Infrared and ab initio studies of hydrogen bonding and proton transfer in the complexes formed by pyrazoles

The results of experimental studies and quantum mechanical calculations of vibrational spectra and structure of hydrogen bonded complexes formed by pyrazole (P) and 3,5-dimethylpyrazole (DMP) are presented. IR spectra of pyrazoles in solutions, gas phase, and solid state have been investigated in wide range of concentrations and temperatures. It has been found that in the gas phase both P and DMP reveal the equilibrium between monomers, dimers, and trimers. In solutions the equilibrium between monomers and trimers dominates, no bands, which can be attributed to dimers were detected. DMP retains the trimer structure in solid state, while in the case of pyrazole P, formation of the crystal provides another type of association. Geometrical and spectral characteristics of dimers and trimers, obtained by ab initio calculations, are presented and compared with experimental data.

IR spectra of solutions containing P and DMP with a number of acids (acetic and trifluoroacetic acids, pentachlorophenol, HBr) have been studied in parallel with ab initio calculations. It has been found that pentachlorophenol forms with pyrazoles complexes with one strong hydrogen bond O–HN, while NH pyrazole group remains unbonded. With carboxylic acids DMP forms 1:1 cyclic complexes with two hydrogen bonds. In the case of acetic acid, the complex in CH₂Cl₂ solution reveals molecular structure with OHN and C=OHN bonds, in accordance with results of the calculations. For trifluoroacetic acid, the calculations predict the molecular structure to be energetically more stable in the case of the isolated binary complex (in gas phase), while the experimental spectrum of CH₂Cl₂ solution gives an evidence of the proton transfer with formation of the cyclic ionic pair with two NH⁺O⁻ bonds. The agreement with experimental results can be improved by taking into account the influence of environment in the framework of Onsager or Tomasi models. The shape of proton potential function of the complexes and medium effect on its parameters, resulted from experimental data and calculations, are discussed. It has been found that the number of potential minima and their relative depth depend strongly on the method of calculations and the basic set. Under excess of trifluoroacetic acid, the formation of 2:1 acid–DMP complex has been detected. Spectral characteristics and results of calculations point to the cyclic structure of this complex, which includes homoconjugated bis-trifluoroacetate anion and DMPH⁺ cation. With HBr both studied pyrazoles were found to form ionic complexes including one or two pyrazole molecules per one acid molecule and correspondingly monocation or homoconjugated cation BHB⁺.     

Spectrophotometric determination of formaldehyde with chromotropic acid in phosphoric acid medium assisted by microwave oven

In the present study, a spectrophotometric method for the determination of formaldehyde by using chromotropic acid was devised, in which the use of potentially hazardous and corrosive concentrated sulfuric acid was eliminated and advantageously replaced by a mixture of H₃PO₄ and H₂O₂. The reaction between formaldehyde and chromotropic acid (CA) in a concentrated phosphoric acid medium was accelerate by irradiating the mixture with microwave energy for 35 s (1100 W), producing a violet-red compound (λ_max=570 nm). Beer's Law is obeyed in a concentration range of 0.8-4.8 mg l⁻¹ of formaldehyde with a good correlation coefficient (r=0.9968). The proposed method was applied in the analysis of formaldehyde in commercial disinfectants. Recoveries were within 98.0-100.4%, with standard deviations ranging from 0.03 to 0.13%.     

Spectroscopic studies on the proton transfer and hydrogen bonding interactions between p-nitrophenol and aliphatic tertiary amines

The interaction of p-nitrophenol with aliphatic tertiary amines, namely triethylamine, tributyl-amine and trioctylamine, has been examined in a variety of solvents by means of electronic absorption spectroscopy. The acid—base interaction in these systems leads to various types of complexes depending upon the dielectric constant of the medium. The equilibrium data for different kinds of equilibria have been evaluated and discussed. The pK-equilibrium constant plots are found to be linear.     

Spectroscopic studies on proton transfer and hydrogen bonding in p-nitrophenol-donor systems

The interaction of p-nitrophenol with several electron-donors has been studied in aprotic and protic solvents by electronic absorption spectroscopy. The equilibrium data for different kinds of equilibria in 1,2-dichloroethane, n-butanol and acetonitrile have been obtained and discussed. The strong hydrogen bonding interaction between p-nitrophenol and a variety of electron donors has been investigated in heptane by electronic absorption spectroscopy. The interesting correlations obtained in terms of Mulliken's charge transfer model have been examined and extended in many cases.     

