Elimination reactions of ions in the gas phase

The loss of water from the molecular ion of 2-adamantanol was investigated using specifically labelled deuterium derivatives, and, in particular that stereospecifically labelled in position 4. Water is lost predominantly in a stereospecific 1, 3 fashion by two clearly distinguishable mechanisms. Determination of metastable ion characteristics proved to be essential for drawing this distinction.     

Assigning structures to ions in the gas phase

The purpose of this short review is critically to assess the important experiments in gas phase ion chemistry whose correct interpretation can lead to the assigning of structures to organic positive ions. The methods fall into two main categories, (i) the measurement of ion enthalpies and transition state energies for their fragmentations and (ii) the detailed examination of the unimolecular and collision-induced fragmentation behaviour of cations, anions and neutral species. It is argued that in general, none of the above methods alone can suffice for an ion structure determination, but that in combination these techniques provide a powerful tool by means of which ion structures may confidently be assigned.     

Formation of thioacylium ions in the gas phase

A study of the ion chemistry of benzenethioic acid using ion cyclotron resonance techniques shows that a long-lived ion of composition C₇H₅S⁺ is formed from the reaction of the neutral acid with primary fragment ions, m/z 77 (phenyl) and m/z 105 (benzoyl). The product is assigned the thiobenzoyl structure on the basis df its mode of formation from benzoyl cations and tbe neutral acid. Other reactant ions (acetylium and thioacetylium) derived from mixtures of benzenethioic acid with ethanethioic acid or acetate esters similarly lead to thiobenzoyl ions as the major product The significance of these results as support for the thioacetylium structure of C₂H₃S⁺ ions from ethanethioic acid is discussed.     

Formation enthalpies of hetaryl ions in the gas phase

Conclusions Photoionization was used to determine the appearance potentials of ions from 2-acetylfuran, 2-acetylthiophene, 2-acetylpyrrole, 2,5-dimethylfuran, 2,5-dimethylthiophene, and 2,5-dimethylpyrrole. The enthalpies of formation were determined for RC0⁺ ions (R=2-furyl, 2-thienyl, and 2-pyrroloyl), pyrilium, and thiopyrilium ions.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 905–907, April, 1988.     

Molecular ions of ionic liquids in the gas phase

Ionic liquids form neutral ion pairs (CA) upon evaporation. The softness of the gas-phase ionization of field ionization has been used to generate "molecular ions," CA(+*), of ionic liquids, most probably by neutralization of the anion. In detail, 1-ethyl-3-methylimidazolium-thiocyanate, [C(6)H(11)N(2)](+) [SCN](-), 1-butyl-3-methylimidazolium-tricyanomethide, [C(8)H(15)N(2)](+) [C(4)N(3)](-), N-butyl-3-methylpyridinium-dicyanamide, [C(10)H(16)N](+) [C(2)N(3)](-), and 1-butyl-1-methylpyrrolidinium-bis[(trifluormethyl)sulfonyl]amide, [C(9)H(20)N](+) [C(2)F(6)NO(4)S(2)](-) were used. The assignment as CA(+*) ions, which has been confirmed by accurate mass measurements and misassignments due to thermal decomposition of the ionic liquids, has been ruled out by field desorption and electrospray ionization mass spectrometry of the residues.     

Reactions of ions with molecules in the gas phase

Existing knowledge concerning the gas phase reactions of ions with molecules is summarized in terms of the identification of the reactions, the rate constants of the reactions, and the energetic properties of the ions observed to be formed and of those inferred as intermediates.     

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.     

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.     

Polymerization in clusters and in the gas phase using metal ions

Consecutive addition and elimination reactions have been observed following the interaction of Ti⁺ with isobutylene beam expansion. Clusters provide a feasible and valuable approach for understanding the mechanism of ionic polymerization, and how the size of polymer chains is controlled in such a process.     

Solvation of propanediol ions by water molecules in the gas phase

The thermochemical properties of protonated hydrates of 1,2- and 1,3-propanediols have been investigated using electrospray ionization-high pressure mass spectrometry. The binding enthalpies, entropies, and free energies of the stepwise hydration of protonated propanediols with one to three waters are reported. The observed negative entropy change [ΔΔS_1,3^o for the addition of the third water to 1,3-propanediol·H⁺(H₂O)₂ suggests a stable structure due to an increased number of hydrogen bonds and the loss of the intramolecular hydrogen bond in the water cluster ion. The thermochemical properties of two isomers of butanediol were also investigated in order to further elucidate the structures of the protonated propanediols.     

