Nitric oxide as an electron donor, an atom donor, an atom acceptor, and a ligand in reactions with atomic transition-metal and main-group cations in the gas phase

The room-temperature reactions of nitric oxide with 46 atomic cations have been surveyed systematically across and down the periodic table using an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Rate coefficients and product distributions were measured for the reactions of first-row cations from K+ to Se+, of second-row cations from Rb+ to Te+ (excluding Tc+), and of third-row cations from Cs+ to Bi+. Reactions both first and second order in NO were identified. The observed bimolecular reactions were thermodynamically controlled. Efficient exothermic electron transfer was observed with Zn+, As+, Se+, Au+, and Hg+. Bimolecular O-atom transfer was observed with Sc+, Ti+, Y+, Zr+, Nb+, La+, Hf+, Ta+, and W+. Of the remaining 32 atomic ions, all but 8 react in novel termolecular reactions second order in NO to produce NO+ and the metal-nitrosyl molecule, the metal-monoxide cation and nitrous oxide, and/or the metal-nitrosyl cation. K+, Rb+, Cs+, Ga+, In+, Tl+, Pb+, and Bi+ are totally unreactive. Further reactions with NO produce the dioxide cations CaO2+, TiO2+, VO2+, CrO2+, SrO2+, ZrO2+, NbO2+, RuO2+, BaO2+, HfO2+, TaO2+, WO2+, ReO2+, and OsO2+ and the still higher order oxides WO3+, ReO3+, and ReO4+. NO ligation was observed in the formation of CaO+(NO), ScO+(NO), TiO+(NO), VO+(NO)(1-3), VO2+(NO)(1-3), SrO+(NO), SrO2+(NO)1,2, RuO+(NO)(1-3), RuO2+(NO)1,2, OsO+(NO)(1-3), and IrO+(NO). The reported reactivities for bare atomic ions provide a benchmark for reactivities of ligated atomic ions and point to possible second-order NO chemistry in biometallic and metal-surface environments leading to the conversion of NO to N2O and the production of metal-nitrosyl molecules.     

Effect of exchangeable cation on the swelling property of 2:1 dioctahedral smectite--a periodic first principle study

We used both localized and periodic calculations on a series of monovalent (Li+, Na+, K+, Rb+, Cs+) and divalent (Mg2+, Ca2+, Sr2+, Ba2+) cations to monitor their effect on the swelling of clays. The activity order obtained for the exchangeable cations among all the monovalent and divalent series studied: Ca2+ > Sr2+ > Mg2+ > Rb+ > Ba2+ > Na+ > Li+ > Cs+ > K+. We have shown that, in case of dioctahedral smectite, the hydroxyl groups play a major role in their interaction with water and other polar molecules in the presence of an interlayer cation. We studied both type of clays, with a different surface structure and with/without water using a periodic calculation. Interlayer cations and charged 2:1 clay surfaces interact strongly with polar solvents; when it is in an aqueous medium, clay expands and the phenomenon is known as crystalline swelling. The extent of swelling is controlled by a balance between relatively strong swelling forces and electrostatic forces of attraction between the negatively charged phyllosilicate layer and the positively charged interlayer cation. We have calculated the solvation energy at the first hydration shell of an exchangeable cation, but the results do not correspond directly to the experimental d-spacing values. A novel quantitative scale is proposed with the numbers generated by the relative nucleophilicity of the active cation sites in their hydrated state through Fukui functions within the helm of the hard soft acid base principle. The solvation effect thus measured show a perfect match with experiment, which proposes that the reactivity index calculation with a first hydration shell could rationalize the swelling mechanism for exchangeable cations. The conformers after electron donation or acceptance propose the swelling mechanism for monovalent and divalent cations.     

Evidences of segregated SnO2 type nanoparticles coating layered double hydroxide at moderate temperature

A series of layered double hydroxide (LDH) materials prepared by classical coprecipitation in the presence of divalent Co2+, trivalent Al3+, and tetravalent Sn4+ cations have been investigated as a function of the temperature. As illustrated for the cation composition (Co; Al; Sn) of (0.75; 0.175; 0.075), the segregation of SnO2-type nanodomains in an interphasing LDH sand-rose region is directly evidenced by transmission electron microscopy and selected area electron diffraction (TEM/SAED). At moderate temperature (400 degrees C) the local environment around the cobalt cations is strongly modified, whereas the local structure is found to be unchanged in the vicinity of the tin cations. It is explained on the basis of the breakdown of the lamellar assembly and of the partial oxidation of Co2+ cations and that tin dioxide domains are still segregated from LDH particles. Even if the tin dioxide component does not participate from a structural point of view in the LDH composition, its beneficial effect on the textural properties is significant, increasing the specific surface area and narrowing the pore size distribution.     

