Determination of the overlapping pKa values of resorcinol using UV-visible spectroscopy and DFT methods

In this paper we determine the overlapping pK(a) values of resorcinol in water, applying a UV-Vis spectroscopic method that uses absorbance diagrams. On the other hand, in order to explain the pK(a) values obtained, we also investigate the molecular conformations and solute-solvent interactions of the resorcinate anions, using ab initio and density functional theory methods. Several ionization reactions and equilibria in protic solvents, which possess a high hydrogen-bond-donor capability, are proposed. The mentioned reactions and equilibria constituted the indispensable theoretical basis to calculate the acidity constants of resorcinol. Basis sets at the HF/6-31 + G(d) and B3LYP/6-31 + G(d) levels of theory were used for calculations. Tomasi's method was used to analyze the formation of intermolecular hydrogen bonds between the resorcinate anions and water molecules. In this way, it was determined that in alkaline aqueous solutions the monoanion and dianion of resorcinol are solvated with two and four molecules of water, respectively. The agreement between the experimentally determined pK(a) values and those reported in the literature demonstrates the applicability and accurateness of the spectroscopic method here used. On the other hand, the agreement between the experimental and theoretically calculated pK(a) values provides solid support for the acid-base reactions proposed in this work.     

Effect of CTAB and SDS micelles on the excited state equilibria of some indole probes

The absorption and fluorescence spectral characteristics of some biologically active indoles have been studied as a function of acidity and basicity (H_/pH/H(o)) in cationic (cetyltrimethylammonium bromide, CTAB), anionic (sodium dodecylsulphate, SDS) and aqueous phases at a given surfactant concentration. The prototropic equilibrium reactions of these probes have been studied in aqueous and micellar phases and apparent excited state acidity constant (pK(a)(*)) values are calculated. The probes show formation of different species on changing pH. Various species present in water, CTAB and SDS have been identified and the equilibrium constants have been determined by Fluorimetric Titration method. The fluorescence spectral data suggest the formation of oxonium ion through the excited state proton transfer reaction in highly acidic media and formation of photoproducts due to the base catalyzed auto-oxidative reaction in basic aqueous solutions. Variations in the apparent pK(a)(*) value have been observed in different media. The change in the apparent pK(a) values depends upon the solubilising power of the micelles, as well as on the location of the protonating site in the molecule. The observation about increase in pK(a)(*) values in SDS and decrease in CTAB compared to pure water for various equilibria is consistent with the pseudophase ion-exchange (PIE) model.     

Ab Initio Quantum Mechanical Study on the Origin of the pK(a) Differences of the Proton Sponges 1,8-Bis(dimethylamino)naphthalene, 1,8-Bis(dimethylamino)-2,7-dimethoxynaphthalene, 1,6-Dimethyl-1,6-diazacyclodecane, and 1,6-Diazabicyclo[4.4.4]tetradecane

Ab initio quantum mechanical calculations were used in studying the origin of the exceptionally high basicities of four diamines (13-16) with pK(a1) values ranging from 12.1 to 25. The computational approach involved the calculation of the gas-phase proton affinities of the molecules studied at the MP2/6-31G//HF/6-31G level and the solvation energies with the polarizable continuum model at the HF/6-31G level. The calculated gas-phase and aqueous-phase proton affinities of a structurally diverse series of amines were compared with the corresponding experimental gas-phase proton affinities and pK(a1) values. The calculated values were found to be in reasonable agreement with the experimental ones. The high basicities of the studied diamines were found to originate from the nitrogen lone-pair repulsion, solvation effects, and strong intramolecular hydrogen bonds. Each of these factors were found to be able to increase the pK(a1) values of the high-basicity diamines by 2-6 pK(a) units. The relative contributions of the factors varied between the compounds. The nitrogen lone-pair repulsion was estimated to be the most important factor in increasing the pK(a1) values. In addition, barriers for proton transfers between the nitrogens of selected diamines were calculated, and comparison was made between the barrier heights and the geometries of the diamines.     

