NMR investigation of hydrogen bonding and 1,3-tautomerism in 2-(2-hydroxy-5-substituted-aryl) benzimidazoles

The 1,3-tautomerism associated with 2-(2-hydroxy-5-substituted-aryl)benzimidazoles was studied in different solvents. The effect of hydrogen bonding involving the hydroxyl group of the 2-aryl ring on the tautomerism was investigated using NMR spectroscopy. The influence of the solvent concentration on 2-(2-hydroxy-5-chloroaryl)benzimidazole was studied in acetone-d6 and DMSO-d6.     

Solvent effects on barrier to rotation of enaminonitriles using inversion transfer (1)H NMR spectroscopy and FTIR spectroscopy

The barrier to rotation and hydrogen bonding interactions of 2,2-dicyano-1-(N,N'-dimethylamino)vinylbenzene (1) were studied in a range of solvents. The barrier to rotation of 1 in chloroform was 14.8 kcal/mol and increased by 1.7 kcal/mol in a protic solvent, trifluoroethanol. FTIR studies showed a shift in the cyano stretch of 1 to a higher wavenumber in trifluoroethanol, which is consistent with 1 participating in a hydrogen bonding interaction at the vinyl carbon (C(v)) of the enaminonitrile group.     

Effects of solvents on the electron configurations of the low-spin dicyano[meso-tetrakis(2,4,6-triethylphenyl)porphyrinato]iron(III) complex: importance of the C-H...N weak hydrogen bonding

There are two types of electron configurations, (d(xy))(2)(d(xz), d(yz))(3) and (d(xz), d(yz))(4)(d(xy))(1), in low-spin iron(III) porphyrin complexes. To reveal the solvent effects on the ground-state electron configurations, we have examined the (13)C- and (1)H-NMR spectra of low-spin dicyano[meso-tetrakis(2,4,6-triethylphenyl)porphyrinato]ferrate(III) in a variety of solvents, including protic, dipolar aprotic, and nonpolar solvents. On the basis of the NMR study, we have reached the following conclusions: (i) the complex adopts the ground state with the (d(xz), d(yz))(4)(d(xy))(1) electron configuration, the (d(xz), d(yz))(4)(d(xy)())(1) ground state, in methanol, because the d(pi) orbitals are stabilized due to the O-H...N hydrogen bonding between the coordinated cyanide and methanol; (ii) the complex also exhibits the (d(xz), d(yz))(4)(d(xy))(1) ground state in nonpolar solvents, such as chloroform and dichloromethane, which is ascribed to the stabilization of the d(pi) orbitals due to the C-H...N weak hydrogen bonding between the coordinated cyanide and the solvent molecules; (iii) the complex favors the (d(xz), d(yz))(4)(d(xy))(1) ground state in dipolar aprotic solvents, such as DMF, DMSO, and acetone, though the (d(xz), d(yz))(4)(d(xy))(1) character is less than that in chloroform and dichloromethane; (iv) the complex adopts the (d(xy))(2)(d(xz), d(yz))(3) ground state in nonpolar solvents, such as toluene, benzene, and tetrachloromethane, because of the lack of hydrogen bonding in these solvents; (v) acetonitrile behaves like nonpolar solvents, such as toluene, benzene, and tetrachloromethane, though it is classified as a dipolar aprotic solvent. Although the NMR results have been interpreted in terms of the solvent effects on the ordering of the d(xy) and d(pi) orbitals, they could also be interpreted in terms of the solvent effects on the population ratios of two isomers with different electron configurations. In fact, we have observed the unprecedented EPR spectra at 4.2 K which contain both the axial- and large g(max)-type signals in some solvents such as benzene, toluene, and acetonitrile. The observation of the two types of signals has been ascribed to the slow interconversion on the EPR time scale at 4.2 K between the ruffled complex with the (d(xz), d(yz))(4)(d(xy))(1) ground state and, possibly, the planar (or nearly planar) complex with the (d(xy))(2)(d(xz), d(yz))(3) ground state.     

Photolysis of <Emphasis Type="Italic">ortho</Emphasis>-azidophenol in various solvents

The photolysis of ortho-azidophenol in water, ethanol, acetonitrile, chloroform, and benzene was studied by IR and electronic spectroscopy and thin-layer chromatography. It was found that an equilibrium between ortho-azidophenol and its quinonoid form occurred in benzene. In the photolysis of ortho-azidophenol in benzene, intramolecular hydrogen bonding facilitates the degradation of the azido group through the mechanism of formation of intermediate triazene structures. In the other solvents, which exclude intramolecular hydrogen bonding, the nitrene mechanism of photolysis yielding ortho-aminophenol, ortho-iminoquinone, and an azo compound is operative. The rate of formation of photolysis products depends on the nature of the solvent.     

