Ion Cyclotron Resonance Experiments Research Articles

Indocyanine green MS/MS investigations using femtosecond laser-pulse photodissociation and collision-induced dissociation.

The dissociation reactions of indocyanine green (also known as cardiogreen) have been studied in a Fourier-transform ion-cyclotron resonance mass spectrometer by the application of unimolecular, collision-induced dissociation and photodissociation using femtosecond laser pulses. Ions were prepared by electrospray ionization (ESI) in various solvents. Depending on the properties of the solvent mixture, the mono sodiated molecule could be measured through cation exchange in various compositions. Using a mixture of methanol/water/formic acid as solvent, protonated ions, formed by exchange of sodium, are predominantly observed. A mixture of isopropanol/formic acid leads to the addition of a further sodium cation to the molecule yielding an intense bi-sodiated ion signal. The photodissociation of the stored ions was achieved using pulses at a wavelength of 790 nm and approximately 150 fs pulse duration. The results show that only fs-photodissociation leads to the fragmentation of the different molecular structures, while in case of sodiated indocyanine green collision induced dissociation fails completely to observe any fragments. In all other investigated ion structures, the collision-induced-dissociation spectra show unspecified and little intense spectra. It is shown that fs-photodissociation mass spectrometry is the only method that leads to substance specific fragmentation for this sample. Furthermore, indocyanine green is known to form aggregation products. In the ion-cyclotron resonance experiment, the reactions of a dimeric cluster were also investigated. The necessary condition and the interference of the fragmentations with those of the monomer are discussed.

Effect of ion clouds micromotion on measured signal in Fourier transform ion cyclotron resonance: Computer simulation.

Particle-in-cell-based realistic simulation of Fourier transform ion cyclotron resonance experiments could be used to generate ion trajectories and a signal induced on the detection electrodes. It has been shown recently that there is a modulation of "reduced" cyclotron frequencies in ion cyclotron resonance signal caused by Coulomb interaction of ion clouds. In this work it was proposed to use this modulation in order to determine frequency difference between an ion of known m/z and all other ions generating signal in ion cyclotron resonance cell. It is shown that with an increase of number of ions in ion cyclotron resonance trap, the modulation index increases, which lead to a decrease in the accuracy of determination of peak intensities by super Fourier transform resolution methods such as filter diagonalization method.

Bond energies of ThO+ and ThC+: A guided ion beam and quantum chemical investigation of the reactions of thorium cation with O2 and CO

Kinetic energy dependent reactions of Th(+) with O2 and CO are studied using a guided ion beam tandem mass spectrometer. The formation of ThO(+) in the reaction of Th(+) with O2 is observed to be exothermic and barrierless with a reaction efficiency at low energies of k/kLGS = 1.21 ± 0.24 similar to the efficiency observed in ion cyclotron resonance experiments. Formation of ThO(+) and ThC(+) in the reaction of Th(+) with CO is endothermic in both cases. The kinetic energy dependent cross sections for formation of these product ions were evaluated to determine 0 K bond dissociation energies (BDEs) of D0(Th(+)-O) = 8.57 ± 0.14 eV and D0(Th(+)-C) = 4.82 ± 0.29 eV. The present value of D0 (Th(+)-O) is within experimental uncertainty of previously reported experimental values, whereas this is the first report of D0 (Th(+)-C). Both BDEs are observed to be larger than those of their transition metal congeners, TiL(+), ZrL(+), and HfL(+) (L = O and C), believed to be a result of lanthanide contraction. Additionally, the reactions were explored by quantum chemical calculations, including a full Feller-Peterson-Dixon composite approach with correlation contributions up to coupled-cluster singles and doubles with iterative triples and quadruples (CCSDTQ) for ThC, ThC(+), ThO, and ThO(+), as well as more approximate CCSD with perturbative (triples) [CCSD(T)] calculations where a semi-empirical model was used to estimate spin-orbit energy contributions. Finally, the ThO(+) BDE is compared to other actinide (An) oxide cation BDEs and a simple model utilizing An(+) promotion energies to the reactive state is used to estimate AnO(+) and AnC(+) BDEs. For AnO(+), this model yields predictions that are typically within experimental uncertainty and performs better than density functional theory calculations presented previously.

