Transform Ion Cyclotron Resonance Study Research Articles

Chemistry of (and on) Transition Metal Clusters: A Fourier Transform Ion Cyclotron Resonance Study of the Reaction of Niobium Cluster Cations with Nitric Oxide

The reactions of niobium cluster cations, Nb(+)(n) (n = 2-19), with nitric oxide have been investigated using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR). The overall reaction rate constants are found to be in reasonable agreement with collision rates calculated using the surface charge capture model. The dominant reaction for small clusters (n <9) involves reaction-induced fragmentation resulting in the loss of either NbO or NbN. By contrast, the main reaction observed for the larger clusters (n> 11) is sequential NO chemisorption. Clusters n = 9, 10 exhibit both extremes of behaviour and are the only clusters upon which there is evidence of NO decomposition with N(2) loss observed whenever multiple NO molecules are co-adsorbed. The rate constants for each process have been determined as a function of cluster size.

Radical cation/radical reactions: A Fourier transform ion cyclotron resonance study of allyl radical reacting with aromatic radical cations

A method for the study of reactions of open-shell neutrals (radicals) and radical cations is described. Pyrolysis (25–1500 °C) of thermally labile compounds, such as, 1,5-hexadiene via a Chen nozzle yields a seeded beam of reactive species in helium. The pyrolysis products are then analyzed by electron ionization (EI) or reacted with stored ions. Electron ionization of the pyrolysis products of 1,5-hexadiene shows that both the allyl radical and allene are generated. Reactions of benzene radical cations and the pyrolysis products of 1,5-hexadiene result in carbon–carbon bond formation. Those reactions of allyl radical with the benzene radical cation yield the C 7H 7 + ion of m/ z 91, permitting an unusual entry into arenium ions. The reaction of allene with benzene radical cation in contrast yields C 9H 10 + and C 9H 9 +.

Understanding of the Structure, Bonding, Formation and Decomposition of Metalloid Aluminum Clusters—A Fourier Transform Ion Cyclotron Resonance Study of Solid AlCp*

The investigation of the tetrahedral Al4Cp*4 cluster with Fourier transform ion cyclotron resonance mass spectrometry using laser desorption /ionization as the ionization method results in a couple of aluminum cluster compounds AlxCp*+y (x > y) with up to eight aluminum atoms. MS(n) experiments are performed in order to understand the formation and reactivity of the different species. Quantum chemical calculations of the structure and energy for all compounds identified complete this study.

Reactions of O −· with methyl benzoate: a negative ion chemical ionization and Fourier transform ion cyclotron resonance study

The reactions of O −· with methyl benzoate have been examined by the measurement of negative ion chemical ionization (NICI) mass spectra using a CI source, with confirmatory studies carried out on a Fourier transform ion cyclotron resonance mass spectrometer. Reaction mechanisms have been elucidated using isotopically labeled esters. Nucleophilic attack at the carbonyl carbon and the aromatic ring were important reaction pathways. Nucleophilic attack at the carbonyl carbon was followed by the production of products (C 6H 5CO 2 − and CH 3OCO 2 −) characteristic of radical, β-fragmentation. Using 18O-labeled methyl benzoate, the S N2 reaction was found to account for a smaller percentage, 21(±1)%, of the benzoate product. Aromatic ring attack resulted in formation of [M + O − H] − and [M − 2H] −· ions. Although aryl hydrogens accounted for most H 2 +· abstracted by O −·, evidence for abstraction of H arylH alkyl +· and H alkylH alkyl +· was also found. Although present at much lower abundance, dehydrobenzoate, dehydrophenoxy, and C 7H 6 −· ([M − 2H − CO 2] −·) radical anions were also observed. An H aryl/H alkyl exchange associated with formation of the benzoate anion was attributed to an H alkyl abstraction that occurred within the methanol/dehydrobenzoate ion–dipole complex. The [M − 2H] −·, dehydrobenzoate, dehydrophenoxy, and [M − 2H − CO 2] −· ion signals were quenched by reaction with O 2. Conditions required for production of O −· spectra under NICI conditions were also examined.

