Kinetic Spectrophotometric Determination of Metoprolol Tartrate in Commercial Dosage Forms

A kinetic spectrophotometric method has been described for the determination of metoprolol tartrate in commercial dosage forms. The procedure is based on the reaction of the drug with 1‐chloro‐2, 4‐dinitrobenzene (CDNB) in dimethylsulfoxide (DMSO) at 100 ± 1 °C. The reaction is investigated by measuring the change in absorbance with time at 420 nm. Fixed‐time (ΔA) and equilibrium methods are chosen for obtaining the calibration curves. Both calibration curves were found to be linear over the concentration range of 5‐60 μg mL^?1. The regression analysis of calibration data resulted in the linear regression equations of ΔA = ?1.608 × 10^?4 + 3.96 × 10^?3 C and A = 7.31 × 10^?4 + 1.90 × 10^?2 C for fixed time (ΔA) and equilibrium methods, respectively. The limit of detection (LOD) for fixed time and equilibrium methods are 1.16 and 0.415 μg mL^?1, respectively. The method has been successfully applied to the quantitation of metoprolol tartrate in commercial dosage forms. Statistical comparison of the results shows that there is no significant difference between the proposed methods and El‐Ries's spectrophotometric method.     

New Spectrophotmetric Methods for Determination of Cephapirine Na

Three sensitive and accurate spectrophotometric procedures were developed for the analysis of cephapirine sodium in pure form and in its pharmaceutical formulation. Method A: A kinetic method based on the observation that in acidic medium cephapirine reduces sodium molybdate to molybdenum blue, the absorbance of which is proportional to the amount of antibiotic present at a fixed time of 40 minutes; the formed product was spectrophotometrically measured at 780 nm. The concentration of drug calculated using its calibration by fixed concentration and rate constant methods is feasible with the calibration equations obtained, but the fixed time method proved to be more applicable. Method B is based on chetale formation with palladium(II) chloride in buffered medium as the interaction between metal ions and ligand anions or moleules capable of the formation of complexes which results in the development of colors suitable for the characterization of quantitative determination of metal or ligand. Metals containing easily excited d or f electrons were suitable for the formation of colored complexes. Method C, is based on the formation of colored complex between palladium(II), eosin and cephapirine Na. Sodium lauryl sulphate is used as surfactant to increase the solubility and intensity of the formed complex. Under optimum conditions, the complexes showed maximum absorption at Δ370 and Δ550 for methods B and C, respectively. Apparent molar absorpitivities were 5.2 × 10³, 5.5 × 10³, 1.4 × 10⁴; Sandell's sensitivities were 1.17 × 10^?3, 1.24 × 10^?3, 3.1 × 10^?3, for methods A, B and C, respectively. The solution of the products obeyed Beer's Law in the concentration ranges 10–70, 20–70, 2–48, μg mL^?1 for methods A, B, and C. The proposed methods were applied to the determination of the drug in pure or pharmaceutical preparations. The results obtained were compared statistically with those given by the official method.     

Multi-spectroscopic Study on the Interaction between CdTe Quantum Dots and Bovine Serum Albumin

The interaction between CdTe quantum dots (QDs) and bovine serum albumin (BSA) was systematically investigated by fluorescence, UV‐vis absorption and circular dichroism (CD) spectroscopy under physiological conditions. The experimental results showed that the fluorescence of BSA could be quenched by CdTe QDs with a static quenching mechanism, indicating that CdTe QDs could react with BSA. The quenching constants according to the modified Stern‐Volmer equation were obtained as 1.710×10⁶, 1.291×10⁶ and 1.010×10⁶ L·mol^?1 at 298, 304, and 310 K, respectively. ΔH, ΔS and ΔG for CdTe QDs‐BSA system were calculated to be ?33.68 kJ·mol^?1, 6.254 J·mol^?1·K^?1 and ?35.54 kJ·mol^?1 (298 K), respectively, showing that electrostatic interaction in the system played a major role. According to F?rster theory, the distance between Trp‐214 in BSA and CdTe QDs was given as 2.18 nm. The UV‐vis, synchronous fluorescence and CD spectra confirmed further that the conformations of BSA after addition of CdTe QDs have been changed.     

