Ion-pair System Research Articles

A One-Dimensional Energy Diffusion Approach to Multidimensional Dynamical Processes in the Condensed Phase

We propose a generalized one-dimensional energy diffusion approach for describing the dynamics of multidimensional dynamical processes in the condensed phase. On the basis of a formalism originally due to Zwanzig, we obtain a one-dimensional kinetic equation for a properly selected relevant dynamical quantity and derive new analytical results for the dynamics of a multidimensional electron-transfer process, nonequilibrium solvation, and diffusive escape from a potential well. The calculated results for electron-transfer reactions in solvent-separated and contact ion pair systems are found to be in good agreement with the experimental results. We are able to explain the rate of the electron-transfer reaction using much smaller and reasonable values of the solvent reorganization energy in contrast to earlier works that had to use a much larger value. The proposed theory is not only conceptually simpler than the conventional approaches but is also free from many of their limitations. More importantly, it provides a single theoretical framework for describing a wide class of dynamical phenomena.

A quality by design approach to impurity method development for atomoxetine hydrochloride (LY139603)

The development of an ion-pairing HPLC method and associated system suitability parameters for the analysis of atomoxetine hydrochloride (LY139603 HCl) using a quality by design approach is described. Potential method conditions were evaluated for their ability to meet design requirements and statistically designed experiments were used to optimize conditions and demonstrate method robustness for the separation of atomoxetine and impurities. The separation of two early eluting impurities, phenyl methylaminopropanol (PMAP (±)3-methylamino-1-phenylpropanol) and mandelic acid is correlated to the separation of other impurities that elute near the main sample component and the resolution of this peak pair is used as a system suitability test without the need for impurity reference standards.

Effect of mobile phase additives on resolution of some nucleic compounds in high performance liquid chromatography

Here we investigate the chromatographic behavior, with reversed-phase high performance liquid chromatography (RP-HPLC) of nucleic compounds (nucleobases, nucleosides, and nucleotides) on a C18 column in several different mobile phase additives, including1-butyl-3-methylimidazolium tetrafuloroborate ([BMIm][BF4]), 1-ethyl-3-methylimidazolium methylsulfate ([EMIm][MS]) ionic liquids, ammonium formate, and potassium phosphate. The effect of the alkyl group length, the imidazolium ring, and the ionic liquid's counterions on retention and resolution of the samples were tested. The results show the potential application of a used buffer system, ion pairing system, and ionic liquid as mobile phase additives in liquid chromatography resolution of nucleic compounds.

Metastable Intermolecular Charge-Transfer Complexes with a Pentavalent Carbon Atom

A novel type of metastable complexes of metal (such as Na) atoms with a super-fluorinated carbon molecule is investigated, with a carbon atom exhibiting a unique, pentavalent character. It is induced by the charge transfer from the alkali metal component, followed by a geometric compression of the ion pair system. Analysis of the electron-density distribution confirms the real chemical nature of the extra C-F bond. Structure and stability of the system are characterized ab initio, and a spectrum of electronic perturbations is considered. The ways of forming the systems are discussed, and the spectroscopic parameters are predicted, facilitating their detection in experiments.

Recent Development in the Study of SN2 Reactions at Heteroatoms and Ion Pair Systems

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RECENT DEVELOPMENT IN THE STUDY OF SN2 REACTIONS AT HETEROATOMS AND ION PAIR SYSTEMS

Present review outlines the experimental and theoretical studies on the SN 2 reactions at heteroatoms ( N , O and S ) and ion pair systems in recent years, especially introduce research carried out at our group in detail. The similarities and differences between SN 2 at heteroatoms and at carbon are discussed. Some new structure–energy relationships were proposed. The accuracy of various theoretical schemes for calculating potential energy surfaces has been assessed.

Control of Diabatic versus Adiabatic Field Dissociation in a Heavy Rydberg System

A novel phenomenon is observed in the dynamics of laser-prepared coherent wave packets, bound by the Coulombic 1/r potential of an ion-pair system. After exciting weakly bound (approximately 3 meV) H(+)(-) wave packets in a Stark field, and permitting them to evolve in time, control of field dissociation via adiabatic and diabatic routes is demonstrated by applying delayed pulsed-electric fields, involving a zero-field crossing. Control manifests itself through the production of ions from each pathway at a different instant in time. This phenomenon is applied to map the oscillatory behavior of an angular momentum wave packet in a heavy Rydberg system. The characteristic frequencies of the observed Stark oscillations verify predicted mass-scaling laws for heavy Rydberg systems.