Differences between positive and negative ion stabilities of metal–sulfur cluster proteins: an electrospray ionization fourier transform ion cyclotron resonance study

The stability of iron–sulfur proteins during electrospray ionization in both positive ion and negative ion modes was investigated using Fourier transform ion cyclotron resonance mass spectrometry. Positive ion and negative ion mode mass spectra of iron and zinc rubredoxin from Clostridium pasteurianum and ferredoxins from Pyrococcus furiosus containing [3Fe–4S], [4Fe–4S], [3FeNi–4S], and [3FeMn–4S] clusters are compared. The results demonstrate that all clusters are stable as negative ions, whereas only the [4M–4S], (M = Fe, Mn, Ni) clusters are stable as positive ions. This is the first direct evidence of the existence of the [3FeMn–4S]-containing cluster from Pyrococcus furiosus. The formal oxidation state of each metal–sulfur cluster is determined by the mass-to-charge measurement, and is found to be the same for both positive and negative ions.

A Fourier Transform Ion Cyclotron Resonance Study of theTemperature and Isotope Effects on the Kinetics of Low-Pressure Association Reactions of Protonated Dimethyl Ether with Dimethyl Ether

The temperature dependence of the low-pressure association reaction of dimethyl ether with protonated dimethyl ether has been investigated using Fourier transform ion cyclotron resonance mass spectrometry. The unimolecular dissociation of nascent proton-bound dimers is complicated by two factors: (1) the presence of another unimolecular dissociation route producing trimethyloxonium cation and methanol through a highenergy isomer of the proton-bound dimer and (2) the presence of at least two high-energy isomers of the proton-bound dimer en route to dissociation of the nascent proton-bound dimer. RRKM modeling of the experimental temperature dependence of the unimolecular dissociation of nascent proton-bound dimers strongly suggests that dissociation of the nascent proton-bound dimer proceeds through a high-energy isomer. The possible existence of such species is also shown by ab initio calculations. The original mechanism for the ion/molecule reaction and analysis of radiative association kinetics used in the past was found to be too simple for accurate modeling of the reaction between protonated dimethyl ether and dimethyl ether. A slightly more complicated mechanism is proposed which more accurately accounts for the temperature dependence of the unimolecular dissociation to re-form reactants. As well, three isotopomeric variants of the protonated dimethyl ether/dimethyl ether reaction were examined experimentally and theoretically.

Insights into the C(4)H(8)(+.) potential energy surface: fourier transform ion cyclotron resonance studies of ion-molecule reactions of D-labeled partners

Reactions of [ethylene](+.) with ethylene and of [acetylene](+.) with ethane were studied by Fourier transform ion cyclotron resonance spectrometry using labeled reactants. The results confirm and clarify the different steps of the mechanism proposed previously and elaborated with other methods. The [[acetylene](+.), ethane] system can either dissociate to give the ethyl cation product, or isomerize into [[ethylene](+.), ethylene]. The latter system can either dissociate to yield ionized ethylene or convert into ionized but-2-ene, which undergoes a complete H-exchange prior to dissociation, leading to methyl radical, hydrogen radical and ethylene losses. The transfers of labeled atoms and the existence of H-exchange prior to formation of the products were used as a probe to check the different steps of the mechanism. The influence of the initial energy of the system on the reaction pathway is discussed. Copyright 1999 John Wiley & Sons, Ltd.

Insights into the C4H8+· potential energy surface: Fourier transform ion cyclotron resonance studies of ion–molecule reactions of D-labeled partners

Reactions of [ethylene]+· with ethylene and of [acetylene]+· with ethane were studied by Fourier transform ion cyclotron resonance spectrometry using labeled reactants. The results confirm and clarify the different steps of the mechanism proposed previously and elaborated with other methods. The [[acetylene]+·, ethane] system can either dissociate to give the ethyl cation product, or isomerize into [[ethylene]+·, ethylene]. The latter system can either dissociate to yield ionized ethylene or convert into ionized but-2-ene, which undergoes a complete H-exchange prior to dissociation, leading to methyl radical, hydrogen radical and ethylene losses. The transfers of labeled atoms and the existence of H-exchange prior to formation of the products were used as a probe to check the different steps of the mechanism. The influence of the initial energy of the system on the reaction pathway is discussed. Copyright © 1999 John Wiley & Sons, Ltd.

