Observation of far-infrared transitions of HCO+, CO+ and HN+2

Rotational transitions of molecular ions HCO⁺, CO⁺, and HN⁺₂ have been observed at frequencies aroud 1 THz. The ions were produced in the negative glow of a hollow cathode discharge cooled by liquid nitrogen. Preliminary results indicate efficient production of ions in an absorption cell of simple construction.     

Structure of gas phase [C5H6]+˙ ions

Charge-stripping spectra have been used to differentiate ionized cyclopentadiene from its acyclic isomers. The minimum amounts of translational energy lost during the charge-stripping processes and the relative charge-stripping efficiencies, which are also structurally important parameters, have been measured for these ionic species. [C₅H₆]⁺˙ ions, formed by dissociative ionization of various precursors in the ion source are found, usually, to be a mixture of cyclic and acyclic ions. In contrast, [C₅H₆]⁺˙ ions, derived from the dissociation of metastable molecular ions from a series of organic compounds, have the cyclopentadienyl structure. This structure was confirmed by collision-induced dissociation of ions formed in the first field-free region of a triple sector mass spectrometer.     

Resonance capture of electrons by substituted pyrazoline molecules

The negative ion mass spectra and photoelectron spectra of substituted pyrazolines were studied. A correlation between the ionization energy of the highest occupied molecular orbital and the yield of [MeNHNH₂ ⁻] ions was found. Isomerization of molecular negative ions was studied by resonance electron capture mass spectrometry. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 701–704, April, 2000.     

Energy dependence of the fragmentation of some isomeric [C6H12]+˙ ions

The charge exchange mass spectra of 14 C₆H₁₂ isomers have been determined using [CS₂]^+˙, [COS]^+˙, [Xe]^+˙, [CO]^+˙, [N₂]^+˙ and [Ar]^+˙ as the major reactant ions covering the recombination energy range from ∼10.2 eV to ∼15.8 eV. From the charge exchange data breakdown graphs have been constructed expressing the energy dependence of the fragmentation of the isomeric [C₆H₁₂]^+˙ molecular ions. The electron impact mass spectra are discussed in relation to these breakdown graphs and approximate internal energy distribution functions derived from photoelectron spectra.     

Formation of O 2 − in charge transfer collisions of fast molecular hydrogen ions with O2 molecules

Cross sections for the production of O ₂ ^? in charge transfer collisions of fast molecular hydrogen ions (H ₂ ⁺ , D ₂ ⁺ , H ₃ ⁺ , and D ₃ ⁺ of 10 to 140 keV kinetic energy) with O₂ molecules have been determined by means of a time-of-flight mass spectrometer analysing the slow negative product ions from the collisions. Within the measuring accuracy equivelocity H ₂ ⁺ and D ₂ ⁺ ions have the same cross sections for the generation of O ₂ ^? . The projectile velocity dependence curve of the cross section passes through a broad maximum with a peak value of about 6.5×10^?18 cm² around the Bohr velocity (25 keV/u) before showing an asymptotic decrease still within the limited energy range under investigation that is in inverse proportion to the square of velocity. Throughout the examined energy range H ₃ ⁺ ions yield a cross section which is about 1.4 times larger than that of H ₂ ⁺ ions of the same velocity. The fragment ion O^? has been found to appear with cross sections between 10^?19 and 10^?18 cm² upon collisional excitation in the energy range under investigation, with ever decreasing intensity when the energy of the positive hydrogen ions, the proton included, was increased.     

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]^?.     

Isomerization and fragmentation of methylfuran ions and pyran ions in the gas phase