Fragmentation patterns of boron-dipyrromethene (BODIPY) dyes by electrospray ionization high-resolution tandem mass spectrometry.

4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene derivatives (BODIPYs) are fluorescent organic dyes that are widely used as non-radioactive labels in biological analyses. The fragmentation behaviour of ten structurally related BODIPYs was studied using tandem mass spectrometry (MS/MS), to support the structural elucidation process during synthesis. The BODIPYs were investigated by electrospray ionization (ESI)-MS/MS, utilizing collision-induced dissociation (CID) data from triple quadrupole MS and high-resolution, accurate mass CID data from Fourier transform ion cyclotron resonance (FTICR) experiments. Unusual radical molecular cations ([M](+•)) were formed directly during the ESI process. These radical species dissociated into a large range of product ions during the subsequent CID experiments. Superimposed dissociations originating from parallel [M](+•) and [M+H](+) decompositions significantly complicated the interpretation of the MS/MS spectra. Detailed dissociation mechanisms were proposed in this study for BODIPY dyes. The elemental formulae of CID product ions were unambiguously assigned using FTICR-MS and unique fragment ions were discovered for the rapid identification of methyl, ethyl, butyl, tert-butyl, and phenyl substituents of individual dyes in BODIPY synthesis mixtures by low-resolution MS.

Determination of Types and Binding Sites of Advanced Glycation End Products for Substance P

Glycation by endogenous dicarbonyl metabolites such as glyoxal is an important spontaneous post-translational (PTM) modification of peptides and proteins associated with structural and functional impairment. The aim of this study was to investigate types and site of PTM of glyoxal-derived advanced glycation end-products-in the neuropeptide substance P by ultrahigh-resolution Fourier transform ion cyclotron resonance (FTICR), mass spectrometry, and tandem mass spectrometry (MS/MS) experiments. The main site of PTM by glyoxal was the side chain guanidine moiety of the arginine residue. Binding site identification has been achieved by electron capture dissociation, double-resonance electron capture dissociation, and collision-activated dissociation, with assignment of the modified amino acid residue with mass error <1 ppm.

Uniform Moment Theory for Charged Particle Motion in Gases

Moment equations for the motion of trace amounts of charged particles through dilute gases are developed from the Boltzmann equation. A new method for truncating the coupled moment equations is used to develop differential equations governing the moments in successive approximations. The first approximation equations are shown to agree completely with equations known to describe ion motion in drift-tube mass spectrometers, ion mobility spectrometers, ion traps, and collision-dominated ion cyclotron resonance experiments. Applications to differential mobility spectrometers and other devices are also described.

Fourier Transform Ion Cyclotron Resonance Mass Resolution and Dynamic Range Limits Calculated by Computer Modeling of Ion Cloud Motion

