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91.
While operational qualification (OQ) is a well-established term within equipment qualification, users of equipment often become unsure when it comes to implementation. The biggest problem is how to select procedures and acceptance criteria. Should these be the vendor's specifications or should the users define their own limits, and, if so, how? Should all instruments of the same type have the same values or should these be optimized for each individual instrument? This article will provide an overall strategy and specific examples for HPLC on how to select procedures and acceptance limits that are based on efficient use of resources, on practicality and on the intended use of the equipment. 相似文献
92.
Cody Loy Jana M. Holthoff Robert Weiss Stefan M. Huber Sergiy V. Rosokha 《Chemical science》2021,12(23):8246
Halogen-bonded (XB) complexes between halide anions and a cyclopropenylium-based anionic XB donor were characterized in solution for the first time. Spontaneous formation of such complexes confirms that halogen bonding is sufficiently strong to overcome electrostatic repulsion between two anions. The formation constants of such “anti-electrostatic” associations are comparable to those formed by halides with neutral halogenated electrophiles. However, while the latter usually show charge-transfer absorption bands, the UV-Vis spectra of the anion–anion complexes examined herein are determined by the electronic excitations within the XB donor. The identification of XB anion–anion complexes substantially extends the range of the feasible XB systems, and it provides vital information for the discussion of the nature of this interaction.Spontaneous formation of “anti-electrostatic” complexes in solution demonstrates that halogen bonding can be sufficiently strong to overcome anion–anion repulsion when the latter is attenuated by the polar medium.Halogen bonding (XB) is an attractive interaction between a Lewis base (LB) and a halogenated compound, exhibiting an electrophilic region on the halogen atom.1 It is most commonly related to electrostatic interaction between an electron-rich species (XB acceptor) and an area of positive electrostatic potential (σ-holes) on the surface of the halogen substituent in the electrophilic molecule (XB donor).2 Provided that mutual polarization of the interacting species is taken into account, the σ-hole model explains geometric features and the variation of stabilities of XB associations, especially in the series of relatively weak complexes.3 Based on the definition of halogen bonding and its electrostatic interpretation, this interaction is expected to involve either cationic or neutral XB donors. Electrostatic interaction of anionic halogenated species with electron-rich XB acceptors, however, seems to be repulsive, especially if the latter are also anionic. Yet, computational analyses predicted that halogen bonding between ions of like charges, called “anti-electrostatic” halogen bonding (AEXB),4 can possibly be formed5–12 and the first examples of AEXB complexes formed by different anions, i.e. halide anions and the anionic iodinated bis(dicyanomethylene)cyclopropanide derivatives 1 (see Scheme 1) or the anionic tetraiodo-p-benzoquinone radical, were characterized recently in the solid state.13,14 The identification of such complexes substantially extends the range of feasible XB systems, and it provides vital information for the discussion of the nature of this interaction. Computational results, however, significantly depend on the used methods and applied media (gas phase vs. polar environment and solvation models) and the solid state arrangements of the XB species might be affected by crystal forces and/or counterions. Unambiguous confirmation of the stability of the halogen-bonded anion–anion complexes and verification of their thermodynamic characteristics thus requires experimental characterization of the spontaneous formation of such associations in solution. Still, while the solution-phase complexes formed by hydrogen bonding between two anionic species were reported previously,15–17 there is currently no example of “anti-electrostatic” XB in solution.Open in a separate windowScheme 1Structures of the XB donor 1 and its hydrogen-substituted analogue 2.To examine halogen bonding between two anions in solution, we turn to the interaction between halides and 1,2-bis(dicyanomethylene)-3-iodo-cyclopropanide 1 (Scheme 1).