Molecular complexes of crown ethers. part 2: Complexes between crown ethers and π acceptors

The intermolecular charge transfer (CT) complexes of four crown ethers (CE), viz. B15C5, B18C6, DB18C6 and DB24C8 (as donors), and three acceptors, i.e. tetracyanoethylene (TCNE) 7,7,8,8-tetracyanoquinodimethane (TCNQ) and chloranil were studied in the UV-Visible region in 1,2-dichloroethane (DCE) at 298.2 K. Solid complexes were also prepared and the ratio of the donor to the acceptor was 1:1. Results indicate that the _max of the CT band decreases whileK _c (formation equilibrium constant) increases following the systematic change of the electron affinity of the acceptors. TheK _c values suggest that the phenyl group in CE plays a major role in the formation of a CT complex.     

Complexation of neptunium(V) by salicylate,phthalate and citrate ligands in a pH 7.5 phosphate buffered system

Conditional stability constants, enthalpies and entropies of complexation at pH 7.5 and ionic strength 0.1 have been determined for neptunium(V) complexes of phosphate, salicylate, phthalate and citrate. Phosphate forms a complex with log β = 2.36 ± 0.42 at 25°C, ΔH°_c = ? 69.9 kJ/mole and ΔS°_c = ? 188 J/mole-K. At pH 7.5 salicylate does not form a complex with neptunium(V) due to the low charge density of the NpO₂⁺ ion and incomplete ionization of the salicylate ion. Phthalate forms a complex with log β = 3.43 ± 0.33 at 25°C, ΔH°_c = 33.5 kJ/mole and ΔS°_c = 182 J/mole-K. Citrate forms a complex with log β = 4.84 ± 0.72 at 25°C, ΔH°_c = 14.0 kJ/mole and ΔS°_c = 140 J/mole-K. In all cases, only 1:1 complexes were identified.     

Kinetics and mechanism of diels-alder additions of tetracyanoethylene to anthracene derivatives—I : Substituent effects

The rates of addition of tetracyanoethylene to a number of 9- and 9,10-substituted anthracenes have been measured spectrophotometrically in solvent CCl₄. Substituent effects correlated well using the extended form of the Hammett equation. The importance of steric effects on the reaction was assessed by a systematic variation of the components of the data used in the above correlation.Activation parameters (ΔG_exp^≠, etc.) and the corresponding overall thermodynamic parameters for adduct formation (ΔG_ad°, etc.) were evaluated. ΔG_exp^≠ was found to be linearly related to ΔG_c°, the free energy of formation of the intermediate complex which confirms the role of the latter as a true reaction intermediate. From correlations between ΔG_exp^≠ and ΔG_ad°, an “early” transition state is suggested. The above thermodynamic and activation data enable detailed reaction profiles to be drawn.     

Synthesis and Diels-Alder Reactivity of 2,3,5,6-Tetramethylidenenorbornane

Pd-catalyzed double carbomethoxylation of the Diels-Alder adduct of cyclo-pentadiene and maleic anhydride yielded the methyl norbornane-2,3-endo-5, 6-exo-tetracarboxylate ( 4 ) which was transformed in three steps into 2,3,5,6-tetramethyl-idenenorbornane ( 1 ). The cycloaddition of tetracyanoethylene (TCNE) to 1 giving the corresponding monoadduct 7 was 364 times faster (toluene, 25°) than the addition of TCNE to 7 yielding the bis-adduct 9 . Similar reactivity trends were observed for the additions of TCNE to the less reactive 2,3,5,6-tetramethylidene-7-oxanorbornane ( 2 ). The following second order rate constants (toluene, 25°) and activation parameters were obtained for: 1 + TCNE → 7 : k₁ = (255 + 5) 10^?4 mol^?1 · s^?1, ΔH≠ = (12.2 ± 0.5) kcal/mol, ΔS≠ = (?24.8 ± 1.6) eu.; 7 + TCNE → 9 , k₂ = (0.7 ± 0.02) 10^?4 mol^?1 · s^?1, ΔH≠ = (14.1 ± 1.0) kcal/mol, ΔS≠ = ( ?30 ± 3.5) eu.; 2 + TCNE → 8 : k₁ = (1.5 ± 0.03) 10^?4 mol^?1 · s^?1, ΔH≠ = (14.8 ± 0.7) kcal/mol, ΔS≠ = (?26.4 ± 2.3) eu.; 8 + TCNE → 10 ; k₂ = (0.004 ± 0.0002) 10^?4 mol^?1 · s^?1, ΔH≠ = (17 ± 1.5) kcal/mol, ΔS≠ = (?30 ± 4) eu. The possible origins of the relatively large rate ratios k₁/k₂ are discussed briefly.     

