Fe(CO)4 flexible as a two‐level system avoided conical intersection

This article reports new square‐planar Fe(CO)₄ D_4h structures that are optimized, using the Hartree–Fock (HF) approach, and multiconfiguration self‐consistent field (MCSCF) theory in active space [2b_2g2e_ga_1ga_2u]⁸, and which energy increased in sequence: ³B_2g TS < ¹A_1g TS < ¹A_1g GS. A triple ζ valence basis set supplemented with 4f for Fe and 3d for C and O polarization shells [TZV (DF)] was used. At the HF/TZV (DF) level, ¹A_1g TS and ³B_2g TS (³B_2g TS energetically more favorable), there are transition states of tetrahedral inversion (defining stereochemical flexibility of Fe(CO)₄) between known equivalent ¹A₁ and ³B₂ Jahn–Teller distorted tetrahedron C_2v structures with activation energy at ～0.96 kcal/mol according to the experimental data. ¹A_1g TS differs from ¹A_1g GS in electronic configuration by occupation of a_1g and a_2u MOs. At the MCSCF/ TZV (DF) level, ¹A_1g TS and ¹A_1g GS are optimized as near‐pure states in different potential energy surfaces (PES) avoided conical intersection with near‐equal interatomic distances, and define electronic flexibility of Fe(CO)₄. Estimation of the energy separation in a two‐level system that avoids a conical intersection from vibrational spectrum is based on the effective Hamiltonian of the perturbation theory. The energy gap between two square‐planar Fe(CO)₄ D_4h ¹A_1g TS < ¹A_1g GS is 0.27 kcal/mol. The energy gap between ¹A_1g GS and ¹A₁ is 1.28 kcal/mol. It is possible to observe ³B₂, ¹A₁ and ¹A_1g GS separately in the course of the experiment. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

A numerical solution of the pair equation of a model two‐electron diatomic system

It has been well‐documented that about 90% of the total correlation energy of atomic systems can be obtained by solving so‐called pair equations. For atoms, this approach requires solving partial differential equations (PDE) in two variables. In case of a diatomic molecule, we face devising a method for treating PDEs in five variables. This article shows how a well‐established finite difference method used to solve Hartree–Fock equations for diatomic molecules can be extended to solve numerically a model two‐electron Schrödinger equation for such systems. We show that using less than 100 grid points in each variable, it is possible to obtain the total energy of the helium atom and hydrogen molecule with a chemical accuracy and the S energy of the helium atom and hydride ion as accurately as the best results available. © 2015 Wiley Periodicals, Inc.     

Population inversion,temperature, and photon distributions of the generalized fermionic Ising ferromagnetic model: Path‐integral representation of the spin system

The quantum partition function and the emerging energy of a fermionic Ising ferromagnetic model involving all possible interactions (generalized Ising model) are obtained from an appropriate tracing of the analytic propagator path integral over Grassmann variable coherent nonorthogonal states in the imaginary time domain. The dynamics derived from the interaction of this system with a single‐mode cavity field in the rotating wave approximation is investigated for nonresonant states within the framework of the Jaynes–Cummings two‐level model consisting of the vacuum state and a thermally averaged manifold of excited states. Time evolution of the population inversion is computed in the nanosecond time scale, assuming that the initial coherent state of the field is given by a Poisson distribution. The limit of high temperatures characterizing the manifold of excited states becomes chaotic with rapid oscillations, whereas the ground state is described correctly in the thermodynamic limit by the vacuum state. A breakup is seen in the photon distribution into a series of peaks because of the detuning between the spin system and the field. However, this structure is smeared out, and the general shape is preserved in the computation emerging from the Laplace transform of the photon distribution. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Preparation and characterization of a novel dual‐retention mechanism mixed‐mode stationary phase with PEG 400 and succinic anhydride as ligand for protein separation in WCX and HIC modes

