The effect of vibrational energy content on the neutralization–reionization mass spectra of [H2]+˙ ions

Neutralization-reionization experiments were performed on beams of [H²]⁺˙ ions of different, known vibrational energy content using a variety of neutralization target gases (Xe, H₂, Ne) and reionization gases (He, O₂). The recovery of [H₂]⁺˙ ions was found to be only weakly dependent on the vibrational energy of the original [H₂]⁺˙ ions. The ion kinetic energy spectra of H⁺ fragments from the neutralization-reionization experiments were independent of the collision gas; the processes by which they were generated were identified.     

Mass spectra of aliphatic dicarboxylic acids and their dimethyl esters: Cyclic structures for the [M − H2O]+˙ ions from the diacids and [M − MeOH]+˙ ions from the dimethyl esters

Methyl 2-oxocycIoalkane carboxylate structures are proposed lor the [M ? MeOH]^+˙ ions from dimethyl adipate, pimelate, suberate and azelate. This proposal is based on a comparison of the metastable ion mass spectra and the kinetic energy releases for the major fragmentation reaction of these species with the same data for the molecular ions of authentic cyclic β-keto esters. The mass spectra of α,α,α′,α′-d₄-pimelic acid and its dimethyl ester indicate that the α-hydrogens are involved only to a minor extent in the formation of [M ? ROH]^+˙ and [M ? 2ROH]^+˙ ions, while these α-hydrogens are involved almost exclusively in the loss of ROH from the [M ? RO˙]⁺ ions (R = H or CH₃). The molecules XCO(CH₂)₇COOMe (X = OH, Cl) form abundant ions in their mass spectra with the same structure as the [M ? 2MeOH]^+˙ ions from dimethyl azelate.     

Study of ions with excess kinetic energy. III—Mass spectra of excess kinetic energy ions of aliphatic amines

The mass spectra obtained from ions having excess kinetic energy (named the excess kinetic energy mass spectra) formed by electron impact of several aliphatic amines have been studied. Comparison with the excess kinetic energy mass spectra of the analogous alcohols shows many similarities. For example, the intensity of the [CH₂OH]⁺ ion of an aminoalcohol is about the same as the intensity of the [CH₂NH₂]⁺ ion. The excess kinetic energy mass spectra give useful information about the presence or absence of various functional groups, such as CH₂NH₂, CH₂OH, CH₃CHNH₂ and alkyl groups, and also about rearrangements. The fragmentation mechanisms are straightforward and applicable to a variety of classes of compounds. The molecular weight dependence of excess kinetic energy mass spectra is described.     

The role of charge-site location in fragmenting ions. 1—[CH3 CHXCOOCH3]+˙ and [CH2XCH2 COOCH3]+˙ ions and the structures of daughter ions derived from the loss of X (X = I,Br, Cl and CH3)

The structures of the m/z 87, [C₄H₇O₂]⁺, ions generated by dissociative ionization of CH₃CGXCOOCH₃ and XCH₂CH₂COOCH₃ (X = CH₃, Cl, Br, and I) have been investigated via their unimolecular and collisionally activated fragmentations and by apperance energy measurements. For both precursors loss of X = CH₃ produced, via H atom transfer, ions of structure \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH_2 = CH}\mathop {\rm C}\limits^{\rm + } \left({{\rm OH}} \right){\rm OCH}_{\rm 3} $\end{document} (a), ΔH_f = 386 kj mol^?1. In marked contrast, loss of I^˙ from ionized CH₃CHICOOCH₃ and ICH₂CH₂COOCH₃ proceeded without rearrangement to yield respectively ions of structure \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm CH_3}\mathop {\rm C}\limits^{\rm + } {{\rm HCOOCH_3}} $\end{document} (b), ΔH_f = 480 kJ mol^?1 and (c), ΔH_f = 450 kJ mol^?1. These different fragmentation behaviours are explained via photoelectron spectra which show that the formal charge site in the precursor ion is at the carbonyl oxygen when X = CH₃ but at the halogen atom when X = I. The precursor molecules X = Cl and Br display both of the above characteristics, CH₃CHXCOOCH₃ yielding mixtures of a and b and XCH₂CH₂COOCH₃ producing a and c ions.     