Volumetric, viscometric and refractive index behavior of some α-amino acids in aqueous tetrapropylammonium bromide at different temperatures

Densities, ρ, viscosities, η, and refractive indices, n _D, of glycine (Gly), DL-alanine (Ala), DL-valine (Val) (0.05, 0.10, 0.15, 0.20, 0.25 mol kg^?1), and L-leucine (Leu) (0.02, 0.05, 0.10 mol kg^?1) in water and in 0.20 mol kg^?1 aqueous tetrapropylammonium bromide (TPAB) have been measured at 298.15, 303.15, 308.15, and 313.15 K. The density data have been utilized to calculate apparent molar volumes, ?_v, partial molar volumes at infinite dilution, ?_v°, and partial molar volumes of transfer, ? _v°(tr) of amino acids. The viscosity data have been analyzed by means of Jones-Dole equation to obtain Falkenhagen coefficient, A, and Jones-Dole coefficient, B, free energy of activation of viscous flow per mole of solvent, Δµ₁°*, and solute, Δµ₂°*, and enthalpy, ΔH*, and entropy of activation, ΔS*, of viscous flow. The refractive index data have been used to calculate molar refractivity, R _D, of amino acids in aqueous tetrapropylammonium bromide solutions. It has been observed that ?_v°, B-coefficient and Δµ₂°* vary linearly with increasing number of carbon atoms in the alkyl chain of amino acids, and they were split to get contributions from the zwitterionic end groups (NH₃ ⁺, COO^-) and methylene group (CH₂) of the amino acids. The behavior of these parameters has been used to investigate the solute-solute and solute-solvent interactions as well as the effect of tetrapropylammonium cation (C₃H₇)₄N⁺ on these interactions.     

Simplified formula for the1 A 1⇄5 T 2 spin transition temperature in Fe(II) complexes

A formula relating the¹A₁?⁵T₂ spin transition temperature (T_c) in Fe(II) complexes to characteristics of the compounds is derived. With certain assumptions, T_c is determined by the splitting parameter Δ_LS of e_g- and t_2g-orbitals for the low-spin complexes and by the frequency ratio of normal vibrations of the low- and high-spin phases. For the group of compounds possessing spin transitions, the values of Δ_LS are found and analyzed. Correlations between T_c and Δ_LS are established; the values of the change in the probability of the Mössbauer effect are correlated with those of entropy of spin transition. The correlations are substantiated. It is concluded that for mononuclear Fe(II) complexes possessing sharp spin transitions, T_c may not be significantly higher than for Fe(Phy)₂(BF₄)₂ (T_c=282 K).     

Amine oxides—XIII: Iodine complexes with non-aromatic amine oxides

EDA-iodine complex with trialkylamine oxides in dichloromethane have been investigated spectrophotometrically. Thermodynamic (K_c^AD, —ΔG°) and spectral (ε_λ^AD) characteristics of these complexes were determined. The absorption band of iodine in the visible region was blue-shifted with a variable maximum, depending on the structure of the amine oxide. The K_c^AD and ε_λ^AD values were calculated using a modified method. A non-linear relationship K_c^AD = f(R), where R is the number of carbon atoms in the alkyl chain, was found.     

