Studies in negative ion mass spectrometry. IX—electron capture by a series of tris- and bis-(dipivaloylmethanato) metal chelates

Electron capture data and negative ion mass spectra are reported for a series of tris- and bis-chelates of the types Metal·L₃, Metal·L₂, where L (or dpm) refers to the ligand or enolate ion of the β-diketone 2,2,6,6-tetramethyl-3,5-heptanedione (dipivaloylmethane) and the metals are: Sc(III), Cr(III), Mn(III), Fe(III), Co(III), Al(III), Ga(III), In(III), Co(II), Ni(II), Cu(II), Zn(II). The spectra were all very simple and the principal ions observed in all cases were the molecular anions and ligand ions. Reaction schemes have been established to account for the formation of ligand and other fragment ions, many of which carried less than 0.1% of the total ion currents. Variations in the negative ion mass spectra are attributed to the influence of the metal atom and its 3d electron configuration on the electron capture process. The simplicity of the negative ion mass spectra, together with the fact that many of these metal chelates gave relatively high total ion currents of c. 10^?9–10^?10 A, indicates the potential value of negative ion mass spectrometry in the area of ultra-trace metal analysis, and some estimates of detection limits for some of the metals considered in this study have been made.     

Gas-phase reactions of doubly charged alkaline earth and transition metal complexes of acetonitrile, pyridine, and methanol generated by electrospray ionization

Formation and low energy collision-induced dissociation (CID) of doubly charged metal(II) complexes ([metal(II)+L _n ]²⁺, metal(II)=Co(II), Mn(II), Ca(II), Sr(II) and L = acetonitrile, pyridine, and methanol) were investigated. Complexes of [metal(II)+L _n ]²⁺ where n≤7 were obtained using electrospray ionization. Experimental parameters controlling the dissociation pathways for [Co(II)+(CH₃CN)₂]²⁺ were studied and a strong dependence of these processes on the collision energy was found. However, the dissociation pathways appear to be independent of the cone potential, indicating low internal energy of the precursor ions. In order to probe how these processes are related to intrinsic parameters of the ligand such as ionization potential and metal ion coordination, low energy CID spectra of [metal(II)+L _n ]²⁺ for ligands such as acetonitrile, pyridine, and methanol were compared. For L = pyridine, all metals including the alkaline earth metals Ca and Sr were reduced to the bare [metal(I)]⁺ species. Hydride transfer was detected upon low energy CID of [metal(II)+L _n ]²⁺ for metal(II)=Co(II) and Mn(II) and L = methanol, and corroborated by signals for [metal(II)+H^?]⁺ and [metal(II)+H^?+CH₃OH]⁺, as well as by the complementary ion [CH₃O]⁺.     

Gas phase electron attachment reactions and negative ion mass spectra of tris- and bis(N,N-diethyldithiocarbamato) metal complexes

Results are presented for the gas phase thermalized electron attachment reactions of transition metal dithiocarbamates Metal·[S₂CN·Et₂]_n, with n = 3 for the trivalent metals Cr, Mn, Fe, Co and n = 2 for the divalent metals Fe, Co, Ni, Pd and Cu. Details of negative ion mass spectra are given, and these were obtained by use of the complementary methods of in-beam direct insertion and desorption chemical ionization of samples into temperature and pressure controlled ion source methane gas plasmas. For the tris-complexes, only small abundances of [M]^? were observed, being consistent with the electron entering a metal-based orbital to give a reduced [Metal^II·L₃]*^? species—the precursor of the observed [Metal^II·L₂]^? or [L]^? product ions. For the bis-complexes, a much higher proportion of the total ion current was carried by [M]^?, with [L]^? being the next most abundant species in all cases and the principal product in the reaction sequence \documentclass{article}\pagestyle{empty}\begin{document}${[{\rm Metal}^{{\rm II}} \cdot {\rm L}_{\rm 2}] + {\rm e} \to [{\rm Metal}^{\rm I} \cdot {\rm L}_{\rm 2}]^{*-} \mathop {\rightarrow}\limits^* [{\rm L]}^{\rm - } + [{\rm Metal}^{\rm I} \cdot {\rm L}].}$\end{document}. Rearrangement ions [L·Metal^I·X]^? with X=SH, SCN, NEt₂ were also identified for this series which can be accounted for by rearrangement reactions occurring in ligands not fulfilling a bidentate function towards the relevant metal.     

