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We present the gas-phase infrared spectra of the phenyl cation, phenylium, in its perprotio C$_6$H$_5^+$ and perdeutero C$_6$D$_5^+$ forms, in the 260-1925 cm$^{-1}$ (5.2-38 $\mu$m) spectral range, and investigate the observed photofragmentation. The spectral and fragmentation data were obtained using Infrared Multiple Photon Dissociation (IRMPD) spectroscopy within a Fourier Transform Ion Cyclotron Resonance Mass Spectrometer (FTICR MS) located inside the cavity of the free electron laser FELICE (Free Electron Laser for Intra-Cavity Experiments). The $^1$A$_1$ singlet nature of the phenylium ion is ascertained by comparison of the observed IR spectrum with DFT calculations, using both harmonic and anharmonic frequency calculations. To investigate the observed loss of predominantly [2C,nH] (n=2-4) fragments, we explored the potential energy surface (PES) to unravel possible isomerization and fragmentation reaction pathways. The lowest energy pathways toward fragmentation include direct H elimination, and a combination of facile ring-opening mechanisms ($\leq2.4$ eV), followed by elimination of H or CCH$_2$. Energetically, all H-loss channels found are more easily accessible than CCH$_2$-loss. Calculations of the vibrational density of states for the various intermediates show that at high internal energies, ring opening is the thermodynamically the most advantageous, eliminating direct H-loss as a competing process. The observed loss of primarily [2C,2H] can be explained through entropy calculations that show favored loss of [2C,2H] at higher internal energies., Comment: 10 pages, 6 figures and 1 table. Accepted for publication on PCCP on 04 Feb 2021; supplementary Material available on PCCP website

With the passing of Prof. Robert C. Dunbar on 31 October 2017, the field of ion chemistry lost one of its modern heroes. Throughout his career in mass spectrometry, two of his main research interests involved the interaction of trapped ions with electromagnetic radiation and the chelation motifs of metal ions with organic ligands. The focus of his early career was on the fundamental processes that take place in molecules upon ultraviolet and infrared excitation. From 2003 to 2017, his scientific interests shifted to more structural questions, notably to resolving the structures and binding motifs of metal ion chelation complexes by application of infrared photodissociation spectroscopy. These experiments were carried out during numerous visits to the (Free Electron Laser for Infrared eXperiments) (FELIX) facility in the Netherlands and were complemented by extensive theoretical investigations by Rob. As a tribute to our friend, we present in this contribution a brief review of this work.

We report on a common fragment ion formed during the electron-ionization-induced fragmentation of three different three-ring polycyclic aromatic hydrocarbons (PAHs), fluorene (C13H10), 9,10-dihydrophenanthrene (C14H12), and 9,10-dihydroanthracene (C14H12). The infrared spectra of the mass-isolated product ions with m/z = 165 were obtained in a Fourier transform ion cyclotron resonance mass spectrometer whose cell was placed inside the optical cavity of an infrared free-electron laser, thus providing the high photon fluence required for efficient infrared multiple-photon dissociation. The infrared spectra of the m/z = 165 species generated from the three different precursors were found to be similar, suggesting the formation of a single isomer. Theoretical calculations using density functional theory (DFT) revealed the fragment's identity as the closed-shell fluorenyl cation. Decomposition pathways from each parent precursor to the fluorenyl ion are proposed on the basis of DFT calculations. The identification of a single fragmentation product from three different PAHs supports the notion of the existence of common decomposition pathways of PAHs in general and can aid in understanding the fragmentation chemistry of astronomical PAH species.

A moderate-resolution infrared multiple photon dissociation (IRMPD) spectrum of protonated bis(2-methoxyethyl) ether (diglyme) was obtained using a grating-tuned CO2 laser. The experimental spectrum compares well with one calculated theoretically at the MP2 level and exhibits defined peaks over the span of the CO2 laser output lines as opposed to a relatively featureless spectrum over this wavelength range obtained using free electron laser infrared radiation. The lowest energy structure corresponding to the calculated vibrational spectrum is consistent with structures previously calculated at the same level of theory. Alternative structures were calculated at lower levels of theory for comparison and investigation of the energetics of proton-heteroatom interactions. Broadening of the IRMPD action spectrum due to energetic phenomena characteristic of proton bridges was not observed and thus did not obscure the correlation between theoretical calculations and experimentally determined spectra as it may have in previous studies.

The thermal decomposition pathways of the tungsten dimethylhydrazido complexes Cl4(RCN)W(NNMe2) (1a: R=CH3; 1b: R=Ph), precursors for single source deposition of WNxCy, were investigated using a combination of experiments and calculations. Raman scattering studies were performed in an impinging-jet, up-flow, aerosol-assisted CVD reactor to identify reaction intermediates. Density Functional Theory calculations (B3LYP/LanL2DZ) were used to estimate Raman active frequencies and explore the reaction surface. Dimethylamine and methylmethyleneimine, products from N–N cleavage of the hydrazido ligand, were observed under deposition conditions and identified by comparison with previously reported Raman shifts and calculated frequencies.

The methyl glycosides of N-acetyl-d-glucosamine (d-GlcNAc) and N-acetyl-d-galactosamine (d-GalNAc) have been used as model glycan analogs to study the effects of lithium cation binding on glycan structure in gas-phase experiments. Infrared multiple photon dissociation (IRMPD) spectra for the two Li+-complexed anomers of methyl-d-GlcNAc revealed a difference of 10 cm−1 between their respective carbonyl stretching band positions. A corresponding 11 cm−1 shift was observed for the two Li+-complexed anomers of methyl-d-GalNAc. Theoretical calculations indicate that the position of the methyl group (α and β, or axial and equatorial, respectively) on carbon 1 of the sugar and its close proximity to the carbonyl of the acetamido group on carbon 2 cause the average orientation of the carbonyl to change in order to minimize steric hindrance. This change in orientation is postulated to be the cause of the observed Cdouble bond; length as m-dashO stretching band shift. The calculations also predict competitive binding of the lithium cation between two or more regions of d-GlcNAc and d-GalNAc. This is primarily due to differences in the spatial arrangement and orientation of lone pairs of electrons among the isomers, and stereochemical differences in hydrogen bonding. From an application point of view, differences in the infrared spectra of lithium adducts of acetamido sugars establish the value of variable-wavelength IRMPD as an alternative to fragmentation patterns in discriminating between these isomers.