A fourier transform ion cyclotron resonance study of two- and one-electron capture reactions between doubly charged rare gas ions and rare gas atoms

A Fourier transform ion cyclotron resonance (FTICR) mass spectrometer has been used to study near thermal collision energy symmetric charge transfer reactions. The systems Xe n+ /Xe, Kr n+ /Kr and Ar n+ /Ar have been measured at ambient temperature, where n = 1, 2. In the interactions between doubly charged rare gas ions and their corresponding neutral gas atoms the double electron capture (DEC) has been observed to be more efficient than the single electron capture (SEC). From the measured overall rate coefficient of 3.4 × 10 −10 cm 3 s −1 and the branching ratio of 1.1 between the DEC reaction and the SEC reaction the rate coefficients for double and single electron transfer from Xe to Xe 2+ have been calculated to be 1.8 × 10 −10 cm 3 s −1 and of 1.6 × 10 −10 cm 3 s −1. In the experiments with doubly charged krypton and doubly charged argon ions in their parent gases the symmetric double charge transfer is observed to be even more favored in comparison with the Xe 2+/Xe system at near thermal collision energies.

Fourier transform ion cyclotron resonance study of multiply charged aggregates of small singly charged peptides formed by electrospray ionization

Aggregates of singly protonated peptides formed with a nanoelectrospray ion source have been observed in the gas phase using Fourier transform ion cyclotron resonance (FT-ICR). Employment of “soft” ion sampling conditions in the source, which were developed previously to generate water clusters of biomolecules, provides significant yields of aggregates of singly protonated GGDPG ([2GGDPG + 2H] 2+), GGEPG ([2GGEPG + 2H] 2+), and VEPIPY (2VEPIPY + 2H] 2+). With peptide mixtures, heteroaggregates, e.g., [GGDPG + GGEPG + 2H] 2+ have also been observed along with the homoaggregates. These weakly bound noncovalent complexes undergo facile exothermic dissociation into the corresponding singly protonated monomer species with normal operation of the electrospray ion source. For example, the aggregates were not observed in FT-ICR experiments utilizing a conventional electrospray ionization (ESI) or fast atom bombardment source or with a quadrupolar ion trap mass spectrometer equipped with a conventional ESI source. The formation and metastability of these aggregates are dependent on highly specific intermolecular hydrogen bonding between the monomers. The amino acid sequence (DPG) of GGDPG mimics the well-known β reverse turn of proteins and semiempirical calculations show that it provides excellent hydrogen bonding sites for a protonated N-terminus amino group. Support for this conjecture is provided by the failure to observe aggregate formation of singly protonated peptides with several larger peptides, including hexaglycine and hexaalanine.

A tandem mass spectrometry and Fourier transform-ion cyclotron resonance study of ionized ethylated acetone and deuterated analogs

The unimolecular dissociation of (CH3)2C+OC2H5 ions (I) and their deuterated analogs, generated by ion-molecule reactions (IMR) in acetone-ethyl iodide mixtures was studied by tandem mass Spectrometry methods. Two significant processes that yielded I ions were identified. The Fourier transform ion cyclotron resonance study showed that the reaction between ionized ethyl iodide and neutral acetone was the principal source of I. This process involved the formation of the stable mixed ionized dimer, [C2H5I·O=C(CH3)2]+· (II), which dissociated by the loss of an I atom. Other important fragmentation pathways of II were the formation of C2H5I+·, (CH3)2CO+·; and (CH3)2COI+ and the loss of CH3CHI·. The major dissociation of I was the loss of C2H4. The activation energy for this reaction was determined by metastable ion appearance energy measurements to be ∼55 kJ mol−1 above the thermochemical minimum. The analysis of the metastable and collision-induced dissociation of D-labeled I showed an unusual time-energy effect on the degree of H/D mixing, with the highest selectivity for the ethene loss [β-H(D)-atom shift] being observed for ions with the lowest internal energies. Collisional excitation could not produce significant H/D mixing among dissociating ions. The results were rationalized by the existence of two species— the classical (2-ethoxypropyl) and nonclassical (proton-bound acetone-ethene pair) isomers of I. The classical structure was originally formed by IMR or from II. The energy barrier for the classical to nonclassical isomerization lay well above the thermochemical threshold for C2H4 loss, providing only limited H-atom mixing in nonclassical ions that were always formed in their dissociative state. The effect of the proton affinity of the carbonyl compound on the H/D mixing in RR′C+OC2H5 ions was studied. It was shown that the selectivity for the ethene loss (β-H-atom shift) generally increased with the increase of the proton affinity of RR′CO. Neutralization-reionization mass spectrometry was applied to a study of (CH3)2C+OR ions, where R = H, I, C2H5. The observation of a recovery signal for the ion I was attributed to the formation of the 2-ethoxypropyl radical. Neutral counterparts of (CH3)2COI+ ions were also generated, being the first example of IO-substituted alkyl radicals.

