Pyridine In Aqueous Solution Research Articles

Molecular clusters in aqueous solutions of pyridine and its methyl derivatives

Small-angle neutron scattering proved that molecules in aqueous solutions of pyridine, 2-methylpyridine and 2,6-dimethylpyridine form clusters. The clusters are dynamic aggregates consisting of hydrogen-bonded water-amine complexes. Strengthening of the hydrogen bonds between water and amine molecules due to the methyl groups in the ortho position in the pyridine ring makes the structures more stable, as was evidenced by relatively long times of the structural relaxation. The strong intermolecular forces affect the thermal expansion of the systems. No aggregates similar to those in aqueous systems are present in the methanolic ones. That points to the crucial role of water in the molecular clustering. A molecule of methanol, although capable of hydrogen bonding with the amines, cannot participate in larger structures because of the lack of protons that could form the enhanced network. Thus, even if the amine-methanol complexes occur, they are incapable of further association. It was shown that the co-operative nature of hydrogen bonds and the propensity of water to association are the main factors that determine the properties of aqueous systems.

Synthesis, characterization, and antibacterial activity of yttrium(III) complexes with 2,6-pyridinedicarboxylate and pyridine

A series of metal coordination complexes of yttrium(III) containing 2,6-pyridinedicarboxylate and pyridine have been prepared with different molar ratios of yttrium(III) to 2,6-pyridinedicarboxylate in aqueous pyridine solutions, and characterized by elemental analysis, infrared spectra, nuclear magnetic resonance, and thermal analyses. The in vitro antibacterial activities of the complexes have also been investigated against microorganisms such as Gram-negative bacteria Bacillus coli and Gram-positive bacteria Staphylococcus aureus by the disc diffusion method in DMSO. When compared to previous results, the yttrium(III) complexes of 2,6-pyridinedicarboxylate and pyridine have a moderate effect on microorganisms due to the presence of the pyridine group.

Controlled Assembly of Janus Nanoparticles

Janus nanoparticles were prepared by interfacial ligand exchange reactions of octanethiolate-protected gold (AuC8) nanoparticles with 3-mercapto-1,2-propanediol (MPD) at the air/water interface. AFM and TEM measurements showed that the resulting particles formed stable aggregates in water with dimensions up to a few hundred nanometers, in sharp contrast to the original AuC8 particles and bulk-exchange counterparts where the aggregates were markedly smaller. Consistent behaviors were observed in dynamic light scattering measurements. FTIR measurements of solid films of the nanoparticles suggested that the octanethiolate ligands were mostly of trans conformation, whereas the MPD ligands exhibited gauche defects as a consequence of the hydrogen-bonding interactions between the hydroxyl moieties of adjacent ligands. Raman spectroscopic measurements in an aqueous solution of pyridine showed that the pyridine ring breathing modes remained practically unchanged and the intensity profiles indicated minimal interactions between pyridine and the gold cores within the three nanoparticle ensembles. However, water bending vibrational features were found to be enhanced substantially with the addition of Janus nanoparticles, which was ascribed to the formation of clusters of water molecules that were trapped within the nanoparticle ensembles. No apparent enhancement was observed with the AuC8 or bulk-exchange particles.

Association in Dilute Aqueous Solution of Pyridine and Its Methyl Derivatives Studied by Cryoscopic Method

Pyridine and its methyl derivatives form complexes with water due to hydrogen bonds. The co-operative nature of the hydrogen bonds leads to the association of the complexes and to various modes of hydration. The degrees of association of monohydrate complexes have been calculated for liquid dilute aqueous solutions of pyridine, 2-, 3-, 4-methylpyridine, and 2,6-dimethylpyridine at temperatures from 273 K to 268.5 K. The association number increases with an increase of the amine concentration. Positive correlation was found between the degree of association of the 1:1 water–amine complexes and the size of microheterogeneities. It was shown that the mechanics of this process involves hydrogen bonding and van der Waals interactions.

