Dimer Ions Research Articles

Ion chain mechanism of catalytic alkylation of 2,6-Di-tert-butylphenol by methyl acrylate

Phenoxides based on sterically hindered phenols and alkali metals are used in the synthesis and industrial production of a number of compounds applied in the production of antioxidants [1]. The shielding of the reaction center by the tert -butyl groups gives rise to the specific properties inherent in sterically hindered phenoxides, which are related to negative charge delocalization in the six-membered ring and the formation of a coordination bond of the metal cation with the oxygen atoms and carbon atoms of the six-membered ring. These features are responsible for the ability of potassium ( 1 ) and sodium ( 2 ) 2,6-di- tert -butylphenoxides to form supramolecular structures as contact or solventseparated ion pairs and dimers [2]. Similar structures based on alkali metal cations and radical anions have been studied previously by EPR [3]. The above data suggest that a nonsolvated cation may be formed in a matrix comprising 2,6-di- tert -butylphenol ( 3 ), methyl acrylate ( 4 ), and catalytic amounts of monomer 1 or 2 . The course of alkylation of phenol 3 with methyl acrylate in the presence of catalytic amounts of monomer 1 or 2 differs from that in the presence of the corresponding phenoxide dimers (associates). In this work, we found that, with a nearly equimolar mixture of 3 and 4 , the catalytic alkylation in the presence of 1.5‐3 mol % of monomer 1 or 2 proceeds at higher rates than those known previously (in the presence of dimers 1 or 2 ) [2‐5] and gives the alkylation product, methyl 3-(4-hydroxy-3,5-di- tert -butylphenyl)propionate ( 5 ), in up to 98% yield. The formation of the monomer species was supported by thermoelectric measurements of the molecular weights of phenoxides 1 and 2 at 115 ° C. The kinetic studies for this reaction in the range 105‐130 ° C showed that the pattern of the kinetic curve does not depend on the reaction temperature (Fig. 1), which may imply a tunneling effect in the catalytic alkylation mechanism. The monomer of phenoxide 1 is formed upon the reaction of phenol 3 with potassium hydroxide at 180‐200 ° C. On cooling to 20 ° C, the monomer is converted into dimer 1 . In the experiment, a mixture of 1 and 3 was cooled to 135‐ 120 ° C and compound 5 was simultaneously added. This gave a reaction mass in which phenoxide 1 or 2 was mainly uniformly distributed throughout the mixture of phenol 3 and acrylate 4 and the reaction between these compounds proceeded under steady-state conditions in a thermostat without stirring the reactants. By sampling the reaction mixture during the process and analyzing the samples by liquid chromatography, the contents of 3 , 5 , and dimethyl α -(4-hydroxy-3,5-ditert -butylbenzyl)glutarate ( 6 ) in the reaction mixture were determined. The yield of 6 in the experimental series did not exceed 1%. Figure 1 shows also the kinetic data for the reaction of phenol 3 with methyl acrylate in the presence of catalytic amounts of dimer 1 , which coincide with published data [2].

Gas-phase acidities of the 20 protein amino acids

The gas-phase acidities of the 20 protein amino acids (PAAs) have been determined using an electrospray ionization–quadrupole ion trap instrument. Three different methods were used to determine both the absolute acidities and the relative acidity ordering of the PAAs. The extended kinetic method was used to determine absolute acidities for all 20 PAAs with substituted carboxylic acids and substituted phenols as reference acids. Acidities were obtained with an average uncertainty of ±10 kJ/mol, which is large compared to some of the differences between amino acids with similar acidities. To determine the relative acidity ordering, single-reference kinetic method experiments were performed using both the reference acids from the absolute acidity studies and tryptophan and threonine as reference acids. Additional ordering information was obtained from kinetic method experiments in which proton-bound dimer ions comprising pairs of amino acids were generated and dissociated in the ion trap. The recommended acidity ordering is Gly < Ala < Pro < Val < Leu < Ile < Lys < Trp < Phe < Tyr < Met < Ser < Thr < Cys < Gln < Gln < Arg < Asn < His < Glu < Asp. Isodesmic acidity values were also obtained at the B3LYP/6-311++G**//B3LYP/6-31+G* level of theory with acetic acid as the reference acid. The theoretical acidities are in excellent agreement with the absolute acidities obtained from the extended kinetic method studies. The calculations predict that the preferred isomer for protonated cysteine and tyrosine is not a carboxylate anion, but rather a thiolate anion and a phenoxide anion, respectively.