Protonated benzofuran,anthracene, naphthalene,benzene, ethene,and ethyne: measurements and estimates of pK(a) and pK(R)

Aqueous solvolyses of acyl derivatives of hydrates (water adducts) of anthracene and benzofuran yield carbocations which undergo competitive deprotonation to form the aromatic molecules and nucleophilic reaction with water to give the aromatic hydrates. Trapping experiments with azide ions yield rate constants k(p) for the deprotonation and k(H2O) for the nucleophilic reaction based on the "azide clock". Combining these with rate constants for (a) the H(+)-catalyzed reaction of the hydrate to form the carbocation and (b) hydrogen isotope exchange of the aromatic molecule (from the literature) yields pK(R) = -6.0 and -9.4 and pK(a) = -13.5 and -16.3 for the protonated anthracene and protonated benzofuran, respectively. These pK values may be compared with pK(R) = -6.7 for naphthalene hydrate (1-hydroxy-1,2-dihydronaphthalene), extrapolated to water from measurements by Pirinccioglu and Thibblin for acetonitrile-water mixtures, and pK(a) = -20.4 for the 2-protonated naphthalene from combining k(p) with an exchange rate constant. The differences between pK(R) and pK(a) correspond to pK(H2O), the equilibrium constant for hydration of the aromatic molecule (pK(H2O) = pK(R) - pK(a)). For naphthalene and anthracene values of pK(H2O) = +13.7 and +7.5 compare with independent estimates of +14.2 and +7.4. For benzene, pK(a) = -24.3 is derived from an exchange rate constant and an assigned value for the reverse rate constant close to the limit for solvent relaxation. Combining this pK(a) with calculated values of pK(H2O) gives pK(R) = -2.4 and -2.1 for protonated benzenes forming 1,2- and 1,4-hydrates, respectively. Coincidentally, the rate constant for protonation of benzene is similar to those for protonation of ethylene and acetylene (Lucchini, V.; Modena, G. J. Am. Chem Soc. 1990, 112, 6291). Values of pK(a) for the ethyl and vinyl cations (-24.8) may thus be derived in the same way as that for the benzenonium ion. Combining these with appropriate values of pK(H2O) then yields pK(R) = -39.8 and -29.6 for the vinyl and ethyl cations, respectively.     

Absorption and emission study of 2',7'-difluorofluorescein and its excited-state buffer-mediated proton exchange reactions

2',7'-Difluorofluorescein (Oregon Green 488) is a new fluorescein-based dye, which has found many applications, above all in biochemistry and neurosciences, and its use has become very popular in the last years. In recent years, we have been investigating the excited-state proton exchange reactions of fluorescein and the effect of suitable proton acceptors and donors which promote these reactions. The excited-state proton transfer reactions may appreciably influence the fluorescence results when using these dyes. We present steady-state emission evidence that acetate buffer species promote an excited-state proton transfer between neutral, monoanionic, and dianionic forms of 2',7'-difluorofluorescein. The time course of the excited species in this reaction was characterized through time-resolved fluorescence measurements, and the kinetics of the reaction was solved by using the global compartmental analysis. A previous identifiability study on the compartmental system set the conditions to design the fluorescence decay surface. This is the first experimental system, studied within this kinetic model, solved under identifiability conditions through global compartmental analysis. The recovered rate constant values for deactivation were 2.94 x 10(8) s(-1) for the monoanion and 2.47 x 10(8) s(-1) for the dianion, whereas the rate constant values of the buffer-mediated excited-state reaction were 9.70 x 10(8) and 1.79 x 10(8) M(-1) s(-1) for the deprotonation and protonation, respectively. With these values, a pK(a) = 4.02 was obtained. In this work, we additionally provide an absorption study, including acid-base equilibria, determination of ground-state pK(a) values (1.02, 3.61, and 4.69), and recovery of molar absorption coefficients of every prototropic species, including absorption and NMR evidence for the existence of three tautomers in neutral species. Steady-state emission spectra of 2',7'-difluorofluorescein in aqueous solution are also described, where the strong photoacid behavior of the cation is noteworthy.     