Vibrational relaxation of C-D stretching vibrations in CDCl3, CDBr3, and CDI3

We present time-resolved transient grating measurements of the vibrational relaxation rates of the C-D stretching vibrations of deuterated haloforms in benzene and acetone. We compare our results with previous measurements of excited C-H stretches in the same solvents to obtain insight into the solvent effect on the vibrational relaxation. In deuterated molecules, there are more low-order-coupled states and the states are closer in energy to the C-D stretch than in the unlabeled isotopologs. Therefore, the relaxation is faster for the deuterated molecules. The relaxation also shows a significant solvent dependence. Bromoform and iodoform form charge-transfer complexes with both benzene and acetone which enhance the relaxation rate. For chloroform, hydrogen bonding to acetone is expected to be a more favorable interaction. Surprisingly, however, the vibrational relaxation of CDCl(3) is slower in acetone than in benzene.     

大孔交联聚(对乙烯基苄基苯胺)树脂对苯酚的吸附

由氯甲基化聚苯乙烯合成了大孔交联聚(对乙烯基苄基苯胺)树脂, 测定了其对正己烷和水中苯酚的吸附等温线, 计算了吸附焓. 结果表明, 苯胺基树脂主要是通过氢键吸附正己烷中苯酚的, 树脂负载的功能基氮原子和苯环都作为氢键受体与苯酚的羟基氢原子形成氢键, 而其对水中苯酚的吸附是基于氢键和疏水作用.     

溶剂在丁腈基聚氨酯中的溶解和扩散

用石英弹簧法和示差扫描量热法 (DSC)、红外分光光度计 (FTIR)研究了苯、乙醇、丙酮、醋酸乙酯和1,2 二氯乙烷五种溶剂在端羟基聚丁二烯 丙烯腈共聚物为软段的聚氨酯中的溶解和扩散行为 .结果表明所有溶剂在丁腈聚氨酯中的扩散均为非费克扩散 ,且随着溶剂蒸汽压增大偏离费克扩散的程度增大 .相同相对蒸汽压下 1,2 二氯乙烷和醋酸乙酯偏离费克 (Fickian)扩散的程度较大 ,而乙醇、丙酮和苯则较小 ,这主要与它们和丁腈软段溶解度参数的极性分量和氢键分量有关 .1,2 二氯乙烷和苯在HTBN PU中的溶解度较高 ,而乙醇 ,醋酸乙酯和丙酮较低 ,主要与它们和丁腈软段溶解度参数的色散分量有关 .所有溶剂均表现出近似Flory Huggins型等温吸收曲线 .红外表明吸收溶剂后 ,氨基甲酸酯基团的氢键化程度有不同程度的下降 ,和溶剂与之形成氢键的能力大小有关 .力学性能表明非极性溶剂苯对材料的力学性能影响较小 ,而乙醇 ,醋酸乙酯和丙酮由于可与氨酯基团形成氢键 ,对原HTBN PU中氨酯键氢键的破坏大 ,力学性能下降大     

Investigation of local order in unreacted DGEBA epoxy resin monomers by light scattering

The existence of local order in two epoxy resins of the diglycidyl ether of bisphenol-A (DGEBA) type has been investigated using Rayleigh scattering and Brillouin spectroscopy. The resins differ in their molecular weight distributions and their relative concentrations of epoxide and hydroxyl groups. The complementary use of both techniques in elucidating the thermal behavior of local order is illustrated, and the use of the latter technique to study thermal acoustic phonons and hypersonic relaxation is discussed. Both techniques independently show that molecular aggregates exist in each resin system. The scattering-envelope dissymmetry shows that the resin with the high epoxide/hydroxyl group ratio contains aggregates up to 20 nm in size, and the low-ratio resin exhibits sizes up to 70 nm. These aggregates are thermally unstable in the temperature range studied (293–443 K). Dissolution in chloroform shows that these aggregates are reduced in size and that further structural changes occur which are dependent on solvent concentration. Aggregate volume fractions were determined for a range of aggregate size. Brillouin spectroscopy indicated that both resins exhibit hypersonic relaxation in the temperature range studied. The complex longitudinal moduli of the resins were superimposable under a WLF temperature transformation comparable to the difference in their static glass transition temperatures. Molecular aggregate size, number, and stability are related to the epoxide/hydroxyl ratio of the resins and the degree of intermolecular hydrogen bonding.     

