Continuous Variation Method Research Articles

Spectroscopic evidence on improvement in complex formation of O2N2 aza-crown macrocyclic ligands with Cu(II) acetate upon incorporation with [60]Fullerene

The present paper reports the spectroscopic investigations on the complexation of Cu(II) with two macrocyclic ligands bonded to [60]Fullerene (L1 and L2) measured in N-methylpyrrolidone (NMP) as solvent. On the basis of UV–vis-NIR spectroscopy applying Jobs method of continuous variation, typical 1:1 stoichiometries were established for the complexes of Cu(II) with L1, and L2. DFT calculations suggested that superior HOMO distributions spread over the nitrogen-donor (as well as somehow oxygen- donor in L2) groups of L1 and L2 macrocycles were the key factor for the observed Kb value enhancement. Thermodynamic stabilities for these complexes have also been determined employing Benesi-Hildebrand equation and the results were compared in terms of their calculated binding constants (Kb). These measurements showed that L1 and L2 bound to these cations stronger than their parent free macrocyclic ligands 1 and 2, respectively. Furthermore, Kb values found for L2 complexes revealed that it could coordinate Cu(II) cation better than L1. Thermodynamic parameters (ΔG, ∆H, and −ΔS) derived from Van't Hoff equation showed that L1 and L2 coordination of Cu(II) cation were occurred due to both enthalpic and entropic factors while the coordination of Cu(II) with their parent macrocyclic ligands 1 and 2 only enjoyed from only enthalpic advantages.

Radar Parameter Design for Geosynchronous SAR in Squint Mode and Elliptical Orbit

Low-inclined elliptical orbits are recommended for geosynchronous synthetic aperture radar (GEO SAR) to map the regions of interest at middle latitudes. However, the radius variation of such orbit results in a time-variant nadir interference and slant range of illuminated regions. These factors shorten the available imaging time for a specific pulse repetition interval (PRI), which may be even shorter than the required integration time. Besides, the squint mode widely used in GEO SAR brings in the nonorthogonal and nonuniform ground resolution, which cannot be completely described by the traditional range and azimuth resolutions. To solve these problems, first, the ground resolution area is proved to be an ellipse, and the axis lengths of the resolution ellipse are used to present the worst and the best resolutions along various directions. Conditions for orthogonal and uniform ground resolutions are investigated, and analytical expressions of the resolution-related parameters are derived. Second, effects of the orbit radius variation on data acquisitions are analyzed, and a continuous PRI variation method is proposed to lengthen the available imaging time. By varying the PRI with a constant time interval, the acquisition window and transmit interference are changed with the time-variant illuminated region, which loosens the constraint of the transmit interference. Finally, the proposed ground resolution expressions and continuous PRI variation method are verified by computer simulations.

Alkene Hydrosilylation Using Tertiary Silanes with α-Diimine Nickel Catalysts. Redox-Active Ligands Promote a Distinct Mechanistic Pathway from Platinum Catalysts

Combination of the readily available α-diimine ligand, ((ArN═C(Me))2 Ar = 2,6-iPr2–C6H3), (iPrDI) with air-stable nickel(II) bis(carboxylates) generated a highly active catalyst exhibiting anti-Markovnikov selectivity for the hydrosilylation of alkenes with a variety of industrially relevant tertiary alkoxy- and siloxy-substituted silanes. A combination of the method of continuous variations with stoichiometric studies identified the formally Ni(I) hydride dimer, [(iPrDI)NiH]2 as the nickel compound formed following reduction of the carboxylate ligands. For the hydrosilylation of 1-octene with (EtO)3SiH, a rate law of [Ni]1/2[1-octene][(EtO)3SiH] in combination with deuterium-labeling studies establish dissociation of the nickel hydride dimer followed by fast and reversible alkene insertion into (iPrDI)NiH, consistent with turnover-limiting C–Si bond formation. The hydrosilylation of 1-octene with triethoxysilane, a reaction performed commercially in the silicones industry on a scale of >5 000 000 kg/year...

Lithium Enolates Derived from Pyroglutaminol: Aggregation, Solvation, and Atropisomerism.

Lithium enolates derived from protected pyroglutaminols were characterized by using (6)Li, (13)C, and (19)F NMR spectroscopies in conjunction with the method of continuous variations. Mixtures of tetrasolvated dimers and tetrasolvated tetramers in different proportions depend on the steric demands of the hemiaminal protecting group, tetrahydrofuran concentration, and the presence or absence of an α-fluoro moiety. The high steric demands of the substituted bicyclo[3.3.0] ring system promote dimers to an unusual extent and allow solvents and atropisomers in cubic tetramers to be observed in the slow-exchange limit. Pyridine used as a (6)Li chemical shift reagent proved useful in assigning solvation numbers.

