Studies of cluster anions C n X− (X=N,P, As,Sb, Bi) produced by laser ablation

A series of cluster anions C _n X^? are produced from laser ablation of appropriate samples, where X is selected as a group-VB element. The recorded mass spectra of these cluster anions display a drastic even/odd alternation on ion intensities: For C _n N^?, only anions with oddn can be observed; For C _n P^? and C _n As^?, cluster anions with evenn are produced but with lower signal intensities; For C _n Sb^?, the signal intensity of clusters does not show even/odd alternation; Finally, for C _n Bi^?, the intensities of cluster anions with evenn are higher than those with oddn. This parity effect can be attributed to the linear structure of the cluster anions, and the parity reversal of C _n X^? from C _n N^? to C _n Bi^? can be explained from the electronegativity decreasing of the heteroatom X as it descends in the group. The Hückel model was applied to account the structural feature of these clusters.     

On the electronic structure of the Ni4+2 ion and its relation to the metal-metal bonding in binuclear Ni(II) complexes

In an attempt to improve the understanding of the electronic structure of the Ni²⁺ thiocarboxylates, we have analized, as a zeroth-order approximation, the electronic structure of the Ni⁴⁺₂ dimer in vacuo. Two small-size Slater-type orbitals (STO) basis sets have been used in order to study basis effects, referring particularly to the details of the d-d interactions. All the electrons have been included and all the molecular integrals have been computed accurately. All the multiplets of the {Ar₂}σ²_gσ²_gπ⁴_gπ⁴_uδ^x_gδ^4?x_u (x = 0 to 4) configurations have been calculated at different values of the Ni²⁺Ni²⁺ distance, R, ranging from 3.20 to 5.20 a.u. in the smaller basis calculation and from 3.20 to 4.20 a.u. in the larger basis calculation. Inclusion of configuration interaction (CI) limited to the δ^x_gσ^4?x_u (x = 0 to 4) configurations yields a ³Σ^?_g as the lowest multiplet. This CI reduces R_e from 2.03 to 1.76 Å, an effect also encountered in much more extensive CI calculations on Ni₂. The bonding in Ni⁴⁺₂ is discussed in terms of the R dependence of the orbital energies; σ, π, and δ interactions seem to make important contributions to the bonding, in view of their orbital splittings and stabilizations with respect to the free-ion values. The present results are compared with previous nonempirical studies on the Ni₂ and Ni⁺₂, as well as with empirical relationships among different molecular constants. The relevance of the results on Ni⁴⁺₂ to the question of the metal-metal interaction in {Ni(RCOS)₂}₂ compounds is difficult to discuss with certainty, but, mainly from the theoretical geometry obtained, it is argued that the Ni²⁺Ni²⁺ interactions might have a very small contribution to the selling of the electronic structure of the thiocarboxylates.     

The influence of uncompensated solution resistance on the determination of standard electrochemical rate constants by cyclic voltammetry,and some comparisons with a.c. voltammetry

a digital simulation analysis is presented of the deleterious effects of uncompensated solution resistance, R_us, on the evaluation of standard rate constant, k^s_ob, by cyclic voltammetry. The results are expressed in terms of systematic deviations of “apparent measured” rate constants, k^s_ob(app), evaluated in the conventional manner without regard for R_us, from the corresponding actual values, k^s_ob(true), as a function of R_us and other experimental parameters. Attention is focused on the effects of altering the electrode area and the double-layer capacitance on the extent of the deviations between k^s_ob(app) and k^s_ob(true), and on comparisons with corresponding simulated results obtained from phase-selective a.c. impedance data. The extent to which k^s_ob(app) <k^s_ob(true) for small R_us values was found to be similar for the cyclic and a.c. voltammetric techniques. The latter method is, however, regarded as being preferable under most circumstances in view of the greater ease of minimising, as well as evaluating, R_us for a.c. impedance measurements. The influence of solution resistance on k^s_ob measurements with microelectrodes and without iR compensation is also considered.     