Enthalpies of solvation of ethylene oxide oligomers CH3O(CH2CH2O)nCH3 (n = 1 to 4) in different H-bonding solvents: Methanol, chloroform, and water. Group contribution method as applied to the polar oligomers

The enthalpies of solution and solvation of ethylene oxide oligomers CH₃O(CH₂CH₂O)_nCH₃ (n = 1 to 4) in methanol and chloroform have been determined from calorimetric measurements at T = 298.15 K. The enthalpic coefficients of pairwise solute–solute interaction for methanol solutions have been calculated. The enthalpic characteristics of the oligomers in methanol, chloroform, water and tetrachloromethane have been compared. The hydrogen bonding of the oligomers with chloroform and water molecules is exhibited in the values of solvation enthalpy and coefficient of solute–solute interaction. This effect is not observed for methanol solvent. The thermochemical data evidence an existence of multi-centred hydrogen bonds in associates of polyethers with the solvent molecules. Enthalpies of hydrogen bonding of the oligomers with chloroform and water have been estimated. The additivity scheme has been developed to describe the enthalpies of solvation of ethylene oxide oligomers, unbranched monoethers and n-alkanes in chloroform, methanol, water, and tetrachloromethane. The correction parameters for contribution of repeated polar groups and correction term for methoxy-compounds have been introduced. The obtained group contributions permit to describe the enthalpies of solvation of unbranched monoethers and ethylene oxide oligomers in the solvents with standard deviation up to 0.6 kJ · mol⁻¹. The values of group contributions and corrections are strongly influenced by solvent properties.     

Effect of intermolecular hydrogen bonding on low-surface-energy material of poly(vinylphenol)

We discovered that poly(vinylphenol) (PVPh) possesses an extremely low surface energy (15.7 mJ/m2) after a simple thermal treatment procedure, even lower than that of poly(tetrafluoroethylene) (22.0 mJ/m2) calculated on the basis of the two-liquid geometric method. Infrared analyses indicate that the intermolecular hydrogen bonding of PVPh decreases by converting the hydroxyl group into a free hydroxyl and increasing intramolecular hydrogen bonding after thermal treatment. PVPh results in a lower surface energy because of the decrease of intermolecular hydrogen bonding between hydroxyl groups. In addition, we also compared surface energies of PVPh-co-PS (polystyrene) copolymers (random and block) and their corresponding blends. Again, these random copolymers possess a lower fraction of intermolecular hydrogen bonding and surface energy than the corresponding block copolymers or blends after similar thermal treatment. This finding provides a unique and easy method to prepare a low-surface-energy material through a simple thermal treatment procedure without using fluoro polymers or silicones.     

Surface Modification of Poly(urea-formaldehyde) Microcapsules and the Effect on the Epoxy Composites Performance

The surfaces of poly(urea-formaldehyde) (PUF) were modified by γ -glycidoxypropyltrimethoxy silane (KH560) in order to improve the interfacial bonding between self-healing PUF microcapsules and epoxy matrix. The modification mechanism between PUF microcapsules and KH560 was studied. X-ray photoelectron spectra (XPS) analyses showed that the silane coupling agent molecular binds strongly to the surfaces of PUF microcapsules. Chemical bond (Si–O–C) and hydrogen bond were formed at interface by the reaction between Si–OH and the hydroxyl group of PUF microcapsules surface. The tensile and impact resistance tests revealed that strength and toughness of the composites was improved significantly. Furthermore, scanning electronic microscopy (SEM) photographs of the fractured surface confirmed that the silane coupling agent plays an important role in improving the interfacial performance between microcapsules and resin matrix.     

由特殊相互作用导致的高分子间的络合

采用结合物理老化技术的示差扫描量热法（ＤＳＣ）以及非辐射能量转移荧光光谱法（ＮＲＥＴ）研究了聚（聚乙烯－ｃｏ－４－乙烯基苯酚与聚甲基丙烯酸乙酯（ＰＥＭＡ）共混体系的相容与络合行为。对ＳＴＶＰｈ／ＰＥＭＡ共混体系,当ＳＴＶＰｈ中的ＯＨ基团含量仅为１ｍｏｌ％时,即可实现相容。随ＯＨ基含量继续增加,共混体系的Ｔｇ值上升并逐渐接近和高于Ｆｏｘ方程计算值,玻璃化转变区域逐渐变窄,ＮＲＥＴ能量转移效率远高于普     