Observation of Pyridine Aggregation in Aqueous Solution Using Neutron Scattering Experiments and MD Simulations

Neutron diffraction with isotopic substitution (NDIS) experiments have been used to examine the structuring of aqueous solutions of pyridine. A new method is described for extracting the structure factors relating to intermolecular correlations from neutron scattering experiments on liquid solutions of complex molecular species. This approach performs experiments at different concentrations and exploits the intramolecular coordination number concentration invariance (ICNCI) to separate the internal and intermolecular contributions to the total intensities. The ability of this method to deconvolute molecular and intermolecular correlations is tested and demonstrated using simulated neutron scattering results predicted from molecular dynamics simulations of aqueous solutions of the polyatomic solute pyridine in which the inter- and intramolecular terms are known. The method is then implemented using neutron scattering measurements on solutions of pyridine. The results confirm that pyridine shows a significant propensity to aggregate in solution and demonstrate the prospects for the future application of the ICNCI approach to the study of large polyatomic solutes using next-generation neutron sources and detectors.

Preferential Interactions of Guanidinum Ions with Aromatic Groups over Aliphatic Groups

Small angle neutron scattering (SANS) and molecular dynamics (MD) simulations were used to characterize the long-range structuring (aggregation) of aqueous solutions of isopropanol (IPA) and pyridine and the effect on structuring of guanidinium chloride (GdmCl). These solutes serve as highly soluble analogs of the nonpolar aliphatic (IPA) and aromatic (pyridine) side chains of proteins. SANS data showed that isopropanol and pyridine both form clusters in water resulting from interaction between nonpolar groups of the solutes, with pyridine aggregation producing longer-range structuring than isopropanol in 3 m solutions. Addition of GdmCl at 3 m concentration considerably reduced pyridine aggregation but had no effect on isopropanol aggregation. MD simulations of these solutions support the conclusion that long-range structuring involves hydrophobic solute interactions and that Gdm(+) interacts with the planar pyridine group to suppress pyridine-pyridine interactions in solution. Hydrophobic interactions involving the aliphatic groups of isopropanol were unaffected by GdmCl, indicating that the planar and weakly hydrated Gdm(+) cation cannot make productive interactions with the highly curved or "lumpy" aliphatic groups of this solute. These observations support the conclusion that the effects of Gdm(+) ions on protein-stabilizing interactions involving aromatic amino acid side chains make significant contributions to the denaturant activity of GdmCl, whereas interactions with the "lumpy" aliphatic side chains are likely to be less important.

Thermodynamic properties of aqueous solutions of pyridine and piperidine

This research was designed to examine the thermodynamic properties of aqueous solutions of pyridine and piperidine. The density, excess molar enthalpy of mixing, H E, and excess molar volume of mixing, v E of piperidine–water and pyridine–water mixtures were studied at three temperatures, ranging from 294 K to 315 K. The H E and v E are negative for both mixtures, minimizing at a water mole fraction of about 0.55. The minimum H E for piperidine–water mixtures is −2.23 kJ/mol which is compared with −1.63 kJ/mol for pyridine–water mixtures. The minimum v E are about −1.7 cm 3/mol and −0.7 cm 3/mol for piperidine–water and pyridine–water mixtures, respectively. The thermal expansion coefficients α T at 303 K, calculated from the density studies, exhibited significant positive deviations (about 20%) from ideality for piperidine–water mixtures. In contrast to this pyridine–water α T only exhibited small positive deviations from ideality of about 2%. The results predict decreases in the absolute magnitude of v E with an increase in temperature which are 5. × 10 −3 cm 3/K mol and 1. × 10 −3 cm 3/K mol, for piperidine–water and pyridine–water mixtures, respectively. The results for v E for piperidine–water mixtures agree with the very precise work of Afzal et al., J. Chem. Thermodyn. 40 (2008) 47–53. The data are interpreted in terms of the greater number of opportunities for hydrogen bonding interactions between the unlike molecules for the piperidine–water mixtures.

Complexation of N-methyl-4-(p-methyl benzoyl)-pyridinium methyl cation and its neutral analogue by cucurbit[7]uril and β-cyclodextrin: a computational study

In this work, molecular dynamics (MD) simulations have been conducted to study the inclusion complexes between cucurbit[7]uril (CB7) and β-cyclodextrin (β-CD) with N-methyl-4-(p-methyl benzoyl)-pyridinium methyl cation, and N-methyl-4-(p-methyl benzoyl)-pyridine in aqueous solutions to gain detailed information about the dynamics and mechanism of the inclusion complexes. The obtained MD trajectories were used to estimate the binding free energy of the studied complexes using the molecular mechanics/Poisson Bolzmann surface area (MM–PBSA) method. Results indicate preference of CB7 to bind to the cationic guest more than the neutral guest, whereas β-CD exhibits more or less the same affinity to complex with either species. Furthermore it was interesting to note that β-CD forms more stable complexes with both guests than CB7. Average structure of each complex and the distances between the center of masses of the guest and the host were also discussed.