A computational study of intramolecular proton transfer in gaseous protonated glycine

The minimum energy paths for intramolecular proton transfer between the amino nitrogen and carbonyl oxygen atoms in gaseous protonated glycine were estimated at the Hartree-Fock (HF) and second-order M?ller-Plesset Perturbation (MP2) levels of theory. Potential energy profiles and their associated reactant, transition state, and product species calculated at the MP2/6-31G* level were shown to differ significantly from those obtained at the HF/6-31G* level. Effects of electron correlation and basis functions on the calculated geometries and energies of relevant species were examined at the HF, MP2, MP4, CCSD, and B3LYP levels using the 6-31G*, 6-31G**, 6-31+G**, 6-311+G**, 6-31+G(2d,2p), 6-311+G(3df,2p), cc-pVDZ, aug-cc-pVDZ, and cc-pVTZ basis sets. The HF and MP2 optimized levels with the 6-31G*, 6-31G**, 6-31+G**, and 6-311+G** bases were used to calculate the thermodynamic and kinetic properties of the proton transfer reaction at 298.15 K and 1 atm, which include enthalpy, entropy, Gibbs free energy, equilibrium constant, potential energy barriers, tunneling transmission coefficients, and rate constants. Results indicate that the proton in a carbonyl O-protonated glycine undergoes a rapid migration to the amino nitrogen atom, while the reverse process is extremely unfavorable. The objective of this work is to develop practical theoretical procedures for studying proton transfer reactions in amino acids and peptides and to assemble physical data from these model calculations for future references.     

Kinetic study of the phenolysis of bis(4-nitrophenyl) carbonate, bis(4-nitrophenyl) thionocarbonate, and methyl 4-nitrophenyl thionocarbonate

The reactions of a homogeneous series of phenols with bis(4-nitrophenyl) carbonate (BNPC), bis(4-nitrophenyl) thionocarbonate (BNPTOC), and methyl 4-nitrophenyl thionocarbonate (MNPTOC) are subjected to a kinetic investigation in water, at 25.0 degrees C and ionic strength of 0.2 M (KCl). Under excess of phenol over the substrate, all the reactions obey pseudo-first-order kinetics and are first order in phenoxide anion. The reactions of BNPC show a linear Br?nsted-type plot with slope beta = 0.66, consistent with a concerted mechanism (one step). In contrast, those of BNPTOC and MNPTOC show biphasic Br?nsted-type plots with slopes beta = 0.30 and 0.44, respectively, at high pK(a), and beta = 1.25 and 1.60, respectively, at low pK(a), consistent with stepwise mechanisms. For the reactions of both thionocarbonates, the pK(a) value at the center of the Br?nsted plot (pK(a)(0)) is 7.1, which corresponds to the pK(a) of 4-nitrophenol. This confirms that the phenolyses of the thionocarbonates are stepwise processes, with the formation of an anionic tetrahedral intermediate. By the comparison of the kinetics and mechanisms of the title reactions with similar reactions, the following conclusions can be drawn: (i) Substitution of S(-) by O(-) in an anionic tetrahedral intermediate (T(-)) destabilizes it. (ii) The change of MeO by 4-nitrophenoxy in T(-) results in an increase of both the rate constant and equilibrium constant, for the formation of T(-), and also in an enlargement of the rate coefficient for the expulsion of 4-nitrophenoxide from T(-). (iii) Substitution of an amino group in a tetrahedral intermediate by ArO destabilizes it. (iv) Secondary alicyclic amines and other amines show greater reactivity toward MNPTOC than isobasic phenoxide anions.     

Modeling acid and cationic catalysis on the reactivity of duocarmycins

Several catalyzed alkylation reactions of 9-methyladenine by a model [CPI, cyclopropa[c]pyrrolo[3,2-e]indol-4(5H)-one (1)] of duocarmycin anticancer drugs have been compared to the uncatalyzed reaction in gas phase and in water solvent bulk, using density functional theory at the B3LYP level with the 6-31+G(d,p) basis set and C-PCM solvation model. The effect on the CPI reactivity induced by water, formic and phosphoric acids (general acid catalysis), H3O+ (specific acid catalysis), sodium, and ammonium cation complexation (cationic catalysis) has been investigated. The calculations indicate that the specific acid catalysis and the catalysis induced by sodium cation complexation are strong in the gas phase, but solvation reduces them dramatically by electrostatic effects. The specific acid catalysis is still operative, but strongly reduced in water solution, where the reaction barrier is reduced by 8.6 kcal mol(-1) in comparison to the uncatalyzed reaction. The general acid catalysis induced by phosphoric acid (-7.3 kcal mol(-1)) and the catalysis induced by Na+ and NH4+ complexation become competitive, with a catalytic effect of -3.6 and -4.1 kcal mol(-1) in water, respectively. With the specific acid catalysis, the high acidity (low pK(a) value) of the conjugated acid of CPI (CPIH+), computed in water solution using both C-PCM (pK(a) = +2.6) and PCM-B3LYP/6-31+G(d,p) (pK(a) = +2.4) solvation models, suggests that the catalytic effects induced by NH4+ complexation could become more important than the specific acid catalysis and the general catalysis by H3PO4 under physiological conditions, due to concentration effects of the catalysts.     

