Anti-cooperativity of the two water OH groups

This paper presents the results concerning anti-cooperativity effects between two H-bonds of a water molecule. The IR OH stretching band shifts Δν—as a measure of the H-bond energy—are compared for HOD with different bases B of 1:1 complexes B…HOD…Cl₄ (Δν₁₁) and 1:2 complexes B…HOD…B (Δν₁₂). We found that Δν₁₂ of the 1:2 complexes for different bases B are 25% smaller than Δν₁₁ for the 1:1 complexes. Corrections for the solvent shifts are introduced. This effect is in line with different observations concerning cooperativity effects of OH H-bonds by polarization with neighbouring molecules. The reduction of Δν₁₂ in 1:2 complexes can be understood on the assumption of a negative polarization by the first H-bond to the second OH or OD group and is called anti-cooperativity. This anti-effect has been already detected by NMR observation on NH₂.

We already observe a decrease of the CCl₄ solvent shift by van der Waals forces OH…CCl₄ of 1:1 complexes, induced by the H-bond of the other OH or OD group. This decrease is measured by the CCl₄ solvent effect for monomers. This indicates a real negative polarization by the H-bond in 1:1 complexes on the second OH/OD. This experiment establishes the real polarization and excludes the importance of repulsions of the bases as the cause. The dependence of intermolecular forces is known on the polarizability. Our method demonstrates directly the polarization by interactions.

The anti-cooperativity of symmetric complexes B₁…HOD…B₁ by a strong base B₁ can be reduced in unsymmetric 1:2 complexes B₁…HOD…B₂ by weaker bases B₂. This weakening of the anti-cooperativity of the stronger base could be predicted quantitatively. Similarly, the anti-cooperativity of the weaker base B₂ is strengthened in unsymmetric 1:2 complexes by stronger bases B₁.

It is known that H-bonds XH…B can be strengthened by cooperativity with a second H-bond XH…XH…B. They can be weakened for water by anti-cooperativity of two H-bonds B…HOH…B. The H-bond B₁…HO of 1:2 complexes B₁…HOH…B₂ can be weakened if the base strength of B₂ is stronger than of B₁ or strengthened if B₂ is weaker than B₁. Nature may use these possibilities in biochemistry.     

Structure and properties of hydrogen bonded complexes of pyridine-N-oxide and its derivatives

Molar Kerr constants and electric dipole moments of hydrogen bonded complexes that pyridine-N-oxide and its derivatives form with phenols are studied (a) experimentally, (b) by the vector/tensor addition scheme, and (c) by AM1 and PM3 semi-empirical quantum chemical methods. The data are used to establish the geometry of the complexes. It is shown that for a series of pyridine-N-oxide derivatives, the logarithm of the complex formation equilibrium constant correlates with the charge on the oxygen atom of the N–O group. A method to calculate the first and second complex formation constants based on the electric properties of the individual components of the complex is proposed. A relationship between the polarity of the hydrogen bond and the sum of the charges on the hydrogen and oxygen atoms forming the bond is established for the complexes under study.     

Vibrational spectra, structure and hydrogen bonding of 5-tert-butylpyrazole and its zinc complexes

New zinc complexes [L₂ZnX₂] with X=Cl (I), Br (II), I (III) and NO₃ (IV), [L₃Zn(OClO₃)]ClO₄ (V) and [L₄Zn](ClO₄)₂ (VI) with 5-tert-butylpyrazole as ligand L were synthesized and characterized by infrared-, Raman-, mass- and NMR-spectroscopy. The assignment of the vibrational frequencies for the complexes in the range of 4000–80 cm⁻¹ is proposed. Analysis of the IR spectra in the range of ν_NH frequencies shows that 5-tert-butylpyrazole forms cyclic associates in the solid using intermolecular NHN hydrogen bonds. This was proven by a crystal structure determination, which showed that L exists as tetramers in the solid state. In solution there is an equilibrium between tetramers and monomers and, probably, dimers or trimers. In the complexes, the NH-groups of L form intramolecular H-bonds with the X-group. The intramolecular H-bond in VI has interionic character and partially dissociates in solution at high dilution. Ligand vibrations that are sensitive to the coordination and the dependence of their frequencies on the anionic group X has been revealed.     

