearest-neighbor M(+) ion. The repulsion due to the water dipole and the more distant X(-) has a much smaller effect. Therefore, the bonding in (MXM-OH(2))(+) for constant M and different X ions changes very little. Similarly, for constant X and different M, the bonding follows the hydration energy trends observed for the naked M(+) ions. The sequential hydration bond energies for MXM(H(2)O)(n)(+) decrease with n in pairs, where for n = 1 and n = 2 the values are almost equal, followed by a drop in the values for n = 3 and n = 4, that again are almost equal. The hydration energies of (MX)(m)M(+) decrease with m. The mass spectra with NaCl, obtained with electrospray and observed in the absence of water vapor, show peaks of unusually high intensities (magic numbers) at m = 4, 13, and 22. Experiments with variable electrical potentials in the mass spectrometer interface showed that some but not all of the ion intensity differentiation leading to magic numbers is due to collision-induced decomposition of higher mass M(MX)(m)(+) and M(2)(MX)(m)(2+) ions in the interface. However, considerable magic character is retained in the absence of excitation. This result indicates that the magic ions are present also in the saturated solution of the droplets produced by electrospray and are thus representative of particularly stable nanocrystals in the saturated solution. Hydration equilibrium determinations in the gas phase demonstrated weaker hydration of the magic ion (NaCl)(4)Na(+).     

Gas-phase kinetic measurements of the ligation of Ni+, Cu+, Ni(pyrrole)1,2+ and Cu(pyrrole)1,2+ with CO2, D2O, NH3 and NO

The rate and equilibrium kinetics of the reactions of the biologically important metal species M(+), M(+)(pyrrole) and M(+)(pyrrole)(2) (M = Ni, Cu) have been investigated with the biological gases CO(2), D(2)O, NH(3) and NO in the gas phase at 295 +/- 2 K in helium buffer-gas at a pressure of 0.35 +/- 0.01 Torr. The measurements were taken with an Inductively Coupled Plasma/Selected-Ion Flow Tube (ICP/SIFT) tandem mass spectrometer. Only ligation was observed for the reactions of bare Ni(+) and Cu(+) with CO(2), D(2)O and NH(3) with rates consistent with the known strengths of the resulting ligand-metal bonds. Both metal cations appeared to be oxidized and produce N(2)O in interesting reactions that are second order in NO. One pyrrole ligand was observed to increase the rate of ligation by as much as a factor of 100 and to switch off the oxidation with NO. Equilibrium was achieved for the ligation of CO(2), D(2)O and NO to both Ni(+)(pyrrole) and Cu(+)(pyrrole), and so it was possible to determine absolute values for the standard free energies of ligation. No ligand substitution was observed with M(+)(pyrrole). M(+)(pyrrole)(2) was observed to be generally unreactive towards the small molecules investigated: a notable exception is ammonia. Very fast ligand substitution reactions were observed for reactions of M(+)(pyrrole)(2) with NH(2).     

Alkali metal ion binding to amino acids versus their methyl esters: affinity trends and structural changes in the gas phase

The relative alkali metal ion (M(+)) affinities (binding energies) between seventeen different amino acids (AA) and the corresponding methyl esters (AAOMe) were determined in the gas phase by the kinetic method based on the dissociation of AA-M(+)-AAOMe heterodimers (M=Li, Na, K, Cs). With the exception of proline, the Li(+), Na(+), and K(+) affinities of the other aliphatic amino acids increase in the order AAAAOMe is already observed for K(+). Proline binds more strongly than its methyl ester to all M(+) except Li(+). Ab initio calculations on the M(+) complexes of alanine, beta-aminoisobutyric acid, proline, glycine methyl ester, alanine methyl ester, and proline methyl ester show that their energetically most favorable complexes result from charge solvation, except for proline which forms salt bridges. The most stable mode of charge solvation depends on the ligand (AA or AAOMe) and, for AA, it gradually changes with metal ion size. Esters chelate all M(+) ions through the amine and carbonyl groups. Amino acids coordinate Li(+) and Na(+) ions through the amine and carbonyl groups as well, but K(+) and Cs(+) ions are coordinated by the O atoms of the carboxyl group. Upon consideration of these differences in favored binding geometries, the theoretically derived relative M(+) affinities between aliphatic AA and AAOMe are in good overall agreement with the above given experimental trends. The majority of side chain functionalized amino acids studied show experimentally the affinity order AAAAOMe. The latter ranking is attributed to salt bridge formation.     