Lithium-cation and proton affinities of sulfoxides and sulfones: A fourier transform ion cyclotron resonance study

The kinetic method was used for the quantitative determination of lithium-cation affinities by Fourier transform ion cyclotron resonance. This method was applied to a series of XYSO and XYSO2 compounds. Proton basicities of the SO and SO2 compounds were also determined. When comparison is made between Li+ basicities and proton basicities, a linear regression encompassing XYSO and XYSO2 families suggests that Li+ may be bonded in a similar way to the SO and SO2 moieties, that is, to only one oxygen on the latter. PM3 calculations support this hypothesis.

Is there a hydride transfer between N2OH+ and saturated hydrocarbons?

Various reaction mechanisms for the reaction between N 2 OH + and CH 4 have been explored theoretically at the MP2 6-31G ∗∗ level. Also computed were the structure and thermodynamic state functions for several neutral molecules, notably N 2 O, CH 4 and C 2 H 6 and ions (CH 5 + , C 2 H 5 + , C 2 H 7 + , N 2 OH + ). These results were combined with a Fourier transform ion cyclotron resonance study of the reactions of N 2 OH + with CH 4 or C 2 H 6 . Both studies suggest that the title hydride transfer, while thermodynamically favorable, is not Kinetically preferred under thermal conditions in the gas phase.

Fourier transform ion cyclotron resonance studies of terpyridine based polyether dendrimers and their iron(II) metallocomplexes by liquid secondary ion mass spectrometry

A detailed analysis is presented of the fragmentation patterns of a series of terpyridine-based polyether dendritic ligands (L) and their corresponding iron(II) complexes [Fe(L) 2][PF6]2 recorded under typical liquid secondary ion mass spectrometry (L-SIMS) conditions. Fragmentation behaviour of these compounds is characterised and is well-correlated to their structural features. In addition to the unexpected reduction phenomenon of the iron(II) metal centre, there also exists a high-order of multiple charging for both the free dendritic ligands and the corresponding metallodendrimers. Molecule-ions with up to ten charges have been observed for metallodendrimers. From the comparison of mass spectral features of the free dendritic ligands and the corresponding metallodendrimers, it is concluded that the observed high-order of multiple charging originates from the multiple loss of electrons from the terpyridine core of the molecules.

Fourier Transform Ion Cyclotron Resonance Studies of Gas-Phase Reactions between Tungsten Ions and Hydrocarbons

Summary: FT ICR spectrometry has been used for gas-phase studies of the interaction between singly-charged positive tungsten ions and hydrocarbons (alkanes C2-C9, alkenes C2-C6, and cycloalkanes C3-C6). It is shown that this interaction results mainly in dehydrogenation products. The number of H2 molecules eliminated depends upon the size and type of hydrocarbon and varies from 1 to 6. Significant amounts of C-C cleavage products have been found in the cases of cyclopropane and 2-methylpropene only.

Fourier transform ion cyclotron resonance study of the dehydrogenative polymerization of allene and propyne induced by W+ in the gas phase

AbstractA Fourier transform ion cyclotron resonance study of the gas‐phase reaction of W+ with allene is reported. W+ successively dehydrogenates at least nine allene molecules, leading to the formation of a series of WC3nH ions, with WC27H as the largest product observed after a reaction delay of 40 s. Starting from the third reaction, there is a competition between elimination of H2 and C2H2 at each step. The activation of propyne was also investigated and found to lead to essentially the same sequence as with allene. The reaction of W+ with 1,1‐d2‐allene and collision‐induced dissociation spectra of product ions are used to discuss a plausible mechanism for the formation and structures of some of the species observed.