NMR study and computational assays of meclofenamic Na salt and β-cyclodextrin inclusion complex

Complexation in solution between meclofenamate sodium (MCF) and β-cyclodextrin was studied using one- and two-dimensional 1H NMR spectroscopy and molecular docking coupled with ab initio calculations. The large variation of chemical shifts from protons located inside the hydrophobic cavity of β-cyclodextrin, provided clear evidence of inclusion complexation. The stoichiometry of the inclusion complex was determined to be 1:1, using the method of continuous variation. To ascertain the solution geometry of the host–guest complex a ROESY experiment was carried out. The results suggested a preferential binding of the dichlorophenyl moiety of the guest molecule within the β-cyclodextrin cavity, conformation which also sustained by the theoretical computational investigations. The association constant of the inclusion complex was determined using 1H NMR titration method in solution followed by a non-linear least-square regression implemented in CONSTEQ, a software developed in our group. After a preliminary molecular docking investigation, the most probable conformation was subjected to ab initio calculations using SIESTA software package, to characterize more precisely the stabilization energy of the 1:1 inclusion complex.

Application of the de Job method in the evaluation of the stoichiometry of uranyl phosphate complexes sorbed on bentonite

For the first time, the continuous variation method was applied for the evaluation of the stoichiometry of uranyl phosphate complexes sorbed on bentonite. Sorption of UO 2 (CH 3 COO) 2 ⋅2H 2 O in the presence of Na 2 HPO 4 ⋅7H 2 O from 0.001 mol/L solutions led to the appearance of maxima in the sorption peaks of U(VI) and P(V) ions at molar ratios of [U(VI)]/[P(V)] s = 1.4, 3.3, 3.6 and 1.2, 1.7. It is suggested, based on complementary XRD and XPS data, that the UO 2 HPO 4 complex is located on aluminols (oAl-OH) whereas the (UO 2 ) 3 (PO 4 ) 2 ⋅4H 2 O complex is precipitated in the interlamellar space of bentonite. The participation of (UO 2 ) 3 (OH) 5 + and (UO 2 ) 4 (OH) 7 + species in the formation of U(VI) surface complexes is suggested, based on the deconvolution of sorption spectra of U(VI) on the bentonite in the presence of phosphates.

Nickel-Catalyzed Asymmetric Alkene Hydrogenation of α,β-Unsaturated Esters: High-Throughput Experimentation-Enabled Reaction Discovery, Optimization, and Mechanistic Elucidation.

A highly active and enantioselective phosphine-nickel catalyst for the asymmetric hydrogenation of α,β-unsaturated esters has been discovered. The coordination chemistry and catalytic behavior of nickel halide, acetate, and mixed halide-acetate with chiral bidentate phosphines have been explored and deuterium labeling studies, the method of continuous variation, nonlinear studies, and kinetic measurements have provided mechanistic understanding. Activation of molecular hydrogen by a trimeric (Me-DuPhos)3Ni3(OAc)5I complex was established as turnover limiting followed by rapid conjugate addition of a nickel hydride and nonselective protonation to release the substrate. In addition to reaction discovery and optimization, the previously unreported utility high-throughput experimentation for mechanistic elucidation is also described.

Recognizing the Limited Applicability of Job Plots in Studying Host-Guest Interactions in Supramolecular Chemistry.

Continuous variation method, known as Job plot, is the most commonly applied method for the determination of stoichiometry of complex chemical entities for over 100 years. Although, the method was proven successful in the analysis of very stable metal-ligand complexes, we demonstrate that its use in supramolecular chemistry often provides false results. We support this statement with multiple simulations as well as cases studies of several real host-guest systems. We propose an alternative, general method relying on the analysis of residual distribution in titration data fitting. The latter method is more convenient compared to the Job plot and unlike it gives correct results in all real cases studied.