‡ Even though this compound features a cationic cyclopropenylium core, it is overall anionic, and calculations have demonstrated that its electrostatic potential is universally negative across its entire surface.13 The solution of 1 (with tris(dimethylamino)cyclopropenium (TDA) as counterion) in acetonitrile is characterized by an absorption band at 288 nm with ε = 2.3 × 104 M−1 cm−1 (Fig. 1). As LB, we first applied iodide anions taken as a salt with n-tetrabutylammonium counter-ion, Bu4NI. This salt does not show absorption bands above 290 nm, but its addition to a solution of 1 led to a rise of absorption in the 290–350 nm range (Fig. 1). Subtraction of the absorption of the individual components from that of their mixture produced a differential spectrum which shows a maximum at about 301 nm (insert in Fig. 1). At constant concentration of the XB donor (1) and constant ionic strength, the intensity of the absorption in the range of 280–300 nm (and hence differential absorbance, ΔAbs) rises with increasing iodide concentration (Fig. S1 in the ESI†). This suggests that the interaction of iodide with 1 results in the formation of the [1, I−]-complex which shows a higher absorptivity in this spectral range (eqn (1)):1 + X− ⇌ [1, X−]1Open in a separate windowFig. 1Spectra of acetonitrile solutions with constant concentration of 1 (0.60 mM) and various concentrations of Bu4NI (6.0, 13, 32, 49, 75, 115 and 250 mM, solid lines from the bottom to the top). The dashed lines show spectra of the individual solutions 1 (c = 0.60 mM, red line) and Bu4NI (c = 250 mM, blue line). The ionic strength was maintained using Bu4NPF6. Insert: Differential spectra of the solutions obtained by subtraction of the absorption of the individual components from the spectra of their corresponding mixtures.To clarify the mode of interaction between 1 and iodide in the complex, we also performed analogous measurements with the hydrogen-substituted compound 2 (see Scheme 1). The addition of iodide to a solution of 2 in acetonitrile did not increase the absorption in the 280–300 nm spectral range. Instead, some decrease of the absorption band intensity of 2 with the increase of concentration of I− anions was observed (Fig. S2 in the ESI†). Such changes are related to a blue shift of this band resulting from the hydrogen bonding between 2 and iodide (formation of hydrogen-bonded [2, I−] complex is corroborated by the observation of the small shift of the NMR signal of the proton of 2 to the higher ppm values in the presence of I− anions, see Fig. S3 in the ESI†).§ Furthermore, since H-compound 2 should be at least as suitable as XB donor 1 to form anion–π complexes with the halide, this finding (as well as solid-state and computational data¶) rules out that any increase in absorption in this region observed with the I-compound 1 may be due to this alternative interaction.Likewise, the addition of NBu4I to a solution of TDA cations taken as a salt with Cl− anions did not result in an increase in the relevant region. Hence, we could also rule out anion–π interactions with the TDA counter-ions as source of the observed changes, which is in line with previous reports on the electron-rich nature of TDA.18All these observations (supported by the computational analysis, vide infra) indicate that the [1, I−] complex (eqn (1)) is formed via halogen bonding of I− with iodine substituents in 1. The changes in the intensities of the differential absorption ΔAbs as a function of the iodide concentration (with constant concentration of XB donor (1) as well as constant ionic strength) are well-modelled by the 1 : 1 binding isotherm (Fig. S1 in the ESI†). The fit of the absorption data produced a formation constant of K = 15 M−1 for the [1, I−] complex (Table 1).|| The overlap with the absorption of the individual XB donor hindered the accurate evaluation of the position and intensity of the absorption band of the corresponding complex which is formed upon LB-addition to 1. As such, the values of Δλmax shown in Table 1 represent a wavelength of the largest difference in the absorptivity of the [1, I−] complex and individual anion 1, and Δε reflects the difference of their absorptivity at this point (see the ESI† for the details of calculations).Equilibrium constants and spectral characteristics of the complexes of 1 with halide anions X−