Spectrophotometric and theoretical studies on complexation of a newly synthesized vic-dioxime derivative with nickel(II) in dimethylformamide

The complexation of a recently synthesized vic-dioxime derivative, 1-({2-[(2-aminoethyl) amino]ethyl}amino)-6-methyl-9H-xanthen-9-one with Ni(II) in N,N-dimethylformamide, was investigated by spectrophotometric and conductometric methods. The possible molecular geometries, binding, and spectroscopic properties for the formed complexes were theoretically studied in detail by the Hyperchem program. The stoichiometry of the complex species was determined from spectrophotometric molar ratio methods at 25°C. The spectral data were further treated by KINFIT to calculate the formation constants of the 1?:?1 and 1?:?2 complexes and their molar absorptivities. Program DATAN was used to calculate the spectral behavior of the complexes in the wavelength range ～380–～550?nm for different mole ratio solutions.     

Determination of absorptivity and formation constant of a chalcone association complex

A UV spectrometric method was developed to determine the molar absorptivity (_C) and formation constant (K_c) of the association complex of unsubstituted chalcone in cyclohexane, in the concentration range from 4.00·10⁻⁴ to 2.00·10⁻² mol dm⁻³. The thermodynamic and spectroscopic magnitudes such as K_c and _C contribute to the understanding of the physicochemical behavior of several ,β-unsaturated carbonylic compounds, of low solubility in water, as it is the case of numerous flavonoids of chemical and biological importance. The studied association complex, formed by two chalcone molecules, is characterized by the constants _C (300.8 nm)=4.98·10⁴ dm³ mol⁻¹ cm⁻¹ and K_c=5.58·10³. The method proposed is convenient for the study of solute–solute molecular associations particularly those due to dipole–dipole interactions.     

Potentiometric Studies of Some Bivalent Metal Ion Complexes of a New Ligand,o-(2-Thiazolylazo)-4-Ethylphenol

Abstract

The complexations of a new ligand, o-(2-thiazolylazo)-4-ethylphenol(TAEP) with Ca(II), Mn(II), Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Hq(II) and Pb(II) have been studied by potentiometric titrations, at 25.0 ± 0.2°C and an ionic strength of 0.1 in 30% v/v dioxane-water mixture. The dissociation constant and spectral data of TAEP and formation constants of the complexes containing various molar ratios of metal ion to ligand, are reported. It is observed that Ca(II) forms only an ML complex in any molar ratios, whereas other metal ions react in two steps forming ML and ML₂ complexes in a 1:3 molar ratio. In the case of 1:1 and 1:2 molar ratios, Mn(II), Co(II), Cd(II) and Hg(II) seemed to form bi- or poly-nuclear complexes because of slightly different formation curves from those of 1:3 molar ratio. The sequence of the first successive formation constant is Cu > Hg > Ni > Pb > Co > Zn > Cd > Mn > Ca, showing Mellor-Maley's order. Further correlation is shown between the formation constants and the second ionization potentials of the metals.     

Hydrogen bonding interactions between <Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>-dimethylformamide and cysteine: DFT studies of structures,properties, and topologies

The hydrogen bonding interactions between cysteine and N,N-dimethylformamide (DMF) were studied at the extended hybrid functional DFT-X3LYP/6-311++G(d,p) level regarding their geometries, energies, vibrational frequencies, and topological features of the electron density. The quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses were employed to elucidate the interaction characteristics in the complexes. The results show that two intermolecular hydrogen bonds (H-bonds) are formed in one complex except few complexes with one intermolecular H-bond. The H-bonds involving O atom of DMF as H-bond acceptor usually are red-shifting H-bonds, while the blue-shifting H-bond usually involve methyl of DMF or methenyl of cysteine moiety as H-bond donors. Both hydrogen bonding interaction and structural deformation play important roles in the relative stabilities of the complexes. Due to the π-bond cooperativity, the strongest H-bond is formed between hydroxyl of cysteine moiety and O atom of DMF, however, the serious deformation counteract the hydrogen bonding interaction to a great extent. The complex involves a stronger hydrogen bonding interaction as well as the smaller deformation is the most stable one. The electron density (ρ_b) as well as its Laplacian (∇²ρ_b) at the H-bond critical point predicted by QTAIM is strongly correlated with the H-bond structural parameter (δR _H···Y) and the second-perturbation energies E(2) in the NBO scheme.     