MP2, DFT‐D,and PCM study of the HMB–TCNE complex: Thermodynamics,electric properties,and solvent effects

Geometry, thermodynamic, and electric properties of the π‐EDA complex between hexamethylbenzene (HMB) and tetracyanoethylene (TCNE) are investigated at the MP2/6‐31G* and, partly, DFT‐D/6‐31G* levels. Solvent effects on the properties are evaluated using the PCM model. Fully optimized HMB–TCNE geometry in gas phase is a stacking complex with an interplanar distance 2.87 × 10^?10 m and the corresponding BSSE corrected interaction energy is ?51.3 kJ mol^?1. As expected, the interplanar distance is much shorter in comparison with HF and DFT results. However the crystal structures of both (HMB)₂–TCNE and HMB–TCNE complexes have interplanar distances somewhat larger (3.18 and 3.28 × 10^?10 m, respectively) than our MP2 gas phase value. Our estimate of the distance in CCl₄ on the basis of PCM solvent effect study is also larger (3.06–3.16 × 10^?10 m). The calculated enthalpy, entropy, Gibbs energy, and equilibrium constant of HMB–TCNE complex formation in gas phase are: ΔH⁰ = ?61.59 kJ mol^?1, ΔS = ?143 J mol^?1 K^?1, ΔG⁰ = ?18.97 kJ mol^?1, and K = 2,100 dm³ mol^?1. Experimental data, however, measured in CCl₄ are significantly lower: ΔH⁰ = ?34 kJ mol^?1, ΔS = ?70.4 J mol^?1 K^?1, ΔG⁰ = ?13.01 kJ mol^?1, and K = 190 dm³ mol^?1. The differences are caused by solvation effects which stabilize more the isolated components than the complex. The total solvent destabilization of Gibbs energy of the complex relatively to that of components is equal to 5.9 kJ mol^?1 which is very close to our PCM value 6.5 kJ mol^?1. MP2/6‐31G* dipole moment and polarizabilities are in reasonable agreement with experiment (3.56 D versus 2.8 D for dipole moment). The difference here is due to solvent effect which enlarges interplanar distance and thus decreases dipole moment value. The MP2/6‐31G* study supplemented by DFT‐D parameterization for enthalpy calculation, and by the PCM approach to include solvent effect seems to be proper tools to elucidate the properties of π‐EDA complexes. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Interaction Between Plumbagin and Human Serum Albumin by Fluorescence Spectroscopy

The interaction of plumbagin (PLU) with human serum albumin (HSA) in physiological buffer (pH=7.4) was studied by fluorescence spectroscopy. Results obtained from analysis of the fluorescence spectra indicated that PLU has a strong ability to quench the intrinsic fluorescence of HSA through a static quenching procedure. Fluorescence quenching data revealed that the quenching constants (K) are 4.43×10⁴, 3.26×10⁴ and 1.69×10⁴ L?mol^?1 at 293, 303 and 313 K, respectively. The thermodynamic parameters ΔH° and ΔS° were calculated to be ?36.63 kJ?mol^?1, and ?35.702 J?mol^?1?K^?1 respectively, which suggested that van der Waals interactions and hydrogen bonds play a major role in the interaction of PLU with HSA. The distance between donor (HSA) and acceptor (PLU) was calculated to be 3.76 nm based on Förster’s non-radiative energy transfer theory. The results of synchronous fluorescence spectra showed that binding of PLU to HSA can induce conformational changes in HSA.     