A novel dual‐retention mechanism mixed‐mode stationary phase based on silica gel functionalized with PEG 400 and succinic anhydride as the ligand was prepared and characterized by infrared spectra and elemental analysis. Because of the ligand containing PEG 400 and carboxyl function groups, it displayed hydrophobic interaction chromatography (HIC) characteristic in a high‐salt‐concentration mobile phase, and weak cation exchange chromatography (WCX) characteristic in a low‐salt‐concentration mobile phase. As a result, it can be employed to separate proteins with both WCX and HIC modes. The resolution and selectivity of the stationary phase was evaluated under both HIC and WCX modes with protein standards, and its performance was comparable to that of conventional ion‐exchange chromatography and HIC columns. The results indicated that the novel dual‐retention mechanism column, in many cases, could replace two individual WCX and HIC columns as a ‘2D column’. In addition, the mixed retention mechanism of proteins on this ‘2D column’ was investigated with stoichiometric displacement theory for retention of solute in liquid chromatography in detail in order to understand why the dual‐retention mechanism column has high resolution and selectivity for protein separation under WCX and HIC modes, respectively. Based on this ‘2D column’, a new 2DLC technology with a single column was developed. It is very important in proteome research and recombinant protein drug production to save column expense and simplify the processes in biotechnology. Copyright © 2013 John Wiley & Sons, Ltd.     

Characterization and quantification of triacylglycerols in peanut oil by off‐line comprehensive two‐dimensional liquid chromatography coupled with atmospheric pressure chemical ionization mass spectrometry

The complexity of natural triacylglycerols (TAGs) in various edible oils is prodigious due to the hundreds of set is of TAG compositions, which makes the identification of TAGs quite difficult. In this investigation, the off‐line 2D system coupling of nonaqueous RP and silver‐ion HPLC with atmospheric pressure chemical ionization MS detection has been applied to the identification and quantification of TAGs in peanut oil. The method was successful in the separation of a high number of TAG solutes, and the TAG structures were evaluated by analyzing their atmospheric pressure chemical ionization mass spectra information. HPLC and MS conditions have been optimized and the fragmentation mechanisms of isomers have been validated. In addition, an internal standard approach has been developed for TAG quantification. Then this system was applied in peanut oil samples and there was a total of 48 TAGs including regioisomers that have been determined and quantified.     

Rapid screening and identification of antioxidants in the leaves of Malus hupehensis using off‐line two‐dimensional HPLC–UV–MS/MS coupled with a 1,1′‐diphenyl‐2‐picrylhydrazyl assay

The leaves of Malus hupehensis have a strong antioxidant activity and are commonly consumed as a healthy tea. However, detailed information about its antioxidants is incomplete. Herein, we developed an effective strategy based on combining off‐line two‐dimensional high‐performance liquid chromatography with ultraviolet and tandem mass spectrometry detection with a 1,1′‐diphenyl‐2‐picrylhydrazyl assay to rapidly screen and identify the antioxidants from the leaves of M. hupehensis. In the orthogonal two‐dimensional liquid chromatography system, a Venusil HILIC column was used for the first dimension, while a Universil XB‐C₁₈ column was installed in the second dimension. As a result, 32 antioxidants, including ten dihydrochalcones, two flavanones, nine flavonols, four flavones, and seven phenolic acids were tentatively identified, out of which 23 compounds, as far as we know, were isolated and characterized from the leaves of M. hupehensis for the first time. To the best of our knowledge, this is the first systematic investigation of the antioxidants from the leaves of M. hupehensis. The results indicated that the proposed method is an efficient technique to rapidly investigate antioxidants, especially for coeluted and minor compounds in a complex system.     

Preparation of novel alkaline pH‐responsive copolymers for the formation of recyclable aqueous two‐phase systems and their application in the extraction of lincomycin

Aqueous two‐phase systems have potential industrial application in bioseparation and biocatalysis engineering; however, their practical application is limited primarily because the copolymers involved in the formation of aqueous two‐phase systems cannot be recovered. In this study, two novel alkaline pH‐responsive copolymers were synthesized and examined for the extraction of lincomycin. The two copolymers could form a novel alkaline aqueous two‐phase systems when their concentrations were both 6% w/w and the pH was 8.4(±0.1)–8.7(±0.1). One copolymer was synthesized using acrylic acid, 2‐(dimethylamino)ethyl methacrylate, and butyl methacrylate as monomers. Moreover, 98.8% of the copolymer could be recovered by adjusting the solution pH to its isoelectric point (pH 6.29). The other copolymer was synthesized using the monomers methacrylic acid, 2‐(dimethylamino)ethyl methacrylate, and methyl methacrylate. In this case, 96.7% of the copolymer could be recovered by adjusting the solution pH to 7.19. The optimal partition coefficient of lincomycin was 0.17 at 30°C in the presence of 10 mM KBr and 5.5 at 40°C in the presence of 80 mM Ti(SO₄)₂ using the novel alkaline aqueous two‐phase systems.     