Collision-induced dissociation of negative ions in the gas phase: [Aryl S]−, [aryl SO]− and [aryl SO2]−

Collision-induced dissociation of the ions [ArS]^?, [ArSO]^? and [ArSO₂]^? has uncovered a rich and varied ion chemistry. The major fragmentations of [ArS]^? are complex and occur without prior ring hydrogen scrambling: for example, [C₆H₅S]^?→[C₂HS]^? and [HS]^?; [p-CD₃C₆H₄S]^?→[C₆H₄S]^?˙, [CD₃C₄S]^? and [C₂HS]^?. In contrast, all decompositions of [C₆H₅CH₂S]^? are preceded by specific benzylic and phenyl hydrogen interchange reactions. [ArSO₂]^? and [ArSO₂]^? ions undergo rearrangement, e.g. [C₆H₅SO]^?→[C₆H₅O]^? and [C₆H₅S]^?; [C₆H₅SO₂]^?→[C₆H₅O] ^?. The ion [C₆H₅CH₂SO]^? eliminates water, this decomposition is preceded by benzylic and phenyl hydrogen exchange.     

Gas phase oxidative-addition reactions of alkyl radicals at complexed cobalt(II) centres

Negative chemical ionisation mass spectrometry is used as a probe to identify reactions between hydrocarbon radicals and cornplexed cobalt(II) centres in the gas phase. Methane NCI mass spectra of a series of cobalt(II) complexes containing O₄, O₂N₂ and N₄ donor atom sets are characterised by adduct ions of the form [M + C_nH_2n+1]^? at m/z values above the molecular ion, [M]^?. Formation of such ionic species has been rationalised in terms of a one-electron oxidative-addition mechanism involving attack by hydrocarbon plasma-derived alkyl radicals at the metal centre prior to electron capture: Co^IIL_n + R^? → RCo^IIIL_n$\text{e}{}^{\mathrm{?}}$ [Co^IL_n]^?. The competing resonance electron attachment reaction: Co^IIL_n$\text{e}{}^{\mathrm{?}}$ also occurs within the ion source.     

Adduct ion formation by metal complexes under methane negative chemical ionization conditions

Negative chemical ionization mass spectrometry is used as a probe to examine reactions between hydrocarbon radicals and metal complexes in the gas phase. The methane negative chemical ionization mass spectra of 27 complexes of cobalt(II ), nickel(II ) and copper(II ) in the presence of O₄, O₂N₂ and N₄ donor atom sets are characterized by two dominant series of adduct ions of the form [M + C_nH_2n]^? and [M + C_nH_2n+1]^? at m/z values above the molecular ion, [M]^?. Insertion of the CH radical into the ligand followed by radical/radical recombination and electron capture is proposed as the major mechanism leading to the formation of [M + C_nH_2n]^? adduct ions. A second pathway involves ligand substitution by C_nH_2n+1 radicals concomitant with H elimination and electron capture. Oxidative addition at the metal followed by ionization is suggested as the principal pathway for the formation of [M + C_nH_2n+1]^? adduct ions.     

Metallkomplexe mit biologisch wichtigen Liganden. CLXVI Metallkomplexe mit Ferrocenylmethyl‐L‐cysteinat und 1,1′‐Ferrocenylbis‐(methyl‐L‐cysteinat) als Liganden

Metal Complexes of Biologically Important Ligands. CLXVI Metal Complexes with Ferrocenylmethylcysteinate and 1,1′‐Ferrocenylbis‐(methylcysteinate) as Ligands A series of complexes of transition metal ions ( Cr³⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺ ) and of lanthanide ions ( La³⁺, Nd³⁺, Gd³⁺, Dy³⁺, Lu³⁺ ) with the anions of ferrocenylmethyl‐L‐cysteine [(C₅H₅)Fe(C₅H₄CH(R)SCH₂CH(NH₃⁺)CO₂^?] (L¹) and with the dianions of 1,1′‐ferrocenylbis(methyl‐L‐cysteine) [Fe(C₅H₄CH(R)SCH₂CH(NH₃⁺) CO₂^?)₂] (R = H, Me, Ph) (L²) as N,O,S‐donors were prepared. With the monocysteine ferrocene derivative L¹ as ligands complexes [M^IIL¹₂] or [Cr^IIIL¹₂]Cl type complexes are formed whereas the bis(cysteine) ligand L² yields insoluble complexes of type [ML²]_n, presumably as coordination polymers. The magnetic moments of [Mn^IIL²]_n, [Pr^IIIL²]_n(OH)_n and [Dy^IIIL²]_n(OH)_n exhibit “normal” paramagnetism.     