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

The chemistry of carbon dioxide has been surveyed systematically with 46 atomic cations at room temperature using an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. The atomic cations were produced at ca. 5500 K in an ICP source and allowed to cool radiatively and to thermalize by collisions with Ar and He 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 products were measured for the reactions of first-row atomic ions from K(+) to Se(+), of second-row atomic ions from Rb(+) to Te(+) (excluding Tc(+)), and of third-row atomic ions from Cs(+) to Bi(+). CO(2) was found to react in a bimolecular fashion by O atom transfer only with 9 early transition-metal cations: the group 3 cations Sc(+), Y(+), and La(+), the group 4 cations Ti(+), Zr(+), and Hf(+), the group 5 cations Nb(+) and Ta(+), and the group 6 cation W(+). Electron spin conservation was observed to control the kinetics of O atom transfer. Addition of CO(2) was observed for the remaining 37 cations. While the rate of addition was not measurable some insight was obtained into the standard free energy change, DeltaG(o), for CO(2) ligation from equilibrium constant measurements. A periodic variation in DeltaG(o) was observed for first row cations that is consistent with previous calculations of bond energies D(0)(M(+)-CO(2)). The observed trends in D(0) and DeltaG(o) are expected from the variation in electrostatic attraction between M(+) and CO(2) which follows the trend in atomic-ion size and the trend in repulsion between the orbitals of the atomic cations and the occupied orbitals of CO(2). Higher-order CO(2) cluster ions with up to four CO(2) ligands also were observed for 24 of the atomic cations while MO(2)(+) dioxide formation by sequential O atom transfer was seen only with Hf(+), Nb(+), Ta(+), and W(+).     

Catalytic oxidation of H2 by N2O in the gas phase: O-atom transport with atomic metal cations

Twenty-five atomic cations, M (+), that lie within the thermodynamic window for O-atom transport catalysis of the oxidation of hydrogen by nitrous oxide, have been checked for catalytic activity at room temperature with kinetic measurements using an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Only 4 of these 25 atomic cations were seen to be catalytic: Fe (+), Os (+), Ir (+), and Pt (+). Two of these, Ir (+) and Pt (+), are efficient catalysts, while Fe (+) and Os (+) are not. Eighteen atomic cations (Cr (+), Mn (+), Co (+), Ni (+), Cu (+), Ge (+), Se (+), Mo (+), Ru (+), Rh (+), Sn (+), Te (+), Re (+), Pb (+), Bi (+), Eu (+), Tm (+), and Yb (+)) react too slowly at room temperature either in their oxidation with N 2O to form MO (+) or in the reduction of MO (+) by H 2. Many of these reactions are known to be spin forbidden and a few actually may lie outside the thermodynamic window. Three alkaline-earth metal monoxide cations, CaO (+), SrO (+), and BaO (+), were observed to favor MOH (+) formation in their reactions with H 2. A potential-energy landscape is computed for the oxidation of H 2 with N 2O catalyzed by Fe (+)( (6)D) that vividly illustrates the operation of an ionic catalyst and qualitatively accounts for the relative inefficiency of this catalyst.     

NO+ formation pathways in dissociation of N2O+ ions at the C2Σ+ state revealed from threshold photoelectron-photoion coincidence velocity imaging