Intermolecular vibrations and diffusive orientational dynamics of Cs condensed ring aromatic molecular liquids

The ultrafast dynamics, including the intermolecular vibrations and the diffusive orientational dynamics, of the neat C(s) symmetry condensed ring aromatic molecular liquids benzofuran, 1-fluoronaphtalene, and quinoline were investigated for the first time by means of femtosecond Raman-induced Kerr effect spectroscopy. To understand the features of these C(s) condensed ring aromatic molecular liquids, reference singular aromatic molecular liquids, furan, fluorobenzene, pyridine, and benzene, were also studied. High quality low-frequency Kerr spectra of the aromatic molecular liquids were obtained by Fourier-transform deconvolution analysis of the measured Kerr transients. The Kerr spectra of the C(s) condensed ring aromatic molecular liquids are bimodal, as are those of the reference singular aromatic molecular liquids. The first moment of the intermolecular vibrational spectrum and the peak frequencies of the high- and low-frequency components in the broad spectrum band were compared with their molecular properties such as the rotational constants, molecular weight, and intermolecular (bimolecular) force. The comparisons show that the molecular volume (related to molecular weight and rotational constants) is a dominant property for the characteristic frequency of the entire intermolecular vibrational spectrum. The observed intramolecular vibrational modes in the Kerr spectra of the aromatic molecular liquids were also assigned on the basis of the ab initio quantum chemical calculation results. In their picosecond diffusive orientational dynamics, the slowest relaxation time constant for both the condensed ring and singular aromatic molecular liquids can be accounted for by the simple Stokes-Einstein-Debye hydrodynamic model.     

Explorations of the nature of the coupling interactions between vitamin C and methylglyoxal: a DFT study

As the first step toward understanding the augment role of vitamin C (Vc) for the anticancer effect of methylglyoxal (MG), the nature of the coupling interactions between Vc and MG has been systematically investigated at the B3LYP/6-311++G** level of theory in combination with the atoms in molecules (AIM) theory, natural bond orbital (NBO) method, and energy decomposition analysis (EDA). The possible stable complexes have been located on their potential energy surface (PES). Most of them are characterized by one or two intermolecular H-bonds with the binding energies varying from −11.1 to −2.0 kcal/mol. AIM analyses suggest that all the intermolecular H-bonds have been predominated by the electrostatic interaction. A good linear correlation between the intermolecular H-bond distance and the electron density as well as its Laplacian at the bond critical point of the intermolecular H-bond has been observed. Depending on the selected coupling modes between Vc and MG, the origin of the blue-shifts of the stretching vibrational frequencies of different C–H bonds has been elucidated. Additionally, the inherent reason for the positive role of Vc in the anticancer process for MG has been verified through the investigation of the one-electron oxidation behaviors of the most stable complex.     

大分子振动的一般价力场力常数计算方法的研究

本文基于力常数矩阵F_s从UBFF到GVFF的可迁性,提出了一个“由UBFF力常数来限定GVFF力常数”的方法;其次,对“运动学确定力常数法”中Λ矩阵元排序的规则作了进一步研究。     

Acid-base dissociation constants of 2,2'-bipyridyl in mixed protic solvents

The acid dissociation constants (K_a), base dissociation constants (K_b) and the autoprotolysis constants (K_s) for 2,2′-bipyridyl in water and in water+alcohol(methanol, ethanol, iso-propanol) mixed solvents have been determined at 25°C and an ionic strength of 0.1 mol l⁻¹, from a direct potentiometric method based on the treatment of the data of a single pH titration. It has been shown that K_a increases, whereas K_b and K_s decrease, with increasing proportion of the alcohol in the mixed solvents. Linear relations between pK_a, pK_b, pK_s and the mole fraction of the alcohol were observed in the composition range investigated. These results are discussed in terms of the properties of solvent and the interactions of the different species existing in dissociation equilibrium with solvents. It is concluded that the higher stabilization of both 2,2′-bipyridyl and its protonated form by dispersion forces and of the proton by its interaction with solvent molecules in the mixed solvents compared with that in water are largely responsible for the observed changes of pK_a with composition. On the other hand, the low stabilization of OH⁻ in the mixed solvents relative to that in water and the electrostatic effect are the main factors in determining the solvent effect on pK_b.     

基于范数指数定量构效关系预测β-环糊精络合常数

环糊精在药剂学领域中是一类重要的包结化合物, 其中络合物稳定常数(logK)是一个关键评价参数. 本研究基于扩展距离矩阵提出了一组范数指数, 利用多种计算方法构建了系列定量构效关系模型, 并对233种化合物与β-环糊精的logK进行了计算预测. 计算结果表明基于扩展距离矩阵范数建立的系列定量构效关系模型均能较好预测logK; 其中利用最小二乘-支撑向量机方法建立的模型预测效果最好, 其预测结果的相关性系数R和留一、留十交叉验证相关性系数(Q_LOO,Q_LTO)分别为0.9587、0.8775和0.8732. 与文献方法对比结果表明, 本工作的预测结果在准确性和稳定性上有着显著的改善, 且能分辨同分异构体. 本课题组前期研究成果和本项工作表明基于范数指数构建的定量构效关系不仅适用于计算化合物的基础物理化学性质, 还能应用到化学反应过程相关常数的准确预测.     