Heavy Rydberg states

This paper discusses the nature of heavy Rydberg states, i.e. quantum states in molecular systems that are bound by the almost pure Coulomb potential between pairs of ions. A theoretical framework is developed in terms of mass–scaling laws for heavy Rydberg systems of certain reduced mass, so that the physics of electronic Rydberg states can be straightforwardly applied to heavier ion–pair systems, or any other system bound by a 1/r potential. The general description of such quantum systems is supported by an experimental investigation of the energy region near the H+H− ion–pair dissociation limit, using a 1 XUV + 1 UV laser excitation scheme. Such a scheme allows for preparation of a single intermediate rovibrational quantum state, from which the ion–pair threshold region can be explored with a narrowband Fourier–transform limited laser. Field–induced lowering of the H+H− dissociation limit was observed in the presence of an electric field. Using a combination of DC and pulsed electric fields two–photon induced threshold ion–pair production spectroscopy (TIPPS) was performed for a variety of field strengths. Below the field–dissociation limit coherent wave packets of bound heavy Rydberg states are excited in an electric field. The coherent evolution of the Stark states, giving rise to oscillations in angular momentum space, can be quantitatively understood in terms of the linear Stark effect in the hydrogen atom, where the light electron is replaced by the heavier H− particle. The dynamics of wave packets of heavy Rydberg states is slower than in ordinary electronic Rydberg states, according to their larger mass. For a wide variety of binding energy and external electric field, the observed oscillation frequencies of the Stark wave packets match this model. An important parameter describing the specific properties of the H+H− Rydberg states is the scaled lifetime, which may be considered as a material constant, and it is determined atτ= (5.8±2.0)×10−21 s n 4.

Aspects of coordination polymerization in heterogeneous and homogeneous catalysis. A computational survey

Theoretical studies on ethylene polymerization by single site homogeneous catalysts, as well as by Ziegler–Natta heterogeneous catalysts, have been reviewed. Studies on cation–anion interactions in ion-pair systems of the type [LL′MR]+[A]− [L, L′=Cp, NCR2, NPR3; M=Ti, Zr; A=MeB(C6F5) 3 − , MAOMe−, B(C6F5) 4 − ] provided indications as to which ancillary ligands on the cation acted as good electron donors, and also revealed the importance of having a weakly coordinating anion to enable the ion-pair to separate more easily in solution. Subsequent calculations of barriers to ethylene insertion into the metal-methyl bond in ion-pair systems of the type LL′MeM-μ-B(C6F5)3[M=Ti, Zr] revealed that the ease of separation of the anion from the cation in solution was an important criterion in determining the activity of the catalysts. The barrier to insertion was found to be the rate determining step during the first insertion of ethylene into the metal–methyl bond, but second insertion studies conducted for two different ion-pair systems showed that the barrier to uptake of the monomer became the rate determining step during the second insertion of the ethylene monomer. Theoretical studies conducted on TiCl4/MgCl2 heterogeneous Ziegler–Natta catalysts provided insights into the nature of the binding of the titanium catalysts (with the Ti in different oxidation states) on to the MgCl2 support. The results indicated that the support worked best when a few Ti atoms replaced Mg atoms in the crystal lattice of the support. Ethylene polymerization studies, along with investigations on chain termination, led to the conclusion that the TiCl3-based edge site on MgCl2 would be the most promising model for the actual catalytic species. Studies on addition of tetrahydrofuran (THF) to the active sites showed that the presence of the base would lead to higher molecular weights of the polymers, as it acted to increase the barrier to termination (by chain transfer to monomer) more than the barrier to insertion.

Possible Thermal Decomposition Routes in [MeB(C6F5)3]-[L2TiMe+] as Deactivation Pathways in Olefin Polymerization Catalysis: A Combined Density Functional Theory and Molecular Mechanics Investigation

Combined quantum mechanical (QM) and molecular mechanical (MM) models (QM/MM) have been used to explore possible decomposition routes for ion pair systems used as catalysts in olefin polymerization...