Bipolar reverse osmosis membrane for separating mono-and divalent ions

Bipolar reverse osmosis membranes that have both negatively and positively charged layers have been prepared to enhance the selectivity towards mono- and divalent ions in respect of both cations and anions. Positively charged layers are formed on low pressure reverse osmosis membranes having negative charge (NTR-7410 and 7450) by an adsorption method using polyethyleneimine (PEI) or a quaternary ammonium polyelectrolyte (QAP). These layers attach to the membrane's dense layer, which is made of sulfonated polyether sulfone. The selectivity of mono- and divalent ions is proven by experimental results for single electrolytes (NaCl, Na₂SO₄ and MgCl₂). Although negatively charged membranes repulse divalent anions more strongly than cations and monovalent anions, bipolar reverse osmosis membranes reject both divalent cations and divalent anions better than monovalent ions. An optimal preparation method for bipolar membranes showing selectivity towards mono- and divalent ions were developed. The bipolar membranes showed good ion selectivity for artificial sea water.     

Self-activated supramolecular reactions: effects of host-guest recognition on the kinetics of the Diels-Alder reaction of open-chain oligoether quinones with cyclopentadiene

Diels-Alder reactions of acyclic oligoether-substituted quinones 1b, 1c, 2b, and 2c with cyclopentadiene were accelerated by the addition of alkali and alkaline earth metal perchlorates, and scandium trifluoromethane sulfonate (k(c)/k(f) = 1.2-23 for univalent cations, 11-1160 for divalent cations, and 1700-192 000 for Sc(3+), where k(c) and k(f) are the rate constants for the metal complexed and uncomplexed quinones, respectively). The shorter-armed 1a, 2a, and 3, however, exhibited no such acceleration effects. The rate accelerations can be rationalized by the FMO consequence in which the bound guest cation withdraws electron density from the quinone dienophile and lowers the LUMO energy suitable for the orbital interaction with the HOMO of cyclopentadiene. Despite the poor cation selectivity, these acyclic oligoether quinones showed larger rate accelerations than the relevant quinocrown ethers 4 (k(c)/k(f) = 1.3-3.0 for univalent cations, 5.0-160 for divalent cations, and 100-2020 for Sc(3+)). The effective electron withdrawal, which leads to the enhanced rate acceleration, can be caused by the direct interaction between the metal cation accommodated in the pseudo-cyclic oligoether linkage and the quinone carbonyl oxygen, as indicated by (1)H NMR spectroscopy. In addition, the larger rate enhancement is rather achieved in the complex with low binding constant K, because the strong encapsulation of metal cation by the oligoether chain diminishes the crucial interaction to the quinone carbonyl oxygen. As a whole, the smaller and higher valent cations tend to bring about notable rate acceleration due to the more enhanced ion-dipole interaction with the quinone carbonyl oxygen. Spectroscopic titration (absorption and (1)H NMR) and kinetic experiments indicated that only the longest di-armed 2c constructs 1:1, and then 1:2, host/guest complexes with Ca(2+), Sr(2+), and Ba(2+). These 1:2 complexes exhibited the most effective acceleration for the respective metal cations.     

Effects of divalent cations on the formation and structure of solid supported lipid films