A joint vibro-electronic mechanism in the dissociation of molecular hydrogen in nonequilibrium plasmas

A new mechanism Of H₂ dissociation in electrical discharges (10¹¹ ? n_e ? 10¹² cm^?3, 2.10^?16 ? E/N ? 3.10^?16 V cm², 300 ? T_g ? 1000 K, 3 ? p ? 30 torr) is presented and discussed. In this mechanism, called joint vibro-electronic mechanism (JVE), the electrons of the discharge create a strong vibrational disequilibrium with respect to the gas temperature (T_g) and promote electronic transitions from all vibrational levels of ¹Σ_g H₂ state to the repulsive ³Σ_u one. Moreover the V-V (vibration-vibration) and V-T (vibration-translation) energy exchanges are considered for building up the vibrational distribution of ¹Σ_g state. A complete set of e - D cross sections (e + H₂(¹Σ_g,ν) → e + H₂ (³Σ_u) → + 2H, ν = 0,14) is calculated by using an extension of the semiclassical Gryzinski theory in combination with the Franck-Condon principle. Dissociation rates calculated according to JVE are larger either than those obtained by the pure vibrational mechanism (PVM) discussed in our previous work or than those from the direct electronic impact mechanism (DEM) from the ground vibrational level. The behaviour of JVE rates as a function of gas temperature (T_g), of E/N, of electron density (n_e) and of pressure is then reported. The results show strong differences as compared, with the corresponding values obtained, with PVM. Finally the influence of the atoms as well as their recombination on the dissociation rates is discussed. The results have been obtained by solving a system of vibrational master equations.     

Gas phase ion/molecule reactions in phosphine/germane mixtures studied by ion trapping

Gaseous mixtures of phosphine and germane have been investigated by ion trap mass spectrometry. Reaction pathways together with rate constants of the main reactions are reported. The mechanisms of ion/molecule reactions have been elucidated by single and multiple isolation steps. The GeH_n⁺ (n = 1–3) ions react with phosphine to give GePH_n⁺ (n = 2–4) ions. The GePH₄⁺ ion further reacts with GeH₄ to yield Ge₂PH₆⁺. The GePH_n⁺ (n = 2–4) mixed ionic family also originates from the P⁺ phosphine primary ion, as well as from the P₂H_n⁺ (n = 0–3) secondary ions of phosphine reacting with neutral germane and from Ge₂H₂⁺ reacting with phosphine. The main reaction pathways of the PH_n⁺ (n = 0–2) ions with GeH₄ lead to the formation of the GeH₂⁺ and GeH₃⁺ ionic species. Protonation of phosphine from different ionic precursors is a very common process and yields the stable phosphonium ion, PH₄⁺. Trends in total abundances of secondary GePH_n⁺ (n = 2–4) ions as function of reaction time for different PH₃/GeH₄ pressure ratios show that excess of germane slightly affects the nucleation of mixed Ge-P ions.     

Structural studies in the BaO-B2O3-TiO2 system by XAS and B-NMR

The structure of barium-titanium-metaborate xBaO-xB₂O₃-yTiO₂ (y=0%, 4%, 8%, 16% and x=50-y/2) amorphous and crystallized powders, obtained using a polymeric precursor method, was investigated by Ti and B K-edge X-ray absorption spectroscopy (XAS) and ¹¹B-NMR high-resolution techniques. XANES study of amorphous samples shows that Ti⁴⁺ ions exist as ^[4]Ti species associated to ^[6]Ti and ^[5]Ti species in a practically equivalent amount. After crystallization, titanium environment is predominately composed by ^[6]Ti species. According to XANES results obtained at the B K-edge, the fraction of boron in tetrahedral sites (^[4]B) reduces as the amount of TiO₂ is increased from x=0% to 4%, with a consequent increase of boron in trigonal sites (^[3]B). By a combination of ¹¹B-NMR spin-echo and triple quantum magic angle spinning (3Q-MAS) techniques, the detailed borate speciation was determined as consisting in ^[4]B and two kind of trigonal sites, ^[3]B_A and ^[3]B_B, corresponding, respectively, to borates sharing three and two O atoms with other boron units. NMR results reveal not only the reduction in boron coordination also seen by XANES but also the simultaneous reduction in the condensation degree of trigonal units, when the Ti content is increased in the glass. In crystallized samples, β-BaB₂O₄ and BaTi(BO₃)₂ phases were identified and quantified by ¹¹B-NMR.     