Steric and electronic contributions to barriers of internal rotation in 1,8-dibenzoylnaphthalenes

Restricted rotation about the naphthalenylcarbonyl bonds in the title compounds resulted in mixtures of cis and trans rotamers, the equilibrium and the rotational barriers depending on the substituents. For 2,7-dimethyl-1,8-di-(p-toluoyl)-naphthalene (1) ΔH° = 3.66 ± 0.14 kJ mol^?1, ΔS° = 1.67 ± 0.63 J mol^?1 K^?1, ΔH^≠_ct = 55.5 ± 1.3 kJ mol^?1, ΔH^≠_ct = 51.9 ± 1.3 kJ mol^?1, ΔS^≠_ct = ?41.3±4.1 J mol^?1 K^?1 and ΔS^≠_ct = ?42.9±4.1 J mol^?1 K^?1. The rotation about the phenylcarbonyl bond requires ΔH^≠ = ?56.9±4.4 kJ mol^?1 and ΔS^≠ = ?20.5±15.3 J mol^?1 K^?1 for the cis rotamer, and ΔH^≠ = 43.5Δ0.4 kJ mol^?1 and ΔS^≠ =± ?22.4Δ1.3 J mol^?1 K^?1 for the trans rotamer. The role of electronic factors is likely to be virtually the same for both these rotamers but steric interaction between the two phenyl rings occurs in the cis rotamer only. Hence, the difference of the activation enthalpies obtained for the cis and trans rotamers, ΔΔH^?1 = 13.4 kJ mol^?1, provides a basis for the estimation of the role of steric factors in this rotation. For the tetracarboxylic acid 2 and its tetramethyl ester 3 the equilibrium is even more shifted towards the trans form because of enhanced steric and electrostatic interactions between the substituents in the cis form. The barriers for the rotation around the phenylcarbonyl bond and the cis-trans isomerization are lowered; an explanation for this result is presented.     

Study of the system manganese(VII)-3,7-bis(dimethylamino)-phenothiazin-5-ium chloride-water-1,2-dichloroethane Using UV-spectrophotometry

The system manganese(VII)-3,7-bis(dimethylamino)-phenothiazin-5-ium chloride (MB)-water-1,2-dichloroethane has been studied using UV-spectrophotometry. The molar absorptivity of the complex is (3.86 ± 0.06) × 10⁴ L mol^?1 cm^?1 at 290 nm and the system obeys Beer??s law in the range 0.1?C0.99 ??g mL^?1 Mn(VII). The detection limit (DL) and quantitation limit (QL) of Mn(VII) determination were found to be 0.0146 and 0.049 ??g mL^?1, respectively. The composition of the complex is established as MB: MnO ₄ ^? = 1: 1. Extraction investigations of the system discussed were carried out. The characteristic values for the extraction equilibrium and the equilibrium in the aqueous phase was determined: extraction constant K_ex = (1.12 ± 0.05) × 10⁵, distribution constant K_D = 75.61 ± 0.1 and association constant ?? = (1.48 ± 0.08) × 10³. A new method has been developed for the microdetermination of manganese(VII) in plants and steels.     

Study on the Interaction of Raloxifene and Bovine Serum Albumin by the Tachiya Model

The interaction of bovine serum albumin (BSA) with raloxifene was assessed via fluorescence spectroscopy. The number of binding sites and the apparent binding constants between raloxifene and BSA were analyzed using the Tachiya model and Stern-Volmer equation, respectively. The apparent binding constant and the number of binding sites at 298 K were 2.33×10⁵ L?mol^?1 and 1.0688 as obtained from the Stern-Volmer equation and 2.00×10⁵ L?mol^?1 and 2.6667 from the Tachiya model. The thermodynamic parameters ΔH and ΔS were calculated to be 69.46 kJ?mol^?1 and 121.12 J?K^?1?mol^?1, respectively, suggesting that the force acting between raloxifene and BSA was mainly a hydrophobic interaction. The binding distance between the donor (BSA) and acceptor (raloxifene) was 4.77 nm according to Förster’s nonradiational energy transfer theory. It was also found that common metal ions such as K⁺, Cu²⁺, Zn²⁺, Mg²⁺ and Ca²⁺ decreased the apparent association constant and the number of binding sites between raloxifene and BSA.     