The interaction of glassy carbon-supported thin wetting films of lecithin with some divalent cations is investigated by impedimetry and voltammetry. The influence of Ca2+, Mg2+, and Mn2+ on the film structure is explored in two different cases--the divalent cations are added to the electrolyte either before or after the formation of the film. When the film has been previously formed, the addition of divalent cations in millimolar concentrations leads to changes in the passive electrical parameters and the blocking properties of the films. On the one hand the dielectric properties of the film measured in 0.1 M KCl seem to improve after the interaction with divalent cations--the film capacitance decreases, the resistance and resistivity of the film increase. On the other hand the increase of the redox current in the presence of 1 mM Fe(CN)6(3-/4-) in the electrolyte suggests the formation of some defects in the lipid structure of the film after the action of divalent cations. It is shown that the amount of these defects could be significantly decreased when the divalent cations are present in the electrolyte solution before the film formation. The effect of divalent cations on the film stability is tested by applying negative potential to the film. In 0.1 M KCl the films are not stable at potential of - 0.8 V (vs. Ag/AgCl) and are destroyed. The addition of divalent cations stabilizes the films and at certain millimolar concentrations the films remain intact after the action of the negative potential. The effect of Mn2+ is more pronounced, the Ca2+ and Mg2+ have smaller commensurate effect. It is proposed that the changes in the films' properties could be related with more tight packing of the lipid molecules with the divalent cations inserted in the film and that some defects could be opened during the rearrangement of the lipids when the film has been previously formed.     

Electronic structure studies of tetrazolium-based ionic liquids

New energetic ionic liquids are investigated as potential high energy density materials. Ionic liquids are composed of large, charge-diffuse cations, coupled with various (usually oxygen containing) anions. In this work, calculations have been performed on the tetrazolium cation with a variety of substituents. Density functional theory (DFT) with the B3LYP functional, using the 6-311G(d,p) basis set was used to optimize geometries. Improved treatment of dynamic electron correlation was obtained using second-order perturbation theory (MP2). Heats of formation of the cation with different substituent groups were calculated using isodesmic reactions and Gaussian-2 calculations on the reactants. The cation was paired with oxygen rich anions ClO4-, NO3-, or N(NO2)2- and those structures were optimized using both DFT and MP2. The reaction pathway for proton transfer from the cation to the anion was investigated.     

Selected ion flow tube study of the reactions between gas phase cations and CHCl2F, CHClF2, and CH2ClF

The branching ratios and rate coefficients have been measured at 298 K for the reactions between CHCl2F, CHClF2, and CH2ClF and the following cations (with recombination energies in the range 6.3-21.6 eV); H3O+, SFx+ (x = 1-5), CFy+ (y = 1-3), NO+, NO2+, O2+, Xe+, N2O+, O+, CO2+, Kr+, CO+, N+, N2+, Ar+, F+, and Ne+. The majority of the reactions proceed at the calculated collisional rate, but the reagent ions SF3+, NO+, NO2+, and SF2+ do not react. Surprisingly, although all of the observed product channels are calculated to be endothermic, H3O+ does react with CHCl2F. On thermochemical grounds, Xe+ appears to react with these molecules only when it is in its higher-energy 2P1/2 spin-orbit state. In general, most of the reactions form products by dissociative charge transfer, but some of the reactions of CH2ClF with the lower-energy cations produce the parent cation in significant abundance. The branching ratios produced in this study and by threshold photoelectron-photoion coincidence spectroscopy agree reasonably well over the energy range 11-22 eV. In about one-fifth of the large number of reactions studied, the branching ratios are in excellent agreement and appreciable energy resonance between an excited state and the ground state of the ionized neutral exists, suggesting that these reactions proceed exclusively by a long-range charge-transfer mechanism. Upper limits for the enthalpy of formation at 298 K of SF4Cl (-637 kJ mol-1), SClF (-28 kJ mol-1), and SHF (-7 kJ mol-1) are determined.     

Carbon disulfide reactions with atomic transition-metal and main-group cations: gas-phase room-temperature kinetics and periodicities in reactivity

