29Si NMR研究TEOS/DDS双前驱体体系的水解动力学

原位引入有机组分对氧化硅体系改性是合成有机-无机杂化硅材料的重要方法. 利用原位的²⁹Si液体核磁, 研究了甲醇为溶剂、氨水催化条件下的四乙氧基硅烷(TEOS)和二甲基二乙氧基硅烷(DDS)原位共水解的动力学过程. 通过改变反应体系中氨和水的浓度, 拟合出单体及中间产物浓度随时间的变化曲线, 得到了TEOS和DDS各自的水解速率常数以及相应各反应物的反应级数. 与单前驱体水解一致的是, 在双前驱体系中TEOS和DDS自身的反应级数仍保持一级, 但是氨和水的反应级数都有不同程度的增大. 与单前驱体水解速率方程相比, 混合体系中TEOS的水解速率常数增大. 同时, DDS在双前驱体中比单前驱体中的水解速率常数有很大程度的减少. 水解动力学表明, TEOS和DDS在双前驱体体系中显示出更平行的水解速率. 利用固体²⁹Si MAS NMR, XPS及小角X射线散射(SAXS)手段对双前驱体体系研究得到的信息显示, 碱催化条件下原位的TEOS水解中间物与DDS中间产物的原位共缩聚程度很弱.     

液态29Si NMR研究TEOS/DDS混合体系的氨催化水解动力学

原位引入有机组分对氧化硅体系改性是合成有机-无机杂化硅材料的重要方法.利用原位的29Si液体核磁,研究了甲醇为溶剂、氨水催化条件下的四乙氧基硅烷（TEOS）和二甲基二乙氧基硅烷（DDS）原位共水解的动力学过程.通过改变反应体系中氨和水的浓度,拟合出单体及中间产物浓度随时间的变化曲线,得到了TEOS和DDS各自的水解速率常数以及相应各反应物的反应级数.与单前驱体水解一致的是,在双前驱体系中TEOS和DDS自身的反应级数仍保持一级,但是氨和水的反应级数都有不同程度的增大.与单前驱体水解速率方程相比,混合体系中TEOS的水解速率常数增大.同时,DDS在双前驱体中比单前驱体中的水解速率常数有很大程度的减少.水解动力学表明,TEOS和DDS在双前驱体体系中显示出更平行的水解速率.利用固体29SiMAS NMR,XPS及小角X射线散射（SAXS）手段对双前驱体体系研究得到的信息显示,碱催化条件下原位的TEOS水解中间物与DDS中间产物的原位共缩聚程度很弱.     

Kinetics of the acid-catalyzed hydrolysis of tetraethoxysilane (TEOS) by <Superscript>29</Superscript>Si NMR spectroscopy and mathematical modeling

Tetraethoxysilane (TEOS) is widely used to synthesize siliceous material by the sol–gel process. However, there is still some disagreement about the nature of the limiting step in the hydrolysis and condensation reactions. The goal of this research was to measure the variation in the concentration of intermediates formed in the acid-catalyzed hydrolysis by ²⁹Si NMR spectroscopy, to model the reactions, and to obtain the rate constants and the activation energy for the hydrolysis and early condensation steps. We studied the kinetics of TEOS between pH 3.8 and 4.4, and four temperature values in the range of 277.2–313.2?K, with a TEOS:ethanol:water molar ratio of 1:30:20. Both hydrolysis and the condensation rate speeded up with the temperature and the concentration of oxonium ions. The kinetic constants for hydrolysis reactions increased in each step k_h1?<?k_h2?<?k_h3?<?k_h4, but the condensation rate was lower for dimer formation than for the formation of the fully hydrolyzed Si(OH)₄. The system was described according to 13 parameters: six of them for the kinetic constants estimated at 298.2?K, six to the activation energies, and one to the equilibrium constant for the fourth hydrolysis. The mathematical model shows a steady increase in the activation energy from 34.5?kJ?mol^?1 for the first hydrolysis to 39.2?kJ?mol^?1 in the last step. The activation energy for the condensation reaction from Si(OH)₄ was ca. 10?kJ?mol^?1 higher than the largest activation energy in the hydrolytic reactions. The decrease in the net positive charge on the Si atom contributes to the protonation of the ethoxy group and makes it a better leaving group.