Mass spectrometric studies of dithiophosphinato metal complexes

Electron impact induced fragmentation reactions of planar, tetrahedral, octahedral and oligomeric metal dithiophosphinates Me(II)L₂ (L=Et₂PS₂^?; Me(II)=Zn, Cd, Hg, Pb, Co, Mn, Ni, Pd, Pt), Me(III)L₃ (Me(III) = Sb, Bi, In, Rh, Ir) and (Me(I)I)_n (Me(I)/n=Tl/1, Au/2, Cu/4) have been studied. Fragmentation patterns, which are in accordance with metastable peak determinations by linked scans, are reported. In the case of the transition metals the spectra of the complexes show abundant [M]^+· predominantly metal containing ions and, the former being weak and the intensities of the latter being considerably reduced in the case of metal complexes with filled d shells. With planar or tetrahedral transition metal complexes no dependence of fragmentation on the coordination geometry can be observed. The dependence of fragmentation on d configuration, ionization energy of the metal and metal ligand π bonding is discussed. In the case of the oligomeric complexes strong metal-metal interaction is observed even under electron impact.     

Determination of Cu(III) in semiconductor ceramics using cationic violet reagent

A method is presented for the determination of Cu(III) ion in semiconductor ceramic materials. It is based on (a) the reaction of copper(III) with Fe(II) ions, (b) the quantitative formation of Fe(III) ions, (c) liquid-liquid extraction of Fe(III) in the form of an ion associate between the FeCl ₄ ^- anion with a Cationic Violet dye, and (d) spectrophotometric quantitation of the ion associate. All processes are executed in a single sequence of steps. The absorbance of the colored extracts obeys Beer's law in the range from 0.16 to 1.92 mg L^?1 of Cu(III). The molar absorptivities range from 38,000 to 82,000 L mol^?1 cm^?1 depending on the kind of extractant used. The effects of other ions on the recovery of Cu(III) were studied, and those that generally occur with copper in semiconductor samples do not interfere.     

Heteronukleare Komplexverbindungen mit Metall—Metall-Bindungen. VIII. Neue heterodinukleare Komplexe mit Bindungen zwischen Kupfer(I) und Mangan(–I), Eisen(–I) oder Cobalt(–I)

Heteronuclear Coordination Compounds with Metal—Metal Bonds. VIII. New Heterodinuclear Complexes with Bonds between Copper(I) and Manganese(?I), Iron(?I), or Cobalt(?I) [(en)Cu? Mn(CO)₅] ( 1a ), [(dien)Cu? Mn(CO)₅] ( 1b ), [(en)Cu? Fe(CO)₃(NO)] ( 2a ), [(dien)Cu? Fe(CO)₃(NO)] ( 2b ), [(en)Cu? Co(CO)₄] ( 3a ), and [(dien)Cu? Co(CO)₄] ( 3b ) are new heterobinuclear metal—metal bonded complexes. The geometry of the [Mn(CO)₅]^?, [Fe(CO)₃(NO)]^?, and [Co(CO)₄]^? ions is distorted only to a less extend in accord with a heteropolar bond to copper.     

Removal of Lead(II), Cadmium(II), and Arsenic(III) from Aqueous Solution Using Magnetite Nanoparticles Prepared by Green Synthesis with Box–Behnken Design