Flow-injection chemiluminescence determinations for human blood lead using controlled reagent release technology

A flow-through CL method for the determination of lead combined with controlled-reagent-release technology has been developed. Chemiluminescence (CL) reagents luminol and potassium permanganate were immobilized on anion exchange resin by electrostatic interaction. Lead ion was determined by its enhancing effect on the CL reaction between luminol and potassium permanganate. Both luminol and potassium permanganate were eluted from the anion exchange resin column by sodium phosphate solution. The linear range of the system was 10 μg mL^?1, and the detection limit was 5?×?10^–9 g mL^?1 lead (3σ). A complete analysis could be performed in 1 min with a relative SD 3.2% (1.0?×?10^–7 g mL^?1, n?=?9). The column shows remarkable stability and can be reused over 350 times and 21 days. The method has been applied to determine lead in human blood samples.     

Investigation of the Removal of Lead by Adsorption onto 1‐(2‐Thiazolylazo)‐2‐Naphthol (TAN) Imbedded Polyurethane Foam from Aqueous Solution

A new preconcentration method is presented for lead on TAN‐loaded polyurethane foam (PUF) and its measurement by differential pulse anodic stripping voltammetry (DPASV). The optimum sorption conditions of 1.29 × 10^?5 M solution of Pb(II) ions on TAN‐loaded PUF were investigated. The maximum sorption was observed at pH 7 with 20 minutes equilibrated time on 7.25 mg mL^?1 of TAN‐loaded foam. The kinetic study indicates that the overall sorption process was controlled by the intra‐particle diffusion process. The validity of Freundlich, Langmuir and Dubinin ‐ Radushkevich adsorption isotherms were tested. The Freundlich constants 1/n and K_F are evaluated to be 0.45 ±0.04 and (1.03 +0.61) × 10^?3 mol g^?1, respectively. The monolayer sorption capacity and adsorption constant related to the Langmuir isotherm are (1.38 ± 0.08) × 10^?5 mol g^?1 and (1.46 ± 0.27) × 10⁵ L mol^?1, respectively. The mean free energy of Pb(II) ions sorption on‐TAN loaded PUF is 11.04 ± 0.28 kJ mol^?1 indicating chemisorption phenomena. The effect of temperature on the sorption yields thermodynamics parameters of ΔH, ΔS and ΔG at 298 K that are 15.0 ± 1.4 kJ mol^?1, 74 ±5 J mol^?1 K^?1 and ‐7.37 ± 0.28 kJ mol^?1, respectively. The positive values of enthalpy (ΔH) and entropy (ΔS) indicate the endothermic sorption and stability of the sorbed complexes are entropy driven. However, the negative value of Gibb's free energy (ΔG) indicates the spontaneous nature of sorption. On the basis of these data, the sorption mechanism has been postulated. The effect of different foreign ions on the sorption and desorption studies were also carried out. The method was successfully applied for the determination of lead from different water samples at ng levels.     

Sequential segregation of Sn and Sb in a Cu(111) single crystal

The sequential segregation of Sn and Sb to the surface of a Cu(111) single crystal was measured in the temperature range 400–1100 K by Auger electron spectroscopy. It was found that Sn with the higher diffusion coefficient first segregates to the surface and then is replaced by the slower‐segregating Sb. The results were fitted by a ternary segregation model yielding segregation energies (ΔG_Sn = 76.3 kJ mol^?1, ΔG_Sb = 95.9 kJ mol^?1), interaction parameters (Ω_SnCu = 3.8 kJ mol^?1, Ω_SbCu = 16.2 kJ mol^?1, Ω_SnSb = ?5.3 kJ mol^?1) and diffusion coefficients (D_0(Sn) = 1.8 × 10^?5 m² s^?1, E_Sn = 173 kJ mol^?1, D_0(Sb) = 6.0 × 10^?5 m² s^?1, E_Sb = 205 kJ mol^?1) for both species. The validity of the interaction coefficients and segregation energies was verified using the Guttman equations for equilibrium segregation in ternary systems. Copyright © 2003 John Wiley & Sons, Ltd.     