Relative calcium‐binding strengths of amino acids determined using the kinetic method

We have measured the relative calcium-binding energies of amino acids using tandem mass spectrometry of Ca(2+)-bound trimeric amino acids. Although calcium-bound dimeric amino acid complexes coordinated too strongly to allow observation of the two competing dissociation products (calcium-bound monomeric ions) required for analysis of their metal binding affinities using the conventional kinetic method, the Ca(2+)-bound trimeric cluster ions dissociated readily to form dimeric cluster ions through simple ligand losses. The calcium-binding energies were obtained by comparing the ratio of the [Ca(2+)(A(1))(2) - H(+)](+) and [Ca(2+) (A(1))(A(2)) - H(+)](+) ions that dissociated from the [Ca(2+) (A(1))(2)(A(2)) - H(+)](+) ion and the ratio of the [Ca(2+)(A(2))(2) - H(+)](+) and [Ca(2+)(A(1)) (A(2)) - H(+)](+) ions that dissociated from the [Ca(2+)(A(1))(A(2))(2) - H(+)](+) ion, where A(1) and A(2) represent two amino acids. The energies deduced from this analysis represent the relative average binding energies of complexes having the form [Ca(2+)(A(1))(2) - H(+)](+). The relative Ca(2+)-binding strengths of the alpha-amino acid complexes follow the order Cys < Ser < Thr < Ile < Leu < Val < Gly < Ala < Pro < Phe < Met < Tyr < Asn < His < Gln < Trp < Lys < Arg. To our knowledge, this report provides the first example of using kinetic methods to determine the relative binding strengths of divalent metal-amino acid complexes.

On the role of the II(1/2g) state in spontaneous dissociation of krypton and xenon dimer ions

We have measured kinetic-energy-release distributions (KERD) for spontaneous dissociation of electronically excited dimer ions of krypton and xenon, formed by electron impact ionization of neutral precursors. The data cannot be reconciled by decay of the strongly bound II(1/2u) state that successfully explains dissociation of Ne 2 + and Ar 2 + . Instead, the KERD is dominated by contributions from the weakly bound II(1/2g) state that has so far escaped a convincing experimental characterization. The present data can be utilized to assess the accuracy of ab initio potential energy curves of this state.

A chemical ionization mass spectrometry technique for airborne measurements of ammonia

A chemical ionization mass spectrometer (CIMS) utilizing protonated acetone dimer ion chemistry to measure gas‐phase ammonia (NH3) from the NOAA WP‐3D aircraft is described. The average sensitivity determined from in‐flight standard addition calibrations ranged from 2.6 to 5 ion counts s−1 pptv−1, depending on flow conditions, for 1 MHz of reagent ion signal. The instrument time response was determined to be 5 s from the 2 e‐folding signal decay time after removal of a standard addition calibration. The instrumental background varied from flight to flight ranging from 0.5 to 1.3 ppbv. The variability between successive background measurements ranged from 50 pptv to 100 pptv. Total uncertainty for the 5 s data was conservatively estimated to be ±(30% + 125 pptv). Two NH3 sources were sampled during the New England Air Quality Study–Intercontinental Transport and Chemical Transformation (NEAQS‐ITCT) 2004 campaign, one urban and one agricultural. During the 25 July flight, enhancements in NH3 mixing ratios were coincident with enhancements in CO, NOx, and SO2 mixing ratios downwind of New York City. The NH3 mixing ratios in the urban outflow plume ranged from 0.4 to 1 ppbv, or enhancements of 0.2 to 0.8 ppbv above local background. During the 15 August flight, NH3 mixing ratios were enhanced 0.3 to 0.45 ppbv above local background directly downwind of an agricultural area northeast of Atlanta, Georgia. The NH3 CIMS instrument has shown the ability to measure sub‐ppbv NH3 levels at high time resolution from an aircraft.

