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.     

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.     

The influence of ligand substituents and donor atom sets on the gas phase electron attachment reactions of bis-chelates of nickel(II)

Electron attachment reactions and negative ion mass spectra which were obtained under negative chemical ionization conditions have been examined for a series of 21 nickel(II) bis-chelates of formula Ni[R¹CXCHCYR²]₂. Three ligand donor atom sets (X, Y), respectively O₄, O₂S₂, S₄ were investigated for each of the substituent combinations, viz.: R¹=CH₃, CF₃ or C₂H₅O, R²=CH₃; R¹=C₆H₅, CH₃ or CF₃, R²=C₆H₅; and R¹ = R² = tert?C₄H₉. While the ligand substituent combinations exerted considerable influence over the various ion decomposition reactions, the relative molecular ion stabilities were largely dependent on the ligand donor atom sets and followed the sequence O₄? O₂S₂>S₄ for most substituent combinations. Rationalizations are offered in terms of reductive electron capture reactions involving metal-based orbitals, as well as the increasing stabilities of reaction products as sulphur is incorporated into the ligand donor atom sets. A comparison is also given of negative ion mass spectral data obtained under electron impact conditions as well as negative chemical ionization conditions when methane was used as an electron energy moderating gas.     

Studies in negative ion mass spectrometry: XIV—Electron attachment reactions of a series of η6-arenetricarbonylchromium(O) complexes

Electron attachment reactions of a series of (η⁶-arene)tricarbonylchromium(O) complexes have been examined in the gas phase. The electron capture process has been shown to be influenced by the structure of the η⁶-arene ligand and its substituents. Whereas (η⁶-benzene)- and (η⁶-mesitylene)tricarbonylchromium(O) undergo dissocative electron capture, or reductive decarbonylation, yielding [M? CO]^?˙ ions of highest abundance in their negative ion mass spectra, [η⁶-(2,2-dimethylindan-1,3-dione)]tricarbonylchromium(O) forms a molecular negative ion which undergoes sequential CO eliminations and finally a demetallation to give the arene radical ion. A localization of charge on the coordinated arene ligand is proposed for the formation of [M]^?˙ in this case. (η⁶-Methylbenzoate)tricarbonylchromium(O) also forms a molecular negative ion by secondary electron attachment which decomposes by simultaneous and consecutive eliminations of up to four CO molecules. The elucidation of a mechanism and sequence for these CO eliminations has been achieved by synthesizing and examining the negative ion mass spectrum of [η⁶-(C₆H₅·¹³CO₂Me)]Cr(CO)₃. The first CO loss in the principal fragmentation pathway occurs solely from the –Cr(CO)₃ group of [M]^?˙. The effect of para substituent groups on the stabilities of molecular negative ions and their fragmentations has been ascertained using a series of para-substituted (η⁶-methylbenzoate)tricarbonylchromium(O) compounds containing the groups NH₂, OH, OCH₃, CL and COOMe. The stabilities of the [M]^?˙ ions have been rationalized in terms of the Hammett and Taft parameters σ_P, σ_I, σ_R^P, σ_P^O and σ_R^O. The overall electronic substituent effect transmitted to the carbonyl groups of the –Cr(CO)₃ unit involves both resonance and inductive components. In this series of compounds the stability of [M]^?˙ decreases as the electron withdrawing capacities of the para substituents increase.     

Positive and negative ion mass spectrometric studies of dioximes of α-diketones and their nickel complexes

The positive and negative ion mass spectra of glyoxime, methylglyoxime, dimethylglyoxime, diphenyl glyoxime and of their nickel(II) complexes are reported. Both the positive and negative ion mass spectra of the dioximes show loss of OH^˙ and H₂O from the molecular ion to give fragment ions which probably have cyclic furazan type structures. The positive ion spectra of the complexes fragment mainly by loss of ligand radicals whereas the negative ion spectra show mainly loss of OH^˙ and H₂O.     