The reactions of 46 atomic-metal cations with CS2 have been investigated at room temperature using an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Rate coefficients and products 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 leading to S-atom transfer, CS2 addition and, with Hg+, electron transfer. S-atom transfer appears to be thermodynamically controlled and occurs exclusively, and with unit efficiency, in the reactions with most early transition-metal cations (Sc+, Ti+, Y+, Zr+, Nb+, La+, Hf+, Ta+, and W+) and with several main-group cations (As+, Sb+) and less efficiently with Se+, Re+ and Os+. Other ions, including most late transition and main-group metal cations, react with CS2 with measurable rates mostly through CS2 addition or not at all (K+, Rb+, Cs+). Traces of excited states (< 10%) were seen from an inspection of the observed product ions to be involved in the reactions with Mo+, Te+, Ba+ and Au+ and possibly Pt+ and Ir+. The primary products YS+, ZrS+, NbS+, HfS+, TaS+, WS+, ReS+ and OsS+ react further by S-atom transfer to form MS2(+), and TaS2(+) reacts further to form TaS3(+). CS2 addition occurs with the cations MCS2(+), MS+, MS2(+), CS2(+), and TaS3(+) to form M+(CS2)(n) (n < or = 4), MS+(CS2)(n) (n < or = 4), MS2(+)(CS2)(n) (n < or = 3), (CS2)2(+) and TaS3(+)(CS2). Up to four CS2 molecules add sequentially to bare metal cations and monosulfide cations, and three to disulfide cations. Equilibrium constant measurements are reported that provide some insight into the standard free energy change for CS2 ligation. Periodic variations in deltaG degrees are as expected from the variation in electrostatic attraction, 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 CS2.     

Speciation of cyclo(Pro-Gly)<Subscript>3</Subscript> and its divalent metal-ion complexes by electrospray ionization mass spectrometry

Electrospray ionization mass spectrometry (ESI-MS) was used to study the binding of selected group II and divalent transition-metal ions by cyclo(Pro-Gly)3 (CPG3), a model ion carrier peptide. Metal salts (CatXn) were combined with the peptide (M) at a molar ratio of 1:10 M/Cat in aqueous solvents containing 50% vol/vol acetonitrile or methanol and 1 or 10 mM ammonium acetate (NH4Ac). Species detected include [M+H]+, [M+Cat-H]+, [M2+Cat]2+, [M+Cat+Ac]+, and [M+Cat+X]+. The relative stabilities of complexes formed with different cations (Mg2+, Ca2+, Sr2+, Mn2+, Co2+, Ni2+, Cu2+, and Zn2+) were determined from the abundance of 1:1 and 2:1 M/Cat species relative to that of the unbound peptide. The largest metal ions (Ca2+, Sr2+, and Mn2+) formed the most stable complexes. Reducing the buffer concentration increased the overall extent of metal binding. Results show that the binding specificity of CPG3 depends upon the size of the metal ion and its propensity for electrostatic interaction with oxygen atoms. Product ion tandem mass spectrometry of [M+H]+ and [M+Cu-H]+ confirmed the cyclic structure of the peptide, although the initial site(s) of metal attachment could not be determined.     

Divalent cations speciation with three phosphonate ligands in the pH-range of natural waters

AMP and HEDP complexation constants for seven divalent cations (Ca, Cu, Cd, Fe, Ni, Pb and Zn) have been determined by acid-base titrations at 25 +/- 0.5 degrees C, at constant ionic strength (0.1 M KNO(3)) with Martell and Motekaitis's computer programs. Speciation diagrams enable us to compare AMP, HEDP and TPP complexing properties. AMP complexes more strongly all seven divalent cations than HEDP and TPP, which have similar ligand behavior. Among the divalent cations studied here, the ligands have lowest affinity with calcium and usually higher with copper. In all the cation/ligand systems, the major species are ML and MHL which are charged species. The uncharged ionic species Ca(2)Y(0), CaH(4)X(0) and CdH(2)Y(0) (H(6)X = AMP and H(4)Y = HEDP) which can potentially exist in solid phase, cannot be neglected. Moreover hydroxide complexes have to be taken into consideration in the complexation constant determinations and in the environmental impact.     

Structural and photophysical properties of adducts of [Ru(bipy)(CN)4]2- with different metal cations: metallochromism and its use in switching photoinduced energy transfer