Spectrophotometric Determination of Cu2+ Metal Ions via Complex Formation with Carboxylated Tris(2-aminoethyl)amine

An aminopolycarboxylic acid chelating agent; tris(2-aminoethyl)aminehexaacetic acid (TAHA) was prepared and characterized. TAHA formed stable complexes with Cu(II) ions and other metal ions at pH 10. The complexation behavior was studied by spectrophotometry at the complex maximum wavelength. Mixtures of Cu(II) and other metal ions (M2+; Ca2+, Co2+, Ni2+, Cd2+)were titrated with TAHA at pH 10. The stoichiometry of ligand - to - metal was found to be 1:1. The stability constant of Cu(II)–TAHA complex was determined to be 1.86×105 by using the continuous variation method. Beer’s law was obeyed over the concentration range 3.0×10–4 M – 1.2×10–2 M for Cu(II) solution. The results of the quantitative determination of Cu(II) gave LOD and LOQ values of 7.285×10–6 M and 2.428×10–5 M respectively. The percent relative standard deviation (%RSD) for five replicate samples was found to be 1.088% and 4.804% for Cu(II) concentrations of 1.2×10–2 M and 3.0×10–4 M respectively.

Extractive Spectrophotometric Determination of Platinum in Cisplatin Injection, Alloys and Catalysts Assisted by 2-nitrobenzaldehydethiocarbohydrazone

A simple, rapid, selective and sensitive spectrophotometric method for the determination of platinum(IV) was developed, based on the color reaction between platinum(IV) and 2nitrobenzaldehydethiocarbohydrazone (2-NBATCH) in the pH range 6.4-7.8. The red colored species have been developed after heating the reaction mixture in boiling water bath for 5 min and it was extracted into chloroform. The complex has an absorption maximum at 440 nm. A 20 fold of excess of reagent was required for complete complex formation. Beer’s law was obeyed for platinum(IV) concentration in the range of 4-12 µg mL-1 and the optimum concentration range was 6-12 µg mL-1 of platinum(IV) as evaluated by Ringbom’s plot. The molar absorptivity and Sandell’s sensitivity were 1.03  104 L mol-1 cm-1 and 0.0189 µg cm-2, respectively. The effect of pH, heating and extraction time, concentration of reagent and interference from various ions were investigated. The stoichiometry of the extracted complex was determined by Job’s method of continuous variation, mole ratio method and slope ratio method. It was found that metal to ligand ratio was 1:2. The developed method has been successfully applied to the determination of platinum(IV) in pharmaceutical samples, catalyst and synthetic alloy sample.

Significance of Jobs Plot in Cyclodextrin Complexation

Jobs plot or continuous variation method is used to estimate the stoichiometry in cyclodextrin complexation. Cyclodextrin complexation is well known as widely acceptable technique for improving the poor physicchemical properties of BCS class II and class IV drugs. The stoichiometry of the inclusion complex is given by the number of guest and host molecules present in the supramolecular complex/inclusion system. The most common stoichiometry is 1: 1, implying the inclusion of a single guest molecule. Now-a-days jobs plot is replaced by phase-solubility study. Jobs plot deals with spectral methods mainly for molecular modelling study of drugs and bioactives.

Development of Extractive Spectrophotometric Method for the Determination of Osmium (IV) with Schiff base 2-[(2-Hydroxy Phenyl Imino) Methyl]-4-Nitro Phenol.

A simple spectrophotometric method has been developed for the determination of Osmium (IV) by using Schiff base 2-[(2-hydroxyphenylimino) methyl]-4-nitrophenol [HPIMNP]. HPIMNP extracts Os (IV) quantitatively (99.94%) into iso-amyl alcohol from an aqueous solution of pH range 0.0–2.0. The iso-amyl alcohol extracts show maximum absorption at 545 nm (λ max). Beer's Law is obeyed over the Os (IV) concentration range of 1.5 to 24.0 μg/ml. The Molar absorptivity and Sandell's sensitivity for Os-HPIMNP system is 3700.00L mole−1 cm −1 and 0.051μg.cm−2 respectively. The composition of extracted species is found to be 1: 3 [Os-HPIMNP] by Job's continuous variation and Mole-ratio method. Interference by various ions has been studied. The proposed method is rapid, sensitive, reproducible and accurate and it has been satisfactory applied for determination of Osmium (IV) in Synthetic mixture samples.

The role of cyclodextrin-tetrabutylammonium complexation on the cellulose dissolution

Cellulose dissolution is a challenging process which is typically very sensitive to the solvent characteristics such as pH, temperature or presence of additives. Regarding the later aspect, it is here reported the interaction between α-cyclodextrin (α-CD) and β-cyclodextrin (β-CD) with the tetrabutylammonium cation (TBA+) by 1H NMR titration experiments. The analysis by the continuous variation method suggests the formation of 1:1 CD:TBA+ complexes. However, the computed apparent association constants reveal that the interaction of TBA+ with the β-CD (K=1580M−1) is unexpectedly stronger than with α-CD (K=106M−1). In both CD cases, the formation of CD:TBA+ complexes decrease the dissolution efficiency of the solvent and this has been rationalized as an effective decrease in the concentration of the amphiphilic cation and concomitant weakening of the hydrophobic interactions in solution influencing the overall performance of the solvent. Additionally, the data also supports the fact that amphiphilic species in solution are beneficial for the enhancement of cellulose solubility.