Thermal Characterization of Polymethyl(α-n-Pentyl)Acrylate

This work presents new results concerning characterization of polymethyl(α-n-pentyl)acrylate polymer by means of thermal analysis. In differential scanning calorimetry investigations, the measured values of T _g, T _f and ΔC _p, i.e. the glass transition temperature, the fictive temperature and the heat capacity step at T _g, show that the polymer can be considered as fragile. Thermogravimetric analysis revealed two mass losses, the first, at low temperature, being associated with the evaporation of water molecules, and the second, at high temperature, corresponding degradation of the polymer. This degradation is a two-step phenomenon. Finally, study of the β and the α transitions by elementary and complex TSDC led to the following values: T _β=?40°C, T _α=36°C, T _c=47°C, τ_c=2.5 s and ΔH=85 to 165 kJ mol^?1.     

Calorimetric determination of the thermodynamic properties involved in the complex formation of silver(I) with some sulphur-containing pyridines

The enthalpy changes involved in the complex formation of Ag⁺ with several sulphur-containing pyridines of general formula

where R = Ch₃, C₂H₅ and CH₂CH₂OH and x=1,2 have been determined by direct calorimetric measurements at 25°C in 0.5 M KNO₃ solution.From the ΔH values obtained and from the free energy changes reported in an earlier publication, the corresponding entropy changes ΔS have been calculated. Changes in ΔH and ΔS are discussed in terms of inductive and steric effects. It is shown that enthalpy values are linearly correlated with the Taft σ^* parameters of the substituents on the ligand.     

Conformational effects in compounds with 6-membered rings—XII : The conformational equilibrium in n-methylpiperidine

The conformational equilibrium at nitrogen in N-methylpiperidines has been determined in the gas phase (ΔG°₂₈₈ ? 13.2 ± 0.4 kJ mol^?1) and for dilute solutions in several solvents (ΔG°₂₉₃ ranging from 12.5 ± 0.4 in dodecane to 10.1 ± 0.4 in chloroform) by kinetically controlled protonation of anancomeric model compounds 6 and 8 at the interface between the pipendine-containing phase and an immiscible strong acid. The conformational energy for N-methylpiperidine determined by this method is strikingly higher than earlier estimates based on less direct methods but is supported by independent evidence from the temperature dependence of ¹³C NMR chemical shifts. Reconsideration of the more important of the earlier methods indicates that these involved invalid or unproven assumptions and that the low values of ΔG° for N-methylpiperidine derived from them are not reliable.     

Methodology of analysis of unseparated mixtures: Error limits in estimating the total analyte concentration recalculated to a standard substance

In estimating the total concentration of similar analytes recalculated to a standard substance X_st, different sensitivities of their determination result in a systematic error δc, depending on the choice of X_st and the composition of the analyzed mixture. Two methods of predicting the limiting values of this error (δc _max), with and without the account of the properties of X_st are proposed and tested on model mixtures. The methods can be used if the total analytical signal linearly depends on the concentrations of analytes and is additive (spectrophotometry, conductometry, etc.). The value of δc _max is independent of the ratio of analytes concentrations in the sample. To reduce δc _max, one should equalize the sensitivities of the determination of all compounds of a given type and use analytes with medium sensitivities as X_st.     

UV Photolysis of a Re(I) Macromolecule, [(4-vinylpyridine)2(4-vinylpyridineRe(C0)3(1,10-phenanthroline)+]200: Addition of C-centered Radicals to Coordinated CO

Abstract

Proteins that bind preferentially to specific recognition sites on DNA also bind more weakly to nonspecific DNA. We have studied both specific and non-specific binding of the EcoRI and BamHI restriction endonucleases, and determined enthalpic and entropic contributions to binding free energy (ΔG°_bind) using both the van't Hoff method and isothermal titration calorimetry. Specific binding is characterized by a strongly negative ΔC°_p and can be either enthalpy-driven or entropy-driven, depending on temperature. Nonspecific binding has ΔC°_p ≈ 0 and is enthalpy-driven. A strongly negative ΔC°_p is the ?thermodynamic signature’ of site-specific binding, because it reflects the characteristics of a tight complementary recognition interface: the burial of previously hydrated nonpolar surface and restriction of configurational-vibrational freedoms of protein, DNA, and water molecules trapped at the protein-DNA interface. These factors are absent in nonspecific complexes. We probed the contributions to ΔC°_p by varying the sequence context surrounding the recognition site. As ΔG°_bind improves, ΔC°_p' ΔH° and ΔS° all become more negative, and there is a linear correlation between ΔH° and ΔS° (enthalpy-entropy compensation). Because these context variations do not change the protein-base or protein-phosphate contacts, the hydrophobic contribution or the number of trapped water molecules at the interface, we conclude that a better sequence context improves the ?goodness of fit’ in the interface and and thus increases the magnitude of the negative configurational-vibrational contribution to ΔC°_p.     