Two types of magnetite (Fe₃O₄) nanoparticles were investigated as adsorbents for the simultaneous removal of Pb(II), Cd(II), and As(III) metal ions from aqueous solution. Magnetite nanoparticles were prepared by two synthesis procedures, both using water as solvent, and are referred to as conventional Fe₃O₄ nanoparticles and green Fe₃O₄ nanoparticles. The latter used Citrus limon (lemon) aqueous peel extract as the surfactant. Box–Behnken experimental design was used to investigate the effects of parameters such as initial concentration (20–150?mg?L^?1), pH (2–9), and biomass dosage (1–5?g?L^?1) on the removal of Pb(II), Cd(II), and As(III) ions. The optimum parameters for removal of the studied metal ions from aqueous solutions, including the initial ion concentration (20?mg?L^?1), pH (5.5) and adsorbent dose (5?g?L^?1), were determined. The pseudosecond-order model exhibited the best fit for the kinetic studies, while adsorption equilibrium isotherms were best described by Langmuir and Freundlich models. The optimum conditions were applied for the treatment wastewater. The removal efficiencies of Pb(II), Cd(II), and As(III) using the conventional and green synthesized Fe₃O₄ nanoparticles were 59.4?±?4.3, 18.7?±?1.9 and 17.5?±?1.6, and 98.8?±?5.6, 46.0?±?1.3, and 48.2?±?2.6%, respectively. These results demonstrate the potential of magnetite nanoparticles synthesized using C. limon peel extract as highly efficient adsorbents for the removal of Pb(II), Cd(II), and As(III) ions from aqueous solution.     

Optimization of the electron spin resonance spectrometric determination of transition-metal ions by variation of temperature

The effect of variation in temperature on the first-derivative electron spin resonance spectra of transition metal ions in aqueous solution is reported. For the aquo ions of Mn(II), Cr(III), Fe(III) and vanadyl, and for hexafluoroferrate(III), an increase in temperature results in a decrease in line width with a concomitant increase in signal amplitude. In contrast, [Cu(H₂O)₆]²⁺ and [Ti (H₂O)₄F₂]⁺ show an increase in line width and a marked decrease in derivative amplitude as the temperature is raised. Consequently, the choice of the optimum temperature for each system results in greatly improved sensitivity and lower limits of detection. Recording spectra at more than one temperature is also useful for detecting a given metal ion in a multicomponent system that has overlapping resonance lines.     

Synthesis and properties of transition and lanthanide complexes with benzyl monohydrazone-3-hydrazine- 4-benzyl-6-phenyl pyridazine ligand (BHP)

Synthesis and characterization of benzyl-monohydrazone-3-hydrazino-4-benzyl-6-phenyl pyridazine (BHP) and its complexes with copper(II), nickel(II), cobalt(II), zinc(II), manganese(II), cadmium(II), thorium(IV), dioxyuranium(VI), samarium(III) and erbium(III) are presented. The protonation equilibrium of BHP ion and complex formation equilibrium with the metal ions have been studied by potentiometry in 75% (v/v) dioxane-water and 0.10M KNO₃ at different temperatures (10, 20, 30 and 40°C). A series of mononuclear complexes [ML _n ]^{(1? z )+} (L^? =?BHP and n =?1 ??z) were found in solution and their formation constants, enthalpies and entropies were determined.

The solid metal complexes and corresponding thermal products were elucidated by elemental analysis, conductance, IR and electronic spectra, magnetic moments, ¹H NMR and TG-DSC measurements as well as by mass spectroscopy. The use of BHP as analytical reagents for the determination of copper(II), nickel(II) and cobalt(II) as well as extracting agents for these metal ions are discussed.     

Experimental and DFT Evidence for the Fractional Non‐Innocence of a β‐Diketonate Ligand

New compounds [Ru(pap)₂(L)](ClO₄), [Ru(pap)(L)₂], and [Ru(acac)₂(L)] (pap=2‐phenylazopyridine, L^?=9‐oxidophenalenone, acac^?=2,4‐pentanedionate) have been prepared and studied regarding their electron‐transfer behavior, both experimentally and by using DFT calculations. [Ru(pap)₂(L)](ClO₄) and [Ru(acac)₂(L)] were characterized by crystal‐structure analysis. Spectroelectrochemistry (EPR, UV/Vis/NIR), in conjunction with cyclic voltammetry, showed a wide range of about 2 V for the potential of the Ru^III/II couple, which was in agreement with the very different characteristics of the strongly π‐accepting pap ligand and the σ‐donating acac^? ligand. At the rather high potential of +1.35 V versus SCE, the oxidation of L^? into L^. could be deduced from the near‐IR absorption of [Ru^III(pap)(L^.)(L^?)]²⁺. Other intense long‐wavelength transitions, including LMCT (L^?→Ru^III) and LL/CT (pap^.?→L^?) processes, were confirmed by TD‐DFT results. DFT calculations and EPR data for the paramagnetic intermediates allowed us to assess the spin densities, which revealed two cases with considerable contributions from L‐radical‐involving forms, that is, [Ru^III(pap⁰)₂(L^?)]²⁺?[Ru^II(pap⁰)₂(L^.)]²⁺ and [Ru^III(pap⁰)(L^?)₂]⁺?[Ru^II(pap⁰)(L^?)(L^?)]⁺. Calculations of electrogenerated complex [Ru^II(pap^.?)(pap⁰)(L^?)] displayed considerable negative spin density (?0.188) at the bridging metal.     