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Aluminum Solvate Complexes Forming in Acidic Methanol-Acetone Mixtures Studied by <Superscript>27</Superscript>Al NMR Spectroscopy

²⁷Al NMR spectroscopy is a powerful tool for the study of coordination and solvation in both aqueous and nonaqueous solutions. In this study, the complexes coexisting upon dissolution of AlCl₃ in acidic acetone + methanol solutions are shown to consist essentially of mixed hexacoordinated species of the general formula [Al(CH₃OH)_6−n (CH₃COCH₃) _n ]³⁺ (n=1,2 and 3), all exhibiting distinctly different ²⁷Al shielding effects. The relative populations of the various mixed species are found to be highly dependent upon the acetone:methanol mole ratio that in the more acetone-rich mixtures with aluminum become appreciably coordinated by acetone. The results demonstrate that the key factor for the formation of acetone-containing species in acidic methanolic solutions is having the CH₃COCH₃:CH₃OH mole ratio at 3:1.     

Accumulation of <Superscript>137</Superscript>Cs and <Superscript>40</Superscript>K in aboveground organs of tropical woody fruit plants

Distribution of ⁴⁰K and ¹³⁷Cs in tissues of the Citrus aurantifolia was measured by gamma spectrometry. A simple theoretical model is also proposed to describe the temporal evolution of ⁴⁰K activity concentration in such tropical woody fruit species. This model exhibits close agreement with the ⁴⁰K experimental results, in the leaf growing and fruit ripening processes of lemon trees.     

Functionalization of one or two methyl groups in the [Cp*<Subscript>2</Subscript>RuBr]<Superscript>+</Superscript>Br<Superscript>−</Superscript> complex in the reaction with bromine

The reaction of [Cp*₂RuBr]⁺Br⁻ with bromine in CH₂Cl₂ (CD₂Cl₂) in an inert atmosphere at room temperature produces the complexes [Cp*Ru(Br)C₅Me₄CH₂Br]⁺Br₃ ⁻ (syn conformer), [Cp*Ru(Br)C₅Me₃(CH₂Br)₂]⁺ (syn and anti conformers), and [Ru(Br)(C₅Me₄CH₂Br)₂]⁺ (syn conformer). All complexes were characterized by ¹H and ¹³C NMR spectroscopy; the former complex, by elemental analysis. These complexes were also prepared by the reaction of [Cp*RuC₅Me₄CH₂]⁺BF₄ ⁻ with bromine in CH₂Cl₂. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2712–2718, December, 2005.     

Effects of La<Superscript>3+</Superscript> on inward K<Superscript>+</Superscript> channels at plasma membrane in guard cells

The effects of La³⁺ on inward K⁺ channels at plasma membrane in vicia guard cells are investigated using the whole-cell patch-clamp recording mode. It is shown that La³⁺ on both sides of plasma membrane blocks inward K⁺ currents in a concentration-dependent manner, indicating that La³⁺ binding sites may exist on both sides of plasma membrane in guard cells in vicia. The dose response is fitted by the Michaelis-Menten relation characterized by an inhibitor constant K _i of 2.56±0.25 μmol · L⁻¹ (outside membrane) and (1.18±0.11)×10⁻¹⁵ mol · L⁻¹ (inside membrane). Intracellular La³⁺ has much stronger inhibitory effect on inward K⁺ currents than extracellular La³⁺ does, suggesting there may exist stronger binding sites inside membrane than outside membrane. Since ion channel activities of guard cells directly affect plant stomatal movement and water status, our results imply that rare earth elements might have potential practical values in regulating plant water status and strengthening plant drought endurance.     