Crystal chemistry of the (Ta6Si4O26)6− and (Ta14Si4O47)8− frameworks

The range of chemical flexibilities of the hexagonal frameworks (Ta₆Si₄O₂₆)^6? and (Ta₁₄Si₄O₄₇)^8? have been partially explored. This has been done with high-temperature preparations as in general ionic mobilities in these frameworks are too low to permit low-temperature ion exchange. Ionic site potential calculations indicate that preferential site-occupancy factors as well as geometric constraints are responsible for the absence of ionic motion. New phases K_6?xNa_xTa₆Si₄O₂₆ (x ? 4), K_8?xNa_xTa₁₄Si₄O₄₇ (x ? 5), and impure Ba_3?xNa_2xTa₆Si₄O₂₆ have been prepared. Introduction of up to 2 moles of Li⁺ and 1 mole of Mg²⁺ ions per formula unit into sites of the framework not normally occupied has been demonstrated as well as the possibility of partially substituting Zr⁴⁺ for Ta⁵⁺ ions. Substitutions designed to introduce large tunnel vacancies in the presence of only monovalent K⁺ or Na⁺ ions (P for Si, W for Ta and F for O) generally proved unsuccessful. Competitive phases also frustrated attempts to substitute either the larger Rb⁺ or the smaller Li⁺ ions into the large-tunnel sites. A large area of solid solution was discovered in the BaONa₂OTa₂O₅ phase diagram; it has a (TaO₃)-framework with the structure of tetragonal potassium tungsten bronze.     

CEPA calculations on open-shell molecules

Ab initio calculations at SCF and CEPA levels using large Gaussian basis sets have been performed for the two lowest electronic states,X ² Σ⁺ andA ² Π, of HeAr⁺. Spin-orbit coupling (SOC) effects have been added using a semiempirical treatment. The resulting potential curves for the three statesX,A ₁, andA ₂ have been used to evaluate molecular constants such as vibrational intervals ΔG(v + 1/2) and rotational constantsB _v as well as — by means of a Dunham expansion — equilibrium constants such asR _e , ω _e ,B _e etc. Comparison with the experimental data from UV emission spectroscopy shows that the calculated potential curves are slightly too shallow and have too large equilibrium distances:D _e = 242 cm^?1 andR _e = 2.66 Å compared to the experimental values of 262 cm^?1 and 2.585 Å, respectively, for theX ²Σ⁺ ground state. However, the ab initio calculations yield more bound vibrational levels than observed experimentally and allow for a more complete Dunham analysis, in particular for theA ₂ state. The experimental value of 154 cm^?1 for the dissociation energyD _e of this state is certainly too low; our best estimate is 180±5 cm^?1. For theA ₁ state our calculations are predictions since this state has not yet been observed experimentally.     

Studies on the pitting corrosion of zinc in aqueous solutions II. Measurement of pitting corrosion currents operating under natural conditions

A simple experimental arrangement was applied for the measurement and the evaluation of pitting corrosion currents operating under natural conditions. The feasibility of the procedure was examined by using Zn as a test metal, K₂CrO₄, Na₂HPO₄ and Na₂WO₄ as inhibitors, and Cl^?, Br^? and I^? as pitting corrosion agents. Both the type and concentration of the inhibiting and corroding agents were varied in a programmed manner. In CrO₄^2? and HPO₄^2? solutions, the pitting corrosion currents started to flow after an induction period, which decreased with increase in the concentration of the attacking agent. In WO₄^2? solutions, on the other hand, initially high currents were recorded due to the reduction of the agent to soluble, non-inhibiting species.In all solutions tested the corrosion current reached steady-state values which depended on the type and concentration of both the inhibiting and the aggressive anions. When keeping the inhibitor concentration constant, the corrosion current varied with the concentration of the aggressor according to: log i_corr = a₁ + b₁ log c_agg On the other hand, in solutions of constant aggressor concentration, with varying inhibitor amounts, the relation was: log i_corr = a₂ ? b₂ log c_inh where a₁(a₂) and b₁(b₂) are constants.The two equations were derived theoretically on the basis of competitive adsorption of the two counteracting agents on the surface of the metal. Comparison between the experimental values of a and b, with the corresponding terms of the theoretical equations was made.The aggressivity of the three tested anions decreases in the order Cl^? > Br^? > I^?, whilst inhibition varied as CrO₄^2? > HPO₄^2? > WO₄^2?.     