Delocalization and Valence Tautomerism in Vanadium Tris(iminosemiquinone) Complexes

To survey the noninnocence of bis(arylimino) acenaphthene (BIAN) ligands (L) in complexes with early metals, the homoleptic vanadium complex, [V(L)₃] ( 1 ), and its monocation, [V(L)₃]PF₆ ( 2 ), were synthesized. These complexes were found to have a very rich electronic behavior, whereby 1 displays strong electronic delocalization and 2 can be observed in unprecedented valence tautomeric forms. The oxidation states of the metal and ligand components in these complexes were assigned by using spectroscopic, crystallographic, and magnetic analyses. Complex 1 was identified as [V^IV(L^red)(L^.)₂] (L^red=N,N′‐bis(3,5‐dimethylphenylamido)acenaphthylene; L^.=N,N′‐bis(3,5‐dimethylphenylimino)acenaphthenesemiquinonate). Complex 2 was determined to be [V^V(L^red)(L^.)₂]⁺ at T<150 K and [V^IV(L^.)₃]⁺ at T>150 K. Cyclic voltammetry experiments reveal six quasi‐reversible processes, thus indicating the potential of this metal–ligand combination in catalysis or materials applications.     

Übergangsmetallkomplexe der Pyrazin-2,6-dicarbonsäure und der Pyridin-2,6-dicarbonsäure: Synthesen und Elektrochemie. Die Kristallstruktur von NH4[RuCl2(dipicH)2]

Transition Metal Complexes Containing the Ligands Pyrazine-2, 6-dicarboxylate and Pyridine-2, 6-dicarboxylate: Syntheses and Electrochemistry. Crystal Structure of NH₄[RuCl₂(dipicH)₂] The coordination chemistry of the tridentate ligand pyrazine-2, 6-dicarboxylate (pyraz-2,6 = L) with transition metals in aqueous solution has been investigated. The reaction of the ligand with metal aqua ions (1:1) affords insoluble precipitates [M^IIL(OH₂)₂] (M = Mn, Fe, Co, Ni, Cu, Zn, Cd). [TiOL(OH₂)₂], [VOL(H₂O)₂] and [UO₂L(H₂O)] were also prepared. [M^IIIL₂]^? complexes (M^III ? Fe^III, Co^III) were isolated as NH₄⁺ and P(C₆H₅)₄⁺ salts; they are strong one electron oxidants (E_1/2 = +0.602 V and +0.795 V vs. NHE, respectively). Redox potentials of analogous complexes containing pyridine- 2, 6-dicarboxylate (L′) ligands have been determined by cyclic voltammetry: [ML′₂]^1-/2?: M = V^III: -0.591 V; Cr^III: -0.712 V. It is shown that pymzine-2,6-dicarboxylate as compared to pyridine-2,6-dicarboxylate stabilizes metal complexes in low oxidation states (+II). The reaction of RuCl₃ · nH₂O with pyridine-2,6-dicarboxylic acid in aqueous solution affords the yellow-green anion [RuCl₂(L′H)₂]^?. The crystal structure of NH₄[RuCl₂(L′H)₂] has been determined. It crystallizes in the monoclinic space group P2₁/c with a = 8.812(2) Å b = 10.551(2) Å, c = 10.068(2) Å, β = 110.03(6)°, Z = 2; 2507 independent reflections; R = 0.032. The ruthenium centers are in an octahedral environment of two Cl^? ligands (trans) and two bidentate pyridine-2, 6-hydrogendicarboxylate ligands which possess each one protonated, uncoordinated carboxylic group.     