Nano-textured metallic surfaces for optical sensing and detection applications

This work describes fabrication of three-dimensionally nano-textured silicon surface structures metalized by thin gold films, and their application for optical sensing and detection of solute molecules utilizing surface-enhanced Raman scattering (SERS) effect. Two types of sensitive surfaces were prepared by different techniques. The first of them was a checkerboard pattern of 3D nanoblocks defined in compact areas on the surface of a silicon wafer using highly accurate electron-beam lithography (EBL) technique, and fabricated by dry reactive-ion etching (RIE) technique with subsequent metalization of the surface by gold sputtering. The second type of sensitive surface consisted of a square array of nano-apertures, defined on a large area of the silicon surface by a less accurate but simpler ultraviolet holographic lithography, and subsequently fabricated in a sequence of dry and wet etching as well as gold sputtering steps. The fabricated structures were found to exhibit significant near-field enhancement, as evidenced by strong SERS signal from the sensors immersed in aqueous pyridine solutions with concentrations as low as 10 −9 M due to the presence of well-defined 3D periodic texture of the structure.

Synthesis of Organo-soluble Tetrapyrazinoporphyrazines and Their Visible Spectra Properties

There were many researches about phthalocyanines for various applications such as chemical sensor, catalyst, etc. The unsubstituted metallophthalocyanines have intense colors but are generally insoluble in organic solvents or water, thereby limiting their use to just a few fields. It is well known that the insolubility of metal phthalocyanine derivatives results from their molecular stacking, which gives rise to strong intermolecular interactions between the macrocycles in the phthalocyanine molecules. There are, however, two methods of achieving solubility in almost all solvents. The first involves the use of a trior tetravalent transition metal as the core of the macrocycle, with the coordination compound having an axial ligand at the central metal. The second method involves the introduction of various peripheral substituents on the macrocycle. This minimizes the formation of aggregates, thereby improving the solubility. We have been studied the syntheses of functional dye materials based on 2,3-dicyanopyrazine chromophores, and correlated their physical properties with structures. In this paper, we designed and synthesized metal and metal-free tetrapyrazinoporphyrazines derived from 2,3-dicyano-5-(4tert-butylphenyl)-6-alkyl pyrazines derivatives for increasing solubility. Treatment of these 4-(tert-butyl)alkyl phenone derivatives with one equivalent of bromine in chloroform at room temperature gave α-brominated 4-(tert-butyl)alkyl phenone derivatives. α-Acetoxylated 4-(tert-butyl)alkyl phenone (obtained by the treatment of the α-brominated ketone with an excess of anhydrous potassium acetate in acetone) was reacted with 10% methanolic NaOH under reflux conditions to produce the corresponding compounds (2). The α-diketones (3) were obtained by the oxidation of α-hydroxyketone with copper sulfate in aqueous pyridine solution. Tetrapyrazinoporphyrazine precursors (4) were synthesized from α-diketones and diaminomaleonitrile, in the presence of a catalytic amount of p-toluenesulfonic acid in methanol. The final tetrapyrazinoporphyrazine magnesium complexes (5) were successfully synthesized using excess magnesium butoxide in n-butanol under reflux conditions. These magnesium complexes were easily demetalated. This was done by stirring them in excess p-toluenesulfonic acid, in THF, at room temperature, for 30 min, to produce 6 (in a yield range of 44-50%). The reaction route is summarized

Prediction of basicity constants of various pyridines in aqueous solution using a principal component-genetic algorithm-artificial neural network