UPLC-ICP-MS—a fast technique for speciation analysis

Ultra performance liquid chromatography is a new development of the HPLC separation technique that allows separations on column materials at high pressures up to 108 Pa using particle diameters of 1.7 μm. This increases the efficiency, the resolution and the speed of the separation. Four aqueous selenium standards were separated within 1.2 min on a 1.00 id × 50 mm reversed phase column in an ion-pair chromatographic system using a flow rate of 200 μL min−1. Hence, analysis times could be reduced to 1/10 compared with ordinary HPLC for aqueous standards. The precision and detection limits were comparable to values obtained by HPLC. Detection limits were better than 0.4 μg Se L−1. A urine sample was analysed on a 1.0 id × 100 mm column within 5 min using a flow rate of 100 μL min−1. The improved separation efficiency, owing to the use of 1.7 μm column particles, allowed the use of short columns. Hence, analysis times could be halved without loss of separation efficiency in this biological sample.

Selenium speciation in human urine samples by LC- and CE-ICP-MS—separation and identification of selenosugars

Human urine samples were analysed by a reversed-phase chromatographic system and an ion-pair chromatographic system. The chromatographic system was connected to the ICP-MS either by a microconcentric nebulizer (MCN) in combination with a cyclonic spraychamber or by a modified direct injection nebulizer (MDIN). The sensitivity of the latter was better than the sensitivity of the MCN, which on the other hand was more robust for the analysis of samples with high concentrations of dissolved solids. Urine sample composition did not seem to change when urine was exposed to evaporation under nitrogen at ambient temperature and methanol extraction. A pre-concentration factor of 10 was achieved with this procedure. On occasions when a pre-concentration factor of 100 was obtained by lyophilsation and methanol extraction, at least 10 selenium compounds were separated in the urine sample. Urine samples were collected from two healthy volunteers who had been supplied with 1000 µg and 2000 µg of selenium, respectively, in the form of selenized yeast. When samples were spiked with 8 different standards, only two standards co-eluted with compounds in urine in both chromatographic systems: the major urinary metabolite Se-methyl-N-acetylgalactosamine and Se-methyl-N-acetylglucosamine. The presence of Se-methyl-N-acetylglucosamine in urine was verified by co-migration with the standard in capillary electrophoresis after fractionation by preparative reversed-phase chromatography. Se-methyl-N-acetylglucosamine is only a minor metabolite as its concentration was less than 2% of the concentration of Se-methyl-N-acetylgalactosamine. The presence of this metabolite in urine has, to our knowledge, not been suggested before. Trimethylselenonium, selenomethionine, Se-methylselenocysteine, Se-methylselenomethionine and selenocystamine were not detected in these samples.

Chromatographic behavior of cadmium in an ion-pair reversed-phase micro HPLC system and its application to the determination of bio-available cadmium in soil samples.

A novel reversed phase ion-pair micro HPLC system with on-line fluorescence detection has been developed systematically and studied for the determination of cadmium in its bio-available fractions of soil samples. In this system, a micro ODS column of 1.0 mm i.d.x150 mm length and a mobile phase containing 6 mmol L(-1) 8-hydroxyquinoline 5-sulphonic acid (HQS), 3 mmol L(-1) cetyltrimethylammonium bromide (CTMABr), 10 mmol L(-1) acetic acid-acetate buffer (pH 5.1) as well as 50% acetonitrile at 50 microL min(-1) flow rate were employed to determine cadmium with a 2 microL flow cell through its fluorescence at 518 nm under 338 nm excitation. Furthermore, the composition of Cd-HQS chelate formed on the column was confirmed to be [Cd(HQS)(2)](2-) through a log-log plot method, and then combined with the ion-pair reagent by the electrostatic force under the chromatographic condition proposed. With such a method, the detection limit of cadmium was 8.48 ng mL(-1) (3sigma) with 1 microL sample injection, and the linear range for the determination of cadmium was 30-800 ng mL(-1) (R(2)=0.992). This method has been successfully applied to determination of cadmium in its bio-available fractions of BCR-483 and soil samples without interference from other coexistent metal ions. The RSD (n=6) was less than 7.3%. The results were in agreement with the indicative value for BCR-483 and those for the soil samples obtained by ICP-MS with a pretreatment of bis(1,1,3,3-tetramethylbutyl) phosphinic acid extraction.