Structures and stabilities of [C3H2]+ ˙ and [C3H2]2+ ions

Ab initio molecular orbital theory using basis sets up to 6-311G^{* *}, with electron correlation incorporated via configuration interaction calculations with single and double substitutions, has been used to study the structures and energies of the C₃H₂ monocation and dication. In agreement with recent experimental observations, we find evidence for stable cyclic and linear isomers of [C₃H₂]^{+ ˙}. The cyclic structure (, a) represents the global minimum on the [C₃H₂]^{+ ˙} potential energy surface. The linear isomer (, b) lies somewhat higher in energy, 53 kJ mol^?1 above a. The calculated heat of formation for [HCCCH]^{+ ˙} (1369 kJ mol^?1) is in good agreement with a recent experimental value (1377 kJ mol^?1). For the [C₃H₂]²⁺ dication, the lowest energy isomer corresponds to the linear [HCCCH]²⁺ singlet (h). Other singlet and triplet isomers are found not to be competitive in energy. The [HCCCH]²⁺ dication (h) is calculated to be thermodynamically stable with respect to deprotonation and with respect to C? C cleavage into CCH⁺ + CH⁺. The predicted stability is consistent with the frequent observation of [C₃H₂]²⁺ in mass spectrometric experiments. Comparison of our calculated ionization energies for the process [C₃H₂]^{+ ˙} → [C₃H₂]²⁺ with the Q_min values derived from charge-stripping experiments suggests that the ionization is accompanied by a significant change in structure.     

The [CH2 = CHOH/H2O]+˙ system: A theoretical study of distonic ions,hydrogen-bridged ions and ion–dipole complexes

Several isomeric forms of the vinyl alcohol/water radical cation have been investigated by high-level ab initio molecular orbital theory calculations, including electron correlation effects. Of the ions considered here, the anti form of the ? O ?H ?O? bridged complex is calculated to be the lowest in energy, having a stabilization energy of 100 kJ mol^?1 with respect to the dissociation products [CH₂CHOH]⁺˙ and H₂O. Although the isomeric ions may formally be represented as distonic ions, hydrogen-bridged ions and ion–dipole complexes, the only significant barrier separating the isomers appears to be the anti?syn isomerization barrier. However, in the ? O ?H ?O? bridged complex this barrier is found to be considerably lowered relative to the anti?syn isomerization barrier for the free vinyl alcohol radical cation.     

(C2H10N2)[BPO4F2] — Structural Relations between [BPO4F2]2— and [Si2O6]4—

(C₂H₁₀N₂)[BPO₄F₂] — Strukturbeziehungen zwischen [BPO₄F₂]^2— und [Si₂O₆]^4— Colourless crystals of (C₂H₁₀N₂)[BPO₄F₂] were prepared from mixture of ethylendiamine, H₃BO₃, BF₃ · C₂H₅NH₂, H₃PO₄ and HCl under mild hydrothermal conditions (220 °C). The crystal structure was determined by single crystal methods (triclinic, P1¯ (no. 2), a = 451.85(5) pm, b = 710.20(8) pm, c = 1210.2(2) pm, α = 86.08(1)°, β = 88.52(2)°, γ = 71.74(1)°, Z = 2) and contains infinite tetrahedral zweier‐single‐chains {[BPO₄F₂]^2—} which are isoelectronic (48e^—) with the polyanions {[Si₂O₆]^4—} of the pyroxene family.     

Characterization of and energy deposition in [C2H4O2]+˙ ions using energy-resolved mass spectrometry

Energy-resolved mass spectrometry (ERMS) has been used to distinguish [C₂H₄O₂]⁺˙ ions generated from seven precursor molecules. The low collision energy ERMS data confirm conclusions reached from previous high-energy experiments; all seven ions have different structures or mixtures of structures. Structural distinctions are at least as easy to make using the two-dimensional ERMS data as through collisional activation at high energy. The average internal energy deposited in low-energy collisions (30 eV) is shown to be similar to the internal energy deposited at 8 keV in these systems.     

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.     

The structures and formation of [C4H6N]+ ions 2—ion structures derived from 2-methylpropanenitrile

The loss of a hydrogen atom from ionized 2-methylpropanenitrile is preceded by a drastic rearrangement of the molecular ion. The result of this fragmentation is the generation of two stable structurally different [C₄H₆N]⁺ ions, formed via different pathways. Their structures can be established by a careful comparison of the metastable ion spectra, collision activation spectra, and charge stripping spectra from the compound and its three deuterium labeled analogues and from [C₄H₆N]⁺ ions generated from reference compounds via electron impact ionization or in selected ion/molecule reactions.     

Translational energy release and stereochemistry of steroids. VII—the loss of angular methyl groups after the dehydration of molecular ions of cholesterol and related C(5)-unsaturated 3β-hydroxy steroids

The translational energy, T, released during the loss of the angular 18- and 19-methyl groups both from metastable molecular ions and metastable [M ? H₂O]⁺ and [M ? 2H₂O]⁺ ions, in C(5)-unsaturated mono-and di-hydroxy steroids, as well as in their 19-nor and deuterated analogues bearing the label in the 19-methyl group, has been measured. It was found that, while the T values for the 19-CH₃ loss, following the dehydration of the molecular ions, are increased substantially when compared to those for the same loss from the molecular ions, the T values for the 18-CH₃ loss are increased much more moderately. Nevertheless, the amounts of translational energy released in the [M ? H₂O]⁺˙ ? 18-CH₃˙ and [M ? 2 H₂O]⁺˙ ? 18-CH₃˙ transitions are still higher than those found for the respective 19-methyl loss, in accordance with the general rule established recently.     