Spectroscopic investigations of hematoxylin–Eu(III) complex interacting with Herring-sperm DNA

The interaction of HE–Eu(III) complex (HE?=?hematoxylin) with Herring-sperm DNA (hsDNA) has been studied by absorption spectra, fluorescence, and viscosity measurements in physiological buffer (pH?=?7.40). The binding constant of HE–Eu(III) complex to hsDNA was obtained by double reciprocal method at 298 and 310?K and the corresponding thermodynamic parameters (Δ_r Hm??=?8.55?×?10⁴?J?mol^?1, Δ_r Gm??=??3.01?×?10⁴?J?mol^?1, Δ_r Sm??=?387.95?J?mol^?1?K^?1) were calculated, showing that the interaction between HE–Eu(III) complex and hsDNA was driven mainly by entropy. The value of K indicated that the binding mode of HE–Eu(III) complex with DNA was not classical intercalation. These results were further supported by viscosity method and competitive binding experiment. Scatchard analysis suggests that the interaction mode was a mixed binding, which contains partial intercalation and groove binding.     

Surface segregation measurements of In and S impurities from a dilute Cu(In,S) ternary alloy

The surface segregation of In and S from a dilute Cu(In,S) ternary alloy were measured using Auger electron spectroscopy coupled with a linear programmed heater. The alloy was linearly heated and cooled at constant rates. Segregation data of a linear heat run showed surface segregation of In that reached a maximum surface coverage of 25% followed by S, which reached a coverage of 30%. It was found that after In had reached a maximum surface coverage, it started to desegregate as soon as the S enriched the surface until In was completely replaced by S. The segregation parameters, namely, the pre‐exponential factor (D₀), activation energy (Q), segregation energy (ΔG?) and interaction energy (Ω) were extracted from the measured segregation data for both In and S segregation in Cu by simulating the measured segregation data with a theoretical segregation model (modified Darken model). The segregation parameters obtained for In segregation in Cu are D₀ = 1.8 ± 0.5 × 10^?5 m² s^?1, Q = 184.3 ± 1.0 kJ.mol^?1, ΔG? = ?61.4 ± 1.4 kJ.mol^‐1, Ω_Cu?In = 3.0 ± 0.4 kJ.mol^?1; for S segregation in Cu the parameters are D₀ = 8.9 ± 0.5 × 10^?3 m² s^?1, Q = 212.8 ± 3.0 kJ.mol^?1, ΔG? = ?120.0 ± 3.5 kJ.mol^?1, Ω_Cu?S = 23.0 ± 2.0 kJ mol^?1 and the In and S interaction parameter is Ω_In?S = ?4.0 ± 0.5 kJ.mol^?1. The initial parameters used for the Darken calculations were extracted from fits performed with the Fick's and Guttmann model. Copyright © 2013 John Wiley & Sons, Ltd.     

Catalytic Square-Wave Voltammetric Determination of Acrylamide in Potato Chips

Abstract

Square-wave voltammetry is a fast technique used for determination of trace amounts of acrylamide. When cobalt(II) ions were added to the acrylamide solution, a catalytic peak at about ?1.35 V vs. Ag/AgCl was observed, which was proportional to acrylamide concentration. The calibration curve showed good linearity in the range of 200–800 ng mL^?1 of acrylamide with a regression coefficient of 0.9989. The limit of detection of the method was 3.52 ng mL^?1, and the relative standard deviations for concentrations of 300 ng mL^?1 and 700 ng mL^?1 were 99.8% × 10^?2 and 79.7% × 10^?2, respectively.     

Cloud-point extraction,preconcentration, and spectrophotometric determination of palladium in water samples

The possibility of using Thio-Michler's Ketone (TMK), 4,4′-bis(dimethylamino) thiobenzophenone, for palladium(II) concentrated by micellar extraction at the cloud-point temperature, and later spectrophotometric determination, was investigated. Under the optimum conditions, preconcentration of 50?mL of water samples in the presence of 0.1% (w/v) octylphenoxy polyethoxy ethanol (Triton X-114), 2?×?10^?6?mol?L^?1?TMK and 1?×?10^–3?mol?L^?1 buffer solution (pH?=?3.0) gave the limit of detection of 0.47?ng?mL^?1, and the calibration graph was linear in the range of 2–50?ng?mL^–1. The recovery under optimum working conditions was higher than 97%. The proposed method has been applied to the spectrophotometric determination of palladium(II) in natural water samples after cloud-point extraction with satisfactory results.     