A minimalist approach to understanding the efficiency of mononuclear Zn(II) complexes as catalysts of cleavage of an RNA analog

Mononuclear complexes between Zn(2+) and the following four macrocycles were prepared: 1,4,7,10-tetraazacyclododecane (1), 1-oxa-4,7,10-triazacyclododecane (2), 1,5,9-triazacyclododecane (3) and 1-hydroxyethyl-1,4,7-triazacyclononane (4). The pH rate profiles of values of the observed second-order rate constant log (k(Zn))(app) for Zn(X)(OH(2))-catalyzed cleavage (X = 1, 2, 3 and 4) of 2-hydroxypropyl-4-nitrophenyl phosphate (HpPNP) show downward breaks centered at the pK(a) for ionization of the respective zinc bound water. At low pH, where the rate acceleration for the catalyzed reaction is largest, the stabilizing interaction between the catalyst and the bound transition state is 5.7, 7.4, 7.4 and 5.9 kcal mol(-1) for the reactions catalyzed by Zn(1)(OH(2)), Zn(2)(OH(2)), Zn(3)(OH(2)) and Zn(4)(OH(2)), respectively. The interactions between the metal cation and the macrocycle cause either a modest increase or reduction in transition state stabilization compared with 6.6 kcal mol(-1) stabilization for catalysis by Zn(OH(2))(6). The best Zn(II)-macrocycle catalysts are those for which the interactions between the metal ion and macrocycle are the weakest. Inhibition studies show that each of the four catalysts form complexes with phosphate and oxalate dianions with a much higher affinity than diethyl phosphate monoanion, consistent with stronger interaction of the catalysts with the transition state dianion compared with the substrate monoanion HpPNP. The pH-dependence of methyl phosphate inhibition of Zn(2) catalyzed cleavage of HpPNP shows that only the Zn(2)(OH(2)) species binds the inhibitor. This result is consistent with a mechanism that has Zn(2)(OH(2)) as the active catalytic species.     

Kinetic study of the aminolysis and pyridinolysis of O-phenyl and O-ethyl O-(2,4-dinitrophenyl) thiocarbonates. A remarkable leaving group effect

The reactions of a series of secondary alicyclic (SA) amines with O-phenyl and O-ethyl O-(2,4-dinitrophenyl) thiocarbonates (1 and 2, respectively) and of a series of pyridines with the former substrate are subjected to a kinetic investigation in water, at 25.0 degrees C, ionic strength 0.2 M (KCl). Under amine excess over the substrate, all the reactions obey pseudo-first-order kinetics and are first-order in amine. The Br?nsted-type plots are biphasic, with slopes (at high pK(a)) of beta(1) = 0.20 for the reactions of SA amines with 1 and 2 and beta(1) = 0.10 for the pyridinolysis of 1 and with slopes (at low pK(a)) of beta(2) = 0.80 for the reactions of SA amines with 1 and 2 and beta(2) = 1.0 for the pyridinolysis of 1. The pK(a) values at the curvature center (pK(a)(0)) are 7.7, 7.0, and 7.0, respectively. These results are consistent with the existence of a zwitterionic tetrahedral intermediate (T++) and a change in the rate-determining step with the variation of amine basicity. The larger pK(a)(0) value for the pyridinolysis of 1 compared to that for 2 (pK(a)(0) = 6.8) and the larger pK(a)(0) value for the reactions of SA amines with 1 relative to 2 are explained by the greater inductive electron withdrawal of PhO compared to EtO. The larger pK(a)(0) values for the reactions of SA amines with 1 and 2, relative to their corresponding pyridinolysis, are attributed to the greater nucleofugalities of SA amines compared to isobasic pyridines. The smaller pK(a)(0) value for the reactions of SA amines with 2 than with O-ethyl S-(2,4-dinitrophenyl) dithiocarbonate (pK(a)(0) = 9.2) is explained by the greater nucleofugality from T(++) of 2,4-dinitrophenoxide (DNPO(-)) relative to the thio derivative. The stepwise reactions of SA amines with 1 and 2, in contrast to the concerted mechanisms for the reactions of the same amines with the corresponding carbonates, is attributed to stabilization of T(++) by the change of O(-) to S(-). The simple mechanism for the SA aminolysis of 2 (only one tetrahedral intermediate, T(++)) is in contrast to the more complex mechanism (two tetrahedral intermediates, T(++) and T(-), the latter formed by deprotonation of T(++) by the amine) for the same aminolysis of the analogous thionocarbonate with 4-nitrophenoxide (NPO(-)) as nucleofuge. To our knowledge, this is the first example of a remarkable change in the decomposition path of a tetrahedral intermediate T by replacement of NPO(-) with DNPO(-) as the leaving group of the substrate. This is explained by (i) the greater leaving ability from T(++) of DNPO(-) than NPO(-) and (ii) the similar rates of deprotonation of both T(++) (formed with DNPO and NPO).     

