The ionization constants of 2‐substituted 4,6‐diamino‐s‐triazines: The applicability to the hammett and taft equations

The ionization (or basicity) constants (pK_b) were determined for many 2‐substituted 4,6‐diamino‐s‐tri‐azines ( I ) by means of the electrometric titration. I includes 2‐alkoxy or aryloxy‐( Ia ), 2‐alkyl‐ or 2‐aryl‐( Ib ), and 2‐alkylamino‐ or 2‐arylamino‐4,6‐diamino‐s‐triazines ( Ic ). For the series with the same alkyl or aryl group, the order of the basicity was found to be Ic < Ib < Ia . A study was made of relationships between the pK_b, values of I , and the substituent constants, σ_p, σ_m, σ_p⁺, σ_m⁺, σ_p^O, σ_m^o, σ_I, σⁿ, and σ*. The Hammett relationships were observed between the pK_a values of I, and the substituent constants σ_m, (or the combination ones, [0.97σ_m + 0.03σ_p] as well as another [0.77σ_I + 0.23σ_R]). The Taft relationships were also found between the pK_a values of Ia , Ib , and Ic and the constants σ_*, respectively. Furthermore, in the case of Ic a linear relationship was observed between the pK_a values and Σσ⁸.     

Thermal and ion transport properties of hydrophilic and hydrophobic polymerized styrenic imidazolium ionic liquids

Polymerized ionic liquids (PILs) are a platform for fundamental studies of structure‐property relationships in single ion conductors, with potential applications in energy storage and conversion. The synthesis, thermal properties, and ionic conductivities of homologous, narrow dispersity styrenic PILs are described. Hydrophilic poly(4‐vinylbenzyl alkylimidazolium chloride) (PVBn(alkyl)ImCl) homopolymers with constant average degrees of polymerization were synthesized by post‐synthetic functionalization of a poly(4‐vinylbenzyl chloride) (M_n = 15.9 kg/mol, M_w/M_n = 1.34) master batch with N‐alkylimidazoles (alkyl = ? CH₃ (Me), ? C₄H₉ (Bu), and ? C₆H₁₃ (Hex)). The chloride counterions of PVBnHexImCl were exhaustively metathesized with BF, PF, and bis(trifluoromethanesulfonyl)imide (TFSI^?) to yield a series of hydrophobic PILs. Thermogravimetric analyses indicate that PVBn(alkyl)ImCl homopolymers are unstable above 220 °C, whereas the hydrophobic PILs remain stable up to 290 °C. The glass transition temperatures (T_g) decrease with both increasing alkyl side‐chain length and increasing counterion size, exemplified by T_g = 9 °C for PVBnHexImTFSI. Hydrophilic PILs exhibit high ionic conductivities (as high as ～0.10 S cm^?1) that depend on the relative humidity, water uptake, and the PIL side chain length. The hydrophobic PILs exhibit lower conductivities (up to ～5 × 10^?4 S cm^?1) that depend predominantly on the polymer T_g, however, counterion size and symmetry also contribute. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1287–1296, 2011     

The basicity of pyridine and its tendency towards cationic homoconjugation in non‐aqueous media

Acid dissociation constant, pK_a, of protonated pyridine, determined in the polar protophobic aprotic solvent, acetone, has been compared with pK_a values of the pyridinium ion in a variety of other polar solvents including aprotic protophobic ones, acetonitrile, benzonitrile, nitrobenzene, nitromethane, and propylene carbonate, in the protophilic aprotic dimethyl sulfoxide and N,N‐dimethylformamide, as well as in the amphiprotic methanol. On the basis of the set of these pK_a values, the effect of the medium on the basicity of pyridine is discussed. Further, based on the cationic homoconjugation constants of pyridine conjugated with the pyridinium cation in the non‐aqueous solvents, the tendency of pyridine towards homoconjugation reactions has also been discussed. Finally, both the basicity of pyridine and its tendency towards cationic homoconjugation has been compared with analogous properties of pyridine N‐oxide.     

