Structures of decomposing and non-decomposing gas-phase [C4H6O]+· radical cations,and their [C2H2O]+· and [C3H6]+· product ions

The structures of gas-phase [C₄H₆O]^+· radical cations and their daughter ions of composition [C₂H₂O]^+· and [C₃H₆]^+· were investigated by using collisionally activated dissociation, metastable ion measurement, kinetic energy release and collisional ionization tandem mass spectrometric techniques. Electron ionization (70 eV) of ethoxyacetylene, methyl vinyl ketone, crotonaldehyde and 1-methoxyallene yields stable [C₄H₆O]^+· ions, whereas the cyclic C₄H₆O compounds undergo ring opening to stable distonic ions. The structures of [C₂H₃O]^+· ions produced by 70-eV ionization of several C₄H₆O compounds are identical with that of the ketene radical cation. The [C₃H₆]^+· ions generated from crotonaldehyde, methacrylaldehyde, and cyclopropanecarboxaldehyde have structures similar to that of the propene radical cations, whereas those ions generated from the remainder of the [C₄H₆O]^+· ions studied here produced a mixed population of cyclopropane and propene radical cations.     

The reactions of the ions [XC]+, [X2C]+·, [X3C]+· and [X4C]+· (where X = H or F) with ethene and propene

The reaction of C1 ions with ethene and propene has been studied in the gas phase using a triple quadrupole mass spectrometer.     

Isomerisation of hydrocarbon ions III–[C8H8]+·, [C8H8]2+, [C6H6]+· AND [C6H5]+ IONS

Collisional activation spectra of [C₈H₈]⁺·, [C₈H₈]²⁺, [C₆H₆]⁺· and [C₆H₅]⁺ ions from fifteen different sources are reported. Decomposing [C₈H₈]⁺· ions of ten of these precursors isomerise to a mixture of mainly the cyclooctatetraene and, to a smaller extent, the styrene structure. Three additional structures are observed with [C₈H₈]⁺· ions from the remaining precursors. [C₈H₈]^2+., [C₈H₈]⁺·, [C₆H₆]⁺· and [C₆H₅]⁺· ions mostly decompose from common structures although some exceptions are reported.     

Formation of [C18H36]+ ˙ and [C7H7O2]+ in the mass spectrum of n-octadecyl benzoate

A series of deuterium-labeled n-octadecyl benzoates serves to extend the literature using lower homologues for study of the mass-spectral reactions characteristic of alkyl esters. The benzoic acid radical cation (a) arises largely via hydrogen migration from C(2), as expected for the conventional mechanism passing through a six-membered quasicyclic intermediate; the presence of contributions from other processes is simply noted in passing. The octadecene radical cation (b) and protonated benzoic acid (c) arise by paths seemingly closely related to each other but differing from the path dominant for a. The data are rationalized in terms of a reaction sequence passing through two ion-neutral complex intermediates. The first, (d), consists of octadecyl carbenium ion and benzoyloxy radical; the second, (e), consists of octadecene radical cation and benzoic acid. The ionized partner in d appears to undergo none to several hydride migrations, but most prominently one; that in e may dissociate promptly to yield b or may persist, undergoing multiple hydride migrations to effect essentially complete scrambling of hydrogens, before dissociating to yield c. The argument for this mechanistic sequence to form b and c rests basically on its ability to account plausibly for the experimental data, and literature precedents cited for specific aspects of it furnish additional support. The low critical energies and small kinetic energy releases observed in this work are in accord with the characteristics listed by McAdoo for decompositions mediated by ion-neutral complexes, and the selection of an octadecyl, as opposed to a smaller alkyl, ester predisposes the molecule to favor such low-energy processes.     

Theoretical study in [C2H4–Tl]+ and [C2H2–Tl]+ complexes

We studied the attraction between [C₂H_n] and Tl(I) in the hypothetical [C₂H_n–Tl]⁺ complexes (n = 2,4) using ab initio methodology. We found that the changes around the equilibrium distance C–Tl and in the interaction energies are sensitive to the electron correlation potential. We evaluated these effects using several levels of theory, including Hartree–Fock (HF), second‐order Møller–Plesset (MP2), MP4, coupled cluster singles and doubles CCSD(T), and local density approximation augmented by nonlocal corrections for exchange and correlation due to Becke and Perdew (LDA/BP). The obtained interaction energies differences at the equilibrium distance R_e (C–Tl) range from 33 and 46 kJ/mol at the different levels used. These results indicate that the interaction between olefinic systems and Tl(I) are a real minimum on the potential energy surfaces (PES). We can predict that these new complexes are viable for synthesizing. At long distances, the behavior of the [C₂H_n]–Tl⁺ interaction may be related mainly to charge‐induced dipole and dispersion terms, both involving the individual properties of the olefinic π‐system and thallium ion. However, the charge‐induced dipole term (R^?4) is found as the principal contribution in the stability at long and short distances. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Competing metastable decompositions of [C7H14]+· ions