Ionization and fragmentation of solid C60 by femtosecond laser ablation

Ionization and fragmentation of solid C(60) dispersed on a silicon plate are investigated by femtosecond laser ablation. Bimodal mass distribution with large fragment ions C(60-2n) (+) (0< or =n< or =11) and small fragment ions C(n) (+) (13< or =n< or =28), formation of dimer ion (C(60))(2) (+), and delayed ionization of C(60) have been observed as reported in gas phase experiments with nanosecond laser excitation. Metastable dissociation of small fragment ions C(n) (+) has been observed for the first time, which suggests different structures of fragment ions compared with those of well-studied carbon cluster ions. From these observations, strong coupling of laser energy to electronic degrees of freedom of solid C(60) has been revealed for femtosecond laser ablation as compared with excitation in the gas phase.

Diastereoselective Synthesis of 4,5‘-Bis-proline Compounds via Reductive Dimerization of N-Acyloxyiminium Ions

A short, practical synthesis of novel, unsymmetrical 4,5'-bis-proline compounds has been achieved, highlighted by the application of an unprecedented samarium diiodide-driven regio- and diastereocontrolled reductive dimerization of N-acyloxyiminium ions generated from readily available 2-methoxy-5-silyloxymethyl-N-Boc pyrrolidines. The title proline dimers proved to be pertinent metal-free catalysts in aldol and Mannich reactions.

The acrylonitrile dimer ion: A study of its dissociation via self-catalysis, self-protonation and cyclization into the pyrimidine radical cation

Large energy barriers prohibit the rearrangement of solitary acrylonitrile ions, CH 2 CHC N +, into their more stable hydrogen-shift isomers CH 2 C C NH + or CH CH–C NH +. This prompted us to examine if these isomerizations occur by self-catalysis in acrylonitrile dimer ions. Such ions, generated by chemical ionization experiments of acrylonitrile with an excess of carbon dioxide, undergo five dissociations in the μs time frame, as witnessed by peaks at m/ z 53, 54, 79, 80 and 105 in their metastable ion mass spectrum. Collision experiments on these product ions, deuterium labeling, and a detailed computational analysis using the CBS-QB3 model chemistry lead to the following conclusions: (i) the m/ z 54 ions are ions CH 2 CHC NH + generated by self-protonation in ion–dipole stabilized hydrogen-bridged dimer ions [CH 2 CHC N⋯H–C(C N)CH 2] + and [CH 2 CHC N⋯H–C(H)C(H)C N] +; the proton shifts in these ions are associated with a small reverse barrier; (ii) dissociation of the H-bridged ions into CH 2 C C NH + or CH CH–C NH + by self-catalysis is energetically feasible but kinetically improbable: experiment shows that the m/ z 53 ions are CH 2 CHC N + ions, generated by back dissociation; (iii) the peaks at m/ z 79, 80 and 105 correspond with the losses of HCN, C 2H 2 and H , respectively. The calculations indicate that these ions are generated from dimer ions that have adopted the (much more stable) covalently bound “head-to-tail” structure [CH 2 CHC N–C(H 2)C(H)C N] +; experiments indicate that the m/ z 79 (C 5H 5N) and m/ z 105 (C 6H 6N 2) ions have linear structures but the m/ z 80 (C 4H 4N 2) ions consist of ionized pyrimidine in admixture with its stable pyrimidine-2-ylidene isomer. Acrylonitrile is a confirmed species in interstellar space and our study provides experimental and computational evidence that its dimer radical cation yields the ionized prebiotic pyrimidine molecule.

Structural and magnetic dimers in the spin-gapped systemCuTe2O5

We investigated the magnetic properties of the system CuTe2O5 by susceptibility and electron spin resonance measurements. The anisotropy of the effective g-factors and the ESR linewidth indicates that the anticipated structural dimer does not correspond to the singlet-forming magnetic dimer. Moreover, the spin susceptibility of CuTe2O5 can only be described by taking into account interdimer interactions of the same order of magnitude than the intradimer coupling. Analyzing the exchange couplings in the system we identify the strongest magnetic coupling between two Cu ions to be mediated by super-super exchange interaction via a bridging Te ligand, while the superexchange coupling between the Cu ions of the structural dimer only results in the second strongest coupling.