Studies in negative ion mass spectrometry. V—A Comparison of positive a positive and negative ion mass spectra of copper(II) complexes of bidentate schiff base ligands

70 eV positive and negative ion mass spectra of a series of copper(II) Schiff base complexes have been obtained consecutively under the same ion source conditions. The characteristic feature of the negative ion spectra is their extreme simplicity relative to the corresponding positive ion spectra, the only ions present in significant abundance being the molecular anions and ligand ions. The influence of substituents (R) on positive and negative ion fragmentation patterns is discussed. Metastable peaks have been obtained in all cases for the transition [Cu(Ligand)₂]^? → [Ligand]^?.     

The formation of metal—carbon bonds via insertion reactions in gaseous negative ion/molecule reactions

The negative ion mass spectra under chemical ionization conditions for a series of copper(II), nickel(II), and cobalt(II) Schiff base complexes have been measured. Reagent gases employed include methane, isobutane, and methane-d₄. For all complexes, molecular negative ions were produced with all three reagent gases via secondary electron capture processes. Ion/molecule reactions between the molecular negative ion and the neutral reactant gas appear to be the dominant processes for the formation of secondary ions. The secondary reactions lead to the incorporation of the CH₂ moiety into the nickel(II) complexes and the CH₃ group into the cobalt(II) compounds.     

Negative ion mass spectra of metal complexes: Secondary electron capture and CS2 elimination from nickel (ii) dithiocarbamates

Negative ion mass spectra of series of bis-(N, N-dithiocarbamato)nickel(II) complexes of formula [NiS₂C·NR¹R²]₂ (where·NR¹R² ? ·NEt₂ ·NPr₂, ·NBu₂, pyrrolidinyl, piperidyl, morpholinyl, and ·NEtPh) have been obtained by secondary electron capture. Intense molecular anions are given for all compounds, with most fragments originating from these ions. Metastable data indicate that CS₂ is eliminated from all molecular anions.     

Studies in negative ion mass spectrometry. X—Electron capture by a series of tris(hexafluoroacetylacetonato) metal chelates

Results of electron capture and negative ion mass spectrometric studies are reported for a series of tris-chelates of the type Metal. L₃, where L refers to the ligand or enolate ion of the β-diketone 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (hexafluoracetylacetone), and the metals are: Sc(III), Ti(III), V(III), Cr(III), Mn(III), Fe(III), Co(III), A1(III), Ga(III), In(III). The negative ion mass spectra were all relatively simple; the most abundant ions being the molecular and ligand ions for all the metals studied. Reaction schemes have been established to account for the appearance of all significant fragment ions, many of which have been formed as a result of fluorine atom transfer processes. For the transition metal complexes, evidence for elimination of neutral divalent metal fluorides comes from the ion decomposition reactions [Metal.L.F₂]^?→[L]^?, and for the Group III metal complexes, [Metal.L₃]^?→[Metal.L₂]^? as well as [Metal.L₂]^?→[L]^? processes indicate that the metals have been reduced as a consequence of the initial electron capture and subsequent fragmentations of metal-containing ions. The influence of the metal atom and its 3d-electron configuration has been shown not to affect significantly the electron capture processes. However, the relative instabilities of molecular anions of the transition metal tris-complexes show an approximately linear dependence on the increasing 3d-electron populations of the metal ions from Ti(III) to Co(III).     

Formation of an unusual MH− ion in the methane negative-ion chemical ionization mass spectra of chlorprothixene and aromatic sulfur compounds