Evaluation of the dipole moments of organic ions in the gas phase

The dipole moments of a series of substituted benzene radical cations were evaluated on the basis of mass displacements from the expected instrumental value by ion trap experiments. Mass displacements represent a direct measure of the total polarizability, giving access to both the electronic and dipolar polarizabilities, depending on the characteristics of the analyte under investigation. A linear relationship was found between the dipolar polarizabilities and the dipole moments of the corresponding neutrals.     

Oxidation of CO by aluminum oxide cluster ions in the gas phase

Small aluminum oxide cluster cations and anions, produced by laser vaporization, were investigated regarding their reactivity toward CO and N2O employing guided-ion-beam mass spectrometry. Clusters with the same stoichiometry as bulk alumina, Al2O3, exhibited atomic oxygen transfer products when reacted with CO, suggesting the formation of CO2. Anionic clusters were less reactive than cations but showed higher selectivity towards the transfer of only a single oxygen atom. Cationic clusters, in contrast, exhibited additional products corresponding to the sequential transfer of two oxygen atoms and the loss of an aluminum atom. To determine if these stoichiometric clusters could be generated from oxygen-deficient species, clusters having a stoichiometry with one less oxygen atom than bulk alumina, Al2O2, were reacted with N2O. Cationic clusters were found to be selectively oxidized to Al2O3(+), while anionic clusters added both one and two oxygen atoms forming Al2O3(-) and Al2O4(-). The oxygen-rich Al2O4(-) cluster exhibited comparable reactivity to Al2O3(-) when reacted with CO.     

Electronic spectroscopy and relaxation of gas phase molecular ions

Techniques for obtaining electronic spectra of singly charged cations are described. Jahn-Teller effects in the spectra of polyhalobenzene cations are discussed. Experimental and theoretical methods for studying radiationless transitions in molecular ions are presented. Recent work on the spectroscopy and intramolecular relaxation of doubly-charged cations is reviewed.     

Visible and ultraviolet spectroscopy of gas phase protein ions

Optical spectroscopy has contributed enormously to our knowledge of the structure and dynamics of atoms and molecules and is now emerging as a cornerstone of the gas phase methods available for investigating biomolecular ions. This article focuses on the UV and visible spectroscopy of peptide and protein ions stored in ion traps, with emphasis placed on recent results obtained on protein polyanions, by electron photodetachment experiments. We show that among a large number of possible de-excitation pathways, the relaxation of biomolecular polyanions is mainly achieved by electron emission following photo-excitation in electronically excited states. Electron photodetachment is a fast process that occurs prior to relaxation on vibrational degrees of freedom. Electron photodetachment yield can then be used to record gas phase action spectra for systems as large as entire proteins, without the limitation of system size that would arise from energy redistribution on numerous modes and prevent fragmentation after the absorption of a photon. The optical activity of proteins in the near UV is directly related to the electronic structure and optical absorption of aromatic amino acids (Trp, Phe and Tyr). UV spectra for peptides and proteins containing neutral, deprotonated and radical aromatic amino acids were recorded. They displayed strong bathochromic shifts. In particular, the results outline the privileged role played by open shell ions in molecular spectroscopy which, in the case of biomolecules, is directly related to their reactivity and biological functions. The optical shifts observed are sufficient to provide unambiguous fingerprints of the electronic structure of chromophores without the requirement of theoretical calculations. They constitute benchmarks for calculating the absorption spectra of chromophores embedded in entire proteins and could be used in the future to study biochemical processes in the gas phase involving charge transfer in aromatic amino acids, such as in the mediation of electron transfer or redox reactions. We then addressed the important question of the sensitivity of protein optical spectra to the intrinsic properties of protein ions, including conformation, charge state, etc., and to environmental factors. We report optical spectra for different charge states of insulin, for ubiquitin starting from native and denaturated solutions, and for apo-myoglobin protein. All these spectra are compared critically to spectra recorded in solution, in order to assess solvent effects. We also report the spectra of peptides complexed with metal cations and show that complexation gives rise to new optical transitions related to charge transfer types of excitation. The perspectives of this work include integrative approaches where UV-Vis spectroscopy could, for example, be combined with ion mobility spectrometry and high level calculations for protein structural characterization. It could also be used in spectroscopy to probe biological processes in the gas phase, with different light sources including VUV radiation (to probe different types of excitations) and ultra short pulses with time and phase modulation (to probe and control the dynamics of de-excitation or charge transfer events), and with the derivatization of proteins with chromophores to modulate their optical properties. We also envision that photo-excitation will play an important role in the future to produce intermediates with new chemical and reactive properties. Another promising route is to conduct activated electron photodetachment dissociation experiments.     