Electron–Ion-Coincidence Measurements for K-Shell Excited Free Krypton Clusters

The decay processes of inner-core excited krypton atom and free krypton clusters up to = 1640, being the average cluster size, have been studied by means of electron ion coincidence (EICO) measurements utilizing hard X-ray. Unlike the soft X-ray absorption measurements, highly charged daughter ions are observed. With increasing cluster size, the relative abundance of the multiply charged ions decreases and that of singly charged dimer or trimer ions increases. The results may be interpreted if one takes site-dependent decay processes into account: when the core hole is generated on the cluster surface, the proliferated holes due to the vacancy cascade are strongly localized within the X-ray absorbing atom, while the charges are migrated to surrounding atoms when the X-ray absorption occurs inside the cluster.

Dimeric Sandwich‐like Ion Pairs in the Crystal Structure of Tetrabutylammonium Ozonide Ammoniate

AbstractThe novel ionic ozonide {[N(C4H9)4](O3)}4·4.75NH3 was synthesized by ion‐exchange reaction in liquid ammonia. The crystal structure was determined by single crystal diffraction at 100 K (monoclinic space group P21, a = 15.014(11) Å, b = 13.696(10) Å, c = 19.890(15) Å, β = 105.407(12)°, V = 3943(5) Å3, Z = 2). The structure consists of a packing of sandwich‐like dimeric ion pairs in which two ozonide anions are interspersed between two tetrabutylammonium cations. Ammonia molecules from the solvent are localized in cavities in the structure. They are involved in hydrogen bonding with the ozonide ions. The desolvated tetrabutylammonium ozonide forms stable solutions in dichloromethane which may open up novel possibilities of tapping into the synthetic potential of the ozonide ion.

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis of cardiolipin extracted from detergent-solubilized mitochondrial electron transfer complexes

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry analysis of cardiolipin extracted from detergent-solubilized mitochondrial electron transfer complexes

Ion Core Structures in Anthracene Anion Tetramers

Anthracene is a typical example of organic molecular crystals for the study of charge carrier states. The weakness of van der Waals intermolecular interaction makes the molecular properties dominant over the crystalline ones, which leads to localized charge states in molecular crystals. The localized charge carrier polarizes the subsystems of the surrounding lattice, where a polaron-like particle emerges due to many-electron interactions in the crystals. As it provides a unique environment to investigate the physics of polaron, the organic molecular crystal has been a subject of extensive studies. In particular, the study of anthracene anion clusters, as a microscopic model of charge localization, will contribute to understand the nature of polaron in organic molecular crystals. Despite numerous studies for the core structures of aromatic cation and neutral clusters, little has been investigated for those of anion clusters. Recently, the multiple ion cores and switching of ion cores in the anthracene anion clusters, Ann − (n = 1-16), were investigated by anion photoelectron spectroscopy and theoretical calculations. The study revealed that a few different ion cores exist depending on the cluster size. The character of ion core changes from monomeric to dimeric to trimeric, as the size increases from n = 1 to 3. For n = 4, coexistence of two ion cores was proposed to account for the anomalous shape of the photoelectron spectrum, which were tentatively assigned as dimeric and trimeric ion cores. Due to lack of further information, the explanation could be mainly deduced from the optimized geometries of neutral clusters. The ion core becomes monomeric again at n = 5 with the completion of half-filled solvation shell. It remains still unclear what ion cores represent the unexpected photoelectron spectrum at n = 4. In this Note, we revisit the ion core structures at n = 4, and report a few ion core structures found in the stable geometries at n = 4. Figure 1 shows the photoelectron spectra of Ann − (n = 15), which are fitted with Gaussian profiles representing the vibration progression of the v6 mode of An molecule. The vertical detachment energies (VDEs) obtained from the fitting are presented as solid bars. As previously reported, the spectral shape of An4 − is distinctly different from those of the others. The anomalous shape is deconvoluted into two separate vibrational progressions with different Figure 1. Photoelectron spectra of anthracene cluster anions, Ann −