Structures and relationship between the 119SnNMR chemical shifts and pKa of their parent acids in organotin (I∇) carboxylates

Ten di-n-butyltin(I∇) carboxylates [(ⁿBu₂Sn-OCOR′)₂O]₂ and ⁿBu₂Sn(OCOR′)₂ (R′ = CCl₃, CHCl₂, CH₂Cl, PhCH = CH, and 2,2,3,3-tetramethylcyclopropyl) were synthesized and characterized by IR, ¹H, ¹³C, ¹¹⁹Sn NMR spectroscopy and elemental analysis. Together with other series of organotin(I∇) carboxylates, their structural features were discussed. The relationship between the ¹¹⁹Sn NMR chemical shifts in the organotin(I∇) carboxylates [(ⁿBu₂SnOCOR′)₂O]₂, ⁿBu₂Sn(OCOR′)₂, ⁿBu₃SnOCOR′, Ph₃SnOCOR′ and the pK_a values of their parent acids R′COOH was studied. The results have shown that the log[-δ(¹¹⁹Sn)] of the same series of carboxylates is linearly related to the pK_a of R′COOH. It seems that the better is the linearity between the log[−δ(¹¹⁹Sn)] and the pK_a, the more analogous are the structures of the same series of carboxylates. © 1996 John Wiley & Sons, Inc.     

The synthesis of 8‐hydroxyquinazoline derivatives and their acid‐base interactions

The reaction of N‐(2‐R¹‐oxyphenyl)benzimidoyl chlorides with cyanamide derivatives in the presence of titanium tetrachloride as a catalyst has yielded some new 4‐amino‐8‐R¹‐oxy‐2‐phenylquinazolines. pK_a values have been determined for these compounds and the influence of substituents at the basicity of the parent system has been discussed. Some investigations on the methyl‐quinazolinyl ether cleavage in different medium have been done yielding the appropriate hydroxyquinazoline derivatives. In those cases when the deprotection of 4‐amino‐8‐methoxy‐2‐phenylquinazoline was carried in aqueous acidic solutions, the formation of the hydrolysis products 3,4‐dihydro‐2‐phenyl‐4‐quinazolone derivatives was observed as well.     

Polynuclear branched tetrazole systems. 3. Acidity of α,ω-ditetrazol-5-ylalkanes

α,ω-Ditetrazol-5-ylalkanes with 1 to 5 methylene groups in the alkyl fragment display the properties of dibasic heterocyclic NH acids with pK_a values lying in the range 3.4–6.1. The pK_a values of these compounds are in a linear dependence on the values of the dielectric permeability of the medium, on the chemical shifts of the signals of the endocyclic carbon atom, and of the carbon atom of the α-methylene group in the ¹³C NMR spectra. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 535–541, April. 2006.     

Self-diffusion in the isotropic phase of the p-alkoxybenzoic acid homologous series

The temperature dependence of the self-diffusion coefficients of those members of the p-alkoxybenzoic acid homologous series with n = 3 to 8 alkyl chain carbon atoms has been measured in the isotropic liquid phase and a comparison has been made between the activation energies for self-diffusion in the liquid and the solid phase. The self-diffusion coefficients at the isotropic-nematic transition temperatures were found to decrease monotonically with increasing number of alkyl chain carbon atoms.     

Beiträge zur Chemie der Pyrrolpigmente, 68. Mitt. Zum Kationentransport mit tripyrrinoiden Ionophoren

A tripyrrinic carrier has been synthesized and its carrier properties for cations measured using a bulk membrane. A remarkable selectivity was found for Zn^{+ +} and Cu^{+ +} ions. ThepK _a values and complexation data of a series of tripyrrinic systems are reported. The basicity of the pyrroleninic nitrogen atom and the geometrical constraints of the ligands are the main reasons for the selectivity and the high efficiency of the tripyrrinic ligand to carry Zn^{+ +} and Cu^{+ +} ions near the diffusion limit.

     

Smectogenic properties of N,N′‐bis[(2‐hydroxy‐4‐alkoxyphenyl)methylene]benzene‐1,4‐diamine liquid crystals with double lateral H‐bonds

A series of eight liquid crystalline compounds, N,N′‐bis[(2‐hydroxy‐4‐alkoxyphenyl)methylene]benzene‐1,4‐diamines, has been synthesized and characterized. These homologous compounds differ in the length of terminal alkyl group C _n H_2n+1 wherein n is an even number ranging from 4 to 18. The spectroscopic techniques, FTIR, ¹H NMR and ¹³C NMR, were employed to characterize the molecular structure. The transition temperatures of all the title compounds and their mesophases were studied by differential scanning calorimetry and polarizing microscopy. All the compounds were smectogenic, exhibiting both tilted and non‐tilted molecular orientation in their smectic phases. Further investigation to ascertain the anisotropic nature of subphases within the smectogenic region was carried out using X‐ray diffraction.     