Time-resolved EPR study on reorganization energies for charge recombination reactions in the systems involving hydrogen bonding

Hydrogen bonding effects on reorganization energies ( λ) in the photoinduced electron donor–acceptor system of duroquinone (DQ)–1,2,4-trimethoxybenzene (TMB) in benzonitrile have been investigated by time-resolved EPR and cyclic voltammetry measurements. The solvent reorganization energies ( λ S) determined for the radical ion pair systems involving the hydrogen-bonded complexes are larger by ∼0.2 eV than those calculated from the Marcus continuum dielectric model. The present results suggest that dissociation of the DQ anion–alcohol hydrogen-bonded complex results in the new component of λ S for the charge recombination reaction in polar solvents.

Simplified high-performance liquid chromatographic determination of residual amprolium in edible chicken tissues.

A simplified determining method for the routine monitoring of residual amprolium in edible chicken tissues (muscle and liver) is developed using a high-performance liquid chromatographic (HPLC) method with a photodiode-array detector after sample cleanup by an Ultrafree-MC/PL centrifugal ultrafiltration unit. For the HPLC determination and identification, a Mightysil RP-4 GP column and a mobile phase of an ethanol-5 mM 1-heptanesulfonic acid sodium salt solution (35:65, v/v) using an ion-pairing system with a photodiode-array detector are used. Average recoveries (spiked at 0.3-3.0 microg/g) are > 90%. The inter- and intravariabilities are 1.9-2.4%. The limits of quantitation are 0.22 microg/g for muscle and 0.25 microg/g for liver. The total time and solvent required for the analysis of one sample are < 20 min and < 2 mL of ethanol, respectively. No toxic solvents and regents are used.

Simplified high-performance liquid chromatographic determination of residual amprolium in edible chicken tissues.

A simplified determining method for the routine monitoring of residual amprolium in edible chicken tissues (muscle and liver) is developed using a high-performance liquid chromatographic (HPLC) method with a photodiode-array detector after sample cleanup by an Ultrafree-MC/PL centrifugal ultrafiltration unit. For the HPLC determination and identification, a Mightysil RP4 GP column and a mobile phase of an ethanol-5 mM 1-heptanesulfonic acid sodium salt solution (35:65, v/v) using an ion-pairing system with a photodiode-array detector are used. Average recoveries (spiked at 0.3-3.0 microg/g) are > 90%. The inter- and intravariabilities are 1.9-2.4%. The limits of quantitation are 0.22 microg/g for muscle and 0.25 microg/g for liver. The total time and solvent required for the analysis of one sample are < 20 min and < 2 mL of ethanol, respectively. No toxic solvents and regents are used.

Selenium speciation in urine by ion-pairing chromatography with perfluorinated carboxylic acids and ICP-MS detection

Five aqueous standards, selenomethionine (SeMet), methylselenomethionine (MeSeMet), methylselenocysteine (MeSeCys), selenogammaaminobutyric acid (SeGaba) and the trimethylselenonium ion (TMSe), were separated in ion-pairing chromatographic systems based on perfluorinated carboxylic acids in methanol. Two different perfluorinated carboxylic acids, heptafluorobutanoic acid (HFBA) and nonafluoropentanoic acid (NFPA), were used as ion-pairing agents in the separation. The selectivities of the ion-pairing agents were different. The separation was performed on a microbore column, which was connected to the ICP-MS via a laboratory-made direct injection nebuliser. This nebulisation system allowed methanol concentrations of 50% in the eluent when a flow rate of 50 µl min−1 was used. The detection limits in urine were between 0.8 and 1.7 µg l−1 corresponding to absolute detection limits of between 2.3 and 5.1 pg. Urine samples from different individuals before and during supplementation with selenomethionine were analysed. Several species were separated in the different urine samples. A major component eluting at the beginning of the chromatogram was predominant in many samples, especially after selenium consumption. This species was not identified and solid phase extraction experiments suggested that it was neutral. When different urine samples were spiked with the available standards, co-elution of species with TMSe, MeSeMet or SeMet was observed in some samples. None of these species were major compounds in urine samples—even after massive consumption of selenium-containing supplements. The selenium species in the urine samples showed a limited stability, as they changed during storage at +4 °C as well as −18 °C.