Infrared multiple photon dissociation (IRMPD) was used to generate vibrational spectra of ions produced with a direct analysis in real time (DART) ionization source coupled to a 4.7 T Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The location of protonation on the nerve agent simulants diisopropyl methylphosphonate (DIMP) and dimethyl methylphosphonate (DMMP) was studied while solutions of the compounds were introduced for extended periods of time with a syringe pump. Theoretical vibrational spectra were generated with density functional theory calculations. Visual comparison of experimental mid-IR IRMPD spectra and theoretical spectra could not establish definitively if a single structure or a mixture of conformations was present for the protonated parent of each compound. However, theoretical calculations, near-ir IRMPD spectra, and frequency-to-frequency and statistical comparisons indicated that the protonation site for both DIMP and DMMP was predominantly, if not exclusively, the phosphonyl oxygen instead of one of the oxygen atoms with only single bonds.

Crosslinking of guanine with tyrosine and glutathione is studied by on-line electrochemistry electrospray ionization mass spectrometry (EC ESI MS) in the positive ion mode with a high resolution FT ICR mass spectrometer without the use of an external oxidizer in the sample. Dimeric adducts of guanine with tyrosine ( m / z 331) and ( m / z 333) and a dimer of guanine with glutathione ( m / z 459) are detected in the mass spectra of guanine–tyrosine and guanine–glutathione mixtures in addition to dimers of guanine ( m / z 303) and tyrosine ( m / z 361) and ( m / z 363) and glutathione dimers ( m / z 613) and ( m / z 615). Guanine tetramers ( m / z 627) and tetramers of guanine with 1 or 2 tyrosines, but not with three tyrosines, were also identified in the mass spectra of guanine–tyrosine mixtures. Formation of radicals and other oxidation products during positive ion mode ESI can drive the formation of covalent dimer adducts of guanine with tyrosine and glutathione. When low EC cell voltage is applied in on-line EC ESI MS, changes in ion intensities reflect changes in the oxidation conditions and are more apparent for 2e, 2H + than for the 1e, 1H + oxidation processes. Efficient oxidation during ESI is indicated for analytes with low redox potentials.

All eight D-aldohexoses and aldohexoses derived from the non-reducing end of disaccharides were investigated by variable-wavelength infrared multiple-photon dissociation (IRMPD) as anions in the negative-ion mode. Spectroscopic evidence supports the existence of a relatively abundant open-chain configuration of the anions in the gas phase, based on the observation of a significant carbonyl absorption band near 1710 cm(-1). The abundance of the open-chain configuration of the aldohexose anions was approximately 1000-fold or greater than that of the neutral sugars in aqueous solution. This provides an explanation as to why it has not been possible to discriminate the anomeric configuration of aldohexose anions in the gas phase when derived from the non-reducing sugar of a disaccharide. Evidence from photodissociation spectra also indicates that the different aldohexoses yield product ions with maximal abundances at different wavelengths, and that the carbonyl stretch region is useful for differentiation of sugar stereochemistries. Quantum-chemical calculations indicate relatively low energy barriers to intramolecular proton transfer between hydroxyl groups and adjacent alkoxy sites located on open-chain sugar anions, suggesting that an ensemble of alkoxy charge locations contributes to their observed photodissociation spectra. Ring opening of monosaccharide anions and interconversion among configurations is an inherent property of the ions themselves and occurs in vacuo independent of solvent participation. (C) 2011 Elsevier Ltd. All rights reserved.

Infrared multiple-photon dissociation (IR-MPD) spectra in the N–H and O–H stretching region (3000–3700 cm −1 ) are reported for gas-phase monomeric M 2+ Trp and dimeric M 2+ Trp 2 complexes (where M = Mg, Ca, Sr, and Ba). The spectra are obtained by irradiating the complexes in the Penning trap of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer, using a tunable continuous-wave (cw) optical parametric oscillator (OPO) laser in combination with a fixed wavelength CO 2 laser (10.6 μm). These spectra are compared with previously recorded mid-IR-MPD spectra, using the free electron laser FELIX, and are interpreted based on harmonic frequency calculations performed with density-functional theory (DFT). The experimental spectra show that a simple assignment of bands can be made to distinguish zwitterionic (ZW) from charge solvation (CS) complexes. In particular, the carboxylic acid O–H stretch at ∼3550 cm −1 identifies the presence of a CS structure, whereas the NH 3 + antisymmetric stretching mode in the 3150–3375 cm −1 range is diagnostic of a ZW structure. For the monomeric Ba 2+ Trp complex, exclusively the ZW structure is observed. Conversely, for the dimeric complexes of M 2+ Trp 2 (M = Sr and Ba) merely CS/CS geometries are confirmed. Surprisingly, for smaller alkaline earth metal dications (M = Mg and Ca), the IR-MPD spectra are consistent with the presence of mixed CS/ZW dimers. This is contrary to most previous trends for alkali metals, where larger cations typically favor ZW stabilization.