Particle-in-Cell (PIC) ion trajectory calculations provide the most realistic simulation of Fourier transform ion cyclotron resonance (FT-ICR) experiments by efficient and accurate calculation of the forces acting on each ion in an ensemble (cloud), including Coulomb interactions (space charge), the electric field of the ICR trap electrodes, image charges on the trap electrodes, the magnetic field, and collisions with neutral gas molecules. It has been shown recently that ion cloud collective behavior is required to generate an FT-ICR signal and that two main phenomena influence mass resolution and dynamic range. The first is formation of an ellipsoidal ion cloud (termed "condensation") at a critical ion number (density), which facilitates signal generation in an FT-ICR cell of arbitrary geometry because the condensed cloud behaves as a quasi-ion. The second phenomenon is peak coalescence. Ion resonances that are closely spaced in m/z coalesce into one resonance if the ion number (density) exceeds a threshold that depends on magnetic field strength, ion cyclotron radius, ion masses and mass difference, and ion initial spatial distribution. These two phenomena decrease dynamic range by rapid cloud dephasing at small ion density and by cloud coalescence at high ion density. Here, we use PIC simulations to quantitate the dependence of coalescence on each critical parameter. Transitions between independent and coalesced motion were observed in a series of the experiments that systematically varied ion number, magnetic field strength, ion radius, ion m/z, ion m/z difference, and ion initial spatial distribution (the present simulations begin from elliptically-shaped ion clouds with constant ion density distribution). Our simulations show that mass resolution is constant at a given magnetic field strength with increasing ion number until a critical value (N) is reached. N dependence on magnetic field strength, cyclotron radius, ion mass, and difference between ion masses was determined for two ion ensembles of different m/z, equal abundance, and equal cyclotron radius. We find that N and dynamic range depend quadratically on magnetic field strength in the range 1-21 Tesla. Dependences on cyclotron radius and Δm/z are linear. N depends on m/z as (m/z)(-2). Empirical expressions for mass resolution as a function of each of the experimental parameters are presented. Here, we provide the first exposition of the origin and extent of trade-off between FT-ICR MS dynamic range and mass resolution (defined not as line width, but as the separation between the most closely resolved masses).

Improved Methods for Feynman Path Integral Calculations of Vibrational−Rotational Free Energies and Application to Isotopic Fractionation of Hydrated Chloride Ions

We present two enhancements to our methods for calculating vibrational-rotational free energies by Feynman path integrals, namely, a sequential sectioning scheme for efficiently generating random free-particle paths and a stratified sampling scheme that uses the energy of the path centroids. These improved methods are used with three interaction potentials to calculate equilibrium constants for the fractionation behavior of Cl(-) hydration in the presence of a gas-phase mixture of H(2)O, D(2)O, and HDO. Ion cyclotron resonance experiments indicate that the equilibrium constant, K(eq), for the reaction Cl(H(2)O)(-) + D(2)O right harpoon over left harpoon Cl(D(2)O)(-) + H(2)O is 0.76, whereas the three theoretical predictions are 0.946, 0.979, and 1.20. Similarly, the experimental K(eq) for the Cl(H(2)O)(-) + HDO right harpoon over left harpoon Cl(HDO)(-) + H(2)O reaction is 0.64 as compared to theoretical values of 0.972, 0.998, and 1.10. Although Cl(H(2)O)(-) has a large degree of anharmonicity, K(eq) values calculated with the harmonic oscillator rigid rotator (HORR) approximation agree with the accurate treatment to within better than 2% in all cases. Results of a variety of electronic structure calculations, including coupled cluster and multireference configuration interaction calculations, with either the HORR approximation or with anharmonicity estimated via second-order vibrational perturbation theory, all agree well with the equilibrium constants obtained from the analytical surfaces.

A critical evaluation of the experimental and theoretical determination of lithium cation affinities

A comprehensive comparison of multiple levels of theory with experimental values for about thirty lithium cation affinities (LCAs) is conducted. The experimental values are largely taken from threshold collision-induced dissociation (TCID) measurements augmented with equilibrium measurements from ion cyclotron resonance experiments. Possible reasons for errors in the experimental TCID results are explored. An examination of the theoretical results reveals that core correlation on the lithium ion(Li-C) is needed to accurately describe these complexes. Several procedures for assessing complete basis set (CBS) extrapolations from MP2(full)/aug-cc-pVnZ(Li-C)//MP2(full)/cc-pVDZ(Li-C), n = D, T, and Q, calculations are completed and compared to experiment and lower levels of theory. It is found that LCAs calculated using CBS methods including core correlation are higher than most other methods and generally in good agreement with experimental values. Because the CBS approach requires use of the computationally intensive aug-cc-pVQZ(Li-C) basis set, we recommend that an adequate level of theory is provided by a MP2(full)/aug-cc-pVTZ(Li-C)//MP2(full)/cc-pVDZ(Li-C) approach excluding basis set superposition errors. Given these theoretical results, discrepant experimental values in the literature for several lithium ion complexes are evaluated.