Consecutive reactions arising from the molecular ions of 14α-equilenin and 14β-isoequilenin

It is shown that two configurational steroidal isomers, 14α-equilenin and 14β-isoequilenin, may be easily distinguished by the unimolecular mass analysed ion kinetic energy spectra of their corresponding molecular ions. Furthermore, many of the peaks in these spectra are found to be due to consecutive reactions, whereby product ions of an initial reaction are themselves reactive and can undergo further fragmentation. In each of these cases, the individual steps of the reaction sequence have been separated in the different field free regions of a reversed geometry, double focusing mass spectrometer. Isotopic labelling with ¹⁸O in the 17-position has also helped to clarify reaction sequences.     

Multiple N—H...NC,C—H...NC and nitrile...π interactions in 4,4′‐bipyridine‐1,1′‐diium bis(1,1,3,3‐tetracyano‐2‐ethoxypropenide): structure determination and DFT calculations of anion...π cation interaction energies

Polynitrile anions are important in both coordination chemistry and molecular materials chemistry, and are interesting for their extensive electronic delocalization. The title compound crystallizes with two symmetry‐independent half 4,4′‐bipyridine‐1,1′‐diium (bpyH₂²⁺) cations and two symmetry‐independent 1,1,3,3‐tetracyano‐2‐ethoxypropenide (tcnoet⁻) anions in the asymmetric unit. One of the bpyH₂²⁺ ions is located on a crystallographic twofold rotation axis (canted pyridine rings) and the other is located on a crystallographic inversion center (coplanar pyridine rings). The ethyl group of one of the tcnoet⁻ anions is disordered over two sites with equal populations. The extended structure exhibits two separate N—H...NC hydrogen‐bonding motifs, which result in a sheet structure parallel to (010), and weak C—H...NC hydrogen bonds form joined rings. Two types of multicenter CN...π interactions are observed between the bpyH₂²⁺ rings and tcnoet⁻ anions. An additonal CN...π interaction between adjacent tcnoet⁻ anions is observed. Using density functional theory, the calculated attractive energy between cation and anion pairs in the tcnoet⁻...π(bipyridinediium) interactions were found to be 557 and 612 kJ mol⁻¹ for coplanar and canted bpyH₂²⁺ cations, respectively.     

Unimolecular rearrangement of α-silylcarbenium ions. An ab initio study

The unimolecular rearrangements of hydrogen, methyl and phenyl groups at the Si atom in α-silylcarbenium ions have been investigated using an ab initio molecular orbital method. MP2/6–31 + G^*//HF/6–31G^* calculations predict that all three groups migrate from the Si to an adjacent C_α with no energy barrier. Thus, the silicenium ion is the only stable species in each potential energy surface. The conformation of the benzylsilicenium ion, (C₆H₅)CH₂−SiH₂⁺, indicates that the phenyl ring is significantly bent toward the silyl cationic center in order to interact with the vacant 3p(Si⁺) orbital. In contrast to MP2 results, Hartree-Fuck calculations (both HF/3–21G^* and HF/6–31G^* levels) predict small energy barriers for 1,2-migrations of H and Me (1.4 kcal mol⁻¹ for H migration, and 1.5 kcal mol⁻¹ for Me migration, respectively, at the HF/6–31G^* level). This difference provides convincing evidence that the incorporation of electron correlation is of particular importance in describing the potential energy surface for the rearrangement of α-silylcarbenium ions to silicenium ions. The results of the calculations have also been applied to the possible rearrangement mechanism of α-chlorosilanes to chlorosilanes, assuming that the experimental conditions are favorable toward the generation of ionic species. Various factors which may govern the migratory aptitudes of various R groups, i.e. (1) activation energies, (2) overall reaction energies and (3) the conformational preference of reactants have been investigated. The calculated activation energy obtained, namely the energy for the generation of the silicenium ion and the C⁻¹ ion from an α-chlorosilane, is consistent with the experimental migratory aptitude in the gas phase observed in mass spectrometers.     

Structure and formation of [C4H6N]+ ions: 1—Ion structures derived from aliphatic nitriles

Structures and formation of the [C₄H₆N]⁺ ions present in the mass spectra of eleven α-substituted and eleven α-unsubstituted nitriles have been investigated from collisional activation and metastable ion spectra. Collisional activation spectra lead to identification of six structures. The [C₄H₆N]⁺ ions from some branched compounds prove to be mixtures. This, as well as the identity of all metastable ion parameters and certain spectral data, shows that energy differences between all structures are small. This is corroborated by MINDO/3 calculations showing a spread from 724 to 891 kJ mol^?1 over the structures. Earlier proposals for two different [C₄H₆N]⁺ ion structures, based on the mass spectra of deuterium labelled compounds, appeared to be correct. A computer program to calculate the contribution of standard spectra in a measured spectrum has been developed.     