Phenylalanine analogs were subjected to hydrogen/deuterium exchange (HDX) in both solution and the gas phase, and gas-phase infrared multiple photon dissociation spectra were obtained for each of the species. For sodium cation-attached N-acetylphenylalanine, gas-phase HDX took place at only one site. Comparison of spectra from both undeuterated and singly deuterated sodiated N-acetylphenylalanine showed band shifts for normal modes that involved mainly vibrations of the O–H group, indicating that gas-phase exchange occurs at the COOH hydrogen and not at the NH hydrogen. Conversely, HDX in solution did result in exchange of the NH hydrogen, even for the protected species O-methyl N-acetylphenylalanine and N-acetylphenylalanine O-methylglycine. Rate coefficients for gas-phase H/D exchange were measured for the single deuteration of sodiated N-acetylphenylalanine and all three deuterations of protonated N-acetylphenylalanine, and found to be in the range (1.5–3.6) × 10−11 cm3/s. Density functional theory calculations predicted that the phenylalanine analogs, although of different size, have relatively similar structural features. These calculations showed that Na+ interacts with the phenyl ring and all available carbonyl oxygens, thus essentially locking the structures into one basic conformation. This behavior is quite distinct from other amino acids which are more flexible, and where gas-phase exchange also occurs at the amine (NH) group.

Lithiated glucose-containing disaccharides with various linkages and anomeric configurations, formed by electrospray ionization (ESI) and trapped in a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer, were fragmented by infrared radiation from a tunable CO 2 laser. Irradiation over the wavelength range from 9.2 to 9.7 μm gave unique fragmentation patterns for each of the disaccharides. These fragmentation patterns can be used to easily determine the monosaccharide linkage position for each disaccharide. Once the linkage is determined, irradiation of the precursor ion ( m / z 349) to produce a specific ratio of peak height for a particular fragment ion to that of the precursor ion is shown to yield unique ratio values that can be used to identify the different anomers of the same linkage at the 95% confidence level.

Direct Analysis in Real Time (DART) is an ambient ionization technique for mass spectrometry that provides rapid and sensitive analyses with little or no sample preparation. DART has been reported primarily for mass analyzers of low to moderate resolving power such as quadrupole ion traps and time-of-flight (TOF) mass spectrometers. In the current work, a custom-built DART source has been successfully coupled to two different Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometers for the first time. Comparison of spectra of the isobaric compounds, diisopropyl methylphosphonate and theophylline, acquired by 4.7 T FT-ICR MS and TOF MS, demonstrates that the TOF resolving power can be insufficient for compositionally complex samples. 9.4 T FT-ICR MS yielded the highest mass resolving power yet reported with DART ionization for 1,2-benzanthracene and 9,10-diphenylanthracene. Polycyclic aromatic hydrocarbons exhibit a spatial dependence in ionization mechanisms between the DART source and the mass spectrometer. The feasibility of analyzing a variety of samples was established with the introduction and analysis of food products and crude oil samples. DART FT-ICR MS provides complex sample analysis that is rapid, highly selective and information-rich, but limited to relatively low-mass analytes.

A cone-shaped MS inlet and on-line electrochemistry (EC) were used to enhance the ionization efficiency in electrospray ionization mass spectrometry (ESI MS) of purine bases. A pathway of positive ion mode ESI may involve oxidation of purine bases, guanine, adenine, xanthine and hypoxanthine, by 1e−, 1H+ processes. The electrospray process generates dimers of purine bases that are detected in ESI MS as protonated ions, except for xanthine, for which a protonated radical dimer is detected. Thus electrochemical oxidation of purine bases during ESI may generate reactive radicals that can subsequently dimerize. Dimer formation is facilitated in ESI MS when the carrier solution pH is high. The positive ion mode ESI MS ionization is consistent with the reactivity of the bases toward oxidation. Furthermore, the formation of the protonated ions, and Na+ and K+ adducts of the bases, expected in positive ion ESI MS, are observed. In addition, unusual H-bonding of purine bases guanine and xanthine is confirmed by ESI MS. Application of low EC voltage to the on-line EC cell in EC/ESI MS improves the sensitivity and correlates with the decrease of the intensity of the dimers, possibly as a result of their further oxidation.

High signal intensities of glutathione (GSH), [GSH+H]+ (m/z 308), cysteine (CySH), [CySH+H]+ (m/z 122), and homocysteine (hCySH), [hCySH+H]+ (m/z 136), are observed in ESI MS with on-line electrochemistry (EC). Dimers formed by H-bonding, which are not electrochemical products, are detected as [2GSH+H]+ (m/z 615), [2CySH+H]+ (m/z 243) and [2hCySH+H]+ (m/z 271) together with disulfide dimers GSSG, CySSCy and hCySSCyh, [GSSG+H]+ (m/z 613), [CySSCy+H]+ (m/z 241) and [hCySSCyh+H]+ (m/z 269). When dopamine is present a thiol/dopamine quinone (DAQ) adduct is observed. Formation of this adduct is proposed to occur by an electrochemical mechanism during ESI. Catalysis of thiol oxidation and analysis of thiol mixtures is addressed.

The ability of Paul and Penning traps to contain ions for time periods ranging from milliseconds to minutes allows the trapped ions to be subjected to laser irradiation for extended lengths of time. In this way, relatively low-powered tunable infrared lasers can be used to induce ion fragmentation when a sufficient number of infrared photons are absorbed, a process known as infrared multiple photon dissociation (IRMPD). If ion fragmentation is monitored as a function of laser wavelength, a photodissociation action spectrum can be obtained. The development of widely tunable infrared laser sources, in particular free electron lasers (FELs) and optical parametric oscillators/amplifiers (OPO/As), now allows spectra of trapped ions to be obtained for the entire "chemically relevant" infrared spectral region. This review describes experiments in which tunable infrared lasers have been used to irradiate ions in Penning traps. Early studies which utilized tunable carbon dioxide lasers with a limited output range are first reviewed. More recent studies with either FEL or OPO/A irradiation sources are then covered. The ionic systems examined have ranged from small hydrocarbons to multiply charged proteins, and they are discussed in approximate order of increasing complexity.