Propericiazine as a Selective and Sensitive Reagent for the Spectrophotometric Determination of Microgram Amounts of Platinium in Minerals

Propericiazine(PPC) is proposed as a new sensitive and selective reagent for the spectrophotometric determination of microgram amounts of platinum. PPC forms a pink complex with platinum(IV) at room temperature (26 + 2 0 C) in hydrochloric acid - sodium acetate buffer medium containing copper (II) ions. The complexation is complete within 10 min. The complex exhibits an absorption maximum at 520 nm with a molar absorptivity of 1.204 X10 4 liters mol -1 cm -1 . Beer’s law is obeyed over the concentration range 0.5-12.2 ppm of platinum. A 50-fold molar excess of the chromogenic regent is necessary for the development of maximum colour Intensity. Job’s method of continuous variation, the molar ratio method, and the slope- ratio method indicate a 1:1 composition for the complex. The effects of pH, time, temperature, reagent concentration, order of addition of reagents and interference for various ions are reported. The reagent has been used successfully for the determination of platinum in minerals and alloys.

Enteric-coated tablet of risedronate sodium in combination with phytic acid, a natural chelating agent, for improved oral bioavailability

The oral bioavailability (BA) of risedronate sodium (RS), an antiresorptive agent, is less than 1% due to its low membrane permeability as well as the formation of non-absorbable complexes with multivalent cations such as calcium ion (Ca2+) in the gastrointestinal tract. In the present study, to increase oral BA of the bisphosphonate, a novel enteric-coated tablet (ECT) dosage form of RS in combination with phytic acid (IP6), a natural chelating agent recognized as safe, was formulated. The chelating behavior of IP6 against Ca2+, including a stability constant for complex formulation was characterized using the continuous variation method. Subsequently, in vitro dissolution profile and in vivo pharmacokinetic profile of the novel ECT were evaluated comparatively with that of the marketed product (Altevia, Sanofi, US), an ECT containing ethylenediaminetetraacetic acid (EDTA) as a chelating agent, in beagle dogs. The logarithm of stability constant for Ca2+–IP6 complex, an equilibrium constant approximating the strength of the interaction between two chemicals to form complex, was 19.05, which was 3.9-fold (p<0.05) and 1.7-fold (p<0.05) higher than those of Ca2+–RS and Ca2+–EDTA complexes. The release profile of RS from both enteric-coated dosage forms was equivalent, regardless of the type of chelating agent. An in vivo absorption study in beagle dogs revealed that the maximum plasma concentration and area under the curve of RS after oral administration of IP6-containing ECT were approximately 7.9- (p<0.05) and 5.0-fold (p<0.05) higher than those of the marketed product at the same dose (35mg as RS). Therefore, our study demonstrates the potential usefulness of the ECT system in combination with IP6 for an oral therapy with the bisphosphonate for improved BA.

Spectrophotometric, FTIR and Theoretical Studies of the Charge-transfer Complexes Between Nicotinamide and the Acceptors (p-Chloranil, Chloranilic Acid and DDQ) in Acetonitrile, Their Association Constants, Thermodynamic Properties and Other Related Properties

Nicotinamide, an important vitamin in our bodies, was found to form beautifully colored charge-transfer complexes with p-chloranil, chloranilic acid and 2,3-dichloro-4,5-dicyano benzoquinone in acetonitrile with absorption maxima at 474.5, 519.0 and 614.0 nm, respectively. The compositions of the complexes were determined to be 1:1 from Job’s method of continuous variation. The measured FTIR spectra of the solid complexes formed between nicotinamide and the acceptors and the corresponding individual constituents showed considerable shifts in absorption peaks, changes in intensities of the peaks and formation of the new bands upon complexation. The thermodynamic association constants K DA and other thermodynamic parameters (ΔG°, ΔH° and ΔS°) of the complexes, determined spectrophotometrically taking D (donor) and (acceptor) A in varying ratios (2:8–8:2) and also in equimolar ratios, showed the complex formation to be spontaneous. The experimental hν CT and the theoretical hν CT values (calculated using density function theory) of the charge-transfer complexes are compared. The hν CT (experimental) values of the transition energies of the complexes in acetonitrile differ from hν CT (theoretical) values in the gaseous state. The $$ I_{\text{D}}^{\text{V}} $$ value of nicotinamide was calculated. A comparative study of the energetics of the partial electrovalent charge-transfer complexes with the organic dative ions D+ and A− (in the excited state) and true electrovalent compounds between M+ and X− is presented.

