A CASSCF/icMRCI study of the electric field gradient in low-lying electronic states of N2/N2

The electric field gradients (EFG’s) at the nucleus are calculated as a function of internuclear separation in the X²Σ_g⁺ and B²Σ_u⁺ electronic states of the nitrogen molecule cation using the internally contracted multireference configuration interaction (icMRCI) method. The EFG’s and potential energy functions (PEF’s) are used to estimate the ¹⁴N nuclear quadrupole coupling constants (NQCC’s) in the two electronic states as functions of vibrational and end-over-end rotational quantum numbers. The dependences of the computed constants on the basis set and reference configuration space are investigated. Since no counterpart for comparison of the calculated NQCC’s exists, the N₂⁺ results are supported by analogous calculations on the X¹Σ_g⁺ and A³Σ_u⁺ states of N₂, for which established data are available. The overall good quality of the icMRCI wave functions is further corroborated by a favorable agreement of spectroscopic constants derived from the corresponding PEF’s and experimental data. Variations of the EFG with internuclear separation are explained in terms of wave function composition, and used for gaining specific insight into the chemical bonding in N₂⁺ and N₂.     

Analysis of the effect of the electronic structure of the Cl atom in complexes of chlorides of group IV elements on Cl NQR frequency and asymmetry parameter

Analysis of changes in ³⁵Cl NQR frequency of complexes MCl₄.L and MCl₄.2L for M = Si, Ge, Sn and Ti was performed. The population of the atomic orbitals was calculated by the quantum chemistry methods PM3 and INDO. The results provided an explanation of the changes in ³⁵Cl NQR frequency and the asymmetry parameter upon complex formation. In the complexes of non-transition elements, a decrease in ³⁵Cl NQR frequency is first of all related to increasing population of p_z orbitals on Cl atoms. In the complexes of Ti, the increase in ³⁵Cl NQR frequency depends on a decrease in the p_π to d_π electron density transfer.     

Analysis of the effect of the electronic structure of the Cl atom in complexes of chlorides of group IV elements on Cl NQR frequency and asymmetry parameter

Analysis of changes in ³⁵Cl NQR frequency of complexes MCl₄.L and MCl₄.2L for M = Si, Ge, Sn and Ti was performed. The population of the atomic orbitals was calculated by the quantum chemistry methods PM3 and INDO. The results provided an explanation of the changes in ³⁵Cl NQR frequency and the asymmetry parameter upon complex formation. In the complexes of non-transition elements, a decrease in ³⁵C1 NQR frequency is first of all related to increasing population of p_z orbitals on Cl atoms. In the complexes of Ti, the increase in ³⁵Cl NQR frequency depends on a decrease in the p_π to d_π electron density transfer.     

Influence of varying electroosmotic flow on the effective diffusion in electric field gradient separations

Recently the use electric field gradient focusing (EFGF) to enhance focusing of proteins has been proposed and explored to provide significant improvement in separation resolution. The objective of EFGF is to focus proteins of specific electrophoretic mobilities at distinct stationary locations in a column or channel. This can be accomplished in a capillary by allowing the electric potential to vary in the streamwise direction. Because the electric field is varying, so also is the electrokinetic force exerted on the proteins and the electroosmotic velocity of the buffer solution. Due to the varying electric field, the Taylor diffusion characteristics will also vary along the column, causing a degradation of peak widths of some proteins, dependent on their equilibrium positions and local velocity distributions. The focus of this paper is an analysis that allows characterization of the local Taylor diffusion and resulting protein band peak width as a function of the local magnitude of the EOF relative to the average fluid velocity for both cylindrical and rectangular channels. In general the analysis shows that as the ratio of the local electroosmotic velocity to the average velocity deviates from unity, the effective diffusion increases significantly. The effectiveness of EFGF devices over a range of protein diffusivities, capillary diameters, flow velocities, and electric field gradient is discussed.     

Forces at the nuclei of a molecule in the presence of non-uniform electric field

The interaction Hamiltonian within the Bloch gauge for the potentials of the electric field has been used to define electric multipole moment operators. Perturbation theory has been applied to evaluate the induced electronic moments and electric field at the nuclei in the presence of spatially non-uniform electric fields of high intensity. Multipole nuclear electric shielding tensors have been defined to describe the contributions arising from a non-homogeneous electric field. These quantities are useful to rationalize linear and non-linear responses of a molecule in the presence of intense external electric perturbations.     