The relative rates of competing metastable decompositions of fourteen isomeric C7H14 monoolefins were measured and compared. In every case except one the most important metastable reaction was loss of either CH3 or C2H4, but the rates of these and the other reactions observed varied over a wide range. It was concluded that the molecular ions of these compounds probably do not isomerize to a common structure prior to metastable decay. It was found that a terminal double bond strongly enhances metastable loss of C2H4 and that the additional presence of a 2-methyl substituent favours this reaction still more. Several possible mechanisms for this transition are discussed, but none was found to explain the observed results satisfactorily.     

Criteria for distinguishing between [M]+· and [M+H]+ ions in field desorption mass spectra

Ten criteria are introduced to distinguish between molecular ions and protonated parent molecules in field desorption mass spectrometry.     

Energy partitioning in the metastable fragmentation [C3H7]+ → [C3H5]+ + H2

The dish-topped metastable peak for the fragmentation [C₃H₇]⁺ → [allyl]⁺ + H₂ is generated by the threshold fragmentation. The fraction of the reverse activation energy which is partitioned as translational energy of the products is 0.9 ± 0.1. It is proposed that a similar partitioning coefficient applies to the excess internal energy above threshold.     

Ion structural studies by ion kinetic energy spectrometry: [C7H7]+, [C7H8]+˙, [C7H7OCH3]+˙

The use of kinetic energy release measurements in the structural characterization of ions formed in the mass spectrometer and in the determination of fragmentation mechanisms is demonstrated. In combination with information on the mode of energy partitioning in some of these reactions this allows the following conclusions: (i) The metastable [C₇H₈]⁸˙ ions formed from toluene, cyclohepatatriene, n-butylbenzene, the three methyl anisoles, methyl tropyl ether and benzyl methyl ether all undergo loss of H˙ from a common structure. (ii) The metastable [C₇H₇]⁺ ions generated from the same sources and from benzyl bromide, benzyl alcohol, p-xylene and ethylbenzene appear to undergo loss of acetylene from both the benzylic and the tropylium structures. (iii) The metastable [C₇H₇OCH₃]⁺˙ ether molecular ions undergo loss of CH₃˙ by two types of mechanism, simple cleavage to give the aryloxy cation (not observed for benzyl methyl ether) and a rearrangement process which appears to lead to protonated tropone as the product. (iv) Loss of formaldehyde from the metastable [C₇H₇OCH₃]⁺˙ molecular ions involves hydrogen transfer via competitive 4- and 5-membered cyclic transition states in the case of the anisoles and in the case of methyl tropyl ether, while for benzyl methyl ether, hydrogen transfer in the nonisomerized molecular ion occurs via a 4-membered cyclic transition state to yield the cycloheptatriene molecular ion.     

Dicarboxylatgruppen als Liganden und Anionen in Aquamagnesiumkomplexen: Kristallstrukturen von [Mg (C4H2O4)(H2O)4] · H2O und [Mg(H2O)6](C4HO4)2 · 2H2O ((C4H2O4)2— = Fumarat; (C4HO4)— = Hydrogenacetylendicarboxylat)

Dicarboxylate Groups as Ligands and Anions in Aquamagnesium Complexes: Crystal Structures of [Mg (C₄H₂O₄)(H₂O)₄] · H₂O and [Mg(H₂O)₆](C₄HO₄)₂ · 2H₂O ((C₄H₂O₄)^2— = Fumarate; (C₄HO₄)^— = Hydrogenacetylenedicarboxylate) Crystals of tetraaqua(fumarato)magnesium‐hydrate ( 1 ) and hexaaquamagnesium‐bis(hydrogenacetylenedicarboxylate)‐dihydrate ( 2 ) were prepared by reacting MgCl₂ with sodium fumarate and acetylenedicarboxylic acid, respectively. In 1 cis‐Mg(H₂O)₄ units are bridged by α, Ö‐bonded fumarate groups. The resulting zig zag chains exhibit the maximum symmetry compatible with space group symmetry C2/c. 2 consists of layers of voluminous [Mg(H₂O)₆]²⁺ cations alternating with layers of C₄HO₄^— anions. The nearly planar anions are held together by parallel stacking and by short hydrogen bonds. Both structures contain efficient H bridging systems. 1 : Space group C2/c, Z = 4, lattice constants at 20 °C: a = 5.298(1), b = 13.178(2), c = 13.374(2)Å; ß = 94.79(2)°, R₁ = 0.024. 2 : Space group P1, Z = 1, lattice constants at 20 °C: a = 5.985(1), b = 6.515(1), c = 11.129(1)Å; α = 105.24(2), ß = 91.87(3), γ = 90.92(1)°, R₁ = 0.034.     