Open in a separate windowaAll measurements performed in CH3CN at 22 °C, unless stated otherwise.bIn CH2Cl2.cWavelength of the maximum of the differential spectra.dDifferences in extinction coefficients of XB [1, I−] complex and individual 1 at Δλmax.Since earlier computational studies demonstrated substantial dependence of formation of the AEXB complexes on polarity of the medium,6–12 interaction between 1 and I− anions was also examined in dichloromethane. The spectral changes in this moderately-polar solvent were analogous to that in acetonitrile (Fig. S4 in the ESI†). * The values for the formation constants of the [1, I−] complex and Δε (obtained from the fitting of the ΔAbs vs. [I−] dependence) in CH2Cl2 are lower than those in acetonitrile (Table 1). This finding is in line with the computational studies,6–12 predicting stronger binding in more polar solvents.The addition of bromide or chloride salts to an acetonitrile solution of 1 caused changes in the UV-Vis range which were generally similar to that observed upon addition of iodide. The variations of the magnitude of the differential absorption intensities with the increase in the bromide or chloride concentrations are less pronounced than that observed upon addition of iodide (in agreement with the results of the DFT computations of the UV-Vis spectra of the complexes, vide infra). Yet, they could also be fitted using 1 : 1 binding isotherms (see Fig. S5 and S6 in the ESI†). The formation constants of the corresponding [1, Br−] and [1, Cl−] complexes resulted from the fitting of these dependencies are listed in Table 1. The values of K (which correspond to the free energy changes of complex formation in a range of −6 to −8 kJ mol−1) are comparable to those reported for complexes of neutral monodentate bromo- or iodosubstituted aliphatic or aromatic electrophiles with halides.19–22 Thus, despite the “anti-electrostatic” nature of XB complexes between two anions, the stabilities of such associations are similar to that observed with the most common neutral XB donors.In contrast to the similarity in thermodynamic characteristics, the UV-Vis spectral properties of the complexes of the anionic XB donor 1 with halides are substantially different from that reported for the analogous associations with the neutral XB donors. Specifically, a number of earlier studies revealed that intermolecular (XB or anion–π) complexes of halide anions are characterized by distinct absorption bands, which could be clearly segregated from the absorption of the interacting species.21–23 If the same neutral XB donor was used, the absorption bands of the corresponding complexes with chloride were blue shifted, and absorption bands of the complexes with iodide as LB were red shifted as compared to the bands of complexes with bromide. For example, XB complexes of CFBr3 with Cl−, Br− or I− show absorption band maxima at 247 nm, 269 nm and 312 nm, respectively (individual CFBr3 is characterized by an absorption band at 233 nm).21 Within a framework of the Mulliken charge-transfer theory of molecular complexes,24 such an order is related to a rise in the energy of the corresponding HOMO (and electron-donor strength) from Cl− to Br− and to I− anions. In the complexes with the same electron acceptor, this is accompanied by a decrease of the HOMO–LUMO gap, and thus, a red shift of the absorption band. The data in Table 1 shows, however, that the maxima of differential absorption spectra for these systems are observed at roughly the same wavelength. To clarify the reason for this observation, we carried out computational analysis of the associations between 1 and halide anions.The DFT optimization†† at M06-2X/def2-tzvpp level with acetonitrile as a medium (using PCM solvation model)25 produced thermodynamically stable XB complexes between 1 and I−, Br− or Cl− anions (they were similar to the complexes which were obtained earlier via M06-2X/def2-tzvp computations with SMD solvation model13). The calculated structure of the [1, I−] complex is shown in Fig. 2 and similar structures for the [1, Br−] and [1, Cl−] are shown in Fig. S7 in the ESI.†Open in a separate windowFig. 