Simple methods for the estimation of ionization constants of substituted pyridine N-oxides in polar aprotic solvents and water

A direct method for the determination of the pK _a values of acids conjugated to substituted pyridine N-oxides has been proposed which is based on the pH measurement of the solution of the basic salt. It has been experimentally shown that the method is reliable when applied to N-oxides of not too low basicity (pK _a >5). Correlation has been performed between the pK _a values in aqueous and aprotic media solutions which shows the great influence of the solvation effect on the acid-base equilibria. The good correlation between the pK _a values in aqueous and non-aqueous solutions enables the pK _a values in water to be estimated with sufficient accuracy, even in the cases when the experimental limitations make the determination impossible which is shown on the basis of selected examples.     

Studies on organophosphorus compounds 61 substituent effects in organophosphorus esters

Examination of several model organophosphorus compounds, bicyclic phosphates ( 1 ), p-substituted phenylphosphonates ( 2 ), and acidic alkylphosphonates ( 3 ) reveals that the polar substituent effect in organophosphorus esters parallels that of carbon compounds. No significant variation in polar effects of different alkyl groups with up to 10 carbon atoms is observed. As demonstrated by us, the commonly used Kabachnik polar constant σ_Φ for alkyl groups of organophosphorus compounds reflects the steric effect associated with steric hindrance to solvation of acidic anions. Therefore, a set of solvation steric parameters E^p_s has been calculated from pK_a values of organophosphorus acids. This parameter can be applied to describe the structural effect on dissociation of organic acids and, after suitable modification, may reflect the steric effect of alkyl groups in hydrolytic reactions of organophosphorus esters.     

The connection between the ionization constants of alkaloids and their basicities

Summary 1. The pK_a values of a number of indole alkaloids ofVinca erecta in aqueous ethanolic solution and in chloroform have been determined.2. It has been shown that the pK_a value of an alkaloid in water or in chloroform is not connected with the pH of the transition from chloroform into a buffer solution.3. It is impossible to estimate the basicity of an alkaloid from its pK_a value.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 57–59, January–February, 1973.     

Ionic Liquids: Dissecting the Enthalpies of Vaporization

We calculate the heats of vaporisation for imidazolium‐based ionic liquids [C_nmim][NTf₂] with n=1, 2, 4, 6, 8 by means of molecular dynamics (MD) simulations and discuss their behavior with respect to temperature and the alkyl chain length. We use a force field developed recently. The different cohesive energies contributing to the overall heats of vaporisations are discussed in detail. With increasing alkyl chain length, the Coulomb contribution to the heat of vaporisation remains constant at around 80 kJ mol^?1, whereas the van der Waals interaction increases continuously. The calculated increase of about 4.7 kJ mol^?1 per CH₂‐group of the van der Waals contribution in the ionic liquid exactly coincides with the increase in the heats of vaporisation for n‐alcohols and n‐alkanes, respectively. The results support the importance of van der Waals interactions even in systems completely composed of ions.     

Thermodynamic dissociation constants of silychristin, silybin, silydianin and mycophenolate by the regression analysis of spectrophotometric data

Mixed dissociation constants of four drug acids, i.e. silychristin, silybinin, silydianin and mycophenolate at various ionic strengths I of range 0.01 and 0.30 and at temperatures of 25 and 37 °C were determined using the SQUAD(84) regression analysis program applied to pH-spectrophotometric titration data. The proposed strategy of an efficient experimentation in a protonation constants determination, followed by a computational strategy for the chemical model with a protonation constants determination, is presented on the protonation equilibria of silychristin. The thermodynamic dissociation constant pK_a^T was estimated by non-linear regression of {pK_a, I} data at 25 and 37 °C: for silychristin pK_a,1^T=6.52(16) and 6.62(1), pK_a,2^T=7.22(13) and 7.41(5), pK_a,3^T=8.96(9) and 8.94(9), pK_a,4^T=10.17(7) and 10.03(8), pK_a,5^T=11.89(4) and 11.63(7); for silybin pK_a,1^T=7.00(4) and 6.86(5), pK_a,2^T=8.77(11) and 8.77(3), pK_a,3^T=9.57(8) and 9.62(1), pK_a,4^T=11.66(3) and 11.38(1); for silydianin pK_a,1^T=6.64(7) and 7.10(6), pK_a,2^T=7.78(5) and 8.93(1), pK_a,3^T=9.66(9) and 10.06(11), pK_a,4^T=10.71(7) and 10.77(7), pK_a,5^T=12.26(5) and 12.14(5); for mycophenolate pK_a^T=8.32(1) and 8.14(1). Goodness-of-fit tests for various regression diagnostics enabled the reliability of parameter estimates to be found.     