The sensitivity of detection of uric acid (H2U) in positive ion mode electrospray ionization mass spectrometry (ESI MS) was enhanced by uric acid oxidation during electrospray ionization. With a carrier solution of pH 6.3>pKa1=5.4 of H2U, protonated unoxidized uric acid [H2U+H]+ (m/z 169) was detected together with the protonated uric acid dimer [2H2U+H]+ (m/z 337). The dimer likely forms by 1e− oxidation of urate (HU−) followed by rapid radical dimerization. A covalent structure of the dimer was verified by H/D exchange experiments. Efficiency of 2e−, 2H+ oxidation of uric acid is low during ESI in pH 6.3 carrier solution and improves when a low on-line electrochemical cell voltage is floated on the high voltage of the ES in on-line electrochemistry ESI MS (EC/ESI MS). The intensity of the uric acid dimer decreases with an increase in the low applied voltage. In a carrier solution with 0.1 M KOH, pH 12.7>pKa2=9.8 of H2U, allantoin (Allnt) (MW 158.04), the final 2e−, 2H+ oxidation product of uric acid, was detected as a potassium complex [K(Allnt)+K]+ (m/z 235) and the [2H2U+H]+ dimer was not detected. In direct ESI MS analysis of 1000-fold diluted urine [NaHU+H]+ (pKsp NaHU=4.6) was detected in 40/60 (vol%) water/methanol, 1 mM NH4Ac, pH ca. 6.3 carrier solution. A new configuration of the ESI MS instrument with a cone-shaped capillary inlet significantly enhanced sensitivity in ESI and EC/ESI MS measurements of uric acid.

Reactive desorption electrospray ionization (DESI), an ambient technique, has been explored as a tool for the development of a fast screening approach for supramolecular complexes capitalizing on the specificity of mass spectrometric detection. A library of twelve potential guests for inclusion by a β-cyclodextrin host was initially screened via DESI using a spray solution incorporating the host directed toward an array of deposited guests. The steroid nortestosterone was used to verify the applicability of reactive DESI for complexation experiments with β-cyclodextrin. Results from the DESI experiment and results from an analogous electrospray ionization (ESI) mass spectral screen were compared with solution-phase data obtained by nuclear magnetic resonance (NMR) spectroscopy. The complexes detected using DESI were identical to those determined using NMR, validating the applicability of the technique to supramolecular applications, but the ESI data exhibited significant disparities, predominantly due to the interference of nonspecific artifacts. Copyright © 2008 John Wiley & Sons, Ltd.

Dopamine [DA]+ (m/z 154), DA dimer [2DA-H]+ (m/z 307) and DA quinone [DAQ]+ (m/z 152) are detected in positive ion mode electrospray ionization mass spectrometry (ESI MS) of dopamine in 50/1/49 (vol%) water/acetic acid/methanol. H/D exchange experiments support a covalent structure of DA dimer. Thus, ESI of DA may involve 1e−, 1H+ oxidation processes followed by rapid radical dimerization. The DA quinone signal is low in ESI MS, which indicates a low efficiency of the 2e−, 2H+ oxidation reaction. On-line electrochemistry ESI MS (EC/ESI MS) with low electrochemical cell voltage floated on high ES voltage increases electrospray current and improves sensitivity for DA. The DA quinone signal increases and DA dimer signal decreases. A new configuration of the ESI MS instrument with a cone-shaped capillary inlet significantly enhanced sensitivity of ESI and EC/ESI MS measurements. A DA quinone-cysteine adduct [DAQ+Cys]+ was detected in solutions of DA with cysteine (Cys). ESI MS and EC/ESI MS indicate formation of the DA quinone-cysteine adduct by 1e− pathway. Oxidation pathways in ESI MS are relevant to biological reactivity of DA and Cys.

The appearance of asparaginase-resistant acute lymphoblastic leukemia (ALL) in transformed cell lines has been correlated with increased expression of asparagine synthetase (ASNS). Recent measurements using mRNA-based assays have raised doubts, however, as to the importance of ASNS protein in the cellular mechanisms that confer drug resistance upon the leukemic cells. Studies aimed at determining the concentration of ASNS protein in human leukemias are therefore needed to resolve this issue. A mass spectrometry (MS)-based procedure is presented for the direct quantification of ASNS protein concentration in complex sample mixtures. This assay is able to distinguish samples from transformed cell lines that express ASNS over a wide dynamic range of concentration. Importantly, this method directly detects ASNS protein, the functional entity that may be synthesizing sufficient asparagine to render leukemia cells resistant to asparaginase-treatment. We also report the successful use of this MS method, which has lower limits of detection and quantification of 30 and 100 attomoles, respectively, for the first direct measurements of ASNS protein concentrations in four patient blast samples.

Class III unconventional myosins are critical for the normal function of auditory hair cells and the function and maintenance of photoreceptors; however, the roles of class III myosins in these sensory cells are unknown. Class III myosins are unique in that they have a kinase domain at their N-terminus; thus, they may have both signaling and motor functions. In the horseshoe crab Limulus polyphemus, enhanced phosphorylation of an abundant, photoreceptor specific class III myosin at night correlates with well-characterized circadian changes in photoreceptor structure and function. Thus, the Limulus visual system may be particularly useful for investigating the properties, modulation, and functions of a class III myosin. Previously, we showed that two sites within the actin interface of full-length Limulus myosin III expressed in baculovirus are substrates for both cyclic AMP-dependent protein kinase and autophosphorylation. In the current study, mass spectrometry was used to show that these same sites are phosphorylated in the endogenous protein extracted from Limulus lateral eye, and that enhanced phosphorylation at these sites occurs in vivo in response to natural circadian clock input to these eyes. These findings demonstrate in vivo changes in myosin III phosphorylation in response to a natural stimulus. This phosphorylation may modulate myosin III-actin interactions.