Hydrogen bond interactions in sulfamerazine: DFT study of the O-17, N-14, and H-2 electric field gradient tensors

Hydrogen bond (HB) interactions are studied in the real crystalline structure of sulfamerazine by density functional theory (DFT) calculations of the electric field gradient (EFG) tensors at the sites of O-17, N-14, and H-2 nuclei. One-molecule (single) and four-molecule (cluster) models of sulfamerazine are created by available crystal coordinates and the EFG tensors are calculated in both models to indicate the influence of HB interactions on the tensors. Directly relate to the experiments, the calculated EFG tensors are converted to the experimentally measurable nuclear quadrupole resonance (NQR) parameters, quadrupole coupling constant (qcc) and asymmetry parameter (η_Q). The evaluated NQR parameters reveal that due to contribution of the target molecule to N–HN and N–HO types of HB interactions, the EFG tensors at the sites of various nuclei are influenced from single model to the target molecule in cluster. Additionally, O2, N4, and H2 nuclei of the target molecule are significantly influenced by HB interactions, consequently, they have the major contributions to HB interactions in cluster model of sulfamerazine. The calculations are performed employing B3LYP method and 6-311++G** basis set using GAUSSIAN 98 suite of program.     

Calculation of the deuterium electric field gradients in HD and LiD using a variation-perturbation method with a Gaussian basis set

Summary We present calculations of the deuterium electric field gradients in the HD and LiD molecules obtained with a variation-perturbation method using Gaussian atomic orbitals. The differences between our theoretical values and the corresponding experimental or best calculated values are 2%. We conclude that high accuracy can be obtained with the variation-perturbation method using either Gaussian or Slater orbitals.     

Numerical simulation of droplet coalescence behavior in gas phase under the coupling of electric field and flow field

The coalescence behavior of droplets in an electric field belongs to the important research contents of electrohydrodynamics. Based on the phase field method of the Cahn–Hilliard equation, the electric field and the flow field are coupled to establish the numerical model of twin droplet coalescence in a coupled field. The effects of flow rate, electric field strength, droplet diameter, and interfacial tension on the coalescence behavior of droplets during the coalescence process were investigated. The results show that the dynamic behavior of the droplets is divided into coalescence, after coalescence rupture, and no coalescence under the coupling of electric field and flow field. The proper increase of the electric field strength will accelerate the coalescence of the droplets, and the high electric field strength causes the droplets to burst after coalescence. Excessive flow rates make droplets less prone to coalescence. Under the coupling field, the larger the droplet interface tension, the smaller the droplet diameter, the smaller the flow rate, and the shorter the droplet coalescence time. The results provide a theoretical basis for the application of electrostatic coalescence in gas–liquid separation technology.     

Numerical integration of overlap and ligand contributions to the electric field gradient

The total electric field gradient (EFG) tensor V _pq is calculated by numerical integration of threedimensional integrals. Each of them is solved a) by integrating over one dimension analytically and b) by integrating over the remaining two dimensions on the basis of a Gauss-type integration rule. The use of 100 abscissas in the twodimensional numerical integration scheme yields satisfactory accuracy which was checked by evaluating overlap integrals; an increase to 400 abscissas does not increase the result drastically. Calculating quadrupole splittings E _Q from numerically integrated electric field gradient tensors V _pq we observe that depending a) on the amount of covalency and b) on the amount of deviation from octahedral or tetrahedral symmetry, involved in a molecular system, overlap and ligand contributions to V _pq play an important role. Especially for the sandwich compound ferrocene, Fe(C₅H₅)₂, we find a significant difference between E _Q ^{num. int.} which follows from the numerical integration method, and E _Q ^conventional which is derived from effective charges.Supported by Deutsche Forschungsgemeinschaft     

Variations of the shape of123Sb NQR tines in phase transitions in K2SbF5

Variations of the shape of¹²³Sb NQR lines in phase transitions in a new ionic conductor, potassium pentafluoroantimonite(ui), in the temperature range from 77 to 430 K were studied. The transition from a paraelectric phase to a commensurate phase occurs via incommensurate and commensurate-modulated phases.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2497–2500, October, 1996.     

Study of the EFG tensor at 75As nuclei in Ge‐As‐Se chalcogenide glasses

Asymmetry parameters of the electric field gradient tensor at ⁷⁵As nuclei were determined for chalcogenide glassy semiconductors (CGS) of the Ge‐As‐Se system by comparing the experimental and simulated ⁷⁵As nuclear quadrupole resonance nutation interferograms. The electric field gradient asymmetry in CGS was analyzed, and it is believed that a structural change in these glassy semiconductors takes place at = 2.425. Electron paramagnetic resonance spectra of the Ge‐As‐Se system were obtained for the first time. A comparison was made between the results of analysis of the Ge‐As‐Se system by nuclear quadrupole resonance and electron paramagnetic resonance methods, and this allowed us to make the supposition that a structural phase transition occurs at = 2.4 from two‐dimensional to three‐dimensional CGS structure. Copyright © 2013 John Wiley & Sons, Ltd.     