We show in this paper how the 3MLCT luminescence of [Ru(bipy)(CN)4]2-, which is known to be highly solvent-dependent, may be varied over a much wider range than can be achieved by solvent effects, by interaction of the externally directed cyanide ligands with additional metal cations both in the solid state and in solution. A series of crystallographic studies of [Ru(bipy)(CN)4]2- salts with different metal cations Mn+ (Li+, Na+, K+, mixed Li+/K+, Cs+, and Ba2+) shows how the cyanide/Mn+ interaction varies from the conventional "end-on" with the more Lewis-acidic cations (Li+, Ba2+) to the more unusual "side-on" interaction with the softer metal cations (K+, Cs+). The solid-state luminescence intensity and lifetime of these salts is highly dependent on the nature of the cation, with Cs+ affording the weakest luminescence and Ba2+ the strongest. A series of titrations of the more soluble derivative [Ru(tBu2bipy)(CN)4]2- in MeCN with a range of metal salts showed how the cyanide/Mn+ association results in a substantial blue-shift of the 1MLCT absorptions, and 3MLCT energies, intensities, and lifetimes, with the complex varying from essentially non-luminescent in the absence of metal cation to showing strong (phi = 0.07), long-lived (1.4 micros), and high-energy (583 nm) luminescence in the presence of Ba2+. This modulation of the 3MLCT energy, over a range of about 6000 cm-1 depending on the added cation, could be used to reverse the direction of photoinduced energy transfer in a dyad containing covalently linked [Ru(bipy)3]2+ and [Ru(bipy)(CN)4]2- termini. In the absence of a metal cation, the [Ru(bipy)(CN)4]2- terminus has the lower 3MLCT energy and thereby quenches the [Ru(bipy)3]2+-based luminescence; in the presence of Ba2+ ions, the 3MLCT energy of the [Ru(bipy)(CN)4]2- terminus is raised above that of the [Ru(bipy)3]2+ terminus, resulting in energy transfer to and sensitized emission from the latter.     

Attachment of alkali cations on beta-D-glucopyranose: matrix-assisted laser desorption/ionization time-of-flight studies and ab initio calculations

In matrix-assisted laser desorption/ionization mass spectrometry, carbohydrates ionize by attachment of an alkali cation, and the ion yield varies with the nature of the cation. In an attempt to contribute to the understanding of the mechanisms involved, we have conducted matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) experiments on the simple glucose molecule with the alkali cations Li+, Na+ and K+, and have also performed ab initio calculations. The calculations show that, for the most stable carbohydrate-cation geometry, the carbohydrate ring is twisted and the cation is coordinated to four oxygen atoms. Calculations also show that in these complexes the positions of the three cations are very similar, and the smaller the cation, the closer it is to the oxygen atoms. Finally, the calculated formation enthalpies of the different complexes agree with the experimental results obtained for the order of stability of these complexes.     

Adsorption isotherms of homologous alkyldimethylbenzylammonium bromides on sodium montmorillonite

The adsorption of homologous alkyldimethylbenzylammonium bromides, [C(6)H(5)CH(2)N(CH(3))(2)R]Br, on sodium montmorillonite from aqueous NaCl solutions at room temperature has been studied. R stands for the methyl-, butyl-, hexyl-, octyl-, decyl-, and dodecyl-group, and the corresponding ammonium cations will be denoted as C1+, C4+, C6+, C8+, C10+, and C12+, respectively. C1+, the reference cation, attains the plateau region of adsorption at a level close to the cation exchange capacity (CEC) of the clay. The chain-length dependence on adsorptivity of the homologous cations exhibits an unexpected peculiarity. In the case of short-chain homologues of C1+ their adsorption onto sodium montmorillonite decreases in the order C1+>C4+>C6+. This behavior is due, presumably, to the growing steric hindrances at the surface of clay, which occur because of the limited area available for the bulky organic cations at the exchange sites. These limitations appear to be out-balanced in the case of higher homologues for which the increasingly growing hydrophobic effects lead to the expected sequence of adsorptivity of the cations, i.e., C1+相似文献   

The non-selective voltage-activated cation channel in the human red blood cell membrane: reconciliation between two conflicting reports and further characterisation

Using the patch-clamp technique, the non-selective, voltage-activated cation channel in the human red blood cell (RBC) membrane was further characterised. Activity of the cation channel could be demonstrated at a range of salt concentrations with the current-voltage characteristics for monovalent cations going from linear to superlinear functions, depending on the cation concentration in the range of 100-500 mM. The non-selective voltage-activated cation channel was demonstrated to be permeable to the divalent cations Ca2+ and Ba2+, and even Mg2+. The current-voltage relations for the divalent cations were superlinear even at 75 mM salt concentration, but indicated outward rectification in contrast to the I-V curve for monovalent cations. The degree of activation at a given membrane potential depended strongly on the prehistory of the channel. The gating exhibited hysteretic-like behaviour, since the quasi steady-state deactivation and activation curves were displaced by approximately 25 mV. This result fully explains apparent discrepancies between V0.5-values previously obtained by slightly different experimental protocols. The possible physiological/pathophysiological role of the channel is discussed in the context of the demonstrated permeability for divalent cations.     