The reactions of the bis(η 5 -cyclopentadienyl)methylzirconium cation ( 1 ) with four imines have been studied in the gas phase by Fourier transform ion cyclotron resonance mass spectrometry. Reactivity of the bis(η 5 -cyclopentadienyl)methylzirconium cation is governed by the availability of a labile hydrogen in the corresponding imine. The products observed differ from those that might be expected in analogy with earlier work on ketones, aldehydes and alkenes (e.g., zirconocene η 3 -enolate and η 3 -allyl complexes): azomethyne/benzylidene species are found instead of enamines; however, the general reaction mechanism resembles that proposed for ketones and alkenes. An elimination reaction leading to final products was shown to be preceded by a fast migratory insertion/deinsertion equilibrium for alkyl-substituted imines. Such an equilibrium is not possible for aryl-substituted imines due to lack of lability of the aryl groups and thus only methane elimination was observed for these imines.

The 14-3-3s are a ubiquitous class of eukaryotic proteins that participate in a second regulatory step in many phosphorylation-based signal transduction systems. The Arabidopsis family of 14-3-3 proteins represents a rather large 14-3-3 gene family. The biological motive for such diversity within a single protein family is not yet completely understood. The work presented here utilizes 14-3-3 micro-affinity chromatography in conjunction with Fourier transform ion cyclotron resonance mass spectrometry to survey the substrate sequence selectivity of two Arabidopsis 14-3-3 isoforms that represent the two major subclasses of this protein family. A method was developed to compare the relative binding of eight synthetic phosphopeptide sequences. The degree to which each phosphopeptide bound to either isoform was assigned a relative value, defined here as the binding ratio. The method provided a simple means for visualizing differences in substrate sequence selection among different 14-3-3 isoforms. A reproducible preference for specific phosphopeptide sequences was measured for both isoforms. This binding preference was consistent among the two classes of isoforms, suggesting that any pressure for isoform selectivity must reside outside the central core that interacts with the phosphopeptide sequence of the client.

The reactions of the bis(η5-cyclopentadienyl)methylzirconium cation (1) with ketones and aldehydes in the gas phase have been studied by Fourier transform ion cyclotron resonance mass spectrometry. Reactions of 1 with a majority of the ketones studied resulted in consecutive addition of one and two substrate molecules and/or elimination of alkanes. Deuterium-labeled substrates and methylzirconocene were used to investigate the mechanistic details. On the basis of DFT calculations, the key products of the elimination reaction(s) were identified as η3-enolate complexes formed via an insertion/elimination mechanism. Similar product ion structures are postulated for the reaction of 1 with aldehydes in cases where these complexes are either the only or the major reaction product. A multiple-step mechanism is proposed, which involves migratory insertion of an aldehyde molecule into the methylzirconocene cation, followed by β-H elimination, and, via a six-membered cyclic transition state, formation of the result...

Infrared spectra were recorded for a series of gas-phase Cr+ complexes using infrared multiphoton dissociation (IRMPD) in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The functionalized aromatic ligands (acetophenone, anisole, aniline, and dimethyl aniline) offer a choice of either aromatic ring-pi or n-donor-base binding sites. Use of the FELIX free electron laser light source allowed convenient, rapid scanning of the chemically informative wavelength range from approximately 500 to 1800 cm(-1), which in many cases characterized the preferred site of metal binding, as well as the electronic spin state of the complex. Mono-complex ions, Cr+(ligand), for anisole, aniline, and dimethyl aniline and bis-complex ions, Cr+(ligand)(2), for anisole, aniline, and acetophenone were produced by ligand attachment to laser-desorbed Cr+ ions in the FT-ICR cell. The photodissociation yields plotted as a function of wavelength were interpreted as approximations to the infrared absorption spectra and were compared with computed spectra of different possible geometries and spin states. Clear-cut diagnostic features in the spectra of the acetophenone, anisole, and aniline complexes showed the sites of Cr+ attachment to be the carbonyl oxygen site for acetophenone (bis-complex) and the ring-pi site for anisole and aniline (both mono- and bis-complexes). The bis-complexes of aniline and anisole are low-spin (probably doublet) states, while the mono-complexes of these same ligands are high-spin (sextet) states. The dimethyl aniline complex gave a cluttered spectrum in poor agreement with calculations, which may reflect a mixture of binding-site isomers in this case.

Benzofluorene (C 1 7 H 1 2 ) cations have been studied in the vapor phase by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The identification of certain fragmented species from electron impact/ matrix optical experiments (see Banisaukas, J.; Szczepanski, J.; Vala, M.; Hirata, S. J. Phys. Chem. A 2004, 108, 3713) provided the background for the present detailed studies of the dissociation pathways of the cation via FT-ICR mass spectrometry. Both hydrogen and acetylene loss pathways along various photodissociation routes have been identified.

Electrospray ionization Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometry has been utilized to study solvates of tris(2,2‘-bipyridine)ruthenium(II). Spontaneous dissociation of solvent (acetonitrile, acetone, or methyl ethyl ketone) from the solvation shell occurs when these ions are trapped for extended periods of time in the ICR cell. The pressures employed in these experiments are low enough (10-9 to 10-8 mbar) to neglect (or partially neglect) collisional activation as a means for dissociation. Therefore, it is suggested that the solvated ruthenium species undergo dissociation following the absorption of blackbody infrared radiation. Solvent−complex dissociation has been studied at several pressures ranging from 10-9 to 10-8 mbar to provide a range of dissociation data in the low-pressure regime. The results reported here demonstrate the consistency of the dissociation rate constants at pressures that differ by an order of magnitude. Temperature dependence studies were performed ...

A miniaturized two-electrode electrochemical (EC) cell was developed and was coupled on-line with an electrospray ionization Fourier transform ion cyclotron resonance mass spectrometer (ESI-FTICR MS). Electrochemistry on-line with mass spectrometry, EC/ESI-FTICR MS, of triphenylamine (TPA), which undergoes one-electron oxidation to form a radical cation (TPA•+), demonstrates a significant sensitivity enhancement compared to ESI-FTICR MS. The on-line EC cell configuration with a stainless steel ES needle as the working electrode produces the highest sensitivity in EC/ESI-MS. The results provide evidence that, during the ES ionization, electrolytic reactions occur mainly in the ES tip region, as previously predicted. The results demonstrate that ESI-MS signal suppression by tetrabutylammonium perchlorate electrolyte, which can be a problem, is minimized in EC/ESI-MS. TPA•+ dimer tetraphenylbenzidine (TPB) can be detected by EC/ESI-MS, together with TPA•+, as TPB•+ and TPB2+. The high mass resolving power of...