The mutual interconversion of the molecular ions [C₅H₆O]⁺ of 2-methylfuran (1), 3-methylfuran (2) and 4H-pyran (3) before fragmentation to [C₅H₅O]⁺ ions has been studied by collisional activation spectrometry, by deuterium labelling, by the kinetic energy release during the fragmentation, by appearance energles and by a MNDO calculation of the minimum energy reaction path. The electron impact and collisional activation mass spectra show clearly that the molecular ions of 1–3 do not equilibrate prior to fragmentation, but that mostly pyrylium ions [C₅H₅O]⁺ arise by the loss of a H atom. This implies an irreversible isomerization of methylfuran ions 1 and 2 into pyran ions before fragmentation, in contrast to the isomerization of the related systems toluene ions/cycloheptatriene ions. Complete H/D scrambling is observed in deuterated methylfuran ions prior to the H/D loss that is associated with an iostope effect k_H/k_D = 1.67–2.16 for metastable ions. In contrast, no H/D scrambling has been observed in deuterated 4H-pyran ions. However, the loss of a H atom from all metastable [C₅H₅O]⁺ ions gives rise to a flat-topped peak in the mass-analysed ion kinetic energy spectrum and a kinetic energy release (T₅₀) of 26 ± 1.5 kJ mol^?1. The MNDO calculation of the minimum energy reaction path reveals that methylfuran ions 1 and 2 favour a rearrangement into pyran ions before fragmentation into furfuryl ions, but that the energy barrier of the first rearrangement step is at least of the same height as the barrier for the dissociation of pyran ions into pyrylium ions. This agrees with the experimental results.     

The mass spectra of isomeric hydrocarbons—I: Norbornene and nortricyclene; The mechanisms and energetics of their fragmentations

The mass spectra of norbornene, nortricyclene and deuterium labeled derivatives thereof have been studied. The appearance potentials of the ions [C₇H₁₀]^+·, [C₇H₉]⁺, [C₆H₇]⁺ and [C₅H₆]^+· have been determined for both compounds and heats of formation of the hydrocarbons have been estimated. Detailed fragmentation schemes are proposed for the molecular ions and it is concluded that they dissociate by essentially different mechanisms which do not involve common intermediates. The structures and energy contents of the primary fragment ions are discussed in detail by comparing energetics, labeling experiments and metastable ion abundances.     

Electron impact,chemical ionization and negative chemical ionization mass spectra,and mass-analysed ion kinetic energy spectrometry–collision-induced dissociation fragmentation pathways of some deuterated 2,4,6-trinitrotoluene (TNT) derivatives

Mass-analysed ion kinetic energy spectrometry (MIKES) with collision-induced dissociation (CID) has been used to study the fragmentation processes of a series of deuterated 2,4,6-trinitrotoluene (TNT) and deuterated 2,4,6-trinitrobenzylchloride (TNTCI) derivatives. Typical fragment ions observed in both groups were due to loss of OR′ (R′ = H or D) and NO. In TNT, additional fragment ibns are due to the loss of R₂′O and 3NO₂, whilst in TNTCI fragment ions are formed by the loss of OCI and R₂′OCI. The TNTCI derivatives did not produce molecular ions. In chemical ionization (Cl) of both groups. MH⁺ ions were observed, with [M – OR′]⁺ fragments in TNT and [M – OCI]⁺ fragments in TNTCI. In negative chemical ionization (NCI) TNT derivatives produced M^?′, [M–R′]^?, [M–OR′]^? and [M–NO]^? ions, while TNTCI derivatives produced [M–R]^?, [M–Cl]^? and [M – NO₂]^? fragment ions without a molecular ion.     

Metastabile Übergänge des Ions C2H3O+ im Massenspektrometer

The metastable transitions of C₂H₃O⁺ ions generated from ten different compounds have been investigated. The intensity ratios of the metastable peaks have been found to be independent of the structure of the initial compound. They are dependent however on the internal energy of the ions. The variation of the intensity ratios with the molecular size is in agreement with the degrees-of-freedom-effect. C₂H₃O⁺ ions decompose from the same structure (mixture of structures) in all cases.

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Mit 2 Abbildungen     

Dissociative double electron capture,dynamics of fragmentation of the water and hydrogen sulfide negative ions

The technique of ion kinetic energy spectrometry has been used to observe the unimolecular decompositions of H₂O^?? and H₂S^?? generated by charge exchange of the corresponding high velocity positive ions. The method involves dissociative double electron capture by a high velocity ion and allows the study of unstable negative ions that may be directly observable by conventional electron capture techniques. Information on the energetics of the reaction is obtained from the kinetic energy of the product ion. The reactions under consideration are shown in (1) and (2) where X = O or S.