Fourier transform-ion cyclotron resonance study of the gas-phase acidities of germane and methylgermane; bond dissociation energy of germane

An accurate gas-phase acidity for germane (enthalpy scale, equivalent to the proton affinity of GeH − 3), δ H° acid(GeH 4) = 1502.0 ± 5.1 kJ mol −1, is obtained by constructing a consistent acidity ladder between GeH 4 and H 2S by using Fourier transform-ion cyclotron resonance spectrometry, and 0 and 298.15 K values for the first bond dissociation energy of GeH 4 are proposed: D° 0(H 3G3-H) = 352 ± 9 kJ mol −1; D°(H 3Ge-H) = 358 ± 9 kJ mol −1, respectively. These results are compared with experimental and theoretical data reported in the literature. Methylgermane was found to be a weaker acid than germane by approximately 35 kJ mol −1: δ H° acid (MeGeH 3) = 1536.6 kJ mol −1.

Fourier transform ion cyclotron resonance study of ion‐molecule reactions of [M – OCH3]+ ions of methyl 2,3,4,6‐tetra‐O‐methyl‐d‐hexopyranosides with ammonia

AbstractGlycosidic oxocarbenium ions A1+ were formed by isobutane chemical ionization from methyl 2,3,4,6‐tetra‐O‐methyl‐β‐D‐mannopyranoside, methyl 2,3,4,6‐tetra‐0‐methyl‐β‐D‐galactopyranoside and methyl 2,3,4,6‐tetra‐O‐methyl‐β‐D‐glucopyranoside (the ring ‐ O‐being converted into  O  ), and then‐ reaction with ammonia was studied by Fourier transform ion cyclotron resonance Spectrometry. Very slow formation (reaction efficiency 0.6‐1.4%) of the adduct ion [A1 + NH3]4 was observed as the main process for carefully thermalized ions A1+. Interestingly, the efficiency of the adduct ion formation depends on the sterochemistry of ions A1+.

On the structure of protonated methane

Results of a Fourier transform-ion cyclotron resonance study are reported concerning the reactivity of protonated perdeuteromethane and deuteronated methane, generated under varying pressure conditions in an external chemical ionization ion source, toward ammonia. The competition between proton and deuteron transfer from both protonated perdeuteromethane and deuteronated methane to ammonia exhibits chemically distinguishable hydrogens. The chemical behavior of protonated methane appears to be compatible with the theoretically predicted stable structure with C S symmetry, involving a three-center two-electron bond associating two hydrogens and the carbon atom. Interconversion of this structure due to exchange between one of these hydrogens and one of the three remaining hydrogens appears to be a fast process that is induced by interactions with the chemical ionization gas.

A fourier transform-ion cyclotron resonance study of steric effects on proton transfer reactions of polyalkyl benzenes

The rates of deprotonation, MH + + B → M + BH + (M = 1, 2), and the rates of protonation, M + BH + → MH + + B (M = 2, 3, 4), have been measured by Fourier transform-ion cyclotron resonance (FT-ICR) spectrometry for the increasingly sterically crowded alkyl benzenes ethylbenzene, 1; 1,3,5-trimethylbenzene (mesitylene), 2; 3,5-di( t-butyl)toluene, 3; 1,3,5-tri( t-butyl) benzene, 4; and a series of n-bases B. In addition, the gas phase basicities of 3 and 4 were determined by proton transfer equilibrium measurements using FT-ICR. The efficiencies of deprotonation and protonation are always large (> 50%) for exoergic reactions and increase with increasing exoergicity, but never reach unit efficiency. For each of the alkylated benzenes 1– 4 the efficiency decreases steeply at the border between exoergic and endoergic reactions. This is the “normal” behaviour of proton transfer reactions and shows the absence of steric effects for proton transfer to and from alkylated benzenes even in the case of 3 and 4 substituted by bulky t-butyl groups, and is in complete contrast to the behaviour of t-butylated pyridines. This difference is explained by different geometries of the transition state for the proton transfer reactions in the case of the alkylated pyridines and alkylated benzenes, and by the critical role of π-complexes in proton transfer reactions of benzene derivatives.