Molecular dynamics simulation of cellobiose in water

The conformational behavior of cellobiose was studied by molecular dynamics simulation in a periodic box of waters. Several different initial conformations were used and the results compared with equivalent vacuum simulations. The average positions and rms fluctuations within single torsional conformations of cellobiose were affected only slightly by the solvent. However, water damped local torsional librations and transitions. The conformational energies of the solute and their fluctuations were also sensitive to the presence of solvent. Intramolecular hydrogen bonding was weakened relative to that observed in vacuo due to competition with solvating waters. All cellobiose hydroxyl groups participated in intermolecular hydrogen bonds with water, with approximately eight hydrogen bonds formed per glucose ring. The hydrogen bonding was predominantly between water hydrogens and solute hydroxyl oxygens. Intermolecular hydrogen bonding to ring and bridge oxygens was seldom present. The diffusion coefficients of both water and solute agree closely with experimental values. Water interchanged rapidly between the solvating first shell and the bulk on the picosecond time scale. © 1993 John Wiley & Sons, Inc.     

Effects of methyl substitution on radiative and solvent quenching rate constants of [Ru(phen)2dppz]2+ in polyol solvents and bound to DNA

Methyl substituents on the distant benzene ring of the dppz ligand in the "light switch" complex [Ru(phen)(2)dppz](2+) have profound effects on the photophysics of the complexes in water as well as in the polyol solvents ethylene glycol, glycerol, and 1,2- and 1,3-propanediol. Whereas 11,12-dimethyl substitution decreases the rate of quenching by diminishing hydrogen bonding by solvent, the 10-methyl substituent in addition also decreases both the radiative and the nonradiative rate constant for decay to the ground state of the non-hydrogen-bonded excited state species. For both the 10-methyl and the 11,12-dimethyl derivatives, the effect of methyl substitution on the equilibrium of solvent hydrogen bonding to the excited state is due to changes in the entropy terms, rather than in the enthalpy, indicating that the effect is a steric perturbation of the solvent cage around the molecule. When intercalated into DNA, the effects of methyl substitution is smaller than those in polyol solvent or water, suggesting that the water molecules that quench the excited state by hydrogen bonding to the phenazine aza nitrogens mainly access them from the same groove as in which the Ru(II) ion resides. Since the Delta-enantiomer of [Ru(phen)(2)10-methyl-dppz](2+) has an absolute quantum yield of up to 0.23 when bound to DNA, a value 7000 times higher than in pure water solution, it is promising as a new luminescent DNA probe.     

Proton spin-lattice relaxation study of polymer solutions

Proton spin-lattice relaxation times of solvent molecules were measured on ternary mixtures of a polymer and two solvents by the adiabatic rapid-passage method. The selective adsorption of a good solvent was verified by this experimental technique for the systems benzene—cyclohexane—polystyrene(PS), benzene—carbon tetrachloride—poly(methyl methacrylate)(PMMA), and chloroform—carbon tetrachloride—PMMA. The number of molecules of adsorbed benzene per monomeric unit of PS was estimated to be about four, which is almost identical with that determined previously by thermodynamic measurements. The number of molecules of benzene and chloroform adsorbed on PMMA were also determined to be about five and four, respectively. It was found that the interaction between chloroform and PMMA has the greatest effect on the molecular motion of the solvent, whereas the benzene—PS interaction is weak.     

Hydrogen bond-free flavin redox properties: managing flavins in extreme aprotic solvents

We report a simple, single step reaction that transforms riboflavin, which is insoluble in benzene, to tetraphenylacetyl riboflavin (TPARF), which is soluble in this solvent to over 250 mM. Electrochemical analysis of TPARF both alone and in complexes with two benzene-soluble flavin receptors: dibenzylamidopyridine (DBAP) and monobenzylamidopyridine (MBAP), prove that this model system behaves similarly to other previously studied flavin model systems which were soluble only in more polar solvents such as dichloromethane. Binding titrations in both benzene and dichloromethane show that the association constants of TPARF with its ligands are over an order of magnitude higher in benzene than dichloromethane, a consequence of the fact that benzene does not compete for H-bonds, but dichloromethane does. The alteration induced in the visible spectrum of TPARF in benzene upon the addition of DBAP is very similar to the difference produced by transfer to dichloromethane, further indicating that the flavin head group engages in a similar degree of hydrogen bonding with dichloromethane as with its ligands. This work underlines the importance of using a truly nonpolar solvent for the analysis of the effects of hydrogen bonding on the energetics of any biomimetic host-guest model system.     