The kinetic energy releases accompanying the reactions given in (1) and (2) have been measured and compared to those for the collision-induced reactions which produce the corresponding positive ions. The results have been used to deduce that the sequence of steps in the formation of the fragment negative ions is that given in (1) and (2). The cross section of OH^? formation is observed to be somewhat greater than for O^? production. This result is in contrast with dissociative electron capture cross sections from the neutral species and is interpreted on the basis of the energetic requirements for the reactions under consideration. H₂O^? reacts from different electronic states in yielding OH^? on the one hand and O^? on the other. The energy partitioning associated with reaction (2) suggests that the neutral productions 2H' rather than H₂. The kinetic energy losses accompanying excitation and kinetic energy releases upon fragmentation were similar for the corresponding reactions of the sulfur and oxygen-containing ions indicating related mechanisms in the two sets of reactions.     

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).     

Studies in negative ion mass spectrometry. XIII—Electron attachment to a series of Nickel(II) β-diketonate complexes

Results of electron attachment reactions and negative ion mass spectra are presented for a group of selected nickel(II) β-diketonate complexes of formula Ni[R¹COCHCOR²]₂, where R¹ is a perfluoroalkyl group and R² either an alkyl or aryl group. Molecular negative ions together with ligand ions are the major contributors to the total ion currents for each compound, and the degree of fragmentation has been shown to be dependent on the substituents R¹ and R². Fragmentation schemes have been elucidated for all the major ion decomposition pathways, and all significant ions have been identified in the negative ion mass spectra of each compound. Bis(1,1,1-trifluoro-5,5-dimethyl-2,4-hexanedionato) nickel(II), with R¹?CF₃ and R²?tert-butyl is the complex which shows considerable potential for analytical quantitation in the negative ion mode, because of the stability of its negative molecular ion, the high negative ion yield given after electron attachment, as well as the volatility of the compound.     

The Chemistry of C6H6O radical cations: A study of rearrangement reactions of halogen substituted ethyl phenyl ethers

New experimental data on the rearrangement reaction of various phenoxyethyl halides to give [C₆H₆O]^+˙ are presented and compared with previous studies so that a coherent picture of this process can be developed. By examining the metastable kinetic energy release for low energy decomposing molecular ions of the phenoxyethyl halides, it has been concluded that formation of [C₆H₆O] occurs by competitive 1,2 and 1,3 hydrogen shifts from the alkyl carbons to oxygen followed by a rate determining C? O bond cleavage. This is substantiated by the absence of a primary hydrogen isotope effect. For more highly activated molecular ions, a new mechanism comes into play as evidenced by the appearance of a small hydrogen isotope effect. It is postulated that this third mechanism involves transfer of the alkyl hydrogen to the ortho position of the ring by a rate determining 1,5 shift, followed by a 1,3 hydrogen shift from the ortho methylene group to oxygen and rapid C? O bond cleavage. This 1,3 hydrogen shift to oxygen appears to be ‘catalysed’ by the halogen atoms yielding phenol ions. No indications have been found for the formation of tautomeric 2,4-cyclohexadienone ions. Furthermore, highly activated molecular ions produce [C₆H₆O]^+˙ which can undergo metastable decomposition to lose carbon monoxide. Kinetic energy release measurements for the latter reaction show that the majority of these [C₆H₆O]^+˙ions have been formed as phenol ions as well. These arguments are supported by energetic measurements and by comparisons with previous ion cyclotron resonance and collisional activation studies.     

Kinetic-energy spectroscopy of high-rydberg fragments from keV collisions of H2+, D2+, N2+ and C2+ ions with rare-gas atoms

A method for measuring the kinetic-energy spectrum of high-Rydberg fragments from collisions of keV molecular ions with rare-gas atoms is described. The kinetic-energy spectra of high-Rydberg fragments from the collisions between D₂⁺, H₂⁺, N₂⁺ and C₂⁺ ions having 8 keV kinetic energy and thermal He and Xe are reported. Two single-collision processes for the generation of high-Rydberg fragments have been identified.     