Calorimetric investigations of association equilibria: the pyridine-iodine complex in cyclohexane and in carbon tetrachloride

The results of two independent calorimetric investigations of the pyridine- iodine complex are reported. “Best” values are reported as K_m = 128 1/mol and ΔH° = -8.4 kcal mol^?1 for the formation of the complex in cyclohexane at 25°C, and K_m = 104 and ΔH° = -7.9 kcal mol^?1 in carbon tetrachloride. Evidence is presented to support the contention that association constants for weak complexes determined by calorimetric methods can be as reliable as those determined by spectrometric methods, and that values of ΔH° determined by the calorimetric method are much more reliable than those derived from the temperature dependence of equilibrium constants.     

Spectrophotometric Method of Determination for Cycloserine Using Chlornil

Abstract

Cycloserine has been determined by a spectrophotometric method based on n → charge-transfer complex formation with chloranil. The method involves addition of cycloserine solution to chloranil reagent in borate buffer of pH 9; and heating at 65°C for 45 min. The complex formed exhibits absorption maximum of 348 nm; and A_max is linearly related to concentration over the range 2–8 μg/ml. The mean percentage found for the authentic sample is 100.32 ± 0.70. When applied to commercial capsules claimed to contain 250 mg each, the mean percentage found is 102.45 ± 1.05. These results have shown good agreement when compared with those obtained by the pharmacopoeial (B.P.) methods.     

Charge transfer molecular complexes of arylidene anthranilic acid derivatives with π-electron acceptors

X-Benzylidenesanthranilic acid molecular complexes with π-acceptors, tetracyanoethylene, 2,3-dichloro-5,6-dicyano-p-benzoquinone and chloranil, have been studied. The intramolecular hydrogen bonding that exists in such compounds greatly inhibits the transition of the nitrogen azomethine n-electrons. The formation constant values and molar extinction coefficients of the p-dimethyl-aminobenzylidenean-thranilic acid-DDQ CT complexes have been determined in CH₂Cl₂, C₂H₄Cl₂ and CHCl₃ in the temperature range 10–30°C. Such CT complexes are of strong n-π type.     

Charge transfer properties of molecular complexes involving carbonyl bonds

The fraction of the dative structure, b² + abS, of hydrogen bonded complexes of N,N-dimethylacetamide with phenol derivatives and of charge transfer complexes of some aliphatic amides with iodine, is computed from the displacement of some vibrational modes. The values of b² + abS are computed from the v_co frequency shifts using a limiting value of 6 · 10⁵ dyne cm^?1 for the value of the force constant of the carbonyl bond in the dative structure.In the case of hydrogen bonded complexes, these results are compared with values of b² + abS computed from the Δv_OH stretching vibrations by the theory of Szczepaniak and Tramer. For the charge transfer complexes, the values of b² + abS are computed from the v_(I-I) stretching vibrations using the equation of Friedrich and Person. The agreement between the two methods is good in both cases.     

A thermodynamic study of interaction of Ag+, Mg2+, Ca2+, and K+ cations with 4-hydroxyphenyl-2,5-bis(2-benzofuranyl)pyridine in some binary mixed non-aqueous solvents