Ultrafast molecular dynamics of liquid aromatic molecules and the mixtures with CCl4

The ultrafast molecular dynamics of liquid aromatic molecules, benzene, toluene, ethylbenzene, cumene, and 1,3-diphenylpropane, and the mixtures with CCl(4) have been investigated by means of femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The picosecond Kerr transients of benzene, toluene, ethylbenzene, and cumene and the mixtures with CCl(4) show a biexponential feature. 1,3-Diphenylpropane and the mixtures with CCl(4) show triexponential picosecond Kerr transients. The slow relaxation time constants of the aromatic molecules and the mixtures with CCl(4) are qualitatively described by the Stoke-Einstein-Debye hydrodynamic model. The ultrafast dynamics have been discussed based on the Kerr spectra in the frequency range of 0-800 cm(-1) obtained by the Fourier transform analysis of the Kerr transients. The line shapes of the low-frequency intermolecular spectra located at 0-180 cm(-1) frequency range have been analyzed by two Brownian oscillators ( approximately 11 cm(-1) and approximately 45 cm(-1) peaks) and an antisymmetric Gaussian function ( approximately 65 cm(-1) peak). The spectrum shape of 1,3-diphenylpropane is quite different from the spectrum shapes of the other aromatic molecules for the low magnitude of the low-frequency mode of 1,3-diphenylpropane and/or an intramolecular vibration. Although the concentration dependences of the low- and intermediate-frequency intermolecular modes (Brownian oscillators) do not show a significant trend, the width of high-frequency intermolecular mode (antisymmetric Gaussian) becomes narrower with the higher CCl(4) concentration for all the aromatics mixtures with CCl(4). The result indicates that the inhomogeneity of the intermolecular vibrational mode in aromatics/CCl(4) mixtures is decreasing with the lower concentration of aromatics. The intramolecular vibrational modes of the aromatic molecules observed in the Kerr spectra are also shown with the calculation results based on the density functional theory.     

锌试剂与牛血清白蛋白作用机理的研究

采用UV光谱法研究了锌试剂与牛血清白蛋白(BSA)在弱酸性溶液中的结合反应,研究了溶液吸光度与BSA浓度的关系。测得其表观摩尔吸光系数ε_P=1.3×10⁶L·mol^-1·cm^-1,最大结合数n=278,表观结合常数K_c=8×10⁷.研究了小分子探针与蛋白质的反应机理及在蛋白质上的结合部位及结合力类型。它们之间主要是以分子间的静电引力结合反应。离子强度对结合反应有显著的影响;不同类型的表面活性剂均以不同的程度和形式对反应有影响。讨论了此结合反应的模式,认为该反应基本符合Scatchard模型。     

Prediction of geometries and interaction energies of complexes formed by small molecules using semiempirical and ab initio methods

The accuracy of the semiempirical quantum mechanics methods (AM1 and PM3), and the ab initio methods (6-31G^** and MP2/6-31G^**) in predicting intermolecular geometries and interaction energies have been evaluated by detailed studies of 17 bimolecular complexes formed by small molecules. Comparisons between calculated and experimental geometries for 12 complexes are presented. It was found that AM1 gave reasonably good predictions of the geometries of complexes such as CH₄ · CH₄, which have very weak interactions, but it is not as good as other methods in predicting intermolecular geometry for complexes where hydrogen bonding interactions play an important role. This is consistent with its inability to reproduce the charge transfer in the formation of hydrogen bonds in these complexes.

PM3 is able to predict intermolecular geometries for most complexes, including those with hydrogen bonding; its major flaw is its tendency to overestimate the strength of the interactions between hydrogen atoms. Care should be taken therefore in using PM3 to study complicated molecular systems with multiple hydrogen atom interactions and the method's weakness in handling complexes in which electrostatic forces are important should also be noted.

Among ab initio methods, both the 6-31G^** and the MP2/6-31G^** were found to outperform AM1 and PM3 in prediction of intermolecular geometry. Both of these ab initio methods showed excellent consistency in geometry prediction for most of the complexes studied, although MP2/6-31G^** is better than 6-31G^**. It is noted that the MP2/6-31G^** did not produce the correct geometry for the CO₂· HF complex.