Observations of magic numbers in gas phase hydrates of alkylammonium ions

Hydration of alkylammonium ions under nonanalytical electrospray ionization conditions has been found to yield cluster ions with more than 20 water molecules associated with the central ion. These cluster ion species are taken to be an approximation of the conditions in liquid water. Many of the alkylammonium cation mass spectra exhibit water cluster numbers that appear to be particularly favorable, i.e., “magic number clusters” (MNC). We have found MNC in hydrates of mono- and tetra-alkyl ammonium ions, NH₃(C _m H_2m+1)⁺(H₂O) _n , m=1–8 and N(C _m H_2m+1) ₄ ⁺ (H₂O) _n , m=2–8. In contrast, NH₂(CH₃) ₂ ⁺ (H₂O) _n , NH(CH₃) ₃ ⁺ (H₂O) _n1 and N(CH₃) ₄ ⁺ (H₂O) _n do not exhibit any MNC. We conjecture that the structures of these magic number clusters correspond to exohedral structures in which the ion is situated on the surface of the water cage in contrast to the widely accepted caged ion structures of H₃O⁺(H₂O) _n and NH ₄ ⁺ (H₂O) _n .     

Meerwein reaction of phosphonium ions with epoxides and thioepoxides in the gas phase

Phosphonium ions are shown to undergo a gas-phase Meerwein reaction in which epoxides (or thioepoxides) undergo three-to-five-membered ring expansion to yield dioxaphospholanium (or oxathiophospholanium) ion products. When the association reaction is followed by collision-induced dissociation (CID), the oxirane (or thiirane) is eliminated, making this ion molecule reaction/CID sequence a good method of net oxygen-by-sulfur replacement in the phosphonium ions. This replacement results in a characteristic mass shift of 16 units and provides evidence for the cyclic nature of the gas-phase Meerwein product ions, while improving selectivity for phosphonium ion detection. This reaction sequence also constitutes a gas-phase route to convert phosphonium ions into their sulfur analogs. Phosphonium and related ions are important targets since they are commonly and readily formed in mass spectrometric analysis upon dissociative electron ionization of organophosphorous esters. The Meerwein reaction should provide a new and very useful method of recognizing compounds that yield these ions, which includes a number of chemical warfare agents. The Meerwein reaction proceeds by phosphonium ion addition to the sulfur or oxygen center, followed by intramolecular nucleophilic attack with ring expansion to yield the 1,3,2-dioxaphospholanium or 1,3,2-oxathiophospholanium ion. Product ion structures were investigated by CID tandem mass spectrometry (MS(2)) experiments and corroborated by DFT/HF calculations.     

Reactions of first-row transition metal ions with propargyl alcohol in the gas phase

The gas-phase reactions with propargyl alcohol (PPA) of all the singly charged ions of the first-row transition metals, generated by laser ablation in an external ion source, were studied by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICRMS.). The reactivities of the metal ions change irregularly across the periodic table, and the reactivity of each ion is a function of its electronic configuration and corresponding metal-oxygen (M-O) bond energies. The 10 metal ions were classified into three categories according to their reactivities: Sc(+), Ti(+) and V(+) are the most reactive ions which react with PPA to give many kinds of oxygen-rich products due to stronger M-O bonds; Fe(+), Co(+) and Ni(+) are less reactive; Cr(+), Mn(+), Cu(+) and Zn(+) are the most unreactive ions, due to the half and completely occupied valence electronic configurations. The order of reactivity is Ti(+) > V(+) > Sc(+) > Co(+) > Fe(+) approximately Ni(+) > Zn(+) > Cr(+) approximately Mn(+) approximately Cu(+).     

Hydroxylation of selected hydrocarbon ions on reaction with methanol in the gas phase

The ion-molecule reactions of selected hydrocarbon cations with methanol which lead to the production of hydroxylated odd-electron molecular ions in the high-pressure ion source of a mass spectrometer or in the central quadrupole of a tandem quadrupole mass spectrometer are described. A wide variety of hydrocarbon cations were investigated, including aliphatic and aromatic cations; of these, only those having a vacant site on an aromatic system appear to undergo the hydroxylation reaction in high yield. A mechanism is proposed for the formation of the molecular ion of phenol from the ion-molecule reaction involving the phenyl cation with methanol. In addition, thermochemical data are provided which support the formation of the postulated species.