In the present work the complexation process between Ag⁺ and Mg²⁺ cations and 4-hydroxyphenyl-2,5-bis(2-benzofuranyl)pyridine (HBFPY) ligand was studied in pure dimethylformamide (DMF), ethanol (EtOH), acetonitrile (AN) and in (DMF-EtOH), (AN-EtOH) and (DMF-AN) binary mixed solvent solutions at different temperatures using the conductometric method. Also in this work the complexation reaction between Ca²⁺, K⁺ cations and HBFPY ligand, was studied in pure dimethylformamide (DMF), propanol (PrOH), 1,4-dioxane (DOX), ethanol (EtOH) and in DMF-PrOH, DMF-DOX and DMF-EtOH binary mixed solvent solutions at different temperatures using the conductometric method. The conductance data show that the stoichiometry of the complexes formed between this ligand and the studied cations is 1 : 1 [ML]. In most cases, addition of HBFPY to solutions of these cations, causes a continuous increase in the molar conductivities which indicates that the mobility of complexed cations is more than the uncomplexed ones. The stability constants of the complexes were obtained from fitting of molar conductivity curves using a computer program, GENPLOT. The stability constant of [Mg(HBFPY)]²⁺ complex in various neat solvents at 15°C decreases in order: EtOH > DMF > AN and the stability constant of [Ag(HBFPY)]⁺ complex in various neat solvents at 35°C decreases in order: DMF > EtOH. The values of standard enthalpy changes (ΔH° _c ) for complexation reactions were obtained from the slope of the Van’t Hoff plots and the changes in standard entropy (ΔS° _c ) were calculated from the relationship ΔH° _c,295.15= ΔH° _c –298.15ΔS° _c .     

Thermodynamics of Ion-Association of NaClO4 in Aqueous-Methanol and Aqueous-Glycerol Using Conductance Measurements

The conductance of NaClO₄ at (25–45)°C in methanol-H₂O, and glycerol-H₂O mixtures in a concentration range up to 10^?2 M has been measured. The association constant, K_A, values have been determined, where the conductometric data were analysed by a M S X computer using the Fuoss-Aprano method. The K_A values for NaClO₄ in methanol-H₂O are higher than in glycerel-H₂O mixtures, Also K_A values increase as the proportional of methanol or glycerol increases in mixture. The thermodynamic parameters ΔH°, ΔG°, and ΔS° were also calculated. It is obvious that the entire process of the ionic association in those systems are endothermic ones. The Walden products, Λ_oη were calculated for all systems investigated.     

Thermodynamic studies of inclusion complex formation between alkylpyridinium chlorides and β-cyclodextrin using conductometric method

Thermodynamics on inclusion complexation of β-cyclodextrin (β-CD) with n-alkylpyridinium chlorides (C _n PC, n = 12, 14, 16) were measured by conductivity technique to evaluate the effects of chain length of C _n PC and temperature. The data obtained indicate that inclusion complexes S(CD) and S(CD)₂ had formed between surfactant and β-CD in aqueous solution. Investigation showed that the K ₁ (first equilibrium constant) for S(CD) formation is greater than K ₂ (second equilibrium constant) for S(CD)₂ formation. It has been found that C₁₂PC forms only the 1:1 complex, while C₁₄PC and C₁₆PC form 1:1 and 1:2 complexes. Thermodynamic parameters of the complexation, i.e. ΔG°, ΔH° and ΔS° have been also calculated. The large values of ΔG° indicate that complexation between surfactant and β-CD is very favorable.     

Spectroscopic investigation of the charge-transfer interactions between 1,4,7-trimethyl-1,4,7-triazacyclononane and the acceptors iodine, TCNE, TCNQ and chloranil

The interaction of the interesting polynitrogen cyclic base 1,4,7-trimethyl-1,4,7-triazacyclononane (TMTACN) with the sigma-acceptor iodine and pi-acceptors tetracyanoethylene (TCNE), 7,7,8,8-tetracyanoquinodimethane (TCNQ) and tetrachloro-p-benzoquinone (chloranil) have been studied spectrophotometrically and cyclic voltametrically in chloroform at 20 degrees C. Based on the obtained data, the formed charge-transfer complexes were formulated as [(TMTACN)I](+).I(3)(-), [(TMTACN)(TCNE)(5)], [(TMTACN)(TCNQ)(3)] and [(TMTACN)(chloranil)(3)] where the stoichiometry of the reactions, donor:acceptor molar ratios, were shown to equal 1:2 for iodine complex, 1:3 for chloranil and TCNQ complexes and 1:5 for TCNE complex.