Cage complexes as a molecular scaffold for polyfunctional and polytopic systems: Synthesis of the first closo-borate iron(II) clathrochelate

The ribbed functionalization of the clathrochelate iron(II) tris-dioximates as a potential “molecular scaffold” for the synthesis of polyfunctional and polytopic complexes with closo-dodecaborate-anion substituents was performed. closo-Dodecaborate-substituted clathrochelate [FeBd₂(Cl(B₁₂H₁₁NH)Gm)(BF)₂]^2? (where Bd^2? is α-benzyldioxime dianion, Gm is glyoxime residue) dianion was prepared starting from dichloride clathrochelate FeBd₂(Cl₂Gm)(BF)₂ precursor with amino-closo-dodecaborate (NBu₄)[B₁₂H₁₁NH₃] in the presence of potassium tert-amylate. This clathrochelate dianion was isolated as a tetra-n-butylammonium salt and characterized using elemental analysis, MALDI-TOF and PD mass, IR, UV-Vis, ⁵⁷Fe Mössbauer spectra as well as by ¹H, ¹¹B and ¹³C{¹H} NMR spectra.     

Stabilization of bis(triphenylphosphine)palladium(0) by chloride ions. Electrochemical generation of highly reactive zerovalent palladium complexes

“Pd⁰(PPh³)₂” has been generated by electroreduction of Cl₂Pd(PPh₃)₂ in the absence of PPh₃. Its structure is consistent with [Cl_xPd⁰(PPh₃)₂]_n^nx−, with values of x and/or n depending on the chloride ion concentration. The half-lives of its reactions with PhI or PhBr have been determined, and found to be 350 and 100 times, respectively, as large as those for Pd(PPh₃)₄.     

Reactions of 2-arsa- and 2-stiba-1,3-dionato lithium complexes with group 4-7 metal halides

The reactions of a range of 2-arsa- and 2-stiba-1,3-dionato lithium complexes with group 4-7 metals have been investigated. These have given rise to several complexes in which an arsadionate acts as a chelating ligand; [V{η²-O,O-OC(Bu^t)AsC(Bu^t)O}₃], [M{η²-O,O-OC(Bu^t)AsC(Bu^t)O}₂(DME)], M=Cr or Mn; or as an η¹-As-diacylarsenide, [MnBr(CO)₄{As[C(O)Bu^t]₂Li(DME)}]₂. In addition, reactions of lithium arsadionates with TaCl₅ have led to metal mediated arsadionate decomposition reactions and arsadionate oxidative coupling reactions to give the known arsaalkyne tetramer, As₄C₄Bu^t₄, and the new tetraacyldiarsane, [{As[C(O)Mes]₂}₂] Mes=mesityl, respectively. The treatment of several lithium arsadionates with [MoBr₂(CO)₂(PPh₃)₂] has also initiated arsadionate decomposition reactions and the formation of the metal carboxylate complexes, [MoBr(CO)₂{η²-O₂C(R)}(PPh₃)₂] R=Bu^t, Ph, Mes. The X-ray crystal structures of six of the prepared complexes are discussed.     

Facile, aprotic degradation of η-CpZrCl3·dme by ternary sodium/group 14 tert-butoxides: From η-CpZrCl3·dme back to NaCp in two easy steps