2Optimized geometries of the [1, I−] complex with (3, −1) bond critical points (yellow spheres) and the bond path (green line) from the QTAIM analysis. The blue–green disc indicates intermolecular attractive interactions resulting from the NCI treatments (s = 0.4 a.u. isosurfaces, color scale: −0.035 (blue) < ρ < 0.02 (red) a.u.).QTAIM analysis26 of these structures revealed the presence of the bond paths (shown as the green line) and (3, −1) bond critical points (BCPs) indicating bonding interaction between iodine substituent of 1 and halide anions. Characteristics of these BCPs (electron density of about 0.015 a.u., Laplacians of electron density of about 0.05 a.u. and energy density of about 0.0004 a.u., see Table S1 in the ESI†) are typical for the moderately strong supramolecular halogen bonds.27 The Non-Covalent Interaction (NCI) Indexes treatment28 produced characteristic green–blue discs at the critical points'' positions, confirming bonding interaction in all these complexes.Binding energies, ΔE, for the [1, X−] complexes are listed in Table 2. They are negative and their variations are consistent with the changes in experimental formation constants measured with three halide anions in Table 1. The ΔE value for [1, I−] calculated in dichloromethane is also negative. Its magnitude is lower than that in acetonitrile, in agreement with the smaller formation constant of [1, I−] in less polar dichloromethane.Calculated characteristics of the [1, X−] complexesa
Open in a separate windowaIn CH3CN, if not noted otherwise.bIn CH2Cl2.cExtinction coefficient for the lowest-energy absorption band of the complex.dPosition and extinction coefficient of the differential absorption (see Fig. 3).The TD DFT calculations of the individual XB donor 1 and its complexes with halides (which were carried at the same level as the optimizations) produced strong absorption bands in the UV range (Fig. 3). The calculated spectrum of the individual anion 1 (λmax = 252 nm and ε = 4.27 × 104 M−1 cm−1) is characterized by somewhat higher energy and intensity of the absorption band than the experimental one, but the differences of about 0.6 eV in energy and about 0.3 in log ε are common for the TD DFT calculations.Open in a separate windowFig. 3Calculated spectra of 1 and its complexes (as indicated). The dashed lines show differential absorption obtained by subtraction of absorption of 1 from the absorption of the corresponding complex.The TD DFT calculations of the XB complexes with all three anions produced absorption bands at essentially the same wavelength as that of the individual XB donor 1, but their intensities were higher (in contrast, the hydrogen-bonded complex of 2 with iodide showed absorption band with slightly lower intensity than that of individual 2). The differential spectra obtained by subtraction of the spectra of individual anion 1 from the spectra of the complexes are shown in Fig. 3, and their characteristics are listed in Table 2. Similarly to the experimental data in Table 1, the calculated values of Δλmax are very close in complexes with different halides, and values of Δε are increasing in the order 1·Cl− < 1·Br− < 1·I−.An analysis of the calculated spectra of the complexes revealed that the distinction in spectral characteristics of the XB complexes of anionic and neutral XB donors with halides are related to the differences in the molecular orbital energies of the interacting species. Specifically, the energy of the highest occupied molecular orbital (HOMO) of the anionic XB donor 1 is higher than the energies of the HOMOs of I−, Br− and Cl−, and the energy of the lowest unoccupied molecular orbital (LUMO) of 1 is lower than those of the halides (Table S2 in the ESI†). As such, the lowest-energy electron excitations (with the substantial oscillator strength) in the AEXB complexes involve molecular orbitals localized mostly on the XB donor (see Fig. S8 in the ESI†). Accordingly, the energy of the absorption bands is essentially independent on the halide. Still, due to the molecular orbital interactions between the halides and 1, the small segments of the HOMOs of the complexes are localized on the halides, which affected the intensity of the transitions.