金属离子选择电极法测定弱酸的离解常数

通过对弱酸难溶盐平衡体系的讨论,导出了弱酸离解常数,金属离子浓度和溶液ｐＨ值之间的关系,从而给出一种利用金属离子选择电极测定弱酸离解常数的方法。采用该法,本文用钙离子选择电极对Ｈ２ＣＯ３,Ｈ２Ｃ２Ｏ４和ＨＦ的离解常数进行了测定,得到与文献值相一致的结果。同时本法可还以用来测定相应的溶度积常数。     

Acid-base equilibria and dynamics in sodium dodecyl sulfate micelles: geminate recombination and effect of charge stabilization

The synthetic flavylium salt 4-carboxy-7-hydroxy-4'-methoxyflavylium chloride (CHMF) exhibits two acid-base equilibria in the range of pH 1-8 in both aqueous and micellar sodium dodecyl sulfate (SDS) solutions. The values of pK(a1) and pK(a2) for the cation-zwitterion (AH(2)(+) <--> Z + H(+)) and the zwitterion-base (Z <--> A(-) + H(+)) equilibria increase from 0.73 and 4.84 in water to 2.77 and 5.64 in SDS micelles, respectively. The kinetic study of the Z <--> A(-) + H(+) ground-state reactions in SDS points to the diffusion-controlled protonation of A(-) in the aqueous phase (k(p2w) = 4.2 x 10(10) M(-)(1) s(-)(1)) and in the micelle (k(p2m) = 2.3 x 10(11) M(-)(1) s(-)(1)). The deprotonation rate of Z did not significantly change upon going from water (k(d2) = 6.3 x 10(5) s(-)(1)) to SDS (k(d2) = 5.2 x 10(5) s(-)(1)), in contrast with the behavior of ordinary cationic flavylium salts, for which k(d2) strongly decreases in SDS micelles. These results suggest that deprotonation of the zwitterionic acid is not substantially perturbed by the micellar charge. Electronic excitation of the Z form of CHMF induces fast adiabatic deprotonation of the hydroxyl group of Z() (2.9 x 10(10) s(-)(1) in water and 8.4 x 10(9) s(-)(1) in 0.1 M SDS), followed by geminate recombination on the picosecond time scale. Interestingly, while recombination in water (k(rec) = 1.7 x 10(9) s(-)(1)) occurs preferentially at the carboxylate group, at the SDS micelle surface, recombination (k(rec) = 9.2 x 10(9) s(-)(1)) occurs at the hydroxyl group. The important conclusion is that proton mobility at the SDS micelle surface is substantially reduced with respect to the mobility in water, which implies that geminate recombination should be a general phenomenon in SDS micelles.     

Thermodynamic and kinetic studies on reactions of Pt(II) complexes with biologically relevant nucleophiles