Dual‐mode hydrophilic interaction normal phase and reversed phase liquid chromatography of polar compounds on a single column

Adopting a stationary phase convention circumvents problematic definition of the boundary between the stationary and the mobile phase in the liquid chromatography, resulting in thermodynamically consistent and reproducible chromatographic data. Three stationary phase definition conventions provide different retention data, but equal selectivity: (i) the complete solid phase moiety; (ii) the solid porous part carrying the active interaction centers; (iii) the volume of the inner column pores. The selective uptake of water from the bulk aqueous‐organic mobile phase significantly affects the volume and the properties of polar stationary phases. Some polar stationary phases provide dual‐mode retention mechanism in aqueous‐organic mobile phases, reversed‐phase in the water‐rich range, and normal‐phase at high concentrations of the organic solvent in water. The linear solvation energy relationship model characterizes the structural contributions of the non‐selective and selective polar interactions both in the water‐rich and organic solvent‐rich mobile phases. The inner‐pore convention provides a single hold‐up volume value for the retention prediction on the dual‐mode columns over the full mobile phase range. Using the dual‐mode monolithic polymethacrylate zwitterionic micro‐columns alternatively in each mode in the first dimension of two‐dimensional liquid chromatography, in combination with a short reversed‐phase column in the second dimension, provides enhanced sample information.     

Kinetics and electron spin resonance study of the radical polymerization of n‐butyl acrylate mediated by a nitroxide precursor: C‐phenyl‐N‐tert‐butylnitrone

The C‐phenyl‐N‐tert‐butylnitrone/azobisisobutyronitrile pair is able to impart control to the radical polymerization of n‐butyl acrylate as long as a two‐step process is implemented, that is, the prereaction of the nitrone and the initiator in toluene at 85 °C for 4 h followed by the addition and polymerization of n‐butyl acrylate at 110 °C. The structure of the in situ formed nitroxide has been established from kinetic and electron spin resonance data. The key parameters (the dissociation rate constant, combination rate constant, and equilibrium constant) that govern the process have been evaluated. The equilibrium constant between the dormant and active species is close to 1.6 × 10^?12 mol L^?1 at 110 °C. The dissociation rate constant and the activation energy for the C? ON bond homolysis are 1.9 × 10^?3 s^?1 and 122 ± 15 kJ mol^?1, respectively. The rate constant of recombination between the propagating radical and the nitroxide is as high as 1.2 × 10⁹ L mol^?1 s^?1. Finally, well‐defined poly(n‐butyl acrylate)‐b‐polystyrene block copolymers have been successfully prepared. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 6299–6311, 2006     

Tuning of crystallization method and ligand conformation to give a mononuclear compound or two‐dimensional SCO coordination polymer based on a new semi‐rigid V‐shaped bis‐pyridyl bis‐amide ligand

With the new semi‐rigid V‐shaped bidentate pyridyl amide compound 5‐methyl‐N,N′‐bis(pyridin‐4‐yl)benzene‐1,3‐dicarboxamide (L) as an auxiliary ligand and the Fe^II ion as the metal centre, one mononuclear complex, bis(methanol‐κO)bis[5‐methyl‐N,N′‐bis(pyridin‐4‐yl)benzene‐1,3‐dicarboxamide‐κN]bis(thiocyanato‐κN)iron(II), [Fe(SCN)₂(C₁₉H₁₆N₄O₂)₂(CH₃OH)₂] ( 1 ), and one two‐dimensional coordination polymer, catena‐poly[[[bis(thiocyanato‐κN)iron(II)]‐bis[μ‐5‐methyl‐N,N′‐bis(pyridin‐4‐yl)benzene‐1,3‐dicarboxamide‐κ²N:N′]] methanol disolvate dihydrate], {[Fe(SCN)₂(C₁₉H₁₆N₄O₂)₂]·2CH₃OH·2H₂O}_n ( 2 ), were prepared by slow evaporation and H‐tube diffusion methods, respectively, indicating the effect of the method of crystallization on the structure type of the target product. Both complexes have been structurally characterized by elemental analysis, IR spectroscopy and single‐crystal X‐ray crystallography. The single‐crystal X‐ray diffraction analysis shows that L functions as a monodentate ligand in mononuclear 1 , while it coordinates in a bidentate manner to two independent Fe(SCN)₂ units in complex 2 , with a different conformation from that in 1 and the ligands point in two almost orthogonal directions, therefore leading to a two‐dimensional grid‐like network. Investigation of the magnetic properties reveals the always high‐spin state of the Fe^II centre over the whole temperature range in 1 and a gradual thermally‐induced incomplete spin crossover (SCO) behaviour below 150 K in 2 , demonstrating the influence of the different coordination fields on the spin properties of the metal ions. The current results provide useful information for the rational design of functional complexes with different structure dimensionalities by employing different conformations of the ligand and different crystallization methods.     