On the electrophilicity of hydroxyl radical: a laser flash photolysis and computational study

The rate coefficients for reactions of hydroxyl radical with aromatic hydrocarbons were measured in acetonitrile using a novel laser flash photolysis method. Comparison of kinetic data obtained in acetonitrile with those obtained in aqueous solution demonstrates an unexpected solvent effect on the reactivity of hydroxyl radical. In particular, reactions of hydroxyl radical with benzene were faster in water than in acetonitrile, and by a significant factor of 65. Computational studies, at the B3LYP and CBS-QB3 levels, have confirmed the rate enhancement of hydroxyl radical addition to benzene via calculation of the transition states in the presence of explicit solvent molecules as well as a continuum dielectric field. The origin of the rate enhancement lies entirely in the structures of the transition states and not in the pre-reactive complexes. The calculations reveal that the hydroxyl radical moiety becomes more anionic in the transition state and, therefore, looks more like hydroxide anion. In the transition states, solvation of the incipient hydroxide anion is more effective with water than with acetonitrile and provides the strong energetic advantage for a polar solvent capable of hydrogen bonding. At the same time, the aromatic unit looks more like the radical cation in the transition state. The commonly held view that hydroxyl radical is electrophilic in its reactions with DNA bases is, therefore, strongly dependent on the ability of the organic substrate to stabilize the resulting radical cation.     

Association of Copper Tetra-4-(n-alkoxycarbonyl)phthalocyanines in Solvents with Low Dielectric Permittivity

Association of copper tetra-4-(n-butoxycarbonyl)- and copper tetra-4-(n-nonoxycarbonyl)phthalocyanines in benzene and chloroform was studied. It was found that the length of the alkyl chain has no noticeable effect on the association; the solvent effect is somewhat stronger. The thermodynamic parameters of dimerization were calculated.     

Miscibility and hydrogen‐bonding interactions in blends of carbon dioxide/epoxy propane copolymer with poly(p‐vinylphenol)

The miscibility and hydrogen‐bonding interactions of carbon dioxide and epoxy propane copolymer to poly(propylene carbonate) (PPC)/poly(p‐vinylphenol) (PVPh) blends were investigated with differential scanning calorimetry (DSC), Fourier transform infrared (FTIR) spectroscopy and X‐ray photoelectron spectroscopy (XPS). The single glass‐transition temperature for each composition showed miscibility over the entire composition range. FTIR indicates the presence of strong hydrogen‐bonding interassociation between the hydroxyl groups of PVPh and the oxygen functional groups of PPC as a function of composition and temperature. XPS results testify to intermolecular hydrogen‐bonding interactions between the oxygen atoms of carbon–oxygen single bonds and carbon–oxygen double bonds in carbonate groups of PPC and the hydroxyl groups of PVPh by the shift of C_1s peaks and the evolution of three novel O_1s peaks in the blends, which supports the suggestion from FTIR analyses. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1957–1964, 2002     

Enthalpies of solvation of ethylene oxide oligomers CH3O(CH2CH2O)nCH3 (n = 1 to 4) in different H-bonding solvents: Methanol,chloroform, and water. Group contribution method as applied to the polar oligomers

The enthalpies of solution and solvation of ethylene oxide oligomers CH₃O(CH₂CH₂O)_nCH₃ (n = 1 to 4) in methanol and chloroform have been determined from calorimetric measurements at T = 298.15 K. The enthalpic coefficients of pairwise solute–solute interaction for methanol solutions have been calculated. The enthalpic characteristics of the oligomers in methanol, chloroform, water and tetrachloromethane have been compared. The hydrogen bonding of the oligomers with chloroform and water molecules is exhibited in the values of solvation enthalpy and coefficient of solute–solute interaction. This effect is not observed for methanol solvent. The thermochemical data evidence an existence of multi-centred hydrogen bonds in associates of polyethers with the solvent molecules. Enthalpies of hydrogen bonding of the oligomers with chloroform and water have been estimated. The additivity scheme has been developed to describe the enthalpies of solvation of ethylene oxide oligomers, unbranched monoethers and n-alkanes in chloroform, methanol, water, and tetrachloromethane. The correction parameters for contribution of repeated polar groups and correction term for methoxy-compounds have been introduced. The obtained group contributions permit to describe the enthalpies of solvation of unbranched monoethers and ethylene oxide oligomers in the solvents with standard deviation up to 0.6 kJ · mol⁻¹. The values of group contributions and corrections are strongly influenced by solvent properties.