The methane negative-ion chemical ionization (NCI) mass spectrum of chlorprothixene shows an unusual MH^? ion. This ion can be accounted for by electron capture followed by H˙ transfer from the reagent gas. The most probable site of electron attachment was concluded to be related to the sulfur atom of the thioxanthene ring based on the observation of analogous ions for structurally related compounds, all containing a heterocyclic sulfur. The MH^? ion observed with methane as the reagent gas was shifted to MD^? when tetradeuteromethane was used in place of methane. The ratio of [M ? H]^? to MH^? did not change with emission current suggesting that the process is independent of the radical concentration in the CI plasma. Consistent with this observation is the lack of CH₃˙ or C₂H₅˙ adduct ions in the NCI mass spectrum and the fact that gold-plating the ion source did not decrease the proportion of MH^?. Also, this mechanism is consistent with thermochemical considerations of reactions of a phenyl radical with various alkanes and observations of ions formed by methane NCI from model compounds. Therefore, unlike other MH^? ions observed in methane NCI mass spectra, the mechanism of formation does not appear to involve a hydrogen radical addition followed by electron capture.     

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.     

Negative chemical ionization mass spectrometry of explosives

The negative chemical ionization mass spectra of nitrobenzene, ethylene glycol dinitrate and nitroglycerine have been obtained using various reagent ions. For nitrobenzene, [OH]^? gives the [M ? H]^?, together with [M]^?˙ ions formed by electron capture, but other reagent ions gave relatively low intensity adduct peaks. Ethylene glycol dinitrate and nitroglycerine gave abundant [M + X]^? ions (X = NO₂, NO₃, Cl, Br, I), together with ions arising from the thermal decomposition of the samples in the heated inlet system. The rate of anion attachment to these compounds is much greater than that to related compounds having only one functional group, and it is suggested that this is due to the participation of the adjacent groups in the bonding between the substrate and anion.     

Stereospecific adduct ion formation in the chemical ionization mass spectra of E- and Z-1,2,3-triaryl-2-propen-1-ones

Stereospecific adduct ion formation has been observed in the chemical ionization mass spectra (positive and negative) of certain E- and Z-1,2,3-triaryl-2-propen-1-ones. The Z isomers are found to give higher relative abundances of adduct ions than the E isomers. This has been interpreted in terms of the differences in the proton affinities of the isomers originating from their different degrees of enone resonance. Halide ion (CI^? and Br^?) attachment spectra of these compounds also show stereochemical differences in the relative abundances of [M]^?˙ and [M+halide]^? ions, though the effect is not as pronounced as in the case of the positive ion spectra.     

Ion-molecule reactions observed for nitroaromatic compounds in negative ion fast atom bombardment mass spectrometry

Analyses of a series of nitroaromatic compounds using fast atom bombardment (FAB) mass spectrometry are discussed. An interesting ion-molecule reaction leading to [M + O ? H]^? ions is observed in the negative ion FAB spectra. Evidence from linked-scan and collision-induced dissociation spectra proved that [M + O ? H]^? ions are produced by the following reaction: M + NO₂^? → [M + NO₂]^? → [M + O ? H]^?. These experiments also showed that M^?˙ ions are produced in part by the exchange of an electron between M and NO₂^? species. All samples showed M^?˙, [M ? H]^? or both ions in their negative ion FAB spectra. Not all analytes studied showed either [M + H]⁺ and/or M⁺˙ in the positive ion FAB spectra. No M⁺˙ ions were observed for ions having ionization energies above ～9 eV.     

AQUEOUS EQUILIBRIA OF COPPER(II)- AND NICKEL(II)-POLYGLYCINE COMPLEXES

Abstract

Magnetic susceptibility measurements, new potentiometric data, optical spectra, and a new statistical method of calculation are combined in the formulation of an equilibrium scheme defining the dilute solution interactions of nickel(II) and copper(II) ions with diglycine, triglycine, and tetraglycine as a function of pH. At low pH appreciable concentrations of a previously unreported complex, MHL²⁺ (HL =polyglycine ligands) are shown to be present in all nickel(II)-polyglycine systems and in the copper(II)-triglycine system. This new protonated species is assigned a structure in which the metal ion is coordinated to the terminal carboxylate and to the adjacent peptide carbonyl oxygen with the proton residing on the terminal amino group. As the pH is raised in the 1:1 systems, MH_?1L, MH_?2L^? and MH_{_3}L^2- are formed in succession depending on the number of peptide linkages in the ligands, HL. The concentration of the monodeprotonated intermediate species NiH_?1L never exceeds 10% of the total metal ion concentration in the triglycine case and is always less than 0.5% when tetraglycine is the ligand. The dideprotonated intermediate NiH_?2L- reaches a maximum of 38% of the total metal concentration in the 1:1 Ni-tetraglycine system. An explanation is presented for this negative deviation from the predictions based on statistical grounds. Complete species distribution diagrams which include the new protonated complexes are presented and are employed to explain the differences in the interactions of copper(II) and nickel(II) ions with polyglycine ligands. Probable coordinate bonding sites suggested for the complexes formed in solution are inferred on the basis of stoichiometry, relative stabilities, and available microscopic information.     