Rearrangements and fragmentations of [C2H5S]+ ions

From deuterium labelling experiments it was concluded that metastable molecular ions of ethyl methyl sulfide lose a methyl radical with the formation of both [CH₃S?CH₂]⁺ amd [CH₃CH?SH]⁺˙ The fragmentation reactions of metastable ions generated with these structure are losses of C₂H₂, H₂S and CH₄. These reactoins and the preceding isomerizations have also been studied by means of deuterium labelling. From the results it is concluded that the three fragmentation reactions most probably occur from ions with a C? C? S skeleton. Appearance energy measurements for ions generated with the two structures above and all give rise to the same ΔH_f value for these three isomeric forms. Ab initio molecular orbitals calculations confirm that these three ions fortuitously have very similar heats of formation. A potential energy diagram rationalizing the isomerizations and the principal fragmentation reaction is presented.     

Multiply charged carbon ions produced from C2H2 and CH4 molecules in energetic,heavy ion impact

Multiply charged carbon ions up to C⁵⁺ ions have been observed from C₂H₂ and CH₄ molecules in 1.05 MeV/amu Ar¹²⁺ ion impact. These ions are believed to be produced through processes where multiply charged molecular ions are produced first by direct ionization and, then, dissociate via Coulomb explosion into atomic ions. The peak positions of ions from C₂H₂ molecules are found to be shifted with respect to those of ions from CH₄ molecules and this shift is understood to be due to the initial kinetic energy provided through the Coulomb potential between the dissociating ions.     

Isomerization and fragmentation of methyl benzofuran ions and chromene ions in the gas phase

The rearrangement of the molecular ions of the isomeric 2- and 3-methyl benzofurans (1 and 2), 2H-chromene (3) and 4H-chromene (4) has been studied as a further example of the isomerization of oxygen-heteroaromatic radical cations via a ring expansion/ring contraction mechanism well documented for molecular ions of alkyl benzenes. The ions 1⁺˙?4⁺˙ fragment mainly by H loss into identical chromylium ions a. The process exhibits consistently a large kinetic energy release and an isotope effect k_H/k_D, which arise from a rate-determining energy barrier of the last dissociation step. Differences of the kinetic energy releases, the isotope effects and the appearance energies of the methyl benzofuran ions and the chromene ions indicate a large energy barrier also for the initial hydrogen migration during the rearrangement of the methyl benzofuran ions. This is substantiated by an MNDO calculation of the minimum energy reaction path. In contrast to the behaviour of alkyl benzene ions, a unidirectional isomerization of the methyl benzofuran ions by ring expansion takes place but no mutual interconversion of the molecular ions of methyl benzofurans and chromenes.     

RIMS analysis of ion induced fragmentation of molecules sputtered from an enriched U3O8 matrix

Resonance ionization mass spectrometry was used to measure the composition of the sputtered flux from 15 keV Ga⁺, Au⁺, Au₂ ⁺ and Au₃ ⁺ primary ions impacting a ²³⁵U enriched U₃O₈ standard. We demonstrate that molecular fragmentation decreases as the primary ion mass and nuclearity increases. Stopping and range of ions in matter calculations show that cluster ions (Au₂ ⁺ and Au₃ ⁺) deposit more of their energy via direct knock-ons with near-surface target atoms, whereas monatomic ions (Ga⁺ and Au⁺) penetrate much deeper into the target sub-surface region. We correlate these results to the experimental observations by showing that increased cluster ion sputter yields partition the projectile energy over a larger number of sputtered molecules. Therefore, while cluster ions deposit more total energy into the near surface region of the target compared to monatomic ions, the energy per molecule decreases with projectile mass and nuclearity. Less energy per molecule decreases the number of U–O bond breaks and, consequently, leads to a decrease in molecular fragmentation. Additionally, the extent of molecular fragmentation as a function of ion dose was evaluated. We show that molecular fragmentation increases with increased ion dose; primarily as a result of sub-surface chemical damage accumulation. The relative intensity of this effect appears to be projectile independent.     

Elektronenstossinduzierte Fragmentierung von Acetylenverbindungen. IX—Struktur der Stabilen [C9H11]+-Ionen aus Isomeren C9H11-Chloriden

The structure of stable [C₉H₁₁]⁺ ions from different model compounds with the molecular formula C₉H₁₁Cl has been estimated by energy measurements. It has further been shown that acetylenic compounds cyclise to form aromatic ions in the range of the appearance potentials.