ESR Studies on the Micellization Behaviors of a Series of Novel Asymmetric Gemini Surfactants

ＩｎｔｒｏｄｕｃｔｉｏｎＩｎｒｅｃｅｎｔｙｅａｒｓ ,ｂｉｓ(ｑｕａｔｅｒｎａｒｙａｍｍｏｎｉｕｍ)ｓｕｒｆａｃ ｔａｎｔｓｏｒｇｅｍｉｎｉｓｕｒｆａｃｔａｎｔｓ ,ｉｎｗｈｉｃｈｔｗｏｃａｔｉｏｎｉｃｓｕｒｆａｃ ｔａｎｔｍｏｉｅｔｉｅｓａｒｅｃｏｎｎｅｃｔｅｄｗｉｔｈｔｈｅａｍｍｏｎｉｕｍｈｅａｄｇｒｏｕｐｂｙａｐｌｏｙｍｅｔｈｙｌｅｎｅｃｈａｉｎ ,ｎａｍｅｌｙ ,ａｓｐａｃｅｒｈａｖｅｂｅｃｏｍｅｏｆｉｎｔｅｒｅｓｔｄｕｅｔｏｔｈｅｉｒｅｘｃｅｐｔｉｏｎａｌｓｕｒｆａｃｅａｃｔｉｖｉｔｙａｎｄｒｅｍ…     

Effects of Aerosol-OT reversed micelles in carbon tetrachloride on acid-base indicator equilibria

ApparentpK _a values of thymol blue solubilized in Aerosol-OT reversed micelles in carbon tetrachloride at 25 °C were determined spectrophotometrically. The effects of the Aerosol-OT concentration and the (water)/(surfactant)R ratio were investigated. The apparentpK _a values increase as a function of increasing (R). All the measuredpK _a values are less than that in water. The decrease ranges from 1.23 units (detergent=0.4 M,R=1.39) to 0.42 units (Aerosol-OT=0.6 M,R=9,25). These results are rationalized in terms of decreased hydronium ion activity in the micellar core due to its binding to the detergent SO₃ ^? groups.     

A New Class of Organosuperbases,N‐Alkyl‐ and N‐Aryl‐1,3‐dialkyl‐4,5‐dimethylimidazol‐2‐ylidene Amines: Synthesis,Structure, pKBH+ Measurements,and Properties

A series of stable organosuperbases, N‐alkyl‐ and N‐aryl‐1,3‐dialkyl‐4,5‐dimethylimidazol‐2‐ylidene amines, were efficiently synthesized from N,N′‐dialkylthioureas and 3‐hydroxy‐2‐butanone and their basicities were measured in acetonitrile. The derivatives with tert‐alkyl groups on the imino nitrogen were found to be more basic than the tBu P₁ (pyrr) phosphazene base in acetonitrile. The origin of the high basicity of these compounds is discussed. In acetonitrile and in the gas phase, the basicity of the alkylimino derivatives depends on the size of the substituent at the imino group, which influences the degree of aromatization of the imidazole ring, as measured by ¹³C NMR chemical shifts or by the calculated ΔNICS(1) aromaticity parameters, as well as on solvation effects. If a wider range of imino‐substituents, including electron‐acceptor substituents, is treated in the analysis then the influence of aromatization is less predominant and the gas‐phase basicity becomes more dependent on the field‐inductive effect, polarizability, and resonance effects of the substituent.     

Alkyl Ether Analogs of Chlorophyll-a Derivatives: Part 1. Synthesis, Photophysical Properties and Photodynamic Efficacy

The synthesis, preliminary in vivo biological activity, singlet oxygen and fluorescence yields of a series of alkyl ether derivatives of chlorophyll-a analogs are described. For short-chain carbon ethers (1–7carbon units), it was observed that the biological activity increased by increasing the length of the carbon chain, being maximum in compounds with n-hexyl and n-heptyl chains. Related sensitizers prepared by reacting 2-(1-bromoethyl)-2-devinylpyropheophorbide-a with (sec)alcohols were found to be less effective. Under similar treatment conditions, photosensitizers containing cis- and trans- 3-hexenyl side chains were ineffective. Thus, both stereochemical and steric factors caused differences in sensitizing activity. In general, pyropheophorbide-a analogs were found to be more active than related chlorin e₆ derivatives, in which the isocyclic ring (ring “E”) was cleaved. Related photosensitizers in the 9-deoxy- series were found to be as effective as the corresponding pyropheophorbide-a analogs. The photosensitizers prepared from pyropheophorbide-a methyl ester and chlorin e₆ trimethyl ester have long wavelength absorption at 660 nm (ε 45000 to 50000). Reduction of the carbonyl group in the pyropheophorbide-a to methylene (ring E) resulted in a blue shift to 648 nm (ε 38000).     