M+(12-crown-4) supramolecular cations (M+ = Na+, K+, Rb+, and NH4+) within Ni(2-thioxo-1,3-dithiole-4,5-dithiolate)2 molecular conductor

Monovalent cations (M+ = Na+, K+, Rb+, and NH4+) and 12-crown-4 were assembled to new supramolecular cation (SC+) structures of the M+(12-crown-4)n (n = 1 and 2), which were incorporated into the electrically conducting Ni(dmit)2 salts (dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate). The Na+, K+, and Rb+ salts are isostructural with a stoichiometry of the M+(12-crown-4)2[Ni(dmit)2]4, while the NH4+ salt has a stoichiometry of NH4+(12-crown-4)[Ni(dmit)2]3(CH3CN)2. The electrical conductivities of the Na+, K+, Rb+, and NH4+ salts at room temperature are 7.87, 4.46, 0.78, and 0.14 S cm-1, respectively, with a semiconducting temperature dependence. The SC+ structures of the Na+, K+, and Rb+ salts have an ion-capturing sandwich-type cavity of M+(12-crown-4)2, in which the M+ ion is coordinated by eight oxygen atoms of the two 12-crown-4 molecules. On the other hand, the NH4+ ion is coordinated by four oxygen atoms of the 12-crown-4 molecule. Judging from the M(+)-O distances, thermal parameters of oxygen atoms, and vibration spectra, the thermal fluctuation of the Na+(12-crown-4)2 structure is larger than those of K+(12-crown-4)2 and Rb+(12-crown-4)2. The SC+ unit with the larger alkali metal cation gave a stress to the Ni(dmit)2 column, and the SC+ structure changed the pi-pi overlap mode and electrically conducting behavior.     

Effects of cations on the hydrogen bond network of liquid water: new results from X-ray absorption spectroscopy of liquid microjets

Oxygen K-edge X-ray absorption spectra (XAS) of aqueous chloride solutions have been measured for Li(+), Na(+), K(+), NH(4)(+), C(NH(2))(3)(+), Mg(2+), and Ca(2+) at 2 and 4 M cation concentrations. Marked changes in the liquid water XAS are observed upon addition of the various monovalent cation chlorides that are nearly independent of the identity of the cation. This indicates that interactions with the dissolved monovalent cations do not significantly perturb the unoccupied molecular orbitals of water molecules in the vicinity of the cations and that water-chloride interactions are primarily responsible for the observed spectral changes. In contrast, the addition of the divalent cations engenders changes unique from the case of the monovalent cations, as well as from each other. Density functional theory calculations suggest that the ion-specific spectral variations arise primarily from direct electronic perturbation of the unoccupied orbitals due to the presence of the ions, probably as a result of differences in charge transfer from the water molecules onto the divalent cations.     

A unique coordination environment for an ion: EXAFS studies and bond valence model approach of the encapsulated cation in the Preyssler anion

X-Ray absorption spectroscopy was used to probe the coordination of different encrypted cations in the Preyssler anions [M(n+)P5W(30)O(110)]((15-n)-)(M(n+)= Sr2+, Am3+, Eu3+, Sm3+, Y3+, Th4+, U4+ in decreasing order of ionic radius, IR), hereafter abbreviated [M(n+)PA](15-n)-. The increase of the M-W distance and the decrease of the M-P distance with increasing M ionic radius reveal that the M cation is displaced along the C5 axis within the Preyssler cavity. The slight change (0.07 A) of the M-O distance that does not correspond to the IR difference of 0.27 A confirms that the cavity retains its rigidity upon cation substitution. Geometric modeling of the encapsulated cation in the channel was performed for comparison to the EXAFS results. The position of the cation in the cavity was calculated as well as the M-O10, -W5 and -P5 distances. This modeling confirms the cation displacement toward the center of the Preyssler anion as the cation size increases, which is understood in terms of the non-homogenous electrostatic potential present within the cavity. The bond valence model approach was applied to obtain experimental bond valences. Only the bond valence sum (BVS) of Am3+ is close to its actual charge. Sums smaller than the actual valences of the +3 and +4 ions (2.39-2.63 for +3 cations, Y, Sm, Eu; 3.17 and 3.38 for +4 cations, U and Th, respectively) were obtained, and a larger sum (2.89) was obtained for Sr2+. The deviations from the formal M sums of the encapsulated ions are attributed to the rigidity of the Preyssler framework. The tendency toward coordinative unsaturation for electroactive cations, such as Eu3+, is thought to be the driving force for facile reduction. Unlike other inorganic chelating ligands, the Preyssler anion provides a unique redox system to stabilize an electroactive cation in a low oxidation state.     