CpZrCl₃·dme was treated with Na[El(O^tBu)₃], El = Ge, Sn, Pb, respectively. The addition of Na[Sn(O^tBu)₃] to CpZrCl₃·dme caused rapid cyclopentadienide loss and the equally rapid appearance of CpSnCl, half of which crystallized as the trinuclear complex {[ZrCl(O^tBu)₃]₂·CpSnCl}. Pristine CpSnCl reacted almost instantly with NaO^tBu to give NaCp and Na[Sn(O^tBu)₃], which co-crystallized as a coordination polymer. Na[Ge(O^tBu)₃] also displaced Cp from zirconium, but with a different product distribution, giving Cp₂Ge, fac-[Ge(μ-^tBuO)₃ZrCl(O^tBu)₂], and ZrCl(O^tBu)₃. By contrast, Na[Pb(O^tBu)₃] only exchanged its tert-butoxide groups with zirconium to furnish CpZr(O^tBu)₃ and PbCl₂. The solid-state structures of {[ZrCl(O^tBu)₃]₂·CpSnCl}, fac-[Ge(μ-^tBuO)₃ZrCl(O^tBu)₂], and {NaCp·Na[Sn(O^tBu)₃]}_n were determined.     

Measurement of k 0 values for caesium and iridium

The OPAL research reactor in Australia has been used to determine k ₀ values for ^134mCs, ¹³⁴Cs, ¹⁹²Ir and ¹⁹⁴Ir. Values for ²⁴Na have also been measured for quality control. The neutron flux at the irradiation positions was very highly thermalised (f > 2,000), resulting in almost negligible activation by epithermal neutrons. As a consequence, the contribution to the total uncertainty of the k ₀ values from epithermal-related factors such as Q ₀ and $ \bar{E}_{\text{r}} $ was very small. The measured caesium k ₀ values have been compared with the library values as well as with recent measurements by St Pierre et al. and Farina Arboccò et al. While there are k ₀ values for ¹⁹⁴Ir in the library, no ¹⁹²Ir values have been measured previously. Despite ¹⁹²Ir having a higher sensitivity than ¹⁹⁴Ir, k ₀ values were not measured during the establishment of the k ₀-method because the nuclear data available at the time indicated that the activation cross-section of ¹⁹¹Ir deviated significantly from 1/v behaviour (g(T _n ) ≠ 1), which would result in unacceptable errors if k ₀ analysis were to be carried out using the Høgdahl convention. However later nuclear data compilations showed that ¹⁹¹Ir has better 1/v behaviour than previously reported, making it suitable for k ₀ analysis using the Høgdahl convention. For completeness, k ₀ values have been determined using both the Høgdahl and modified-Westcott conventions and these have been compared with library (¹⁹⁴Ir) and calculated values.     

The novel Cs4Nb6F8.5I3.5I6 octahedral niobium cluster fluoro-iodide: a step towards the Nb6F12 cluster core excision

The solid state synthesis of Cs₄Nb₆Fⁱ_8.5Iⁱ_3.5I^a₆ starting from Nb₆F₁₅ binary fluoride, as well as its crystal structure determined by X-ray single crystal diffraction, are presented in this work. This novel cluster compound is based on a Nb₆Iⁱ₃Fⁱ₆Lⁱ₃I^a₆ (L=F, I) discrete unit and crystallizes in the monoclinic system (space group C2/m; Z=4 ; a=10.4363(4) Å, b=18.1227(7) Å, c=19.5102(9) Å β=101.223(1)°, V=3619.5(3) Å³, R₁=0.057; wR₂=0.159). This halide is the first octahedral niobium cluster compound containing unshared terminal I^a ligands together with ordered μ₂-Iⁱ and μ₂-Fⁱ ligands on nine inner positions whilst the three last ones (Lⁱ) are slightly affected by a I/F random occupancy. The structural findings are discussed and compared with those of Nb₆F₁₅, Nb₆I₁₁, CsNb₆I₁₁ and the fluorochlorides and fluorobromides recently reported.     

The crystal chemistry of the M+VO3 (M+ = Li,Na, K,NH4, Tl,Rb, and Cs) pyroxenes

The crystal structures of M⁺VO₃(M⁺ = K, NH₄, and Cs) have been refined using three-dimensional counter-diffractometer X-ray data and full-matrix least-squares methods. The structure of these compounds is characterized by a (V⁵⁺O^2?₃)^?_∞ chain extending along the c-axis (Pbcm orientation), with adjacent chains linked by the alkali metal cation. The structure may be considered as a variant of the pyroxene structure, and standard atom nomenclature is proposed in order to facilitate comparison with silicate pyroxenes. Structural variation across this series is discussed in detail and is compared with the analogous M⁺M³⁺Si₂O₆ (M⁺ = Li, Na; M³⁺ = Al, Cr, Fe, Sc, In) series.     