Nanoparticles of type Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>-SiO<Subscript>2</Subscript>-graphene oxide and coated with an amino acid-derived ionic liquid for extraction of Al(III), Cr(III), Cu(II), Pb(II) prior to their determination by ICP-OES

An amino acid derived ionic liquid, Fe₃O₄ nanoparticles and graphene oxide (GO) were used to prepare a material for the magnetic solid phase extraction (MSPE) of the ions Al(III), Cr(III), Cu(II) and Pb(II). The material was characterized by Fourier transform infrared spectral (FT-IR), scanning electron microscopy (SEM), thermal gravimetric analysis (TGA), magnetic analysis and isoelectric point (pI) analysis. It is shown to be a viable sorbent for the separation of these metal ions. Single factor experiments were carried out to optimize adsorption including pH values, ionic strength, temperature and solution volume. Following desorption with 0.1 M HCl, the ions were quantified by inductively coupled plasma optical emission spectrometry. Under the optimum conditions, the method provides a linear range from 10 to 170 μg· L^?1 for Al(III); from 4.0 to 200 μg· L^?1 for Cr(III); from 5.0 to 170 μg· L^?1 for Cu(II); and from 5.0 to 200 μg· L^?1 for Pb(II). The limits of detection (LOD) are 6.2 ng L^?1 for Al(III); 1.6 ng L^?1 for Cr(III); 0.52 ng L^?1 for Cu(II); and 30 ng L^?1 for Pb(II). Method performance was investigated by determination of these ions in (spiked) environmental water and gave recoveries in the range of 89.1%–117.8%.

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Graphical abstract The graph shows that Al(III), Cr(III), Cu(II), Pb(II) are not adsorbed quantitatively by Fe₃O₄-SiO₂. On the other hand, Cr(III) and Pb(II) are adsorbed quantitatively by Fe₃O₄-SiO₂-GO while Al(III) and Cu(II) are not quantitatively retained. However, 3D–Fe₃O₄-SiO₂-GO-AAIL adsorb all these 4 metal ions quantitatively.

     

Some lanthanide metal complexes of n-isonicotinamidosalicylaldimine

Trivalent lanthanide complexes of the type K[ML ₂] whereM=La(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III) and Dy(III) and H₂ L=N-isonicotinamidosalicyladimine, have been prepared and characterised. The nephelauxetic ratio (β), covalency (δ) and bonding parameter (b ²) of K[NdL ₂] have been calculated. Infrared spectral studies reveal that N-isonicotinamidosalicylaldimine acts as a dibasic tridentate ligand. A coordination number six has been proposed for the lanthanide metal ions.     

A Ce3+-imprinted functionalized potassium tetratitanate whisker sorbent prepared by surface molecularly imprinting technique for selective separation and determination of Ce3+

A surface molecular imprinting technology was developed to adsorb Ce(III) ions that showed much higher adsorption affinity and selectivity for than for other metal ions. The batch adsorption process was studied with respect to effects of pH value, residence time, temperature, and initial concentration of Ce(III) ion. The maximum adsorption capacity is 43 mg g^?1 at an initial Ce(III) concentration of 300 mg L^?1 and at a sorbent dosage of 1.0 g L^?1. A Langmuir isotherm fits the experimental data. The imprinted sorbent exhibits a much higher separation and selectivity for the target imprinted ion than the non-imprinted polymer. Cerium ion can be desorbed with 1M hydrochloric acid solution which is also proven by scanning electron microoscopy and X-ray diffraction experiments. The limit of detection is 37 ng mL^?1. The sorbent has been applied to the determination of trace cerium in different environmental samples with satisfactory results.     

Studies in negative ion mass spectrometry. VII—Negative ion mass spectra of copper (II) β-diketonate complexes

Electron capture processes in a series of copper (II) β-diketonate complexes of formula Cu[R¹COCHCOR²]₂ (where R¹ is an alkyl, perfluoroalkyl or aryl group and R² either an alkyl or aryl group) have been examined. Molecular anions, ligand ions and some novel rearrangement ions have been observed with these compounds. Relative intensities of fragment ions were dependent on the substituents R¹, R² as well as the electron energy and compound pressure in the ion source. By operating the mass spectrometer at compound pressures of c. 4×10^?6 Torr and higher, reproducible negative ion mass spectra (free from any significant ion-molecule contributions) have been obtained for all compounds of the series.