Flow-injection determination of hydrogen peroxide based on fluorescence quenching of chromotropic acid catalyzed with Fe(II)

Flow-injection analysis system (FIA system), which was based on Fe(II)-catalyzed oxidation of chromotropic acid with hydrogen peroxide, was developed for the determination of hydrogen peroxide. The chromotropic acid has a fluorescence measured at λ_em = 440 nm (emission wavelength) with λ_ex = 235 nm (excitation wavelength), and the fluorescence intensity at λ_em = 440 nm quietly decreased in the presence of hydrogen peroxide and Fe(II), which was caused by Fe(II)-catalyzed oxidation of chromotropic acid with hydrogen peroxide. By measuring the difference of fluorescence intensity, hydrogen peroxide (1.0 × 10⁻⁸-1.0 × 10⁻³ mol L⁻¹) could be determined by the proposed FIA system, whose analytical throughput was 40 samples h⁻¹. The relative standard deviation (RSD) was 1.03% (n = 10) for 4.0 × 10⁻⁸ mol L⁻¹ hydrogen peroxide. The proposed FIA technique could be applied to the determination of hydrogen peroxide in rain water samples.     

Intra and intermolecular hydrogen bonding in formohydroxamic acid

The presence of hydrogen bonding interactions in several tautomeric forms of formohydroxamic acid (FHA) and 1:1 association among the tautomeric forms and water‐coordinated tautomeric forms of FHA is explored theoretically. Out of the seven equilibrium structures, four tautomeric forms have been selected for aggregation with single water molecule and dimer formation. Fifteen aggregates of FHA with H₂O have been optimized at MP2/AUG‐cc‐PVDZ level and analyzed for intramolecular and intermolecular H‐bond interactions. Twenty‐seven dimers of the four tautomeric forms have been obtained at MP2/6‐31+G* level. The stabilization energies associated with dimerization and adduct formation with water are the result of H‐bond interactions and range from very weak to medium. The atomic charges and NBO analysis indicate that the electrostatic and the charge transfer are the important components favoring H‐bond formation. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

曙红B及变色酸与核酸作用的光谱学性质研究

利用荧光光谱法、紫外-可见吸收光谱法、圆二色谱法、凝胶电泳法对曙红B及变色酸与核酸的作用机理进行了探讨. 当ct-DNA浓度增加时,曙红B和变色酸的荧光光谱呈荧光增强趋势;紫外-可见光谱呈下降趋势;并使ct-DNA圆二色谱正负峰吸收强度呈现一定的降低;琼脂糖凝胶电泳实验中,二者在泳道中与DNA作用发出的荧光在紫外灯下皆清晰可见. 并将所得结果与溴化乙锭-DNA体系性质进行对比,为寻找高效低毒的DNA分子荧光探针提供了实验基础.     

Flow injection direct spectrophotometric assay for the speciation of trace chromium(III) and chromium(VI) using chromotropic acid as chromogenic reagent

A new rapid and sensitive FI assay is reported for the simultaneous direct spectrophotometric determination of trace Cr(VI) and Cr(III) in real samples. The method is based upon the reaction of Cr(VI) with chromotropic acid (CA) in highly acidic medium to form a water-soluble complex (λ_max = 370 nm). Cr(III) reacts with CA only after its on-line oxidation to Cr(VI) by alkaline KIO₄. The determination of each chromium species in the sample was achieved by absorbance differences. The calibration curves were linear over the range 3-4000 μg l⁻¹ and 30-1200 μg l⁻¹ for Cr(VI) and Cr(III), respectively, while the precision close to the quantitation limit was satisfactory in both cases (s_r = 3.0% for Cr(VI) and 4.0% for Cr(III) (n = 10) at 10 and 50 μg l⁻¹ level, respectively). The method developed proved to be adequately selective and sensitive (c_L = 1 and 10 μg l⁻¹ for Cr(VI) and Cr(III), respectively). The application of the method to the analysis of water samples (tap and mineral water) gave accurate results based on recovery studies (93-106%). Analytical results of real sample analysis were in good agreement with certified values.     

