Selected examples of gas-phase ion chemistry studies.

Gas-phase ion chemistry is an area in mass spectrometry that has received much research interest since the mid fifties of the last century. Although the focus of mass spectrometric research has shifted the last twenty years largely to life science studies, including proteomics, genomics and metabolomics, there are still several groups in the world active in gas-phase ion chemistry of both positive and negative ions, either unimolecularly and/or bimolecularly. In this tutorial lecture the formation and determination of tautomeric ion structures and intra-ionic catalyzed tautomerization in the gas phase will be discussed. In addition, an example of formation of different tautomeric structures in protic and aprotic solvents under electrospray ionization conditions will be given, as established by gas-phase infrared multiphoton dissociation spectroscopy. This will be followed by presenting an example of time-resolved MS/MS which enables to identify the structure of an ion, generated at a particular molecular ion lifetime. At the end of the lecture the power of ion mobility will be shown in elucidating the mechanism of epimerization of bis-Tröger bases having chiral nitrogen centers.

Gas-phase reactions of the atomic oxygen radical cation with halogenated compounds.

Rate constants and product distributions have been measured for the reactions of O(+)˙((4)S) with the methyl halides (CH(3)F, CH(3)Cl, CH(3)Br, CH(3)I) and three perfluorinated compounds (CF(4), SF(6), SF(5)CF(3)) at 300 K in a flowing afterglow-selected ion flow tube (FA-SIFT). The reactions occur with high efficiency, despite the necessity for spin conversion in some processes. The mechanisms include charge transfer, atom abstraction, and fragmentation reactions. Computational modeling was carried out to provide insight into these processes.

Localization of fatty acyl and double bond positions in phosphatidylcholines using a dual stage CID fragmentation coupled with ion mobility mass spectrometry.

A high content molecular fragmentation for the analysis of phosphatidylcholines (PC) was achieved utilizing a two-stage [trap (first generation fragmentation) and transfer (second generation fragmentation)] collision-induced dissociation (CID) in combination with travelling-wave ion mobility spectrometry (TWIMS). The novel aspects of this work reside in the fact that a TWIMS arrangement was used to obtain a high level structural information including location of fatty acyl substituents and double bonds for PCs in plasma, and the presence of alkali metal adduct ions such as [M + Li](+) was not required to obtain double bond positions. Elemental compositions for fragment ions were confirmed by accurate mass measurements. A very specific first generation fragment ion m/z 577 (M-phosphoryl choline) from the PC [16:0/18:1 (9Z)] was produced, which by further CID generated acylium ions containing either the fatty acyl 16:0 (C(15)H(31)CO(+), m/z 239) or 18:1 (9Z) (C(17)H(33)CO(+), m/z 265) substituent. Subsequent water loss from these acylium ions was key in producing hydrocarbon fragment ions mainly from the α-proximal position of the carbonyl group such as the hydrocarbon ion m/z 67 (+H(2)C-HC = CH-CH = CH(2)). Formation of these ions was of important significance for determining double bonds in the fatty acyl chains. In addition to this, and with the aid of (13)C labeled lyso-phosphatidylcholine (LPC) 18:1 (9Z) in the ω-position (methyl) TAP fragmentation produced the ion at m/z 57. And was proven to be derived from the α-proximal (carboxylate) or distant ω-position (methyl) in the LPC.

The gas phase Smiles rearrangement of anions PhO(CH(2))(n)O(-) (n = 2-4). A joint theoretical and experimental approach.

A combination of experimental data [using (18)O labelling fragmentation data together with metastable ion studies in a reverse sector mass spectrometer (from a previous study)] and ab initio reaction coordinate studies at the CCSD(T)/6-31++G(d,p)//B3LYP/6-31++G(d,p) level of theory, have provided the following data concerning the formation of PhO(-) in the gas-phase from energized systems PhO(CH(2))(n)O(-) (n = 2-4). All DeltaG values were calculated at 298 K. (1) PhO(CH(2))(2)O(-) effects an ipso Smiles rearrangement (DeltaG(r) = +35 kJ mol(-1); barrier to transition state DeltaG(#) = +40 kJ mol(-1)) equilibrating the two oxygen atoms. The Smiles intermediate reverts to PhO(CH(2))(2)O(-) which then undergoes an S(N)i reaction to form PhO(-) and ethylene oxide (DeltaG(r) = -24 kJ mol(-1); DeltaG(#) = +54 kJ mol(-1)). (2) The formation of PhO(-) from energized PhO(CH(2))(3)O(-) is more complex. Some 85% of the PhO(-) formed originates via a Smiles intermediate (DeltaG(r) = +52 kJ mol(-1); DeltaG(#) = +61 kJ mol(-1)). This species reconverts to PhO(CH(2))(3)O(-) which then fragments to PhO(-) by two competing processes, namely, (a) an S(N)i process yielding PhO(-) and trimethylene oxide (DeltaG(r) = -27 kJ mol(-1); DeltaG(#) = +69 kJ mol(-1)), and (b) a dissociation process giving PhO(-), ethylene and formaldehyde (DeltaG(r) = -65 kJ mol(-1); DeltaG(#) = +69 kJ mol(-1)). The other fifteen percent of PhO(-) is formed prior to formation of the Smiles intermediate, occurring directly by the S(N)i and dissociation processes outlined above. The operation of two fragmentation pathways is supported by the presence of a composite metastable ion peak. (3) Energized PhO(CH(2))(4)O(-) fragments exclusively by an S(N)i process to form PhO(-) and tetrahydrofuran (DeltaG(r) = -101 kJ mol(-1); DeltaG(#) = +53 kJ mol(-1)). The Smiles ipso cyclization (DeltaG(r) = +64 kJ mol(-1); DeltaG(#) = +74 kJ mol(-1)) is not detected in this system.