For 12 complexes for which experimental geometry data are available, AM1, PM3, 6-31G^**, and MP2/6-31G^** successfully predicted the geometry in 10, 12, 12, and 11 cases, respectively. The average errors given by AM1 in the predicted intermolecular distances were 0.264, 0.272, 0.091, and 0.061 Å, respectively. In comparison to the ab initio methods, AM1 and PM3 commonly underestimated the molecular interaction energy in such complexes by ˜ 1–2 kcal mol⁻¹.     

Optical constants and integrated intensities of liquid hexafluorobenzene between 4000 and 250 cm at 25 °C

The optical constants (real and imaginary refractive indices) of hexafluorobenzene were determined at 25 °C via transmission measurements. Experimental absorbance spectra measured on a Nicolet Impact 410 FTIR were converted to imaginary refractive indices using methods described in the literature. The real refractive indices were obtained by Kramers–Kronig transformation of the imaginary refractive indices. From the complex refractive indices, the molar absorption coefficient (E_m) and complex molar polarizability spectra were calculated. The integrated intensities for the E_1u fundamentals were obtained from the areas under the bands in the spectrum. These integrated intensities are compared to those for benzene and benzene-d₆ in the literature.     

Synthesis,spectral, thermal,potentiometric and antimicrobial studies of transition metal complexes of tridentate ligand

A series of metal complexes of Cu(II), Ni(II), Co(II), Fe(III) and Mn(II) have been synthesized with newly synthesized biologically active tridentate ligand. The ligand was synthesized by condensation of dehydroacetic acid (3-acetyl-6-methyl-(2H) pyran-2,4(3H)-dione or DHA), o-phenylene diamine and fluoro benzaldehyde and characterized by elemental analysis, molar conductivity, magnetic susceptibility, thermal analysis, X-ray diffraction, IR, ¹H-NMR, UV–Vis spectroscopy and mass spectra. From the analytical data, the stoichiometry of the complexes was found to be 1:2 (metal:ligand) with octahedral geometry. The molar conductance values suggest the non-electrolyte nature of metal complexes. The IR spectral data suggest that the ligand behaves as a dibasic tridentate ligand with ONN donor atoms sequence towards central metal ion. Thermal behaviour (TG/DTA) and kinetic parameters calculated by the Coats–Redfern and Horowitz–Metzger method suggest more ordered activated state in complex formation. To investigate the relationship between stability constants of metal complexes and antimicrobial activity, the dissociation constants of Schiff bases and stability constants of their binary metal complexes have been determined potentiometrically in THF–water (60:40%) solution at 25 ± 1 °C and at 0.1 M NaClO₄ ionic strength. The potentiometric study suggests 1:1 and 1:2 complexation. Antibacterial and antifungal activities in vitro were performed against Staphylococcus aureus, Escherichia coli and Aspergillus niger, Trichoderma, respectively. The stability constants of the metal complexes were calculated by the Irving–Rosotti method. A relation between the stability constant and antimicrobial activity of complexes has been discussed. It is observed that the activity enhances upon complexation and the order of antifungal activity is in accordance with stability order of metal ions.     

Ab initio/DFT and AIM studies on dual hydrogen-bonded complexes of 2-hydroxypyridine/2-pyridone tautomerism

The second-order M?ller-Plesset (MP2) and density functional theory (DFT) calculations have been carried out to investigate the structures and stabilities of hydrogen (H-) bonded 2-hydroxypyridine (2HP)/2-pyridone (2PY) dimeric forms as well as 2HP-2PY complexes. The results on single-point counterpoise (CP) correction of these complexes were compared against CP-optimized correction. The nature of the intermolecular contacts in the sense of normal H-bond or blue-shifting H-bond was determined on the basis of harmonic vibrational, atom-in-molecule (AIM), and natural bond orbital (NBO) analysis. A blue-shifting C-H...N H-bond was found and NBO analysis revealed a slight decrease in the population of the contacting sigmaC-H* antibonding orbital as the primary reason of the C-H contraction. Good correlations have been established between the interaction energies and the H-bond distances versus other characteristic H-bond parameters.     

双冠醚研究(Ⅴ)——Schiff碱型和仲胺型双-(苯并-18-冠-6)对碱金属苦味酸盐的萃取性质

用两类结构不同的Schiff碱型、仲胺型双-(苯并-18-冠-6)在氯仿-水体系中对碱金属苦味酸盐进行萃取,测量了配合物组成比和萃取平衡常数。