‡‡ In contrast, in the XB complexes with the neutral halogenated electrophiles, the energies of the HOMOs and LUMOs of the halides are higher than the energies of the corresponding orbitals of the XB donors. As such, the HOMO of such complexes (as well as the other common molecular complexes) is localized mostly on the XB acceptors (electron donor), and the LUMO on the XB donor (electron acceptor). Accordingly, their lowest energy absorption bands represent in essence charge-transfer transition, and its energy vary with the energies of the HOMO of halides (the TD DFT calculations suggest that similar charge-transfer transitions in complexes of halides with 1 occur at higher energies, and they are overshadowed by the absorption of components).In summary, combined experimental (UV-Vis spectral) and computational studies of the interaction between halides and 1 demonstrated spontaneous formation of the anion–anion XB complexes in moderately-polar and polar solvents (which attenuate the electrostatic anion–anion repulsion and facilitate close approach of the interacting species§§). To the best of our knowledge, this constitutes the first experimental observation of AEXBs in solution. Stabilities of such “anti-electrostatic” associations are comparable to that formed by halide anions with the common neutral bromo- and iodo-substituted aliphatic or aromatic XB donors. These findings confirm that halogen bonding between our anionic XB donor 1 and halides is sufficiently strong to overcome electrostatic repulsion between two anions. It also supports earlier conclusions29 that besides electrostatics, molecular-orbital (weakly-covalent interaction) play an important role in the formation of XB complexes. Since the HOMO of 1 is higher in energy than those of the halides, the lowest-energy absorption bands in the anion–anion complexes is related mostly to the transition between the XB-donor localized MOs (in contrast to the charge transfer transition in the analogous complexes with neutral XB donors). Therefore, the energies of these transitions are similar in all complexes and the interaction with halides only slightly increase their intensities. 相似文献
| Complexa | K [M−1] | Δλmaxc [nm] | 10−3Δεd [M−1 cm−1] |
|---|---|---|---|
| 1·I− | 15 ± 2 | 302 | 9.0 |
| 1·I−b | 8 ± 2 | 303 | 8.0 |
| 1·Br− | 17 ± 2 | 302 | 3.7 |
| 1·Cl− | 40 ± 8 | 302 | 3.0 |
| Complex | ΔE, kJ mol−1 | λ max,c nm | 10−4ε,c M−1 cm−1 | Δλmax,d M−1 cm−1 | 10−3Δε,d M−1 cm−1 |
|---|---|---|---|---|---|
| 1·I− | −14.2 | 252 | 5.70 | 255 | 14 |
| 1·I−b | −4.7 | 253 | 6.07 | — | — |
| 1·Br− | −14.8 | 252 | 5.02 | 253 | 7.4 |
| 1·Cl− | −16.2 | 251 | 4.78 | 249 | 5.3 |
93.
A new fast high-precision on-line technique is described for the determination of hydrogen isotope ratios of water by continuous-flow mass spectrometry. For the first time H(2)/H(2)O-equilibration using a platinum catalyst has been used in a fully continuous process. A significant reduction in the H(2)/H(2)O-equilibration time is achieved by a complete vaporization of the water and by increasing the exchange temperature to 100 degrees C. The analysis time is only approximately 5 min/sample which includes equilibration and processing. Measurement precision and accuracy are better than 1 per thousand and sample consumption is only approximately 5 microL. This new technique allows the measurement of a wide range of aqueous samples either in a semi-continuous way (discrete samples are injected one after another) or in a fully continuous way. This allows us, for the first time, to make continuous measurements of ice cores. 相似文献
94.
The measured partition data of 26 steroids for six different compositions of ternary liquid-liquid systems consisting of 2,2,4-trimethylpentane, ethanol and water are used in calculations of partition data for new phase system compositions. It was possible to reproduce the original data and to verify new experimental data by this calculation with high accuracy. Calculations were done in three ways. In the first, non-linear regression of the partition coefficients as a function of the phase system composition was examined. With this classical approach it is possible to calculate the values of the partition coefficients, solute by solute, with good accuracy. In the second approach, principal component factor analysis was used. The phase system-specific factors for further phase system compositions were calculated with a non-linear regression of these factors on the phase system composition. In combination with the solute-specific factors, the values of the partition coefficients for all solutes in any phase system composition could be predicted with excellent accuracy. In the third approach, target transformation factor analysis was applied. A relationship between the partition coefficient and the phase system composition was established on the basis of either the phase system composition itself or a simple function of it. This method also gave very accurate predictions of known and unknown partition coefficients. 相似文献
95.