Principal component-genetic algorithm-multiparameter linear regression (PC-GA-MLR) and principal component-genetic algorithm-artificial neural network (PC-GA-ANN) models were applied for prediction of the basicity constant (pK b ) for various pyridines (91 compounds) dissolved in water at 25°C. A large number of theoretical descriptors were calculated for each compound. The first 54 principal components (PCs) were found to explain more than 99.9% of variances in the original data matrix. From the pool of these PCs, the genetic algorithm was employed for selection of the best set of extracted PCs for PC-MLR and PC-ANN models. The models were generated using eight principal components as variables. For evaluation of the predictive power of the models, pK b values of 18 compounds in the prediction set were calculated. Mean percentage deviation (MPD) for PC-GA-MLR and PC-GA-ANN models are 21.096 and 3.541. Comparison of the results obtained by the models reveals superiority of the PC-GA-ANN model relative to the PC-GA-MLR model. The improvements are due to the fact that the pK b of the pyridines demonstrate non-linear correlations with the principal components.

Modelling spectroscopic properties of large molecular systems. The combined Density Functional Theory/Molecular Mechanics approach

In this presentation we review some theoretical aspects of the combined Density Functional Theory/Molecular Mechanics (DFT/MM) approach designed to describe large molecular systems in an effective and efficient manner. We will introduce the concept of (polarizable) DFT/MM response theory thereby enabling calculation of a variety of molecular properties. Here we will focus on spectroscopic properties described by linear response with respect to the applied magnetic or electric field. Of special interest will be UV and NMR properties since these are ideal probes for extracting important information concerning the nature of the intermolecular interactions between a solute and the solvent. The methods developed will be applied to acetone and pyridine in aqueous solution. In all cases we find, upon comparison to experimental data, the DFT/MM scheme to be accurate and at the same time cost-effective.

Calculation of acidity constants of pyridines in aqueous solution

With the specific aim of calculating the acidity constants of pyridines in aqueous solution we investigated the molecular conformations and solute–solvent interactions of the bases and their corresponding conjugate acids, applying the B3LYP/6-311++G(d,p) method with the Tomasi’s model. Several ionization reactions and equilibria in protic solvents, which possess a high hydrogen-bond-acceptor capability, are proposed. The agreement between the acidity constants calculated theoretically and the experimental values reported in the literature demonstrates the utility of the method here used. Furthermore, this fact constitutes a strong support for the reactions and equilibria of ionization in aqueous medium proposed in this work.

Kinetics and mechanism of the pyridinolysis of 4-nitrophenyl and 2,4-dinitrophenyl S-methyl thiocarbonates.

The reactions of 4-nitrophenyl and 2,4-dinitrophenyl S-methyl thiocarbonates (1 and 2, respectively) with a series of 3- and/or 4-substituted pyridines in aqueous solution, at 25.0 degrees C and an ionic strength of 0.2 M (KCl), are subjected to a kinetic investigation. The reactions are studied by following spectrophotometrically the release of 4-nitrophenoxide (400 nm) or 2,4-dinitrophenoxide (360 nm) anions. Under amine excess, pseudo-first-order rate coefficients (kobsd) are found. Plots of kobsd vs [pyridine] are linear and pH-independent, with slope kN. The Brønsted-type plot (log kN vs pKa of pyridinium ions) for the reactions of 1 is linear, with slope beta = 1.1, in contrast to the plot for the reactions of 2, which is biphasic, with slopes beta1 = 0.25 (high pKa) and beta2 = 0.90 (low pKa) and the curvature center at pKa = p = 7.3. The latter Brønsted plot is consistent with a stepwise mechanism, through a zwitterionic tetrahedral intermediate (T+/-) on the reaction path, and a change of the rate-determining step, from breakdown to formation of T+/-, as pyridine basicity increases. For the reactions of 1 the beta value indicates that the mechanism is also stepwise with expulsion of the nucleofuge from T+/- as the rate-determining step. By comparison of the reactions under investigation among each other and with similar aminolyses, the following conclusions can be drawn. (i) Thiocarbonate 2 is more reactive than 1 toward pyridines. (ii) The pka0 value for the pyridinolysis of 2,4-dinitrophenyl methyl carbonate (4) is larger than that for thiocarbonate 2. (iii) The k1 values (pyridine attack to form T+/-) are smaller for thiocarbonates 1 and 2 than the corresponding oxy carbonates 3 and 4, respectively. This is not in accordance with the electronic effects of MeS and MeO and could be attributed to steric hindrance of the MeS group toward pyridine attack. (iv) The kN values for the pyridinolysis of carbonates 3 and 4 are larger than those for thiocarbonates 1 and 2, respectively, when the k2 step is rate-limiting.