The effect of different N-N spectator ligands on the reactivity of platinum(II) complexes was investigated by studying the water lability of [Pt(diaminocyclohexane)(H2O)2]2+ (Pt(dach)), [Pt(ethylenediamine)(H2O)2]2+ (Pt(en)), [Pt(aminomethylpyridine)(H2O)2]2+ (Pt(amp)), and [Pt(N,N'-bipyridine)(H2O)2]2+ (Pt(bpy)). Some of the selected N-N chelates form part of the coordination sphere of Pt(II) drugs in clinical use, as in Pt(dach) (oxaliplatin), or are models, regarding the nature of the amines, with higher stability in terms of substitution and hydrolysis of the diamine moiety, as in Pt(en) (cisplatin) and Pt(amp) (AMD473). The effect of pi-acceptors on the reactivity was investigated by introducing one (Pt(amp)) or two pyridine rings (Pt(bpy)) in the system. The pK(a) values for the two water molecules (viz., Pt(dach) (pK(a1) = 6.01, pK(a2) = 7.69), Pt(en) (pK(a1) = 5.97, pK(a2) = 7.47), Pt(amp) (pK(a1) = 5.82, pK(a2) = 6.83), Pt(bpy) (pK(a1) = 4.80, pK(a2) = 6.32) show a decrease in the order Pt(dach) > Pt(en) > Pt(amp) > Pt(bpy). The substitution of both coordinated water molecules by a series of nucleophiles (viz., thiourea (tu), L-methionine (L-Met), and guanosine-5'-monophosphate (5'GMP-) was investigated under pseudo-first-order conditions as a function of concentration, temperature, and pressure using UV-vis spectrophotometric and stopped-flow techniques and was found to occur in two subsequent reaction steps. The following k1 values for Pt(dach), Pt(en), Pt(amp), and Pt(bpy) were found: tu (25 degrees C, M(-1) s(-1)) 21 +/- 1, 34.0 +/- 0.4, 233 +/- 5, 5081 +/- 275; L-Met (25 degrees C) 0.85 +/- 0.01, 0.70 +/- 0.03, 2.15 +/- 0.05, 21.8 +/- 0.6; 5'GMP- (40 degrees C) 5.8 +/- 0.2, 3.9 +/- 0.1, 12.5 +/- 0.5, 24.4 +/- 0.3. The results for k2 for Pt(dach), Pt(en), Pt(amp), and Pt(bpy) are as follows: tu (25 degrees C, M(-1) s(-1)) 11.5 +/- 0.5, 10.2 +/- 0.2, 38 +/- 1, 1119 +/- 22; L-Met (25 degrees C, s(-1)) 2.5 +/- 0.1, 2.0 +/- 0.2, 1.2 +/- 0.3, 290 +/- 4; 5'GMP- (40 degrees C, M(-1) s(-1)) 0.21 +/- 0.02, 0.38 +/- 0.02, 0.97 +/- 0.02, 24 +/- 1. The activation parameters for all reactions suggest an associative substitution mechanism. The pK(a) values and substitution rates of the complexes studied can be tuned through the nature of the N-N chelate, which is important in the development of new active compounds for cancer therapy.     

Acidity of anilines: calculations vs experiment

The gas-phase acidities of ca. 60 monosubstituted anilines (with acidity span of almost 50 kcal mol(-1)) have been calculated using density functional theory (DFT) at the B3LYP/6-311+G** level. At this relatively simple level of theory the calculated (ΔG(calc)) and available experimental (ΔG(exp)) acidities are in reasonable quantitative correlation according to the following equation: ΔG(obs) = a + bΔG(calc), where a=20.79, b=0.942, n=27, R(2)=0.990, and s=0.78 kcal·mol(-1). The slope is not far from its ideal value. Substituent effects on the acidities were dissected separately into those operating in the neutral acid molecule and in its conjugated anion using the isodesmic homodesmotic reactions. All in all, both forms, neutral and anionic, are contributing in combination to make up the gross acidity of anilines. However, the contributions of the anions into the gross substituent effects are much larger than the substituent effects in the neutral anilines. Some of the systems were used in testing a relatively new theoretical model, COSMO-RS (conductor-like screening model for real solvents), using it for the prediction of pK(a) values in DMSO. The method proved to be rather accurate for showing pK(a) trends (R(2)=0.980 in DMSO). However, the predicted absolute pK(a) values were all somewhat lower (rmsd=2.49 kcal·mol(-1)) than the respective experimental values.     