Theoretical modeling of postionization fragmentation of rare-gas trimer cations

The dynamics of ionic rare-gas trimers (Ar(3) (+), Kr(3) (+), and Xe(3) (+)) produced by a sudden ionization of neutral precursors is investigated theoretically with a hybrid classical-quantum method for solving the equations of motion governed by a Hamiltonian obtained from a previously tested diatomics-in-molecules model. Initial conditions are selected with Monte Carlo sampling. Two possibilities for generating the initial electronic state are considered: diabatic (local) and adiabatic (delocalized). The dynamics generally leads to fragmentation, producing either monomer ions or dimer ions in a relatively short time; however, a large number of long-lived metastable trimer ions are also seen in some cases. We have analyzed the dynamics with respect to the fraction of monomer ions produced, the distribution of the kinetic energy of the products, and the distribution of fragmentation times of the trimers. Initial diabatic ionization is associated with much faster fragmentation than adiabatic ionization. Spin-orbit coupling plays an important role in the fragmentation dynamics.

Size measurements of gasborne poly(amidoamine) (PAMAM) dendrimers using a differential mobility analyzer (DMA)

Poly(amidoamine) (PAMAM) dendrimers generations 2 through 6 were aerosolized by an electrospray aerosol generator and their electrical mobility spectra observed using a differential mobility analyzer (DMA). We succeeded in measuring the electrical mobilities of the monomer and dimer ions of these dendrimers. The experimental results suggested that PAMAM dendrimers are useful as standard particles in the range of a few nanometers.

Thermodynamic features of the Cu-ZSM-5 catalyzed NO decomposition reaction

Over a Cu-ZSM-5 catalyst with a quantified amount of the active Cu2+-dimers (Cu2+-O2--Cu2+), the kinetics of the catalytic NO decomposition to N2 and O2 was derived on the basis of the proposed reaction mechanism, and such thermodynamic data as adsorption enthalpies of NO and O2 onto the Cu ion dimer sites were evaluated. It was revealed that the enthalpy of the adsorption of NO (δH=-34.1 kcal/mol) onto a reduced Cu+-dimer, as the initiating step of NO decomposition catalysis, was higher than that (δH=-27.8 kcal/mol) onto an oxidized Cu2+-dimer, or that (δH=-27.4 kcal/mol) of the dissociative adsorption of O2 onto the two reduced Cu+-dimers in neighbor. The strong inhibition effect of gas phase oxygen on the kinetic rate of NO decomposition at 400–600 ‡C could be explained by the thermodynamic predominance of the oxidized Cu2+-dimers against the active reduced Cu+-dimers on the catalyst even at high temperature and under the low partial pressure of oxygen. It was also found that the maximum catalytic activity at temperatures around 500 ‡C, which was commonly observed in the Cu-ZSM-5 catalyzed NO decomposition reaction, was attributed to the relatively large enthalpy of NO adsorption onto the reduced Cu+-dimers as compared to that of the reaction activation energy (=19.5 kcal/mol), resulting in less favored NO adsorption at the higher temperatures than 500 ‡C.

Fragmentation dynamics of argon clusters (Arn, n=2 to 11) following electron-impact ionization: Modeling and comparison with experiment

The fragmentation dynamics of argon clusters ionized by electron impact is investigated for initial cluster sizes up to n = 11 atoms. The dynamics of the argon atoms is modeled using a mixed quantum-classical method in which the nuclei are treated classically and the transitions between electronic states quantum mechanically. The potential-energy surfaces are derived from a diatomics-in-molecules model with the addition of the induced dipole-induced dipole and spin-orbit interactions. The results show extensive and fast fragmentation. The dimer is the most abundant ionic fragment, with a proportion increasing from 66% for n = 2 to a maximum of 95% for n = 6 and then decreasing down to 67% for n = 11. The next abundant fragment is the monomer for n < 7 and the trimer otherwise. The parent ion dissociation lifetimes are all in the range of 1 ps. Long-lived trajectories appear for initial cluster sizes of seven and higher, and favor the formation of the larger fragments (trimers and tetramers). Our results show quantitative agreement with available experimental results concerning the extensive character of the fragmentation: Ar+ and Ar2(+) are the only ionic fragments for sizes up to five atoms; their overall proportion is in quantitative agreement for all the studied sizes; Ar2(+) is the main fragment for all sizes; stable Ar3(+) fragments only appear for n > or = 5, and their proportion increases smoothly with cluster size from there. However, the individual ionic monomer and dimer fragment proportions differ. The experimental ones exhibit oscillations with initial cluster size, with a slight tendency to decrease on average for the monomer. In contrast our results show a monotonic, systematic evolution, similar to what was found in our earlier studies on neon and krypton clusters. Several hypotheses are discussed in order to find the origin of this discrepancy. Finally, the metastable II(1/2)u and II(1/2)g states of Ar2(+) are found to decay with a lifetime of 3.5 and 0.1 ps, respectively, due to spin-orbit coupling. The difference with the commonly accepted microsecond range value for rare-gas dimer ions could originate from the role of autoionizing states in the formation of the parent ions.