Heavy water reactions with atomic transition-metal and main-group cations: gas phase room-temperature kinetics and periodicities in reactivity

Reactions of heavy water, D(2)O, have been measured with 46 atomic metal cations at room temperature in a 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 ICP source and were allowed to decay radiatively and thermalize by collisions with Ar and He 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+), and of sixth-row atomic cations from Cs+ to Bi+. Primary reaction channels were observed leading to O-atom transfer, OD transfer, and D2O addition. O-Atom transfer occurs almost exclusively (>or=90%) in the reactions with most early transition-metal cations (Sc+, Ti+, V+, Y+, Zr+, Nb+, Mo+, Hf+, Ta+, and W+) and to a minor extent (10%) with one main-group cation (As+). OD transfer is observed to occur only with three cations (Sr+, Ba+, and La+). Other cations, including most late transition and main-group cations, were observed to react with D2O exclusively and slowly by D2O addition or not at all. O-Atom transfer proceeds with rate coefficients in the range of 8.1 x 10(-13) (As+) to 9.5 x 10(-10) (Y+) cm3 molecule(-1)(s-1) and with efficiencies below 0.1 and even below 0.01 for the fourth-row atomic cations V+ (0.0032) and As+ (0.0036). These low efficiencies can be understood in terms of the change in spin required to proceed from the reactant to the product potential energy surfaces. Higher order reactions are also measured. The primary products, NbO+, TaO+, MoO+, and WO+, are observed to react further with D(2)O by O-atom transfer, and ZrO+ and HfO+ react further through OD group abstraction. Up to five D(2)O molecules were observed to add sequentially to selected M+ and MO+ as well as MO2+ cations and four to MO(2)D+. Equilibrium measurements for sequential D(2)O addition to M+ are also reported. The periodic variation in the efficiency (k/k(c)) of the first addition of D(2)O appears to be similar to the periodic variation in the standard free energy (DeltaG degrees) of hydration.     

Promotion effects in the oxidation of CO over zeolite-supported Pt nanoparticles

Well-defined Pt-nanoparticles with an average diameter of 1 nm supported on a series of zeolite Y samples containing different monovalent (H+, Na+, K+, Rb+, and Cs+) and divalent (Mg2+, Ca2+, Sr2+, and Ba2+) cations have been used as model systems to investigate the effect of promotor elements in the oxidation of CO in excess oxygen. Time-resolved infrared spectroscopy measurements allowed us to study the temperature-programmed desorption of CO from supported Pt nanoparticles to monitor the electronic changes in the local environment of adsorbed CO. It was found that the red shift of the linear Pt-coordinated CO vibration compared to that of gas-phase CO increases with an increasing cation radius-to-charge ratio. In addition, a systematic shift from linear (L) to bridge (B) bonded CO was observed for decreasing Lewis acidity, as expressed by the Kamlet-Taft parameter alpha. A decreasing alpha results in an increasing electron charge on the framework oxygen atoms and therefore an increasing electron charge on the supported Pt nanoparticles. This observation was confirmed with X-ray absorption spectroscopy, and the intensity of the experimental Pt atomic XAFS correlates with the Lewis acidity of the cation introduced. Furthermore, it was found that the CO coverage increases with increasing electron density on the Pt nanoparticles. This increasing electron density was found to result in an increased CO oxidation activity; i.e., the T(50%) for CO oxidation decreases with decreasing alpha. In other words, basic promotors facilitate the chemisorption of CO on the Pt particles. The most promoted CO oxidation catalyst is a Pt/K-Y sample, which has a T(50%) of 390 K and a L:B intensity ratio of 2.7. The obtained results provide guidelines to design improved CO oxidation catalysts.