Theoretical analysis on the hydrogen bonding and reactivity that associated with the proton transfer reaction of carboxylic acid dimers and their monosulfur derivatives

Carboxylic acid dimers and their monosulfur derivatives are investigated by density functional theory calculations. Basis set superposition error (BSSE) counterpoise correction is included to compare the influence of BSSE on the interaction energies as well as on the geometries. The nature of hydrogen bond is determined on the basis of atoms in molecules (AIM) and natural bond orbital (NBO) analyses. Good correlations have been established between H‐bond length versus AIM topological parameter, orbital interaction, and barrier height for proton transfer. The reactivity behavior along the reaction path of the double proton transfer reaction has also been studied. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Studies on solid-state proton transfer along hydrogen bond of pyrazolone-ring

Evidence from the time-dependent UV-vis reflection spectra studies indicates the compound 1-phenyl-3-methyl-4-(4-methylbenzal)-5-pyrazolone 4-ethylthiosemicarbazone (PM4MBP-ETSC) undergoes a solid-state photochromism. The reaction rate constant was studied by the first-order kinetics curves. X-ray single crystal structural analysis shows that the pyrazolone-ring stabilizes in the keto form. The conclusion can be made that its photochromism in crystalline is associated with a photoinduced proton transfer reaction (inter- and intra-molecular hydrogen transfer) along hydrogen bond leading to a colored tautomer as the compound crystallizes in H-bonded supramolecular configuration.     

细菌紫红质中氢键相互作用和质子转移机理的分子模拟

用CHARMM程序以细菌紫红质1R84晶体为模型, 模拟了在等温定容条件下细菌紫红质在1 ps过程中的变化, 分析了与质子传递相关的ASP85, ASP212和水分子与视黄醛间氢键的结构变化情况. 考虑到氨基酸残基和席夫碱质子的不同距离, 考察了EC和PC两种结构的变化情况, 探讨了紫红质中质子传递的可能途径. 模拟结果表明1R84中可能的质子连续传递的机理是质子由席夫碱向水传递, 再由水向ASP85传递. 发现Asp212在模拟过程中保持EC结构, 这样可能更有利于顺序质子传递.     

Chromotropic acid-formaldehyde reaction in strongly acidic media. The role of dissolved oxygen and replacement of concentrated sulphuric acid

The procedure for formaldehyde analysis recommended by the National Institute for Occupational Safety and Health (NIOSH) is the Chromotropic acid spectrophotometric method, which is the one that uses concentrated sulphuric acid. In the present study the oxidation step associated with the aforementioned method for formaldehyde determination was investigated. Experimental evidence has been obtained indicating that when concentrated H₂SO₄ (18 mol l⁻¹) is used (as in the NIOSH procedure) that acid is the oxidizing agent. On the other hand, oxidation through dissolved oxygen takes place when concentrated H₂SO₄ is replaced by concentrated hydrochloric (12 mol l⁻¹) and phosphoric (14.7 mol l⁻¹) acids as well as by diluted H₂SO₄ (9.4 mol l⁻¹). Based on investigations concerning the oxidation step, a modified procedure was devised, in which the use of the potentially hazardous and corrosive concentrated H₂SO₄ was eliminated and advantageously replaced by a less harmful mixture of HCl and H₂O₂.     

Theoretical studies of electron and hydrogen transfer reactions between semiquinone radicals and oxygen

 To explore the interactions between ubiquinones and oxygen in living organisms, the thermodynamics of a series of electron and hydrogen transfer reactions between semiquinone radicals, as well as their corresponding protonated forms, and oxygen, singlet or triplet, were studied using the hybrid Hartree–Fock–density functional theory method Becke's three parameter hybrid method with the Lee, Yang, and Parr correlation functional. Effects of the solvent and of the isoprenyl tail on the electron and hydrogen transfer reactions were also investigated. It is found that semiquinone radicals (semiquinone anion radicals or protonated semiquinone radicals) cannot react with triplet oxygen to form the superoxide anion radical O₂ ⁻. In contrast, neutral quinones can scavenge O₂ ⁻ efficiently. In the gas phase, only protonated semiquinone radicals can react spontaneously with singlet oxygen to produce peroxyl radical (HO₂). However, both semiquinone anion radicals and protonated semiquinone radicals can react with singlet oxygen to produce harmful oxygen radicals (O₂ ^{− a l l b u l l} and HO₂, respectively) in aqueous and protein environments. The free-energy changes of the corresponding reactions obtained for different ubiquinone systems are very similar. It clearly shows that the isoprenyl tail does not influence the electron and hydrogen transfer reactions between semiquinone radicals and oxygen significantly. Results of electron affinities, vertical ionization potentials, and proton affinities also show that the isoprenyl tail has no substantial effect on the electronic properties of ubiquinones. Received: 3 July 2000 / Accepted: 6 September 2000 / Published online: 21 December 2000     