Speciation analysis of organoarsenical compounds in biological matrices by coupling ion chromatography to atomic fluorescence spectrometry with on-line photooxidation and hydride generation

The optimisation of an on-line decomposition based on UV photooxidation for the analysis of organoarsenic species by coupling cation-exchange chromatography and atomic fluorescence spectrometry with hydride generation, is described. In this study, special consideration is given to the compatibility of mobile phases with post-column treatments. Results show that the most commonly used mobile phase, aqueous pyridine solutions, decreases species conversion efficiency, leading to a significant loss of sensitivity. New fully-compatible chromatographic conditions are proposed to separate arsenobetaine, arsenocholine, trimethylarsine oxide and tetramethylarsonium ion within 20min. The very low absolute limits of detection, 4–12pg(As), allow speciation at trace levels. Analysis of a certified reference fish tissue (DORM-2) and other seafood samples (French and Chilean oysters and mussel) highlights the robustness and the accuracy of the optimised system.

Small-angle neutron scattering and Kirkwood–Buff integral study of aqueous solutions of pyridine

Solutions of pyridine in heavy water were studied by small-angle neutron scattering at 25 °C in the pyridine mole fraction range of 0.02–0.25. From the experimental data Kirkwood–Buff integrals and the closeness to phase separation parameters were calculated and compared with those obtained for pyridine – light water solutions from thermodynamic data available in the literature. The detailed analysis shows that a significant clustering of similar species occurs in the solution. In spite of the strongly non-ideal behavior these solutions are still far away from macroscopic phase separation.

Kinetics of hydrolysis of 4-methoxy-2-[2-hydroxy-3(4-phenyl-1-piperazinyl)]propyl-2,3-dihydro-6-methyl-1,3-dioxo-1 H-pyrrolo[3,4-c]pyridine in aqueous solutions

The kinetics of hydrolysis of 4-methoxy-2-[2-hydroxy-3(4-phenyl-1-piperazinyl)]propyl-2,3-dihydro-6-methyl-1,3-dioxo-1 H-pyrrolo[3,4-c]pyridine ( I) in aqueous solutions at 333, 343, 353 and 363 K over the pH range 0.4–4.8 was investigated. The decomposition was observed with the HPLC method (LiChrosorb ® 100 RP-18 column 250 × 4.0 mm ID, dp = 5 μm; mobile phase: acetonitrile—0.01 M phosphate buffer (H 3PO 4 + KH 2PO 4; pH 3) (50:50, v/v—phase A) or (30:70, v/v—phase B). The pH-rate profile indicated specific acid- and base-catalyzed reactions as well as spontaneous water-catalyzed decomposition of both dissociated and undissociated molecules of I. Various buffer substances exhibited general acid and base catalysis of the degradation. The ionic strength effect, the pK a value (4.2), the pH min value (2.0) and thermodynamic parameters of the reaction (energy and enthalpy of activation and the frequency coefficient for the specific rate constants) were determined.

Enthalpic interaction between α-amino acids and pyridine and methylpyridine in aqueous solutions

Enthalpies of dilution of aqueous L-serine, pyridine and methylpyridine solutions and their enthalpies of mixing have been determined by a mixing-flow microcalorimeter at 298.15 K. The data have been analyzed in terms of McMillan-Mayer formalism to fit to virial polynomials from which the heterotactic enthalpic pairwise interaction coefficients, hxy, betweenL-serine and pyridine and methylpyridine isomers have been evaluated. The results obtained in the present paper are compared with those reported in the earlier paper about glycine and L-alanine in the same organic solvent aqueous solutions, giving a global insight of the interaction mechanism between the a-amino acids and pyridine and methylpyridine from the point of view of solute-solute interactions and substituent effects of methyl groups introduced into the pyridine ring.