1,2-Carbon shifts in [C4H8O]+˙ and [C5H10O]+˙ ions

Present results demonstrate that α,β-shifts of the functional group carbon strongly dominate β,α-methyl shifts in [C₄H₈O]⁺˙ and [C₅H₁₀O]⁺˙ ions, paralleling observations of others on methyl isobutyrate ions.     

Isomeric [C3H4]+· ions: Their identification and generation in dissociative ionizations

The collisional activation (CA) and charge stripping (CS) mass spectra of the three [C₃H₄]^+· isomers, allene, propyne and cyclopropene, are reported. The extent of isomerization among these ions prior to collisional excitation depends on their internal energy content, but is small. Each [C₃H₄]^+· ion structure also can uniquely be generated via appropriate dissociative ionizations. Analysis of mixtures of [C₃H₄]^+· (daughter) ion structures is, in general, not possible from CA and CS mass spectra alone but may be aided by appearance energy measurements.     

Thermochemistry and metastable energy release for the dissociation [C3H7]+ → [C3H5]+ + H2

The reverse activation energy, E_rev, for the dissociation [C₃H₇]⁺ → [C₃H₅]⁺ + H₂ has been determined as 0.24 ± 0.06 eV from measurements of the AP of [C₃H₅]+ produced by electron-impact from thermally generated sec-C₃H₇ radicals. The energy release observed in the corresponding metastable dissociation is 0.21 ± 0.01 eV, indicating that virtually all of E_rev is partitioned as translational Kinetic energy of the fragmentation products. The metastable ion peak shape is also discussed with respect to the evaluation of the energy release.     

The decompositions of metastable [C4H8O]+˙ ions and the [C4H8O]+˙ potential surface

Collisionally activated decomposition (CA) spectra of [C₄H₈O]⁺˙ ions and the products of their metastable decompositions are used to refine a previously presented picture of the reactions of [C₄H₈O]⁺˙ ions. Metastable [C₄H₈O]⁺˙ isomers predominantly rearrange to the 2-butanone ion and decompose by loss of methyl and ethyl, although up to 38% of the methyl losses take place by other pathways to form \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm{CH}}_{\rm{2}} = {\rm{CHCH = }}\mathop {\rm{O}}\limits^{\rm{ + }} {\rm{H}}{\rm{.}} $\end{document} . The CA spectra of many of the [C₄H₈O]⁺˙ ions with the oxygen on the first carbon are very similar, consistent with those ions isomerizing largely to common structures before or after collision. However, several of these ions have unique CA spectra, so they must remain structurally distinct from the majority of the [C₄H₈O]⁺˙ ions below energies required for decomposition. The CA spectra of ions with the oxygen on the second carbon are distinct from those of ions with the oxygen on the first carbon, so there is limited interconversion of the non-decomposing forms of the two types of ions. A potential energy diagram for the reactions of metastable [C₄H₈O]⁺˙ ions is constructed from appearance energy measurements. As would be expected, the relative importances of most of the [C₄H₈O]⁺˙ isomerizations seem to be inversely related to the activation energies for those processes. Some parallels between the isomerizations of [C₄H₈O]⁺˙ ions and those of related ions are pointed out.     

The structure of [C3H4N2]+· and [C5H5N2]+ ions formed from vinylimidazoles,studied by collisionally activated dissociation mass spectrometry

Mass spectra of the three isomeric vinylimidazoles have been compared and the structures of the fragment ions [C₃H₄N₂]^+· and [C₅H₅N₂]⁺ have been investigated by collisionally activated dissociation mass spectrometry. The greater part of the non-decomposing ions m/z 68 from 2-vinylimidazole and from 2-imidazolecarboxylic acid methyl ester, and a minor part of this ion formed from the free acid, all have the same structure: the imidazole ring system, with hydrogens at both nitrogen atoms but none at C(2). An analogous structure, with an ethyl group at C(2), is proposed for the m/z 93 ion from 2-vinylimidazole.     