Theoretical study of intermolecular proton transfer reaction in isolated 5-hydroxyisoxazole–water complexes

A systematic investigation in isolated 5-hydroxyisoxazole–water complexes (5-HIO · (H₂O)_nn = 1–3) is performed at the DFT level, employing B3LYP/6-31G(d, p) basis set. Single-point energy calculations are also performed at the MP2 level using B3LYP/6-31G(d, p) optimized geometries and the 6-311++G(d, p) basis set. The computational results show that the keto tautomer K₂ is the most stable isomer in the gas phase, and the tautomer K₁ to be the next most stable tautomer. Hydrogen bonding between HIO and the water molecule(s) will dramatically lower the barrier by a concerted multiple proton transfer mechanism. The proton transfer process of 3WE_cis ↔ 3WK₁ and 2WE_trans ↔ 2WK₂ is found to be more efficient in two tautomerization, and the barrier heights are 7.03 and 14.15 kcal/mol at B3LYP/6-31G(d, p) level, respectively. However, the proton transfer reaction between E_cis and K₁ cannot happen without solvent-assisted.     

FTIR studies of intermolecular hydrogen bonding in halogenated ethanols

The effect of halogen substitution on intermolecular hydrogen-bonding in ethanol is studied. Specifically, Fourier-transform infrared (FTIR) spectra of ethanol, 2,2,2-trifluoroethanol (TFE), and 2,2,2-trichloroethanol dissolved in carbon tetrachloride are reported as a function of temperature and concentration. The spectral intensities corresponding to monomer, dimer, and multimer formation are used to determine the effect of halogen substitution on intermolecular hydrogen-bonding. The enthalpy for dimerization was found to evolve from -4.2+/-0.3 kcal/mol in ethanol to -6.8+/-1.0 kcal/mol in TFE. An opposite trend was observed for multimer formation with enthalpies of -3.7+/-0.5 in ethanol and -2.1+/-1.4 kcal/mol in TFE. The majority of this evolution is assigned to the ability of ethanols to form intramolecular hydrogen bonds involving the hydoxyl proton and the halogen substituents.     

甲醇为氢源的连续化催化转移加氢合成芳胺

开发了一种温和高效的以甲醇为氢源,以Ru-Fe双金属催化剂催化的硝基芳烃连续化转移加氢方法。采用浸渍法制备Ru-Fe双金属催化剂,通过电感耦合等离子体-质谱(ICP-MS)、透射电子显微镜(TEM)、X射线衍射(XRD）、氢气程序升温还原(H₂-TPR)对催化剂进行表征。结果表明催化剂具有较小的粒径和较好的分散性。在Ru-Fe双金属催化剂上,成功实现了硝基芳烃与甲醇在无外加氢源条件下的连续化转移加氢合成芳胺。通过对反应条件的调控,成功得到了一系列产率较高的胺类化合物。特别地,该方法对不饱和基团(醛基、羰基或炔基)取代的硝基芳烃的加氢表现出优异的选择性和转化率。     

甲醇为氢源的连续化催化转移加氢合成芳胺

开发了一种温和高效的以甲醇为氢源,以Ru-Fe双金属为催化剂的硝基芳烃连续化转移加氢方法。采用浸渍法制备Ru-Fe双金属催化剂,通过电感耦合等离子体-质谱(ICP-MS)、透射电子显微镜(TEM)、X射线衍射(XRD）、氢气程序升温还原(H₂-TPR)对催化剂进行表征。结果表明催化剂具有较小的粒径和较好的分散性。在Ru-Fe双金属催化剂上,成功实现了硝基芳烃与甲醇在无外加氢源条件下的连续化转移加氢合成芳胺。通过对反应条件的调控,成功得到了一系列产率较高的胺类化合物。特别地,该方法对不饱和基团(醛基、羰基或炔基)取代的硝基芳烃的加氢表现出优异的选择性和转化率。     

Interaction between biimidazole complexes of ruthenium and acetate: hydrogen bonding and proton transfer