Study of the volume phase transition of poly( N-isopropylacrylamide) gels in aqueous solutions of pyridine by depolarized Rayleigh and Raman scattering

The volume phase transition of neutral poly( N-isopropylacrylamide) gels in aqueous solutions of pyridine was studied by means of depolarized Rayleigh and Raman scattering. The Rayleigh reorientational relaxation time, τ Ray, of pyridine, calculated from the half-width of the depolarized Rayleigh spectrum, attains a maximum at approximately x=0.2 at which the gel undergoes the abrupt volume change. As the gel shrinks, the ratio, I G/ I S, of the intensity of the spectrum of the gel to that of the aqueous solution abruptly decreases, reflecting the decrease in the number of free pyridine in the gel network. A new Raman band, which shifts by 130 cm −1, appears for the shrunken gels. This new band is assigned to the C–H stretching vibrations of pyridine bound to the PNIPA chains by the hydrogen-bonding interaction. As the gel shrinks, the intensity, I B, of the new Raman band abruptly increases, on the contrary, the intensity, I F, of the free pyridine band decreases. The intensity ratio, I F/( I F+ I B), which represents the number fraction of free pyridine in the network, excellently agrees with I G/ I S. The results show that the hydrophilic solutes such as pyridine in PNIPA gels are bound by the hydrogen-bonding interaction between the solutes and the chains.

Cooperative ligation, back-bonding, and possible pyridine-pyridine interactions in tetrapyridine-vanadium(II): a visible and X-ray spectroscopic study.

The binding of pyridine by V(II) in aqueous solution shows evidence for the late onset of cooperativity. The K(1) governing formation of [V(py)](2+) (lambda(max) = 404 nm, epsilon(max) = 1.43 +/- 0.3 M(-1) cm(-1)) was determined spectrophotometrically to be 11.0 +/- 0.3 M(-)(1), while K(1) for isonicotinamide was found to be 5.0 +/- 0.1 M(-1). These values are in the low range for 3d M(2+) ions and indicate that V(II).py back-bonding is not significant in the formation of the 1:1 complex. Titration of 10.5 mM V(II) with pyridine in aqueous solution showed an absorption plateau at about 1 M added pyridine, indicating a reaction terminus. Vanadium K-edge EXAFS analysis of 63 mM V(II) in 2 M pyridine solution revealed six first-shell N/O ligands at 2.14 A and 4 +/- 1 pyridine ligands per V(II). UV/vis absorption spectroscopy indicated that the same terminal V(II) species was present in both experiments. Model calculations showed that in the absence of back-bonding only 2.0 +/- 0.2 and 2.4 +/- 0.2 pyridine ligands would be present, respectively. Cooperativity in multistage binding of pyridine by [V(aq)](2+) is thus indicated. XAS K-edge spectroscopy of crystalline [V(O(3)SCF(3))(2)(py)(4)] and of V(II) in 2 M pyridine solution each exhibited the analogous 1s --> (5)E(g) and 1s --> (5)T(2g) transitions, at 5465.5 and 5467.5 eV, and 5465.2 and 5467.4 eV, respectively, consistent with the EXAFS analysis. In contrast, [V(py)(6)](PF(6))(2) and [V(H(2)O)(6)]SO(4) show four 1s --> 3d XAS transitions suggestive of a Jahn-Teller distorted excited state. Comparison of the M(II)[bond]N(py) bond lengths in V(II) and Fe(II) tetrapyridines shows that the V(II)[bond]N(py) distances are about 0.06 A shorter than predicted from ionic radii. For [VX(2)(R-py)(4)] (X = Cl(-), CF(3)SO(3)(-); R = 4-Et, H, 3-EtOOC), the E(1/2) values of the V(II)/V(III) couples correlate linearly with the Hammett sigma values of the R group. These findings indicate that pi back-bonding is important in [V(py)(4)](2+) even though absent in [V(py)](2+). The paramagnetism of [V(O(3)SCF(3))(2)(py)(4)] in CHCl(3), 3.8 +/- 0.2 mu(B), revealed that the onset of back-bonding is not accompanied by a spin change. Analysis of the geometries of V(II) and Fe(II) tetrapyridines indicates that the ubiquitous propeller motif accompanying tetrapyridine ligation may be due to eight dipole interactions arising from the juxtaposed C-H edges and pi clouds of adjoining ligands, worth about -6 kJ each. However, this is not the source of the cooperativity in the binding of multiple pyridines by V(II) because the same interactions are present in the Fe(II)-tetrapyridines, which do not show cooperative ligand binding. Cooperativity in the binding of pyridine by V(II) is then assigned by default to V(II)-pyridine back-bonding, which emerges only after the first pyridine is bound.