Evaluation and application of a mixed‐mode chromatographic stationary phase in two‐dimensional liquid chromatography for the separation of traditional Chinese medicine

In this study, two mixed‐mode chromatography stationary phases (C8SAX and C8SCX) were evaluated and used to establish a two‐dimensional liquid chromatography system for the separation of traditional Chinese medicine. The chromatographic properties of the mixed‐mode columns were systematically evaluated by comparing with other three columns of C8, strong anion exchanger, and strong cation exchanger. The result showed that C8SAX and C8SCX had a mixed‐mode retention mechanism including electrostatic interaction and hydrophobic interaction. Especially, they were suitable for separating acidic and/or basic compounds and their separation selectivities could be easily adjusted by changing pH value. Then, several off‐line 2D‐LC systems based on the C8SAX in the first dimension and C8SAX, C8SCX, or C8 columns in the second dimension were developed to analyze a traditional Chinese medicine—Uncaria rhynchophylla. The two‐dimensional liquid chromatography system of C8SAX (pH 3.0) × C8SAX (pH 6.0) exhibited the most effective peak distribution. Finally, fractions of U. rhynchophylla prepared from the first dimension were successfully separated on the C8SAX column with a gradient pH. Thus, the mixed‐mode stationary phase could provide a platform to separate the traditional Chinese medicine in practical applications.     

Preparation of a monolithic column with a mixed‐mode stationary phase of reversed‐phase/hydrophilic interaction for capillary liquid chromatography

A monolithic capillary column with a mixed‐mode stationary phase of reversed‐phase/hydrophilic interaction chromatography was prepared for capillary liquid chromatography. The monolith was created by an in‐situ copolymerization of a homemade monomer N,N‐dimethyl‐N‐acryloxyundecyl‐N‐(3‐sulfopropyl) ammonium betaine and a crosslinker pentaerythritol triacrylate in a binary porogen agent consisting of methanol and isopropanol. The functional monomer was designed to have a highly polar zwitterionic sulfobetaine terminal group and a hydrophobic long alkyl chain moiety. The composition of the polymerization solution was systematically optimized to permit the best column performance. The columns were evaluated by using acidic, basic, polar neutral analytes, as well as a set of alkylbenzenes and Triton X100. Very good separations were obtained on the column with the mixed‐mode stationary phase. It was demonstrated that the mixed‐mode stationary phase displayed typic dual retention mechanisms of reversed‐phase/hydrophilic interaction liquid chromatography depending on the content of acetonitrile in the mobile phase. The method for column preparation is reproducible.     

Development of Imidazoline‐2‐Thiones Based Two‐Photon Fluorescence Probes for Imaging Hypochlorite Generation in a Co‐Culture System

We designed and prepared the imidazoline‐2‐thione containing OCl^? probes, PIS and NIS , which operate through specific reactions with OCl^? that yield corresponding fluorescent imidazolium ions. Importantly, we demonstrated that PIS can be employed to image OCl^? generation in macrophages in a co‐culture system. We have also employed two‐photon microscopy and PIS to image OCl^? in live cells and tissues, indicating that this probe could have wide biological applications.     