Structural Characterization of Mn2+—ß″—Al2O3(Mn0.77Al10.46Mg0.54O17)

The structure of completely exchanged Mn²⁺—ß″—Al₂O₃(Mn_0.77Al_10.46Mg_0.54O₁₇) crystals has been investigated by single—crystal X—ray diffraction methods at room temperature (trigonal, R3¯, Z = 3, a = 560.65(7), c = 3329.3(9) pm). The manganese ions (Mn²⁺) are found to occupy Beevers‐Ross (56 %) and mid—oxygen positions (44 %) in nearly the same amounts. The crystal composition was confirmed by electron probe microanalyses on various crystals.     

Exploring the intrinsic polar [4 + 2+] cycloaddition reactivity of gaseous carbosulfonium and carboxonium ions

Gas‐phase reactions of model carbosulfonium ions (CH₃‐S⁺ = CH_2; CH₃CH₂‐S⁺ = CH₂ and Ph‐S⁺ = CH₂) and an O‐analogue carboxonium ion (CH₃‐O⁺ = CH₂) with acyclic (isoprene, 1,3‐butadiene, methyl vinyl ketone) and cyclic (1,3‐cyclohexadiene, thiophene, furan) conjugated dienes were systematically investigated by pentaquadrupole mass spectrometry. As corroborated by B3LYP/6‐311 G(d,p) calculations, the carbosulfonium ions first react at large extents with the dienes forming adducts via simple addition. The nascent adducts, depending on their stability and internal energy, react further via two competitive channels: (1) in reactions with acyclic dienes via cyclization that yields formally [4 + 2⁺] cycloadducts, or (2) in reactions with the cyclic dienes via dissociation by HSR loss that yields methylenation (net CH⁺ transfer) products. In great contrast to its S‐analogues, CH₃‐O⁺ = CH₂ (as well as C₂H₅‐O⁺ = CH₂ and Ph‐O⁺ = CH₂ in reactions with isoprene) forms little or no adduct and proton transfer is the dominant reaction channel. Isomerization to more acidic protonated aldehydes in the course of reaction seems to be the most plausible cause of the contrasting reactivity of carboxonium ions. The CH₂ = CH‐O⁺ = CH₂ ion forms an abundant [4 + 2⁺] cycloadduct with isoprene, but similar to the behavior of such α,β‐unsaturated carboxonium ions in solution, seems to occur across the C = C bond. Copyright © 2012 John Wiley & Sons, Ltd.     

Electron-impact-induced rearrangements of organic ions: IV—Hydrogen and carbon scrambling in [t-C3H8N]+ metastable ions

Independent loss of positional identity of the hydrogen and carbon atoms in [t-C₃H₈N]⁺ metastable cations prior to fragmentation in the gas phase with loss of ethylene, has been demonstrated by ²H and ¹³C labelling experiments. The scrambling reaction is interpreted as a rearrangement in competition with the high activation energy unimolecular decompositions. The study of this fragmentation process indicates the occurrence of a primary isotope effect discriminating in favour of ²H and ¹³C retention in the original methylene group.     

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.     

Destabilized carbenium ions: α-carbomethoxy-α,α-dimethyl-methyl cations

Tertiary α-carbomethoxy-α,α-dimethyl-methyl cations a have been generated by electron impact induced fragmentation from the appropriately α-substituted methyl isobutyrates 1–4. The destabilized carbenium ions a can be distinguished from their more stable isomers protonated methyl methacrylate c and protonated methyl crotonate d by MIKE and CA spectra. The loss of I^— and Br˙ from the molecular ions of 1 and 2, respectively, predominantly gives rise to the destabilized ions a, whereas loss of Cl˙ from [3]^{+ ˙} results in a mixture of ions a and c. The loss of CH₃˙ from [4]⁺˙ favours skeletal rearrangement leading to ions d. The characteristic reactions of the destabilized ions a are the loss of CO and elimination of methanol. The loss of CO is associated by a very large KER and non-statistical kinetic energy release (T₅₀ = 920 meV). Specific deuterium labelling experiments indicate that the α-carbomethoxy-α,α-dimethyl-methyl cations a rearrange via a 1,4-H shift into the carbonyl protonated methyl methacrylate c and eventually into the alkyl-O protonated methyl methacrylate before the loss of methanol. The hydrogen rearrangements exhibit a deuterium isotope effect indicating substantial energy barriers between the [C₅H₉O₂]⁺ isomers. Thus the destabilized carbenium ion a exists as a kinetically stable species within a potential energy well.