Deuterium isotope effects and mechanism of the gas phase reaction [C3H7]+→[C3H5]+ + H2

The deuterium kinetic isotope effect and the deuterium isotope effect upon kinetic energy release have been calculated for the loss of H₂ from the [C₃H₇]⁺ ion. The calculations are based on the transition state structure suggested recently from ab initio calculations on the reaction pathway. The results obtained are in good agreement with experimental data.     

[C]+· and [CH]+ from doubly charged ions formed from malononitrile

The metastable ions [M]²⁺, [M – H]²⁺· and [M – H₂]²⁺ from malononitrile fragment by loss of [CH]⁺, [C]⁺· and [C]⁺·, respectively. The reaction of the molecular ion involves the methylene and nitrile carbon atoms in the statistical probability ratio, while that of [M – H]²⁺· involves exclusively the nitrile carbon and that of [M ? H₂]²⁺ involves an approximately equal contribution, from both sources. It is suggested that the metastable molecular ion fragments through a bipyrimidal intermediate.     

A mass spectrometry/mass spectrometry investigation of the nature of [C10H10]+˙, [C9H7]+ and [C10H8]+˙ gas phase ions

Additional evidence for the rearrangement of the 1- and 3-phenylcyclobutene radical cations, their corresponding ring-opened 1,3-butadiene ions and 1,2-dihydronaphthalene radical cations to methylindenetype ions has been obtained for the decomposing ions by mass analysed ion kinetic energy spectroscopy (MIKES). The nature of the [C₉H₇]⁺ and [C₁₀H₈]^+˙ daughter ions arising from the electron ionization induced fragmentation of these [C₁₀H₁₀]^+˙ precursors has been investigated by collisionally activated dissociation (CAD), collisional ionization and ion kinetic energy spectroscopy. The [C₉H₇]⁺ produced from the various C₁₀H₁₀ hydrocarbons are of identical structure or an identical mixture of interconverting structures. These ions are similar in nature to the [C₉H₇]⁺ generated from indene by low energy electron ionization. The [C₁₀H₈]^+˙ ions also possess a common structure, which is presumably that of the maphthalene radical cation.     

Piaselenol—Piaselenolium—Pentaiodid (C6H4N2Se · C6H5N2Se+I3− · I2), eine Struktur mit Polyiodid-schichten

Piaselenole—Piaselenolium—Pentaiodide (C₆H₄N₂Se · C₆H₅N₂Se⁺ I₃^? · I₂), a Structure with Polyiodide Layers The title compound crystallizes in the monoclinic space group P2₁/n with a = 9.320(3), b = 13.812(2), c = 17.159(3) Å, β = 96.11(2)°, V = 2196.3 ų, Z = 4. There occur no isolated I^5? anions but layer-shaped polyiodide aggregates built up by linear, asymmetric I₃^? anions and I₂ molecules. Almost linear triiodide chains are connected by I₂ molecules in a novel type of arrangement to form slightly puckered layers. The polyiodide layers contain several substructures known from other examples. The piaselenole and its conjugated acid, the piaselenolium cation, form a ribbon-like quasi-polymer in which the two components are alternating. They are connected in turns by a linear NH? N hydrogen bridge (N? N: 2.844 Å) and by a so called (SeN)₂-connectivity parallelogram, in which Se? N bonds and Se? N contacts are adjacent. Here we found a very short Se? N contact distance of 2.691 Å. The bond distances of piaselenole (Se? N: 1.787(3) Å, N? C: 1.318(5) Å, C? C: 1.453(8) Å) and also the angles are equal or similar to those occuring in other 1,2,5-selenadiazoles. The protonation of one N in the SeN₂ unit results in a loss of symmetry and significant changes in bonding distances and angles.     

[CdCl2(C2H4(OH)2)]3 · C2H4(OH)2 — ein neuer Typ von Ethandiol-1,2-Koordinationsverbindungen

[CdCl₂(C₂H₄(OH)₂)]₃ · C₂H₄(OH)₂ — a New Type of Ethanediol-1,2 Coordination Compounds The coordination compound crystallizes with triclinic symmetry in space group P1 ; a = 10.418(2), b = 10.739(2), c = 11.053(2) Å, α = 86.04(2), β = 67.68(1), γ = 88.23(2)°; Z = 2, d_calc. = 2.32 g/cm³, R(F) = 0.031. The structure consists of Cd(C₂H₆O₂)Cl_4/2 octahedra with two edges (Cl double bridges) in common which form endless chains along [11 1]. A new orientational sequence of the unique ligand C₂H₆O₂ in the chain is found. The C₂H₆O₂ solvent molecule is located between the chains. Hydroxyl groups are involved in different H bridging systems which cause for all C₂H₆O₂ molecules a different conformation. This is confirmed by the results of IR and Raman spectra.