Quantitative method for analysis of tobacco‐specific N‐nitrosamines in mainstream cigarette smoke by using heart‐cutting two‐dimensional liquid chromatography with tandem mass spectrometry

A heart‐cutting two dimensional liquid chromatography coupled with tandem mass spectrometry method was developed for the analysis of tobacco‐specific N‐nitrosamines (TSNAs) at low concentration level in Virginia‐type cigarette smoke. A strong cation exchange column was utilized for the first dimensional separation, which effectively removed acidic and neutral components in the smoke, followed by a reversed phase liquid chromatography coupled with tandem mass spectrometric analysis. To capture components of the TSNAs in the effluent on the trapping column, a compensating pump was applied for online dilution and pH adjustment during the period of the TSNAs fraction transferring and enrichment. Highly sensitive determination of the TSNAs in mainstream cigarette smoke was achieved by isotope deuterated internal standards under the multiple reaction monitoring mode. Compared with traditional methodologies, the method was almost no matrix interference. Limits of quantity for the TSNAs were within 0.027–0.094 ng/mL, and the results showed good reproducibility and accuracy. Finally, the new method was applied for analysis of the Kentucky reference cigarettes and the results agreed well with joint experiments of Cooperation Centre for Scientific Research Relative to Tobacco.     

A two‐state reactivity rationale for the reaction of ta atom with acetonitrile in the gas phase

The spin‐forbidden reaction mechanism of Ta (⁴F, 5d³6s²) with CH₃CN, on two different potential surfaces (PESs) has been investigated at the B3LYP, MP2, and CCSD level of theory. Crossing points between the PESs are located using different methods, and possible spin inversion processes are discussed by means of spin‐orbit coupling calculations. As a result, the reaction system will change its spin multiplicities near this crossing seam, leading to a significant decrease in the barrier of ^2‐4TS3 from 24.17 to 5.36 kcal/mol, which makes the reaction access to a lower energy pathway and accelerate the reaction rate. © 2012 Wiley Periodicals, Inc.     

Use of a recently developed thermal modulator within the context of comprehensive two‐dimensional gas chromatography combined with time‐of‐flight mass spectrometry: Gas flow optimization aspects

The present research is based on the use of a recently developed comprehensive two‐dimensional gas chromatography thermal modulator, which is defined as solid‐state modulator. The transfer device was installed on top of a single gas chromatography oven, while benchtop low‐resolution time‐of‐flight mass spectrometry was used to monitor the compounds exiting the second analytical column. The solid‐state modulator was first described by Luong et al. in 2016, and it is a moving modulator that does not require heating and cooling gases to generate comprehensive two‐dimensional gas chromatography data. The accumulation and remobilization steps occur on a trapping capillary, this being subjected to thermoelectric cooling and micathermic heating. In this study, the effects of the gas linear velocity on the modulation performance were evaluated by using two different uncoated trapping capillaries, viz., 0.8 m × 0.25 mm id and 0.8 m × 0.20 mm id. Solid‐state modulator applications were carried out on a standard solution containing n‐alkanes (C_9, C_10, C₁₂), and on a sample of diesel fuel. The results indicated that the type of trapping capillary and gas velocity have a profound effect on modulation efficiency.     

Preparative separation of isoquinoline alkaloids from Corydalis impatiens using a middle‐pressure chromatogram isolated gel column coupled with two‐dimensional liquid chromatography

We established a two‐dimensional strong cation exchange/reversed‐phase liquid chromatography protocol to isolate and purify isoquinoline alkaloids from Corydalis impatiens. Isoquinoline alkaloids were first enriched from a C. impatiens extract in which liposoluble components were removed using a medium‐pressure chromatographic tower containing middle chromatogram isolated gel. A strong cation exchange column was employed to separate and obtain 30 fractions. We chose fractions 22–29 for reversed‐phase liquid chromatography purification using characteristic isoquinoline alkaloid ultraviolet absorption spectra. Several isoquinoline alkaloid fractions (22–29) were further separated, and those of low resolution were isolated via two‐dimensional liquid chromatography in the orthogonal plane. A total of eight novel isoquinoline alkaloids with characteristic ultraviolet spectra were obtained from C. impatiens. We thus demonstrate the benefits of off‐line two‐dimensional strong cation exchange/reversed‐phase liquid chromatography to isolate isoquinoline alkaloids from C. impatiens.