Miniature Differential Mobility Spectrometry Using Atmospheric Pressure Photoionization

Positive and negative ion spectra have been obtained with a miniature differential mobility spectrometer equipped with a photoionization source operating at atmospheric pressure. With benzene as a dopant, providing C6H6+ as reactant ion, protonated molecular ions and proton-bound dimer ions were obtained with dimethyl methylphosphonate and butanone. The spectra obtained from gas chromatographic injections of aromatic hydrocarbons, benzene, toluene, and the xylenes, produced the molecular ions when the moisture level was very low, but at a high level the hydrated proton was also present. Possible mechanisms for the formation of protonated products are discussed. Negative ions were produced from electron capture by sulfur hexafluoride using benzene or acetone as dopant. Photoionization of nitrogen dioxide led to the formation of the nitrate ion whose yield was a nonlinear function of concentration. The use of a suitable dopant enhanced ion formation by up to 2 orders of magnitude, and limits of detection in both the positive and negative modes were all at the sub ppm(v) level. The study makes a strong case for the use of a photoionization source as an alternative to the radioactive 63Ni source.

Charge-state dependence of energy loss of MeV dimers inGaAs(100)

Carbon and oxygen dimers with charge states $1+$ and $3+$ were implanted into $\mathrm{GaAs}$ along the [100] direction at an energy of $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}∕\mathrm{atom}$. The defect depth profiles are extracted from Rutherford backscattering spectrometry and channeling. The depth profile of carbon is extracted from secondary ion mass spectrometry measurements. The defect density produced by dimer ions is larger than monomer ions. The depth profile of carbon in dimer implanted $\mathrm{GaAs}$ is deeper than that of monomer implanted $\mathrm{GaAs}$ showing negative molecular effect. The defect depth profile of oxygen dimer implanted $\mathrm{GaAs}$ is deeper for $3+$ than that for $1+$ charge state. This indicates that energy loss of ${\mathrm{O}}_{2}^{3+}$ is smaller than that of ${\mathrm{O}}_{2}^{+}$. It is attributed to charge asymmetry and a higher degree of alignment of ${\mathrm{O}}_{2}^{3+}$ along the [100] axis of $\mathrm{GaAs}$.

Letter: Influence of Experimental Conditions on Electrospray Ionization Mass Spectrometry of Ferrocenylalkylazoles

Influence of experimental conditions on electrospray/ionization (ESI) mass spectra of ferrocene derivatives FcCHRAz (Fc = eta5-C5H5-Fe-eta5- C5H4; R = H, Az = benzimidazole; R = Ph, Az = 2-methylimidazole) has been investigated. The spectra of all the compounds revealed [M]+*, product of its fragmentation [FcCHR]+ as well as products of ion/molecular interactions (protonated molecule [MH]+, binuclear ion [(FcCHR)2 Az]+, dimeric ion [M2]+* and its protonated form [M2H]+). Relative abundances of these ions appreciably (more than one order) depend on experimental conditions: analyte concentration, temperature of heated capillary, spray voltage, flow rate of mobile phase and polarity of solvents. Established correlations allow the selection of optimum experimental conditions for registration of ESI mass spectra, as required by the application. If an unknown compound is to be identified, it is necessary to operate by using polar solvents, small concentration, low temperature of heated capillary, high spray voltage and flow rates. There are high-intensity binuclear and protonated dimeric ions in mass spectra under other conditions. It can give rise to wrong interpretation of the structure of investigated compound. At the same time, for study of ion-molecular processes by ESI-MS it is necessary to use concentrated samples in non-polar solvents. In this case the dependence of reaction products yields on temperature and flow rate of mobile phase must be investigated.