Spectrophotometric study on the charge transfer reaction between 2-amino-4-methylpyridine with chloranilic acid in polar solvents

ABSTRACTCharge transfer (CT) complexes formed between 2-amino-4-methylpyridine as electron donor, chloranilic acid as electron acceptor was investigated spectrophotometrically in acetonitrile (AN), methanol (MeOH) and binary mixture of acetonitrile 50% + methanol 50% (MeOH-AN). Minimum–maximum absorbance method has been used for estimating the formation constants of the CT reactions (KCT). Job’s method of continuous variation and photometric titration studies were used to detect the stoichiometric ratios of the formed complexes, and they showed that 1:1 complexes were produced. The molar extinction coefficient (e), oscillator strength (f), dipole moment (l), CT energy (ECT), ionisation potential (IP) and the dissociation energy (W) of the formed complexes were estimated; they reached acceptable values suggesting the stability of the formed CT complexes. The solid CT complexes were synthesised and characterised by elemental analyses, 1H NMR and FTIR spectroscopies where the formed complexes included proton and electron transfer.

Solid-State and Solution Structures of Glycinimine-Derived Lithium Enolates.

A combination of crystallographic, spectroscopic, and computational studies was applied to study the structures of lithium enolates derived from glycinimines of benzophenone and (+)-camphor. The solvents examined included toluene and toluene containing various concentrations of tetrahydrofuran, N,N,N',N'-tetramethylethylenediamine (TMEDA), (R,R)-N,N,N',N'-tetramethylcyclohexanediamine [(R,R)-TMCDA], and (S,S)-N,N,N',N'-tetramethylcyclohexanediamine [(S,S)-TMCDA]. Crystal structures show chelated monomers, symmetric disolvated dimers, S4-symmetric tetramers, and both S6- and D3d-symmetric hexamers. (6)Li NMR spectroscopic studies in conjunction with the method of continuous variations show how these species distribute in solution. Density functional theory computations offer insights into experimentally elusive details.

Spectrophotometric Determination of Selenium in Various Samples Using 5,6-Diaminouracil Hydrochloride (DAUH)

A simple, rapid and sensitive spectrophotometric method was developed for the determination of Se (IV) using 5,6-diaminouracil hydrochloride (DAUH) as an analytical reagent. The reagent has been synthesized and characterized using IR, 1H NMR. The metal ion in aqueous medium forms yellow colored complex with DAUH showing maximum absorbance at 343 nm. Hence, analytical studies were further carried out at 343 nm. The reagent reacts with selenium in acidic medium to form yellow colored 1:1 (M:L) complex . The color reactions are instantaneous and absorbance values remain constant for one week. The composition of the Se (IV) complex with DAUH was studied by the method of job’s continuous variation and molar ratio method. Beer’s law was obeyed in the range 0.2-15.0 μg mL−1of Se (IV). The molar absorptivity and Sandell’s sensitivity of the method were found to be 0.5×10+4 L.mol-1.cm-1and 0.031μg.cm-2 respectively. Since DAUH method is more sensitive, it was applied for the determination of selenium in samples.

Spectrophotometric Determination of Selenium in Various Samples Using 5,6-Diaminouracil Hydrochloride (DAUH)

A simple, rapid and sensitive spectrophotometric method was developed for the determination of Se (IV) using 5,6-diaminouracil hydrochloride (DAUH) as an analytical reagent. The reagent has been synthesized and characterized using IR, 1H NMR. The metal ion in aqueous medium forms yellow colored complex with DAUH showing maximum absorbance at 343 nm. Hence, analytical studies were further carried out at 343 nm. The reagent reacts with selenium in acidic medium to form yellow colored 1:1 (M:L) complex . The color reactions are instantaneous and absorbance values remain constant for one week. The composition of the Se (IV) complex with DAUH was studied by the method of job’s continuous variation and molar ratio method. Beer’s law was obeyed in the range 0.2-15.0 μg mL−1of Se (IV). The molar absorptivity and Sandell’s sensitivity of the method were found to be 0.5×10+4 L.mol-1.cm-1and 0.031μg.cm-2 respectively. Since DAUH method is more sensitive, it was applied for the determination of selenium in samples.