Trityl salt—initiated polymerization of α-methylstyrene

The initiation reaction of the polymerization of α-methylstyrene by trityl tetrachloroferate and tritylhexachloroantimonate in 1,2-dichloroethane at 20°C was studied. The rate constants were 14 × 10^?3 and 27 × 10^?3 L mol^?1s^?1, respectively. The dissociation constants of tritylterachloroferate (K_d = 0.88 × 10^?4M^?1) and tritylhexachloroantimonate (K_d = 2.64 × 10^?4M^?1) was determined. The effect of electron acceptors and donors on the dissociation equilibrium and initiation rate was investigated. It was shown that in strongly dissociated ion pairs such as stable carbenium salts the electron donors and acceptors have no appreciable effect on the magnitude of the dissociation. The temperature dependence of the rate constants in the ?20–+20°C range yielded the following thermodynamic parameters for trityltetrachloroferate: E_i = 8.54 kcal/mol; A = 3.2 × 10⁴ mol^?1s^?1; ΔH* = 8 kcal/mol; and S* = ?39.8 eu.     

ULTRASONIC RELAXATION KINETICS OF PLANAR:OCTAHEDRAL EQUILIBRIA OF A NICKEL MACROCYCLIC COMPLEX

Abstract

The equilibrium between the diamagnetic, planar nickel(II) macrocyclic complex known as NiCR²⁺(CR is 2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-1(17),2, 11,13,15pentaene) and its paramagnetic, six-coordinate dimethanol adduct has been examined in methanol solution as the tetrafluoroborate salt. From the temperature dependence of the electronic absorption spectrum, thermodynamic parameters of ΔH°₄₆ = ?4.35 kcal mol^?1 and ΔS°₄₆ = ?8.72 cal deg^?1 mol^?1 were determined. From the excess ultrasonic absorption a relaxation frequency of 15 MHz was observed from which rate constants of k₄₆ of 8.6 × 10⁷ s^?1 and k₆₄ of 7.9 × 10⁶ s^?1 were calculated. The rate constant k₄₆ is nearly the same as the rate constant for solvent exchange which had previously been determined by NMR. This implies that the solvent exchange is effected by the octahedral-planar equilibrium and that a two-step mechanism with a five-coordinate intermediate can be eliminated.     

Mechanistic Aspects of Ligand Substitution on the Hydroxopentaaquarhodium(III) Ion in Aqueous Solution by Sulfur‐Containing Bioactive Ligands

The kinetics of the interactions between three sulfur‐containing ligands, thioglycolic acid, 2‐thiouracil, glutathione, and the title complex, have been studied spectrophotometrically in aqueous medium as a function of the concentrations of the ligands, temperature, and pH at constant ionic strength. The reactions follow a two‐step process in which the first step is ligand‐dependent and the second step is ligand‐independent chelation. Rate constants (k₁ ～10^?3 s^?1 and k₂ ～10^?5 s^?1) and activation parameters (for thioglycolic acid: ΔH₁^≠ = 22.4 ± 3.0 kJ mol^?1, ΔS₁^≠ = ?220 ± 11 J K^?1 mol^?1, ΔH₂^≠ = 38.5 ± 1.3 kJ mol^?1, ΔS₂^≠ = ?204 ± 4 J K^?1 mol^?1; for 2‐thiouracil: ΔH₁^≠ = 42.2 ± 2.0 kJ mol^?1, ΔS₁^≠ = ?169 ± 6 J K^?1 mol^?1, ΔH₂^≠ = 66.1 ± 0.5 kJ mol^?1, ΔS₂^≠ = ?124 ± 2 J K^?1 mol^?1; for glutathione: ΔH₁^≠ = 47.2 ± 1.7 kJ mol^?1, ΔS₁^≠ = ?155 ± 5 J K^?1mol^?1, ΔH₂^≠ = 73.5 ± 1.1 kJ mol^?1, ΔS₂^≠ = ?105 ± 3 J K^?1 mol^?1) were calculated. Based on the kinetic and activation parameters, an associative interchange mechanism is proposed for the interaction processes. The products of the reactions have been characterized from IR and ESI mass spectroscopic analysis. A rate law involving the outer sphere association complex formation has been established as      