Previous Fourier transform ion cyclotron resonance mass spectrometry (FTICR/MS) experiments have shown that UV/visible photolysis of the fluorene cation leads primarily to sequential loss of one to five hydrogens. Subsequent photolysis of the odd mass dehydrogenated species induces further fragmentation to lower mass products. In the present paper, results from density functional calculations are used to explain the experimental findings. These results show that dehydrogenation is predicted to occur first from the sp3 carbon on the five-membered ring and then from only one of the six-membered rings. The predicted infrared spectrum of this C13H5+ (m/z 161) species is shown to match well with a matrix isolation spectrum of a photolyzed fluorene sample. The conclusion is drawn that the C13H5+ (m/z 161) ion retains its fluorene-like framework and does not isomerize upon dehydrogenation. Photolysis of this C13H5+ (m/z 161) ion does appear to lead to isomerization. Plausible photodecomposition pathways leading ...

The current state of knowledge concerning mixed-terminal-ligand oxo-centered carboxylate-bridged trinuclear complexes is summarized and the application of mass spectrometric techniques to the study of trinuclear basic carboxylates is briefly reviewed. The exchange/substitution reactions of terminal ligands with other molecules in both liquid and gas phase have been investigated using electrospray ionization Fourier transform-ion cyclotron resonance mass spectrometry (ESI FT-ICR MS), studying the clusters [Fe3O(pivalato)6(H2O)3]+ClO4 − (1), [Cr3O(RCOO)6(L)3]+ClO4 − (R=C(CH3)3, L=H2O (2); R=C6H5, L=H2O (3), CH3OH (4), or piperidine (5)) as well as [(C2H5)2NH2]+ 2[Fe3O((CH3)2CHCH2COO)6(F)3]2−·CH3CN·2CHCl3 (6) and K+[Cr3O(C6H5COO)6(F)2(H2O)]−·2(CH3)2CO (7). It was established that factors influencing the quantitative and qualitative yields of products in the ion–molecule collisions/reactions occurring in the ESI source are: skimmer voltage, capillary voltage, ion accumulation time in the hexapole, capillary temperature, pressure in the hexapole region, and the properties of the atomic or molecular system used for concomitant creation of background pressure and providing reagent for ion–molecule reactions. Under ‘soft’ ESI conditions coordinatively saturated gaseous ions [(core)(L)3]+ (where [core]+=[Cr3O(RCOO)6]+) could be generated and supramolecular self-assembled entities [(Anion)(core)2(L)6]+ (where Anion−=ClO4 − for coordination compounds 2–5 or F− for 7) and [(ClO4){(core)(H2O)(L)2}3]2+ (for compound 3 in methanol) pre-existing in solution were detected. The ESI FT-ICR MS study revealed that the [(core)(L)3]+ ions lose L molecules as a result of undergoing dissociation by the absorption of blackbody infrared radiation. Based on studies of exchange/substitution reactions in the liquid phase, two new methods of condensed phase synthesis of mixed-terminal-ligand species are proposed: (a) an ‘aged solutions’ strategy; and (b) site-directed gradual replacement of terminal ligands via supramolecular assemblies. Also, a new compound, K+[Cr3O(C6H5COO)6(F)2(H2O)]−·2(CH3)2CO, was synthesized and X-ray crystallographic analysis (at 120 K) was employed to confirm its ‘unsymmetric’ structure.

The photodecomposition of the vapor phase fluorene cation has been studied using Fourier transform ion cyclotron resonance (ICR) mass spectrometry. Broad-band visible and ultraviolet irradiation for relatively short times (∼0.5 s) is shown to lead predominantly to sequential hydrogen atom loss, with one to five hydrogens lost. A relatively minor decomposition route for several precursor ions is the loss of H2 (or 2H atoms). Longer irradiation times lead to various products resulting from multiple C−C bond cleavages. The “action” spectrum for the m/z 166 → 165 process reveals two major electronic absorption bands which correlate well with a previous matrix absorption spectrum of the parent ion. The C13H+10-n (n = 0−5) species have been isolated in the ICR mass spectrometer, irradiated further, and their photodecomposition followed kinetically. Solution of the appropriate coupled differential equations that govern the kinetics has enabled fits to the experimental data. Rate constants and branching ratios fo...

Fourier transform ion cyclotron resonance mass spectrometry has been found to produce reliable exact mass measurements using two different internal calibration methods. For these measurements, electrospray ionization (ESI) and matrix-assisted laser desorption/ionization (MALDI) were utilized both individually and in tandem. For internal calibration with a co-dissolved polyethylene glycol standard, measurements of 41 compounds resulted in an average absolute mass determination error of 0.7 ppm, with a standard deviation of 0.9 ppm. For comparison, internal calibration was effected through the simultaneous use of ESI and MALDI, with the former being used for the introduction of analyte ions and the latter for formation of polymethylmethacrylate calibrant ions. This technique led to mass measurements with an average absolute error of 0.8 ppm and a standard deviation of 1.0 ppm. In addition, exact mass measurements of tandem mass spectrometry fragment ions were made for 35 compounds using external calibration with a single internal mass standard. The observed average absolute error was 0.7 ppm with a standard deviation of 1.0 ppm.