Coordination geometry of tetradentate Schiff’s base nickel complexes: the effects of donors, backbone length and hydrogenation

The X-ray crystal structures of (N,N′-bis-(o-amidobenzilidene)-1,3-diaminopropane)nickel (Niambpr), (N,N′-bis-(o-amidobenzilidene)-1,4-diaminobutane)nickel (Niambut), (N,N′-bis-(o-thiobenzilidene)-1,4-diaminobutane)nickel(II) (Nitsalbut), bis-acetonitrile-(N,N′-bis-(o-aminobenzyl)-1,2-diaminoethane) nickel(II) tetrafluoroborate [Ni(H₄amben)(MeCN)₂] [BF₄]₂, bis-O-acetato-(N,N′-bis-(o-aminobenzyl)-1,2-diaminoethane) nickel(II) [Ni(H₄amben)(OAc)₂ · H₂O] and bis-O-acetato-(N,N′-bis-(o-aminobenzyl)-1,3-diaminopropane) nickel(II) [Ni(H₄ambpr)(OAc)₂] are presented. These structures complete the structural characterisation of the simple unsubstituted Schiff’s base complexes with N₄ and N₂S₂ donor sets and allow us to assess the effects of donor groups and polymethylene chain length on the coordination geometries of nickel(II). The hydrogenated N₄ complexes offer an insight into the effects of increased flexibility and character of the internal nitrogen donors. Unlike the parent N₄ imine species the hydrogenated amine species do not deprotonate at the peripheral nitrogen donors and do not seem to be restricted to the meridial plane of the nickel.     

Determination of polycyclic aromatic hydrocarbons in water samples by gas chromatography coupled to triple quadrupole tandem mass spectrometry using macroporous resin solid‐phase extraction

A method for the determination of 16 polycyclic aromatic hydrocarbons in water has been developed. First, we made a solid‐phase extraction column. After this, the parameters affecting the efficacy of the experimental method were optimized, including appropriate selection of a solid‐phase extraction column and cleanup conditions on columns. The separation was achieved by gas chromatography and detection with triple quadrupole tandem mass spectrometry. The method showed satisfactory linearity (R² > 0.999) over the range assayed (0.01–1 μg/mL), and limits of quantification ranging from 0.0011 to 0.0199 μg/L. The recoveries ranged from 83 to 113%. The relative standard deviation is in the range 0.86–3.1%. The results indicated that this method had high selectivity and precision that was suitable for the simultaneous determination of 16 polycyclic aromatic hydrocarbons in water.     

Simultaneous Determination of Nitrite and Hydrogen Peroxide Using Hemoglobin Modified Pencil Lead Electrode as a Novel Biosensor: Application to the Analysis of Mother’s Milk

In the present paper simultaneous determination of nitrite and hydrogen peroxide using hemoglobin modified pencil lead electrode (Hb/PLE) prepared by a simple and rapid electroless method was described. In the first part of the work the reduction of NO₂ and H₂O₂ at the modified electrode was investigated by cyclic voltammetry. Then under optimal conditions using differential pulse voltammetry, the biosensor could be used for the determination of H₂O₂ at concentration ranging from 5 to 240 M and NO₂ at concentration ranging from 10 to 240 M. The detection limits were 3×10^?6 and 5×10^?6 M, for NO₂ and H₂O₂ respectively. Differential pulse voltammetry also used for the simultaneous determination of NO₂ and H₂O₂. This modified electrode successfully used for the determination of NO₂ and H₂O₂ in tap water and mother’s milk samples.     

Effect of electric field on the partitioning behavior of solutes in entropic interaction chromatography

In this study, a novel column design with a round cross‐section was proposed to be suitable for a transverse electric field (EF). Additionally, two beads for entropic interaction chromatography (EIC) were prepared by grafting glycidyl methacrylate onto Toyopearl HW‐65F (T65F) beads. Solute partitioning was then investigated to elucidate the role of graft polymerization with and without an EF. In a T65F column, solute partitioning was attributed to the distinct pore structure in the beads and was governed by pore flow. Under EF, partition coefficients (K_p) for solutes decreased with increasing EF strength. In the two EIC columns, a decrease of K_p was also observed without an EF while the fractionation windows were extended. It was more pronounced in the EIC column with a high grafting density (T65F‐H). This was explained by the decrease in the effective pore size of solutes caused by the steric hindrance of polymer chains. Under an EF, the solutes showed different partitioning behaviours in the T65F‐H column. With increasing EF strength, K_p for vitamin B₁₂ and myoglobin was decreased. In contrast, K_p for large solutes increased as a result of concentration polarization on the bead surface. Both behaviors were related to the modulation of graft polymerization to residual charge on the matrix and the pore size of the solutes.     