EXCITED STATES OF SIX-MEMBERED N-HETEROCYCLES. FLUORESCENCE, PHOSPHORESCENCE AND ACID—BASE EQUILIBRIA OF ELECTRONICALLY EXCITED PHENAZINE

Abstract— An account of a systematic study of the acid-base equilibria of phenazine in the two lowest excited (π,π) states is presented. Pure electronic levels of the free base and of both its protonated forms have been located by spectroscopic methods. Fluorescence, phosphorescence and corresponding absorption spectra have been measured. The O-O energies of the free base, of the singly-protonated species and of the doubly protonated form in the lowest triplet state (³L_α(π, π)) are: 15, 475 cm^-1, 14, 175 cm^-1 and about 9300cm^-1, respectively. This last value has been estimated from the experimentally determined S-T splitting in the other two forms. Corresponding energies of the lowest singlet state (^IL_α(π,π)) are: 23,500 cm^-1, 21,250cm^-1 and 17,300 cm^-1. The fluorescence of the free base has been found in polar as well as in non-polar solvents and has been checked by the fluorescence excitation spectrum. Fluorescence quantum yields for the free base have been measured: 8.6 times 10^-4 and 3.0 × 10^-5 in ethanol and hexane solutions, respectively. Emission in ethanol has been ascribed to (π,π), that in hexane —to (π, π). fluorescence. The changes of pK_α's under excitation, calculated from the Forster's cycle, are equal: δpK_a¹=+2.8±0.3; δpK_a¹¹?+10±1.5 in the lowest (π, π) triplet state and δpK_a¹=+4.8±0.5; δpK_a¹¹=+8.4 ± 0.5 in the lowest (π,π) singlet state. The δpK_a¹¹ in the triplet state is at least as high as that in the ¹L_a(π, π) state. P P P calculations of the electronic levels and of the molecular diagrams have been performed. The energies obtained exceed experimental values by not more than 0.5 eV. An increase of the net charge on nitrogen δp under excitation has been found to be +50, +70 and +19 per cent in the ¹L_a, ¹L_b and ³L_a states, respectively. A good correlation has been found between δpK_a¹ and δp in both excited states, which have been studied experimentally.     

A Novel QSAR Approach for Estimating Toxicity of Phenols

Abstract

Toxicity values (log IGC^?1 ₅₀) for 60 phenols tested in the 2-d static population growth inhibition assay with the ciliate Tetrahymena pyriformis were tabulated. Each chemical was selected so the series formed uniform coverage of the hydrophobicity/ionization surface. A high quality hydrophobicity-dependent (log K _ow) structure-toxicity relationship (log IGC^?1 ₅₀ = 0.741 (log K_ow) ?1.433; n = 17; r² = 0.970; s = 0.134; F = 486.55; Pr > F = 0.0001) was developed for phenols with pK_a values > 9.8. Similarly, separate hydrophobicity-dependent relationships were developed for phenols with pK_a values of 4.0, 5.1, 6.3, 7.5, and 8.7. Comparisons of intercepts and slopes, respectively, revealed phenols with pK_a values of 6.3 to be the most toxic and the least influenced by hydrophobicity. These relationships were reversed for the more acidic and basic phenols. Plots of toxicity versus pK_a for nitro-substituted phenols and phenols with log K_ow values of either 1.75 or 2.50 further demonstrated bilinearity between toxicity and ionization. In an effort to more accurately model the relationship between toxicity and ionization, the absolute value function |6.3-pK_a| was used to model ionization affects for derivatives with pK_a values between 0 and 9.8. For derivatives with pK_a value > 9.8, a value of 3.50 was used to quantitate ionization effects. The use of log K_ow in conjunction with this modified pK_a (ΔpK_a) resulted in the structure-toxicity relationship (log IGC^?1 ₅₀ = 0.567 (log K_ow)-0.226 (ΔpK_a-0.079; n = 54; r² = 0.926; s = 0.215; F = 321.06; Pr > F = 0.0001). Derivatives with a nitro group in the 4-position typically did not model well with the above equation.