Collision-induced fragmentation pathways including odd-electron ion formation from desorption electrospray ionisation generated protonated and deprotonated drugs derived from tandem accurate mass spectrometry.

The rapid desorption electrospray ionisation (DESI) of some small molecules and their fragmentation using a triple-quadrupole and a hybrid quadrupole time-of-flight mass spectrometer (Q-ToF) have been investigated. Various scanning modes have been employed using the triple-quadrupole instrument to elucidate fragmentation pathways for the product ions observed in the collision-induced dissociation (CID) spectra. Together with accurate mass tandem mass spectrometry (MS/MS) measurements performed on the hybrid Q-ToF mass spectrometer, unequivocal product ion identification and fragmentation pathways were determined for deprotonated metoclopramide and protonated aspirin, caffeine and nicotine. Ion structures and fragmentation pathway mechanisms have been proposed and compared with previously published data. The necessity for elevated resolution for the differentiation of isobaric ions are discussed.

Four decades of joy in mass spectrometry.

Tremendous developments in mass spectrometry have taken place in the last 40 years. This holds for both the science and the instrumental revolutions in this field. In chemistry the research was heavily focused on organic molecules that upon electron ionization fragmented via complex mechanistic pathways as shown by isotopic labeling experiments. These studies, including ion structure determinations, were performed with use of double focusing mass spectrometers of both conventional and reversed geometry, and equipped with various types of metastable ion scanning and collision-induced dissociation techniques developed by physical and analytical chemists. Time-resolved mass spectrometry by use of the field ionization kinetics method, developed by physical chemists, was another powerful way to unravel details of unimolecular gas phase ion dissociations. Then the development of new ionization methods, such as desorption chemical ionization, field desorption, and fast atom bombardment permitted not only to analyze unvolatile, thermally labile and higher molecular weight compounds, but also to study their chemical behavior in the gas phase, initially with use of double focusing instruments and later on with multisector and hybrid mass spectrometers. These ionization methods also enabled to study organometallic compounds and increasingly the field of medium-sized to large biomolecules, the latter being exploded in the last decade by the development of electrospray- and matrix-assisted laser desorption ionization/time-of-flight mass spectrometry. Another area of research concerned the bimolecular chemistry of organic ions with organic molecules in the gas phase. Initially this was performed with use of among others drift-cell ion cyclotron resonance spectroscopy, that later on was replaced by the developed method of ion trapping and Fourier transform ion cyclotron resonance. Combination of the latter with the afore-mentioned ionization methods has shifted also in this case the research on organic molecules to organometallic/inorganic systems, and predominantly to biomolecules in the last decade. This invited review will describe the research efforts made by the author's group over the last 40 years together with some personal experiences during his career.

The gas phase acidity of oligofluorobenzenes and oligochlorobenzenes: about the additivity or non-additivity of substituent effects.

The deprotonation energies of benzene, fluorobenzene, all di-, tri-, and tetrafluorobenzenes, pentafluorobenzene, chlorobenzene, all di-, tri-, and tetrachlorobenzenes, and pentachlorobenzene have been calculated at various levels of second-order Moller-Plesset and density functional theory. Taking the previously determined experimental data as a benchmark, good agreement was achieved in the chloro series even with moderate computational effort, whereas more extended basis sets have to be used to obtain meaningful numbers in the fluoro series. Apparently, most extensive electron correlation is required to avoid artifacts caused by the proximity of nonbonding lone pairs at the carbanionic center and at the fluorine atoms. When two or more fluorine substituents were introduced in the same aromatic ring, their individual effects (as defined by position-dependent acidity increments) proved to be perfectly additive in the entire series. In contrast, the acidifying effect of chloro substituents was found to level off when the number of such halogens increases. Additivity or non-additivity of element effects cannot be ascertained after having merely compared the acidity of mono- and disubstituted substrates, but only after having moved to higher degrees of substitution.