Using the novel threshold photoelectron-photoion coincidence (TPEPICO) velocity imaging technique, the dissociative photoionization of N(2)O molecule via the C(2)Σ(+) ionic state has been investigated. Four fragment ions, NO(+), N(2)(+), O(+), and N(+), are observed, respectively, and the NO(+) and N(+) ions are always dominant in the whole excitation energy range of the C(2)Σ(+) ionic state. Subsequently, the TPEPICO three-dimensional time-sliced velocity images of NO(+) dissociated from the vibrational state-selected N(2)O(+)(C(2)Σ(+)) ions have been recorded. Thus the kinetic and internal energy distributions of the NO(+) fragments have been obtained directly as the bimodal distributions, suggesting that the NO(+) fragments are formed via both NO(+)(X(1)Σ(+)) + N((2)P) and NO(+)(X(1)Σ(+)) + N((2)D) dissociation channels. Almost the same vibrational population reversions are identified for both dissociation pathways. Interestingly, the obtained branching ratios of the two channels exhibit some dependence on the excited vibrational mode for N(2)O(+)(C(2)Σ(+)), in which the excited asymmetrical stretching potentially promotes dissociation possibility along the NO(+)(X(1)Σ(+)) + N((2)D) pathway. In addition, the measured anisotropic parameters of NO(+) are close to 0.5, indicating that the C(2)Σ(+) state of N(2)O(+) is fully predissociative, indeed, with a tendency of parallel dissociation, and therefore, the corresponding predissociation mechanisms for the N(2)O(+)(C(2)Σ(+)) ions are depicted.     

Is collision-induced dissociation of low-energy carbonyl sulfide cations adversely affected by asymmetry?

We have measured relative abundances of fragment ions resulting from collision-induced dissociation of OCS(+) ions in collision with xenon neutrals as a function of ion kinetic energy and scattering angle. The lowest energy dissociation product, S(+), dominates at all energies up to 53 eV kinetic energy studied here. Surprisingly, the second most abundant dissociation channel is CS(+) and not CO(+) even though the thermochemical threshold for CO(+) is lower than that for CS(+) and CO(+) is more abundant than CS(+) in the normal mass spectrum of OCS. We do not observe any significant abundance of CO(+) in this energy range, suggesting that collision-induced excitation and dissociation of OCS(+) is significantly different to that of symmetric triatomic ions. A possible role of asymmetry in the molecular ion's collisional activation via neutral collision is suggested for the different behavior.     

Dissociation of gas-phase dimeric complexes of lactic acid and transition-metal ions formed under electrospray ionization conditions; the role of reduction of the metal ion

Dimeric complex ions of the type [M(A-H)A]+, where M=metal ion (Co, Ni, Cu, and Zn) and A=ligand (lactic acid, methyl lactate or ethyl lactate), were generated in the gas phase under electrospray ionization conditions. The collision-induced dissociation spectra of [M(A-H)A]+ ions were recorded to study the behaviour of ligand and metal ions in decomposition of these dimeric complex ions. Based on the fragmentation pathways observed for complex ions of lactic acid, it is found that both the carboxylic and hydroxyl groups of lactic acid are involved in the complex formation following displacement of a proton by the metal ion. The dimeric complex ions of Co, Ni, and Zn dissociated to yield similar types of ions, whereas that of Cu behaved differently. The dissociations of Co-, Ni-, and Zn-bound dimeric complexes involved losses of neutral molecules while keeping the oxidation state of the metal ion unchanged. However, elimination of radicals is found in the dissociation of dimeric complex ions of Cu, and the oxidation state of copper is reduced from Cu(II) to Cu(I) in the resulting fragment ions. The deprotonated ligand is involved in the fragmentation pathway of Cu complexes, whereas it is intact in other complexes. The oxidation state of the metal ion, nature of the ligand, and site of attachment to the metal ion are found to control the dissociation of these dimeric complex ions.     

Photodissociation of yttrium and lanthanum oxide cluster cations

Transition metal oxide cations of the form M n O m (+) (M = Y, La) are produced by laser vaporization in a pulsed nozzle source and detected with time-of-flight mass spectrometry. Cluster oxides for each value of n form only a limited number of stoichiometries; MO(M2O3)x(+) species are particularly intense. Cluster cations are mass selected and photodissociated using the third harmonic (355 nm) of a Nd:YAG laser. Multiphoton excitation is required to dissociate these clusters because of their strong bonding. Yttrium and lanthanum oxides exhibit different dissociation channels, but some common trends can be identified. Larger clusters for both metals undergo fission to make certain stable cation clusters, especially MO(M2O3) x (+) species. Specific cations are identified to be especially stable because of their repeated production in the decomposition of larger clusters. These include M3O4(+), M5O7(+), M7O10(+), and M9O13(+), along with Y6O8(+). Density functional theory calculations were performed to investigate the relative stabilities and structures of these systems.