To improve the analytical usefulness of Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS), an extensive survey of various methods for quantitation of peak magnitudes has been undertaken using a series of simulated transient response signals with varying signal-to-noise ratio. Both peak height (five methods) and peak area (four methods) were explored for a range of conditions to determine the optimum methodology for quantitation. Variables included dataset size, apodization function, damping constant, and zero filling. Based on the results obtained, recommended procedures for optimal quantitation include: apodization using a function appropriate for the peak height ratios observed in the spectrum (i.e., Hanning for ratios of about 1:10, three-term Blackman-Harris for ratios of ∼1:100, or Kaiser-Bessel for ratios of ∼1:1000); zero filling until the peaks of interest are represented by 10–15 points (generally obtained with one order of zero filling); and use of the polynomial y = (ax2 + bx + c)n and the three data points of highest intensity of the peak to locate the peak maximum, Ymax = (−b2/4a + c)n. In this peak fitting procedure, which we have termed the “Comisarow method,” n is 5.5, 9.5, and 12.5 for the Hanning, three-term Blackman-Harris, and Kaiser-Bessel apodization functions, respectively. Accuracy of quantitation using an optimal peak height determination is about equal to that for peak area measurements. These recommendations were found to be valid when tested with real FTICR-MS spectra of xenon isotopes.

The combination of a glow discharge ionization source with a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer provides several advantages for elemental analysis; among these is an isotope ratio measured at the highest possible mass resolving power. In this report, lead ratios (206Pb/208Pb, 207Pb/208Pb, and 207Pb/206Pb) in a sample composed of 10% lead oxide, 10% mercuric oxide, and 80% silver powder (w/w) were observed to be precise to better than ±3% relative standard deviation (RSD at 1 σ) in a commercial FTICR instrument that used a conventional elongated cell. Upon incorporation of a cell whose excitation voltages approximated those in an ideal cell of infinite length, the precision improved to better than ±0.4% RSD. The isotopic bias between measured and known values was also evaluated. Using a cathode composed of 5% National Institute of Standards and Technology (NIST) SRM 987 SrCO3 in silver powder, biases that averaged less than 2.5% were detected using the cell of improved design. These results compared favorably with those obtained using a commercial magnetic sector glow discharge mass spectrometer, although it is still unclear how glow discharge mass spectrometry biases vary in general.

Seven different tetraphenylpyridinium esters were chosen as model compounds for studying gas phase pyrolysis reactions by Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FTICR/MS) Collisionally Activated Dissociation (CAD) using cation tagging. In this technique a stable positive charge (tetraphenyl pyridinium) is introduced to one end of the system so the reaction that occurs at the other end can be monitored easily using mass spectrometry. Threshold fragmentation energies and rate constants were calculated for all seven starting materials and a comparison of their observed relative fragmentation rates was made.

Gas phase SN2 intramolecular displacements are reported in which neutral nucleophiles displace neutral leaving groups within cationic substrates. In the intramolecular SN2 reaction of N-(2-piperidinoethyl)-2,4,6-triphenylpyridinium cation (11), the cationic product 12 was detected directly. Intramolecular reactions of N-(ω-aminoalkyl)-5,6,8,9-tetrahydro-7-phenyldibenzo[c,h]acridinium salts 13a–c, N-(5-hydroxypentyl)-5,6,8,9-tetrahydro-7-phenyldibenzo[c,h]acridinium salt 14 and N-(ω-aminoalkyl)-2,4,6-triphenylpyridinium salts 20a,b afforded the corresponding protonated acridinium cation (15) or pyridinium cation (21) and, presumably, a neutral heterocycle. This interpretation is supported by isotopic labeling control experiments. No evidence has been obtained for any intermolecular gas phase SN2 reaction with a pyridine as leaving group; theoretical calculations suggest an explanation for these experimental results.

Relative fragmentation energies of a variety of N-acylpyridinium cations have been measured in the gas phase by electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS). N-Aroyl- and N-heteroaroyl-pyridinium cations dissociated to give free ArCO+ ions with activation energies which are strongly decreased/increased by stabilizing/destabilizing electron-donor/acceptor substituents in the aroyl group. N-Alkylcarbonyl- and N-phenoxycarbonyl-pyridinium cations fragment via ion molecule complexes which dissociate to pyridinium cation and ketene or CO2 + aryne. N-Alkoxycarbonylpyridinium cations form pyridinium cations via detectable N-carboxypyridinium cations. The propensity of those cations to undergo SN1 or SN2 reactions is discussed.

Twelve sulfonium salts were studied by electrospray Fourier transform ion cyclotron resonance mass spectrometry. Collisionally activated dissociation, CAD, reactions, using neutral argon as the collision gas, were performed to investigate dissociation pathways in the gas phase and to study the stabilizing/destabilizing effects of electron donating/withdrawing substituents on the sulfonium ions. Ion-molecule reactions were attempted to identify SN2 reaction mechanisms, but to date we have detected exclusively SN1-type reactions in the gas phase. Literature reports on the mass spectrometry of sulfonium cations are reviewed.

A model experiment for the ‘on-line’ screening of substrate libraries by enzymes using combinatorial libraries in combination with electrospray ionization-Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometry has been performed. The reaction between the electrophilic substrate 1-chloro-2,4-dinitrobenzene and components of a H-A-Glu-Cys-Xxx-OH library, catalyzed by glutathione-S-transferase, has been monitored. It shows the feasibility of ‘two-dimensional’ screening of substrate libraries by ESI-FTICR mass spectrometry. © 1997 John Wiley & Sons, Ltd.

Initial results obtained by incorporation of an inductively coupled plasma (ICP) ion source with a Fourier transform ion cyclotron resonance (FTICR) mass spectrometer are reported, along with spectra which demonstrate sub-milligram-per-liter detection limits and high mass resolution (mass resolving power, m/Δm10%V = 7000−10 000). Also shown is an ICP-FTICR spectrum with ultrahigh resolving power (m/Δm10%V = 88 000). Both instrumental configurations used to conduct this work, one at the University of Florida, and the other at the National High Magnetic Field Laboratory, are described. Also included are concise results from a computer aided interference study designed to enumerate mass spectral interferences requiring mass resolving power in excess of 10 000.