Applying a new algorithm for obtaining site specific rate constants for H/D exchange of the gas phase proton-bound arginine dimer

A new algorithm has been developed for extracting site-specific rate constants for hydrogen/deuterium (H/D) exchange in gas phase protonated amino acids, their clusters and peptides. The algorithm minimizes the mutual entropy or the Kullback–Leibler information divergence between the observed concentrations and the model. Electrospray ionization-mass spectrometry (ESI-MS) results from fast flow tube and Fourier transform ion cyclotron resonance (FT-ICR) experiments, respectively, were modeled. The results for protonated glycine were in excellent agreement with previous literature data. Four rate constants were found, three of them identical corresponding to the three equivalent hydrogen atoms of the protonated amine group and a fourth higher one corresponding to the single carboxyl hydrogen. New results for the proton-bound dimer of arginine demonstrated a single high site-specific rate constant and fourteen low ones. These results are in agreement with the ion–zwitterion structure of (arginine) 2H + that has a single carboxyl hydrogen atom.

Cyclopentenyl cation: its thermochemistry and its characterized formation from C 6H 10[rad]+ species

The formation of cyclopentenyl cation from (i) dissociation of ionized cyclohexene, and, (ii) ion–molecule reaction between ionized 1,3-butadiene and ethylene is confirmed by means of the corresponding thermochemistry. Measurement of the Gibbs free energy and enthalpy for deprotonation of cyclopentenyl cation is provided by Fourier transform ion cyclotron resonance experiments and use of the thermokinetic method. The values (GB=795.0±1.2 and PA=824.9±1.1 kJ/ mol , respectively) allow the determination of the heat of formation of the cyclopentenyl cation to be 840.6±4.1 kJ/ mol in good agreement with existing data.

Ter-body intermediates in the gas phase: reaction of ionized enols with tert-butanol

In the gas phase, the CH 2CHOH ·+ enol radical cation 1 as well as its higher homologues CH 3CHCHOH ·+ 2 and (CH 3) 2CCHOH ·+ 3, undergo exactly the same sequence of reactions with tert-butanol, leading to the losses of isobutene, water and water plus alkene. Fourier transform ion cyclotron resonance (FT-ICR) experiments using labeled reactants as well as ab initio calculations show that independent pathways can be proposed to explain the observed reactivity. For ion 1, taken as the simplest model, the first step of the reaction is formation of a proton bound complex which gives, by a simple exothermic proton transfer, the ter-body intermediate [CH 2CHO ·, H 2O, C(CH 3) 3 +]. This complex, which was shown to possess a significant lifetime, is the key intermediate which undergoes three reactions. First, it can collapse to yield tert-butylvinyl ether with elimination of water. Second, by a regiospecific proton transfer, this complex can isomerize into three different ter-body complexes formed of water, isobutene and ionized enol. Within one of these complexes, which does not interconvert with the others, elimination of isobutene leads to the formation of a solvated enol ion. Within the others, a cycloaddition–cycloreversion process can proceed to yield the ionized enol 3 (loss of water and ethylene channel).

“Proton-transport” catalysis in the gas phase. Keto-enol isomerization of ionized acetaldehyde