A potentiometric and calorimetric study of the protonation and complexation with Cu2+ and Ni2+ of some sulphur-containing α,ω-amino acids

The behaviour in aqueous solution of some aminocarboxylates of the type NH₂(CH₂)_nS(CH₂)_m-1COO^? (abbreviated as n,m-NSO; n and m = 2 or 3) in equilibria with H⁺, Cu²⁺ and Ni²⁺ ions has been investigated potentiometrically and calorimetrically at 25° C in a 0.5 M KNO₃ medium.The protonation of the amino function and especially the carboxylate function is attended by strong desolvation effects, which are characterized by low exothermic enthalpies and strongly positive entropies of protonation.In [Cu(n,m-NSO)]⁺ and [Ni(n,m-NSO)]⁺ the aminocarboxylates act as tridentate ligands, forming complexes with a strong hard—hard character. The biligand species [Ni(n,m-NSO)₂] behave as six-coordinated complexes whereas in [Cu(n,m-NSO)₂] the second ligand is bound only through the N and S donor, forming a five-coordinated species.Finally, the n,m-NSO ligands also form protonated species with the Cu²⁺ ion.     

The effect of the Co-anion,SCN−, on macrocycle-facilitated selective transport of Cd (II) over Ag (I) in an emulsion membrane

Emulsion membrane systems consisting of (1) an aqueous source phase containing 0.001 M Cd (NO₃)₂ and/or 0.001 M AgNO₃ and varying concentrations of SCN⁻, (2) a toluene membrane containing dicyclohexano-18-crown-6 (DC18C6) (0.02 M) and the surfactant Span 80 (sorbitan monooleate) (3% v/v), and (3) an aqueous receiving phase containing MgS₂O₃ or Mg (NO₃)₂ were studied with respect to the disappearance of Cd (II) and/or Ag (I) from the source phase as a function of time. The transport rate of Cd (II) was highest when a maximum amount of the Cd(II) in the source phase was present as Cd(SCN)₂ ([SCN⁻] =0.4 M). Cadmium(II) was transported over Ag(I), which is present mainly as Ag(SCN)₄³⁻, by 5- and 55-fold in 5 minutes with 0.4 M SCN⁻ in the source phase and 0.3 M S₂O²⁻₃ and 0.3 M NO⁻₃, respectively, in the receiving phase. In these competitive experiments, the total percent of Cd (II) transported was 98 and 84, respectively. The results are explained using the various equilibrium constants for cation-DC18C6, cation-SCN⁻ and cation-S₂O²⁻₃ interactions. These results indicate that rapid transport occurs when a cation is present in the source phase as a neutral complex. Selectivity for neutral species can be designed into these membrane systems when other cations interact with the source-phase anion to form charged species. Emulsion systems like those above were studied with respect to the appearance of Li⁺ in the source phase as a measure of membrane breakage. Maximum membrane stability was obtained when the ionic strengths of the source and receiving phases were equal.     

Comparative mass spectrometric investigation of transition metal polymethylcyclopentadienylcarbonyl complexes: II mass spectra of RnC5H5–nMN(CO)3 (R = CH3, n = 0–5; R = t-C(CH3)3, n = 1)

Investigation of π-(CH_3nC₅H_5–nMn(CO)₃ (Me_n-CpMnT) n = 0−5 and t-butyl CpMnT mass spectra showed that Me_nCpMnT molecular ion (M⁺) fragmentation occurs by a simpler scheme than that for Me_nCpReT M⁺ molecular ions. The reason is that MnCp and MnCO bonds are not as strong as the ReCp and ReCO bonds, and the relative “inertness” (compared to Re) of the Mn atom (ion), coordinated to the methylcyclopentadienyl ligand. Variations of M⁺ molecular ion intensity with different values of n are probably due to a complexity of electronic and spatial methyl-carbonyl group interactions in M⁺.