Low temperature 13C NMR study of 1-(3-pentyloxyphenylcarbamoyloxy)-2-dialkyl- aminocyclohexanes

Cyclohexane and piperidine ring reversal in 1-(3-pentyloxyphenylcarbamoyloxy)-2-dialkylaminocyclohexanes was investigated by ¹³C NMR. An unusually low conformational energy ΔG = 0.59 kJ mol^?1 and activation parameters ΔG^≠₂₁₈ = 43.8 ± 0.4 kJ mol^?1, ΔH^≠ = 48.9 ± 2.5 kJ mol^?1 and ΔS^≠ = 23 ± 9 J mol^?1 K^?1 were found for the diequatorial to diaxial transition of the cyclohexane ring in the trans-pyrrolidinyl derivative. In the trans-piperidinyl derivative, ΔG₂₂₂ = 44.7 ± 0.5 KJ mol^?1, ΔH^≠ = 55.7 ± 6.3 kJ mol^?1 and ΔS^≠ = 51 ± 21 J mol^?1 K^?1 was found for the piperidine ring reversal from the non-equivalence of the α-carbons.     

IONIC STRENGTH EFFECTS ON THE GROUND STATE COMPLEXATION and TRIPLET STATE ELECTRON TRANSFER REACTION BETWEEN ROSE BENGAL and METHYL VIOLOGEN

Abstract— The equilibrium constants, K_c, for complexation between methyl viologen dication (MV²+) and Rose Bengal, or Eosin Y, decrease with increasing ionic strength. At zero ionic strength K_c is 6500 (± 500) mol^?1 dm³ for Rose Bengal and 3200 (± 200) mol^?1 dm³ for Eosin Y, and these values decrease to 1500 (± 100) and 680 (± 40) mol^?1 dm³, respectively, at an ionic strength of 0.1 mol dm^?3. K_c is independent of pH between 4.5 and 10. ΔH is -25 (± 1) kJ mol^?1 for complexation with either dye, whereas ΔS is -15 (± 3) J K^?1 mol^?1 for Rose Bengal, and - 23 (± 3) J K^?1 mol^?1 for Eosin Y. The complexation constant for Rose Bengal and the neutral viologen, 4,4'-bipyridinium-N, N'-di(propylsulphonate), (4,4'-BPS), is 420 (± 35) mol^?1 dm³, and independent of ionic strength. No complexation could be observed for either Rose Bengal or Eosin with another neutral viologen, 2,2'-bipyridinium-N,N'-di(propylsulphonate), (2,2'-BPS). MV²+ quenches the triplet state of Rose Bengal with a rate constant of 7 × 10⁹ mol^?1 dm³ s^?1, and this rate constant decreases slightly as ionic strength increases. The cage escape yield following quenching, Φ_cc is very low (Φ_cc= 0.02 (± 0.005), and independent of ionic strength. 4,4'-BPS quenches the triplet state of Rose Bengal with a rate constant of 2.2 (± 0.1) × 10⁹ mol^?1 dm³ s^?1, and gives a cage escape yield of 0.033 (± 0.006). 2,2'-BPS quenches the Rose Bengal triplet with a rate constant of 6 (± 1) × 10⁸ mol^?1 dm³ s^?1 and gives a cage escape yield of 0.07 (± 0.01). Conductivity measurements indicate that MV²+(Cl^?)₂ is completely dissociated at concentrations below 2 × 10^?2 mol dm^?3.