Fourier transform ion cyclotron resonance experiments show that a variety of molecules catalyze the hydrogen transfer which converts ionized acetaldehyde CH 3CHO ·+ 1 to its vinyl alcohol counterpart CH 2CHOH ·+ 2. Each of these ions has been characterized by its specific bimolecular reactions with selected reactants. Calculations show that two pathways, for which the rate determining barriers have almost the same energy, are feasible. The first transition state involves a direct catalyzed 1,3-H transfer, while the second involves two successive 1,2-H transfers. A detailed experimental study, using methanol as a catalyst as well as labeled reactants, indicates that only the first pathway operates in the isomerization process. The different steps of these two independent pathways were elucidated. The first begins with the formation of a highly stabilized complex 3, involving a two-center-three-electron interaction between the two oxygen atoms and an interaction between a hydrogen of the methyl group of 1 and the oxygen of methanol. This complex isomerizes into a complex 4, which in turn gives the complex 5, via a transition state located 6.3 kcal mol −1 below the energy of the reactants. This complex 5 corresponds to ionized vinyl alcohol hydrogen bonded to the oxygen of methanol, which dissociates to yield ion 2. The second pathway begins with the interaction between the hydrogen of the CHO group and the oxygen of methanol and gives the complexes 6 and then 7, which correspond to protonated methanol hydrogen bonded to a CH 3CO · radical. Dissociation of 7 to give protonated methanol is favoured with respect to further isomerization leading to ionized vinyl alcohol. Compared to the unimolecular conversion between energetic ions 1 and 2, which can occur either by a direct 1,3-H transfer or by a double 1,2-H transfer, the reaction of 1 with methanol catalyzes the first pathway while inhibiting the second one. In the case studied, catalysis is perhaps better described as a hydrogen atom transport.

Gas‐phase stability of tertiary carbenium ions and rates of solvolysis of tertiary derivatives

The stability of tertiary carbenium ions was determined in the gas phase by ion cyclotron resonance (ICR) and by dissociative proton attachment (DPA). The rate constants for solvolysis of bridgehead derivatives correlate well with the stabilities of bridgehead carbenium ions, as determined by DPA and by ICR, but the ICR data of strained ions do not correlate, indicating rearrangements under the conditions of the ICR experiment. Simple acyclic tertiary derivatives solvolyze faster than predicted on the grounds of the stability of the respective carbenium ions. The effect of nucleophilic solvent participation on the rate of methanolysis of tertiary derivatives was investigated with (R)-3-chloro-3,7-dimethyloctane (17), which reacts with 77% inversion and 23% racemization. Copyright © 2000 John Wiley & Sons, Ltd.

Instrumentation and methodology for simultaneous excitation/detection of ions in an FTICR mass spectrometer

A method for direct and continuous detection of ion motion during different perturbation events of the fourier transform ion cyclotron resonance (FTICR) experiment is demonstrated. The modifications necessary to convert an ordinary FTICR cell into one capable of performing simultaneous excitation/detection (SED) using a capacitive network are outlined. With these modifications, a 200-fold reduction in the detection of the coupled excitation signal is achieved. This allows the unique ability not only to observe the response to the perturbation but to observe the perturbation event itself. SED is used successfully to monitor the ion cyclotron transient during single-frequency excitation, remeasurement and exciter-excite experiments.

The C 2H 7+ potential energy surface: a Fourier transform ion cyclotron resonance investigation of the reaction of methyl cation with methane

Fourier transform ion cyclotron resonance experiments have been carried out to probe the nature of the potential energy surface for the reaction of methyl cation (CH 3 +) with methane (CH 4) to give the ethyl cation (C 2H 5 +). Product distributions for reactions of CH 3 + and CD 3 + with CD 4 and CH 4, respectively, are found to give a near statistical distribution of ethyl cation products, in good agreement with previous work. When the methyl cation is initially coordinated to HF in the form of a methylfluoronium ion, however, the product distributions are decidedly nonstatistical and are indicative of a reaction which is very nearly thermoneutral. Thermochemical data for relevant species suggest that the reaction is very slightly endothermic. All of the experiments support the intermediacy of a C 2H 7 + complex in the reactions. Ab initio calculations, in conjunction with all of the experimental data, reveal that there are likely three different stable forms of C 2H 7 + involved in the potential energy surface for the reaction. One of the forms, a nonclassical C 2H 5 + coordinated to H 2 in a proton bound dimerlike structure, has not previously been considered to play a role in this reaction. The existence of this structure is supported by infrared multiphoton dissociation experiments on C 2H 7 + previously carried out by Lee and co-workers (J. Am. Chem. Soc. 111 (1989) 5597) and the high pressure mass spectrometric experiments of Hiraoka and Kebarle (J. Am. Chem. Soc. 98 (1976) 6119).