The hydrogen bonding and deprotonation processes between four ruthenium biimidazole complexes, namely [Ru(bpy)(2)(BiimH(2))](PF(6))(2) (1, bpy is bipyridine, BiimH(2) is 2,2'-biimidazole), [Ru(bpy)(2)-(BbimH(2))](PF(6))(2) (2, BbimH(2) is 2,2'-bibenzimidazole), and [Ru(bpy)(2)(DMBbimH(2))](PF(6))(2) (3, DMBbimH(2) is 7,7'-dimethyl-2,2'-bibenzimidazole) and [Ru(bpy)(2)(TMBbimH(2))](2+) (4, TMBbimH(2) is 5,6,5',6'-tetramethyl-2,2'-bibenzimidazole), and acetate are investigated. Their hydrogen bonded adducts are indeed trapped and observed by absorption spectra and electrochemical experiments in acetonitrile solution in the presence of an excess of acetic acid for the first time. The binding constants log K(B) for these adducts are 6.74 for 1·OAc, 7.11 for 2·OAc, 7.26 for 3·OAc, and 6.99 for 4·OAc. A new approach to calculate the deprotonation constant is also developed by establishing a set of circular equilibria. The equilibrium constants for the first deprotonation step of the complexes log K(A) are 2.74 for 1, 5.19 for 2, 4.54 for 3, and 3.78 for 4. The pK(a1) values of the complexes in acetonitrile solution are calculated by subtracting log K(A) from pK(a) (HOAc in acetonitrile), giving 19.6 for 1, 17.1 for 2, 17.8 for 3, and 18.5 for 4. The degree of proton transfer (D(PT)) can be quantified by the calculation of absorption spectral and redox data, which is 0.41 for 1·OAc, 0.53 for 2·OAc, 0.57 for 3·OAc, and 0.47 for 4·OAc. Interestingly, the binding constant log K(B) (7.26) and D(PT) value (0.57) both reach their maxima at a critical point, where pK(a1) for the complex is 17.8 and ΔpK(a) for the adduct is 4.5 (ΔpK(a) = pK(a)(HOAc) - pK(a1), in acetonitrile solution). Moreover, the binding constant log K(B) shows linear correlation with the degree of proton transfer D(PT).     

Spectroscopic study of hydrogen exchange processes and structure of intermediate complexes with intermolecular hydrogen bonds

The kinetics of hydrogen exchange in molecular systems with H-bonds has been studied by means of kinetic IR spectroscopy and low-temperature NMR spectroscopy. The experimental values of the rate constants and activation energies for molecules capable of forming H-bonds as both proton donors and proton acceptor are collected and analyzed from the point of view of the influence of H-bond formation ability of the molecules-partners. The evidence available testifies to a molecular mechanism of the H-exchange reactions in inert solvents and in the gas phase via the formation of cyclic bimolecular intermediates. The different mechanisms and the structure of intermediate complex of molecular H-exchange process in inert media are discussed and the possible paths of experimental elucidation of reaction mechanism are offered.     

Experimental and computational studies of hydrogen bonding and proton transfer to [Cp*Fe(dppe)H

The present contribution reports experimental and computational investigations of the interaction between [Cp*Fe(dppe)H] and different proton donors (HA). The focus is on the structure of the proton transfer intermediates and on the potential energy surface of the proton transfer leading to the dihydrogen complex [Cp*Fe(dppe)(H2)]+. With p-nitrophenol (PNP) a UV/Visible study provides evidence of the formation of the ion-pair stabilized by a hydrogen bond between the nonclassical cation [Cp*Fe(dppe)(H2)]+ and the homoconjugated anion ([AHA]-). With trifluoroacetic acid (TFA), the hydrogen-bonded ion pair containing the simple conjugate base (A-) in equilibrium with the free ions is observed by IR spectroscopy when using a deficit of the proton donor. An excess leads to the formation of the homoconjugated anion. The interaction with hexafluoroisopropanol (HFIP) was investigated quantitatively by IR spectroscopy and by 1H and 31P NMR spectroscopy at low temperatures (200-260 K) and by stopped-flow kinetics at about room temperature (288-308 K). The hydrogen bond formation to give [Cp*Fe(dppe)H]HA is characterized by DeltaH degrees =-6.5+/-0.4 kcal mol(-1) and DeltaS degrees = -18.6+/-1.7 cal mol(-1) K(-1). The activation barrier for the proton transfer step, which occurs only upon intervention of a second HFIP molecule, is DeltaH(not equal) = 2.6+/-0.3 kcal mol(-1) and DeltaS(not equal) = -44.5+/-1.1 cal mol(-1) K(-1). The computational investigation (at the DFT/B3 LYP level with inclusion of solvent effects by the polarizable continuum model) reproduces all the qualitative findings, provided the correct number of proton donor molecules are used in the model. The proton transfer process is, however, computed to be less exothermic than observed in the experiment.