The McLafferty rearrangement: a personal recollection.

In this article, dedicated to Professor Fred McLafferty, I wish to recollect early observations with regard to the rearrangement named after him for 1-nitropropane and the methyl ester of gamma-nitrobutyric acid. This rearrangement occurs for both cases clearly in a stepwise fashion, although for the ester, it occurs as a hidden rearrangement that catalyzes the tautomerization of the nitro group into its aci-form.

Manipulating internal energy of protonated biomolecules in electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry.

The internal energy of protonated leucine enkephalin has been manipulated in electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry with two newly designed pump-probe experiments. Blackbody infrared radiation was applied to pump an ion population into a well-defined internal energy distribution below the dissociation threshold. Following this pumping stage, the internal energy distribution was probed using on-resonance collisional activation to dissociate the ions. These pump-probe experiments were carried out in two different ways: (a) using on-resonance collisional activation with variable kinetic energies to dissociate the ions at a constant initial ion temperature (determining the precursor ion survival percentage as a function of kinetic energy) and (b) using on-resonance collisional activation with a constant kinetic energy to dissociate the ions at variable initial ion temperatures (to investigate the ion survival yield-initial ion temperature dependence). Using this approach, a detailed study of the effects of the initial ion temperature, the probing kinetic energy and the internal energy loss rate on the effective conversion efficiency of (laboratory-frame) kinetic energy to internal energy was conducted. This conversion efficiency was found to be dependent on the initial ion temperature. Depending on the experimental conditions the conversion efficiency (for collisions with argon) was estimated to be about 4.0 +/- 1.7%, which agrees with that obtained from a theoretical modeling. Finally, the reconstructed curves of the ion survival yield versus the mode of the (final) total internal energy distribution of the activated ion population (after pump and probe events) at different pump-probe conditions reveal the internal energy content of the activated ions.

A computational study of the double hydrogen migration in the molecular ions of endo tricyclo[5.2.1.0(2,6)]decene.

Previous field ionization kinetic experiments have supported strongly that the double hydrogen migration prior to the elimination of C5H8 from the molecular ions of the endo isomers of 8,9-disubstituted tricyclo[5.2.1.0(2,6)]decenes, which is not observed for the exo isomers, proceeds in a concerted, i.e., dyotropic way. This paper describes the results of ab initio calculations at the ROHF/4-31G and DFT level of theory, performed on the double hydrogen migration for the ionized endo isomer of unsubstituted tricyclo[5.2.1.0(2,6)]decene. An intrinsic reaction coordinate calculation at the DFT level has shown indeed a direct connection between the structures of the molecular ions of the endo isomer before and after the double hydrogen migration, thus corroborating the earlier suggested concerted, i.e., dyotropic pathway of this double hydrogen rearrangement.

Zipping up 'the crushed fullerene' C60H30:C60 by fifteen-fold, consecutive intramolecular H2 losses.

MALDI TOF-MS of tribenzo[l:1':1"]benzo[1,2-e:3,4-e':5,6-e"]triacephenanthrylene (1a, C60H30) gives C60.+ by multiple intramolecular cyclodehydrogenation reactions.

Metastable ion study of organosilicon compounds. Part XIV-trimethylsilylacetic acid, (CH(3))(3)SiCH(2)COOH, and its methyl ester, (CH(3))(3)SiCH(2)COOCH(3).

The unimolecular metastable decompositions of trimethylsilylacetic acid, (CH(3))(3)SiCH(2)COOH (1), and its methyl ester, (CH(3))(3)SiCH(2)COOCH(3) (2), were investigated by mass-analyzed ion kinetic energy (MIKE) spectrometry in conjunction with thermochemical data. The abundance of the molecular ions of both compounds, generated by electron ionization, is extremely low. However, the abundance of the ions generated by the loss of (.)CH(3) and observed at m/z 117 and 131 is moderate. These fragment ions further decompose to form the most abundant m/z 75 and 89 ions, respectively, by the loss of CH(2)CO through a (CH(3))(2)Si group migration. The loss of CH(2)CO is also observed to occur from 2(+.) and its fragment ion at m/z 115 generated by the loss of (.)OCH(3). The former reaction is proposed to occur via an ion-radical complex.

Characterization of Hydrogen-Bonded Supramolecular Assemblies by MALDI-TOF Mass Spectrometry after Ag+ Labeling.

The high affinity of Ag+ ions for aromatic π donors and cyano groups is exploited in a novel MALDI-TOF mass spectrometric method for the identification of hydrogen-bonded assemblies. The interaction with the Ag+ ions-which, for example, can be complexed by two phenyl groups in a sandwich-type manner (see drawing on the right)-provides positively charged assemblies in a nondestructive way.