Two different pathways for unimolecular and for catalyzed keto-enol isomerization of ionized acetophenone

Studies of the unimolecular reactions in the gas phase of the C 6H 5COCH 3 +· ( 1) and C 6H 5C(OH)CH 2 +· ( 2) ions have shown (1) that ion 1 does not convert to ion 2 prior to methyl radical loss, (2) that ion 2 isomerizes into ion 1 prior methyl radical loss, and (3) that this keto-enol isomerization does not occur by a direct 1,3-H transfer but by two successive 1,4-H transfers. Fourier transform ion cyclotron resonance experiments show that acetone catalyses the isomerization 1 → 2. Further, by using labeled reactants, it is demonstrated that this isomerization occurs by a direct catalyzed 1,3-H transfer whereas the less energy demanding pathway connecting bare ions 1 and 2 is a double 1,4-H transfer. This represents the first description of a system for which the pathways connecting two isomeric ions are different for the unimolecular and for the catalyzed isomerizations.

Gas-phase stability of tertiary carbenium ions and rates of solvolysis of tertiary derivatives

The stability of tertiary carbenium ions was determined in the gas phase by ion cyclotron resonance (ICR) and by dissociative proton attachment (DPA). The rate constants for solvolysis of bridgehead derivatives correlate well with the stabilities of bridgehead carbenium ions, as determined by DPA and by ICR, but the ICR data of strained ions do not correlate, indicating rearrangements under the conditions of the ICR experiment. Simple acyclic tertiary derivatives solvolyze faster than predicted on the grounds of the stability of the respective carbenium ions. The effect of nucleophilic solvent participation on the rate of methanolysis of tertiary derivatives was investigated with (R)-3-chloro-3,7-dimethyloctane (17), which reacts with 77% inversion and 23% racemization. Copyright © 2000 John Wiley & Sons, Ltd.

Single- and Two-State Reactivity in the Gas-Phase C−H Bond Activation of Norbornane by `Bare' FeO+

The potential-energy surface for C−H bond activation of norbornane by `bare' FeO+ is examined at the B3LYP/6-31G** level of theory. The free reactants combine to form norbornane/FeO+ ion-dipole clusters in which the FeO+ unit can bind at either the exo or endo face of norbornane. The transition structures for insertion of FeO+ into the exo and endo C−H bonds are located at least 9 kcal⋅mol−1 below the entrance channel, thus accounting for the observed unit efficiency of the C−H bond activation reported in previous gas-phase ion-cyclotron resonance experiments (Helv. Chim. Acta1995, 78, 1013). Interesting features of the reaction profiles are crossovers of the high-spin sextet (S=5/2) and low-spin quartet (S=3/2) states en route to the transition structures (TS); this type of behavior has been termed two-state reactivity (Helv. Chim. Acta1995, 78, 1393). The branchings between the endo and exo pathways are simulated by Rice-Ramsperger-Kassel-Marcus (RRKM) theory with the calculated harmonic frequencies. Additionally, hydrogen/deuterium kinetic isotope effects are computed using RRKM theory and compared with the experimental data. The simulated KIEs differ for high-spin and low-spin TSs, suggesting that isotope effects can be used as sensitive probes for diagnosing spin-crossover mechanisms.

Stability of a highly charged noncovalent complex in the gas phase: holomyoglobin.

Gas phase holomyoglobin (hMb) ions in charge states +7 to +21 were formed by electrospray ionization in combination with a continuous-flow mixing apparatus. Collision cross section measurements show that the highly charged ions are somewhat unfolded in comparison to low charge states but still retain a considerable degree of folding. A new collision model is presented which calculates the relative energies transferred to complexes in tandem mass spectrometry. Tandem mass spectrometry and ion trapping experiments both show that the energies required to dissociate heme from the highly charged heme-protein complexes in the gas phase are similar to those of low charge states, previously shown in literature ion cyclotron resonance experiments to be 0.7-1.0 eV. These energies are comparable to those of the heme binding energy in solution. The results suggest that even for the highly charged hMb ions which have unfolded somewhat, the heme-protein interactions remain relatively unperturbed.