Mass spectral fragmentation pathways in RDX and HMX. A mass analyzed ion kinetic energy spectrometric/collisional induced dissociation study

A collisional induced dissociation study of 1,3,5-trinitro-1,3,5 triazacyclohexane (RDX) and 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX) was carried out using mass analyzed kinetic energy spectrometry. High resolution mass spectra and mass analyzed ion kinetic energy/collisional induced dissociation spectra of RDX and HMX were recorded in the electron impact, chemical ionization and negative ion chemical ionization modes. Fragmentation pathways of the compounds investigated were determined in all three modes of ionization. It was found that a major part of the fragment ions in RDX and HMX originate from formation of the aduct ions [M+NO]⁺ and [M+NO₂]⁺ in electron impact and chemical ionization, and from [M+NO]^? and [M+NO₂]^? in negative chemical ionization, followed by dissociation.     

High energy collisional spectroscopy of [RCOO]− anions from negative ion chemical ionization of fatty acid methyl esters

Saturated and mono-unsaturated fatty acid methyl esters as well as corresponding underivatized acids gave abundant carboxylate anions [RCOO]^? under negative ion chemical ionization (OH^? and NH₂ ^?) and electron attachment conditions. B/E or mass-analysed ion kinetic energy spectra of fragments arising from high energy collision activated dissociation (CAD) of these [RCOO]^? species contained decisive information on the original structure of the neutrals. From an analytical point of view, the method can utilize the gas chromatographic mass spectrometric combination and, in the B/E mode, can be used in practice on any double-focusing mass spectrometer.     

Negative and positive secondary ion mass spectra of organic acids

Organic compounds can be ionized by sputtering the solid sample. The resulting negative and positive secondary ions provide mass spectra which characterize both the molecular weights and the structures of the compounds. Ionization occurs either by direct ejection of charged species from the solid into vacuum or by electron or proton transfer. The sputtered secondary ions dissociate unimolecularly to give fragment ions. These reactions are identical to those which occur when the secondary ions are independently generated by chemical ionization, selected by mass and dissociated in a high energy gas phase collision. The negative ion SIMS spectra show molecular ions (M^?.) or (M-H)^? ions as the dominant high mass species together with fragments due to decarboxylation, dehydration and losses of other simple molecules. Stronger acids show larger (M-H)^?/M^?.abundance ratios. The positive ion spectra are complementary and also useful in characterizing molecular structures. Attachment of cations to organic molecules (cationization) occurs much more readily than anion attachment and this makes negative SIMS spectra simpler than these positive ion counterparts.     

The negative ion mass spectra of mixtures of sulphur and organic compounds. I—The formation of [M+Sn]− from the mixture of sulphur and dinitriles

The negative ion mass spectra of sulphur, dinitriles and their mixtures were studied. The abundance of negative ions of sulphur was almost comparable to that of positive ions. In the negative ion mass spectrum of a mixture of sulphur and dinitrile, intense and characteristic peaks of [M + S_n]^? (n = 2, 3, 4, etc.) were observed resulting from ion–molecule association of dinitrile with the S_n^? anions. As a standard sample of a negative ion mass spectrum, sulphur itself has proved useful in the lower mass region (below m/e 256: S₈^?).