Photodetachment in a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer was used to measure the binding energies of (CH3OH)X- (X = F, Cl, Br, I), (ROH)Br- (R = C2H5, i-C3H7, n-C3H7), and (CH3CN)Br-. The methanol complexes of F- and Cl- were prepared by sequential ion/molecule reactions, while equilibrium solvent exchange was used for preparation of the Br- and I- complexes. The photodetachment process was observed over the wavelength range of 260−350 nm using a tunable dye laser with a frequency-doubling accessory. Assuming that the photodetachment process leads to complete dissociation to the neutral solvent plus halogen atom, the binding energies were calculated by subtracting the known electron affinities of the halogens from the measured photodetachment thresholds. The binding energy values obtained for (CH3OH)F-, (CH3OH)Cl-, (CH3OH)Br-, (CH3OH)I-, (C2H5OH)Br-, (i-C3H7OH)Br-, (n-C3H7OH)Br-, and (CH3CN)Br- are 29.6, 18.7, 15.1, 14.4, 15.2, 16.5, 16.7, and 13.4 kcal/mol, respectively. ...

Protonated molecules of analytes in matrix-assisted laser desorption/ionization (MALDI) are frequently the most intense ions observed, especially when the concentration of alkali metal cations is low. Examination of the laser desorption mass spectra of MALDI matrices usually shows the presence of both molecular radical ions M+• and [M + H]+ ions. With some matrices, the intensity of the [M + H]+ ion is greater than that of the molecular radical ion, e.g. with 2,5-dihydroxybenzoic acid. A logical source for the ions of protonated analyte in MALDI is proton donation from the [M + H]+ ions of the matrix, but donation could also occur from the radical molecular ions. A knowledge of the proton affinities of the common MALDI matrices might be helpful in understanding why some matrices are ‘hotter’ than others and lead to more post-source as well as prompt decay. The ground-state proton affinity of eight common MALDI matrices were determined. For each matrix, the [M + H]+ ion was generated by methane chemical ionization, trapped and isolated in a Fourier transform ion cyclotron resonance mass spectrometer, allowed to cool for 5 s and reacted with reference compounds of known proton affinities. In some cases, the matrix proton affinities are low enough that proton transfer can occur from the ground state [M + H]+ ion to MALDI analytes; in other cases, the matrix proton affinities are so high that some other mechanism for proton transfer is required. © 1997 John Wiley & Sons, Ltd.

The infrared absorption spectra of several gas-phase ions solvated by a single methanol molecule have been determined indirectly by infrared multiple photon dissociation (IRMPD) spectroscopy. A modified White-type multi-pass cell was used in a Fourier transform ion cyclotron resonance mass spectrometer to record the disappearance of the parent ion as a function of the wavelength of a tunable CO2 laser operating in the range of 920–1060 cm−1. Spectra were obtained for fluoride, chloride and methoxide anions to which either a methanol or a d-methanol molecule was attached. In addition, spectra of proton-bound methanol and d-methanol dimer cations are presented. The shifts in the IRMPD peak frequencies are compared with the respective neutral infrared spectra. A consistent and interesting splitting is observed in the case of ions to which a methanol-d1 molecule is attached.

Complexes of 18-crown-6, 15-crown-5, and 12-crown-4 with Na+, K+, Rb+, Cs+ and H3O+ were observed and characterized using infrared multiphoton dissociation (IRMPD) in conjunction with Fourier transform ion cyclotron resonance mass spectrometry. The complexes were formed in methanol-water solution and were subsequently transported into the gas phase using two different electrospray ionization sources. Trapped complexes were dissociated utilizing IRMPD to obtain fragmentation and binding information on selected crown ether complexes. Dissociation of crown ether-alkali metal complexes proceeded via the loss of the alkali metal ion at low laser fluences; dissociation of the crown was not observed. For all of the alkali metal-crown ether complexes investigated, it was found that Na+ binds to the crown ether more strongly than any of the larger alkali metal ions. In contrast to the alkali metal complexes, IRMPD of (18-crown-6)H3O+ proceeded via loss of H2O to produce protonated 18-crown-6 ions. At higher laser fluences, (15-crown-5)H3O+ and (12-crown-4)H3O+ lost water and then fragmented by sequential loss of C2H4O units.

Fourier transform ion cyclotron resonance mass spectrometry was used to study ion/molecule reactions of six cationic methyl metallocenes: [Cp2ZrCH3]+ (1; Cp = η5-cyclopentadienyl), [(Ind)(Cp)ZrCH3]+ (2; Ind = η5-indenyl), [Ind2ZrCH3]+ (3), [ansa-(CH3)2Si(η5-C5H4)2ZrCH3]+ (4), [(Flu)2ZrCH3]+ (5; Flu = η5-fluorenyl), and [ansa-(CH3)2Si(η5-C5Me4)(N-t-C4H9)ZrCH3]+ (6). Rate constants and product distributions for reactions with dihydrogen and various deuterium-labeled and unlabeled alkenes were determined. The electrophilicity of [L2Zr−CH3]+ ions in the gas phase is strongly dependent on the nature of the ancillary ligands L, with electrophilicity decreasing in the order Cp2 > Cp,Ind> Ind2 > Flu2 for reactions with dihydrogen and 1-alkenes. Silyl-bridging of Cp ligands as in 4 appears to increase the electrophilicity somewhat, and the 10-electron ion 6 also has higher electrophilicity than 1. In each reaction used to assess electrophilicity, the probable rate determining transition state involves a 4-center/...

Electrospray ionization coupled with Fourier transform ion cyclotron resonance (FTICR) mass spectrometry has been used to provide information about complete combinatorial libraries of small peptides containing 10(3)-10(4) components. The fidelity of attempted synthesis steps can be ascertained rapidly, and, when the extremely high resolution FTICR mass spectra are combined with appropriate computer simulation, both diversity and degeneracy of the libraries as synthesized can be assessed.