96.
97.
Monolithic capillary columns were prepared by copolymerization of styrene and divinylbenzene inside a 200 microm i.d. fused silica capillary using a mixture of tetrahydrofuran and decanol as porogen. Important chromatographic features of the synthesized columns were characterized and critically compared to the properties of columns packed with micropellicular, octadecylated poly(styrene-co-divinylbenzene) (PS-DVB-C18) particles. The permeability of a 60 mm long monolithic column was slightly higher than that of an equally dimensioned column packed with PS-DVB-C18 beads and was invariant up to at least 250 bar column inlet pressure, indicating the high-pressure stability of the monolithic columns. Interestingly, monolithic columns showed a 3.6 times better separation efficiency for oligonucleotides than granular columns. To study differences of the molecular diffusion processes between granular and monolithic columns, Van Deemter plots were measured. Due to the favorable pore structure of monolithic columns all kind of diffusional band broadening was reduced two to five times. Using inverse size-exclusion chromatography a total porosity of 70% was determined, which consisted of internodule porosity (20%) and internal porosity (50%). The observed fast mass transfer and the resulting high separation efficiency suggested that the surface of the monolithic stationary phase is rather rough and does not feature real pores accessible to macromolecular analytes such as polypeptides or oligonucleotides. The maximum analytical loading capacity of monolithic columns for oligonucleotides was found to be in the region of 500 fmol, which compared well to the loading capacity of the granular columns. Batch-to-batch reproducibility proved to be better with granular stationary phases compared to monolithic stationary phase, in which each column bed is the result of a unique column preparation process. 相似文献
98.
The particle scattering behaviour of a pearl necklace chain is derived. The chain is composed of sphere-like pearls, separated by rod-like segments of fixed length, which have no angular restrictions. By calculating several series of model scattering curves, the important structural features are retrieved. The model is believed to be useful in interpreting intermediate structures of collapsing macromolecules or polyelectrolytes. A first application to a shrinking polyelectrolyte coil generated by molecular dynamic simulations (Limbach and Holm, J.Phys.Chem. 2003) is presented and used to discuss the potentials and limits of the model. 相似文献
99.
Thomas Huber Andrew E. Torda Wilfred F. van Gunsteren 《Journal of computer-aided molecular design》1994,8(6):695-708
Summary The concept of memory has been introduced into a molecular dynamics algorithm. This was done so as to persuade a molecular system to visit new areas of conformational space rather than be confined to a small number of low-energy regions. The method is demonstrated on a simple model system and the 11-residue cyclic peptide cyclosporin A. For comparison, calculations were also performed using simulated temperature annealing and a potential energy annealing scheme. Although the method can only be applied to systems with a small number of degrees of freedom, it offers the chance to generate a multitude of different low-energy structures, where other methods only give a single one or few. This is clearly important in problems such as drug design, where one is interested in the conformational spread of a system. 相似文献
100.
Summary This paper reports the results of a study on the use of a new polymer-based, strong anion-exchange, stationary phase for rapid
and selective separation of carbohydrates and related compounds by high-pH, anion-exchange chromatography with pulsed amperometric
detection. The new adsorbent has been obtained by direct nitration of 2.8 μm, spherical non-porous highly cross-linked, styrene-divinylbenzene
copolymer beads, followed by reduction of superficially introduced nitro groups with nascent hydrogen and quaternization of
the resultant amino groups with iodomethane. It is reported that by optimizing the ionic strength of the mobile phase, columns
packed with the new anion-exchanger can be successfully employed to separate, either in isocratic or gradient elution mode,
oligosaccharides, positional isomers of gluco-disaccharides, as well as uronic acids and sugar monophosphates. 相似文献