Theoretical Study of the Interactions between Cations and Anions in Group IV Transition-Metal Catalysts for Single-Site Homogeneous Olefin Polymerization

Density functional theory has been used to investigate the interaction between a series of cationic polymerization catalysts and their anionic counterions. The catalyst systems include (NPR3)2TiMe+, (Cp)(NCR2)TiMe+, (CpSiR2NR‘)TiMe+, (Cp)OSiR3TiMe+, and (Cp)NPR3TiMe+. The counterions studied are B(C6F5)4-, MeB(C6F5)3-, TMA-MAOMe-, and MAOMe-, where TMA = trimethylaluminum and MAO = methylalumoxane. Two simplified model structures, which have been proposed as the counterions for the active (TMA-MAOMe-) and dormant (MAOMe-) ion pairs in single-site catalysts activated by MAO, were used for the last two counterions. The interaction between the cation and anion will be discussed in terms of ion-pair formation and separation energies. Full quantum-mechanical (QM) calculations demonstrate that, for the same catalyst system but different anions, the ion-pair separation energies increase in the order B(C6F5)4- < MeB(C6F5)3- < TMA-MAOMe- < MAOMe-. For the same counterion but different cations, the (NPR3)2TiMe+ system has the lowest separation energy. Increasing the size of the R group decreases the ion-pair separation energy. Combined quantum-mechanical (QM) and molecular-mechanical (MM) models (QM/MM) for MeB(C6F5)3- and TMA-MAOMe- have also been developed and examined by comparing the ion-pair formation and separation energies to the full QM results. The QM parts of MeB(C6F5)3- and TMA-MAOMe- are represented by MeBCl3- and MeBMe2Cl-, respectively. The other parts of the anions are replaced by MM atoms. Preliminary studies on olefin insertion reactions for the (NPH3)2TiMe−μMe−A (A = B(C6F5)3 and TMA-MAO) systems suggest that the QM/MM models satisfactorily reproduce the behavior of the ion-pair system in the insertion process.

A comparative study of commercial liquid chromatographic detectors for the analysis of underivatized amino acids

This study compares the main commercial detectors that can detect amino acids in their underivatized form. The detectors tested are: the chemiluminescent nitrogen detector (CLND), the evaporative light scattering detector (ELSD), the nuclear magnetic resonance spectrometer, conductivity detector, refractive index, UV, and electrospray quadrupole mass spectrometry (in simple and tandem MS mode). As ELSD, CLND and MS require a volatile mobile phase, an ion-pair reversed-phase liquid chromatographic system was selected, consisting of an octadecyl column and an aqueous mobile phase containing pentadecafluorooctanoic acid as volatile ion-pairing reagent. Underivatized taurine, hypotaurine, aspartic acid, hydroxyproline, asparagine, serine, glycine, glutamine, cysteine, glutamic acid, threonine and alanine were simultaneously analysed with each detector. In order to test the applicability of these detectors to “real world” samples, the amino acid stoichiometry of the tetrapeptide Gly–Gly–Asp–Ala was determined with each detector after acid hydrolysis. The detectors were compared in terms of linearity, limit of detection, advantages and disadvantages as well as special features (capacity to provide structural information, specificity, quantification with single calibration curve, etc.).

Determination of electron self-exchange rate constants with MARY spectroscopy: Dependence on the fluorophore

The influence of electron self-exchange on the magnetic field effect of various exciplex–radical ion pair (RIP) systems (1,2- and 1,4-dicyanobenzene in combination with the fluorophores pyrene, N-methylcarbazole and N-ethylcarbazole in tetrahydrofurane) has been investigated using magnetic-field-effect-on-reaction-yield (MARY) spectroscopy. By varying the concentration of the diamagnetic counterpart of one of the radicals in the RIP, the dependence of the linewidth on the electron self-exchange rate has been studied. The functional behaviour of the linewidth-related parameter B1/2 with respect to the molecule concentration is governed by two opposing effects: lifetime uncertainty broadening of spin levels leads to a linear increase at low concentrations, whereas weakened hyperfine coupling of the corresponding radical ion at high concentrations causes B1/2 to decrease again until a saturation value is reached. From the linear relationship at low concentrations an estimate of the self-exchange rate constant is obtained.