Raman and FTIR spectroscopies of fluorescein in solutions

Raman and Fourier transform-infra red (FT-IR) spectroscopies of fluorescein in aqueous solutions have been investigated in the pH range from 9.1 to 5.4. At pH 9.1 fluorescein is in the dianion form. At pH 5.4, fluorescein is a mixture of monoanion (approximately 85%), dianion and neutral forms (together approximately 15%). The fluorescence quantum yield drops from 0.93 for the dianion form to 0.37 for the monoanion form. The Raman and FT-IR studies focused on the frequency range from 1000 to 1800 cm(-1) which contains the skeletal vibrational modes of the xanthene moiety of fluorescein. At pH 9.1, the spectroscopic feature of fluorescein dianion are consistent with a picture of an electron delocalized among the xanthene moiety and two identical oxygens attached to opposite ends of the xanthene moiety, forming a very symmetric structure. The characteristic of fluorescein dianion is the presence of the phenoxide-like stretch at 1310 cm(-1). At pH 5.4, fluorescein monoanion has lost the symmetric structure characteristic of the dianion. The spectra of the monoanion have distinctive contributions from the phenolic bend at 1184 cm(-1). The assignments of the vibrational bands shown in Raman and FT-IR spectra are given based on both literature and the ab initio calculations at the Hartree-Fock level with HF/6-31 + +G* basis set. Excellent correlation is found between the experimental and calculated spectra.     

Study of heterogeneous equilibria in saturated solutions of some sparingly soluble 8-hydroxyquinolines

Heterogeneous equilibria in saturated aqueous solutions of 8-hydroxyquinoline (A), 2-methyl-8-hydroxyquinoline (B) and 5-chloro-8-hydroxyquinoline (C) were investigated at constant ionic strength (1M sodium chloride), at 25.0 +/- 0.1 degrees , by the application of the following methods: the formation function method, the method of bound protons, the method of free proton sites and the solubility method. The following equilibrium constants were determined: K(s1)=[H(2)A(+)]/[H(3)O(+)], K(s2)=[HA] and K(s3)=[H(3)O(+)] [A(-)]. Their mean pK values are: -2.88, 2.42 and 12.09 for A, -3.27, 2.71 and 12.73 for B, and -0.34, 3.88 and 12.75 for C. In addition, the acidity constants of investigated 8-hydroxyquinolines were determined by pH-metric titrations. The values obtained for these constants were consistent with those calculated on the basis of corresponding equilibrium constants determined in heterogeneous systems.     

An acidity scale of tetrafluoroborate salts of phosphonium and iron hydride compounds in [D2]dichloromethane

Equilibrium constants (K) for reactions between acids and the conjugate base forms of a number of phosphonium salts, [HPR3][BF4], and iron hydrides, [Fe(CO)3H(PR3)2][BF4], in CD(2)Cl(2) have been determined by means of 31P and 1H NMR spectroscopy at 20 degrees C. The anchor compound chosen for pK(CD(2)Cl(2)) determinations was [HPCy3][BF4] with a pK(CD(2)Cl(2)) value of 9.7, as assigned by literature convention (Cy: cyclohexyl). A continuous scale of pK(CD(2)Cl(2)) values covering the range from 9.7 to -3 was created and correlated with the DeltaH values reported by Angelici and co-workers and literature pK(a) values. The pK(CD(2)Cl(2)) values for 15 other hydride or dihydrogen complexes of the iron group elements and of diethyl ether were also placed on this scale. The crystal structures of [Fe(CO)3H(PCy(2)Ph)2][BF4] and [Fe(CO)3(PCy(2)Ph)2] revealed that the trans-oriented, bulky, unsymmetrical phosphane ligands distort the equatorial plane of the complexes. The acidity of iron carbonyl hydrides is an important feature of the reactions of iron hydrogenase enzymes.     

Acid-base equilibria in gamma-butyrolactone studied by use of pH-ISFETs

pH-ISFETs were used in the study of acid-base equilibria in gamma-butyrolactone (GBL). After the spectrophotometric determination of the pK(a) value of 3,5-dichloropicric acid, the pK(a) values and homo-conjugation constants of various acids (including the conjugate acids of bases) were determined potentiometrically using a Ta(2)O(5)-type pH-ISFET. The values of pK(a) in GBL were in a linear relation with those in propylene carbonate (PC) and 1.0 units smaller on average. The difference in pK(a) between GBL and PC was mainly attributable to the difference in proton solvation. The autoprotolysis constant of GBL, roughly estimated by a rapid titration with a Si(3)N(4)-ISFET, was about 30 on the pK(SH) scale. A comparative study was made of the response speeds of the Ta(2)O(5)- and Si(3)N(4)-type pH-ISFETs and a conventional pH-glass electrode. The result was Si(3)N(4)-ISFET > Ta(2)O(5)-ISFET > glass electrode. Because GBL is not stable against acids and bases, the use of pH-ISFETs was much more convenient than the use of the conventional glass electrode.