Determination of neonicotinoid insecticides in environmental samples by micellar electrokinetic chromatography using solid‐phase treatments

A sensitive and reliable method based on MEKC has been developed and validated for trace determination of neonicotinoid insecticides (thiamethoxam, acetamiprid, and imidacloprid) and the metabolite 6‐chloronicotinic acid in water and soil matrices. Optimum separation of the neonicotinoid insecticides was obtained on a 58 cm long capillary (75 μm id) using as the running electrolyte 40 mM SDS, 5 mM borate (pH 10.4), and 5% (v/v) methanol at a temperature of 25°C, a voltage of 25 kV and with hydrodynamic injection (10 s). The analysis time was less than 7 min. Prior to MEKC determination, the samples were purified and enriched by carrying out extraction‐preconcentration steps. For aqueous samples, off‐line SPE with a sorptive material such as Strata‐X (polymeric hydrophobic sorbent) and octadecylsilane (C₁₈) was carried out to clean up and preconcentrate the insecticides. However, for soil samples, matrix solid‐phase dispersion (MSPD) was applied with C₁₈ used as the dispersant. Good linearity, accuracy, and precision were obtained and the detection limits were in the range between 0.01 and 0.07 μg mL^?1 for river water and 0.17 and 0.37 μg g^?1 for soil samples. Recovery levels reached greater than 92% for all of the assayed neonicotinoids in river water samples with Strata‐X. In soil matrices, the best recoveries (63–99%) were obtained with MSPD.     

Dual gradient LC method for the determination of pharmaceutical residues in environmental samples using a monolithic silica reversed phase column

Detailed below is a simple reversed-phase liquid chromatography (RP-LC) method for the simultaneous separation of up to 21 acidic, basic, and neutral pharmaceuticals using Merck Chromolith Performance RP-C18e monolithic columns with direct ultraviolet (UV) absorption detection. By simultaneously applying a solvent elution gradient program with a mobile phase flow gradient, both a decrease in the overall analysis time and a general increase in peak efficiencies were observed. Mobile phase pH and buffer concentration were optimised using the overall resolution product under applied gradient conditions. Under optimised conditions peak area reproducibility (n?=?6) ranged between 0.4 and 9.3%, determined at the method LOQ level. For real sample analysis pharmaceutical residues were extracted using an optimised solid phase extraction (SPE) procedure, utilising Strata-X extraction cartridges, which overall provided the highest relative recovery data in comparison with four other commercially available SPE sorbents (17 out of 20 residues investigated had recoveries over 70%). Complete method precision, including all sample pre-treatment and LC analysis for six spiked river water samples at the 1 and 2?µg?L^?1 level was between 10 and 29%. Using 1?L volumes of 1?µg?L^?1 spiked estuarine water samples, the majority of detection limits were found to be in the 10–50?ng?L^?1 range.     

Determination of free formaldehyde in vaccines and biological samples using solid‐phase microextraction coupled to GC–MS

A headspace solid‐phase microextraction method coupled to GC–MS was successfully developed for the trace determination of formaldehyde in veterinary bacterial and human vaccines, and diphtheria–tetanus antigen. The formaldehyde was derivatized by means of the Hantzsch reaction prior to extraction and subsequent determination. Three different types of solid‐phase microextraction fibers, polar, and nonpolar poly(dimethylsiloxane) and polyethylene glycol were prepared by using a sol–gel technique. The effects of different parameters such as type of fiber coating, extraction time and temperature, desorption conditions, agitation rate, and salt effect were investigated. Under the optimized conditions, the detection limit of the method was 979 ng/L using the selected ion‐monitoring mode. The interday and intraday precisions of the developed method under the optimized conditions were below 13%, and the method shows linearity in the range of 1.75–800 μg/L with a correlation coefficient of 0.9963. The optimized method was applied to the determination of formaldehyde from some biological products. The results were satisfactory compared to the standard method.     

One‐Pot Semisynthesis of Exon 1 of the Huntingtin Protein: New Tools for Elucidating the Role of Posttranslational Modifications in the Pathogenesis of Huntington’s Disease

The natural enzymes involved in regulating many of the posttranslational modifications (PTMs) within the first 17 residues (Nt17) of Huntingtin exon 1 (Httex1) remain unknown. A semisynthetic strategy that allows the site‐specific introduction of PTMs within Nt17 by using expressed protein ligation (EPL) was developed. This strategy was used to produce untagged wild‐type (wt) and T3‐phosphorylated (pT3) Httex1 containing 23 glutamine residues (Httex1‐23Q). Our studies show that pT3 significantly slows the oligomerization and fibrillization of Httex1‐23Q and that Httex1 variants containing polyQ repeats below the pathogenic threshold readily aggregate and form fibrils in vitro. These findings suggest that crossing the polyQ pathogenic threshold is not essential for Httex1 aggregation. The ability to produce wt or site‐specifically modified tag‐free Httex1 should facilitate determining its structure and the role of N‐terminal PTMs in regulating the functions of Htt in health and disease.