Quantitative <Superscript>2</Superscript>H NMR spectroscopy

The selectivity of deuterium distribution between the nonequivalent positions in 3-carene (1), 4-α-acetyl-2-carene (2), and 4-(1-hydroxyethyl)-2-carene (3) has been measured by ²H-{¹H} NMR spectroscopy at the natural abundance of deuterium. These “H/D-isotope portraits” were shown to be typical of terpenes and terpenoids produced in plants via the biosynthetic DXP pathway. The mechanism of acylation of 1 was studied by the density functional theory method (PBE functional, TZ2p basis set). The six-membered ring in compound 1 is planar. However, the endo attack of electrophiles on this ring is more favorable both kinetically and thermodynamically. It was shown both experimentally and theoretically that the elimination of a hydrogen atom in the second reaction step proceeds stereoselectively at the C(2) atom from the anti position with respect to the three-membered ring and occurs with pronounced nucleophilic assistance from the carbonyl group. For Part 2, see Ref. 1. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1657–1664, August, 2008.     

Hydridosilazanes hydrolysis-condensation reactions studied by <Superscript>1</Superscript>H and <Superscript>29</Superscript>Si liquid NMR spectroscopy

Hydridosilazane compounds containing Si–N and Si–H bonds can be used as precursors of SiO_x materials. The hydrolysis-condensation reactions of tetramethyldisilazane, as a polyhydridosilazane model compound, were investigated by ¹H and ²⁹Si liquid NMR spectroscopy. These reactions were carried out at room temperature for up to 120 min in presence of water. The identified products are short linear siloxane species (hydride terminated polydimethylsiloxanes M^HD_xM^H) and cyclosiloxanes. Silicon hydride persistence in the reactional mixture suggested that silazane group is more sensitive to hydrolysis reaction than silicon hydride group. Moreover, additional experiments evidenced that the low steric hindrance of the silicon hydride influences the silazane hydrolysis kinetic. Hence the presence of ammonia released during silazane hydrolysis reaction was demonstrated to be a catalyst of the silicon hydride hydrolysis reaction.     

Ionic mobility in ammonium-rubidium heptafluorozirconate (NH<Subscript>4</Subscript>)<Subscript>2.4</Subscript>Rb<Subscript>0.6</Subscript>ZrF<Subscript>7</Subscript> by (<Superscript>1</Superscript>H, <Superscript>19</Superscript>F) NMR data

NMR (¹⁹F, ¹H) methods are used to study ionic mobility in heptafluorozirconate (NH₄)_2.4Rb_0.6ZrF₇ in a range of temperatures from 150 K to 430 K. Types of ionic movements are determined, and their activation energy is evaluated. As a result of a phase transition a modification forms in which diffusion in the ammonium sublattice and isotropic reorientations of ZrF ₇ ^3? complex anions are observed. According to preliminary data, due to diffusion of ammonium ions the compound has relatively high ionic conductivity (σ ≈ 8.3 × 10^?5 S/cm at 423 K).     

Styrene oxide DNA adducts: quantitative determination using <Superscript>31</Superscript>P monitoring

The reaction of styrene oxide, a potential carcinogen in humans, with DNA constituents has been used to develop an improved method for quantification of DNA adducts. To enable monitoring of DNA adducts caused by xenobiotics at physiological relevant levels, a robust, reliable and powerful method based on monitoring of phosphorus in nucleotides is described. An efficient enzymatic digestion step and a sample-preconcentration procedure are essential, and enable separation of alkylated nucleotides from the large excess of native nucleotides. The adducts are detected by means of the phosphorus signal measured at mass m/z=31 with an inductively-coupled-plasma mass spectrometer. Bis(4-nitrophenyl)phosphate (BNPP) serves as internal standard for quantification of the adducts. The absolute limit of detection, 45 fmol, corresponds to detection of three modified nucleotides among 10⁷ native nucleotides (the calculation is based on use of 50 g calf thymus DNA). An adduct formation ratio at the DNA of 3.6 adducts per 1000 nucleotides was measured, which is 75% lower than for reaction with monomeric 2-deoxy-nucleotides. In addition, a substantial amount of phosphate adducts were detected, but in DNA the rate of phosphate formation was lower than with monomeric nucleotides. Most probably these adducts escaped unnoticed when ³¹P-post-labelling was employed.     

Combination of <Superscript>13</Superscript>C/<Superscript>113</Superscript>Cd NMR,potentiometry, and voltammetry in characterizing the interactions between Cd and two models of the main components of soil organic matter

This work allowed the characterization of the Cd-binding sites of two compounds taken as models for exudates, the main components of soil organic matter (SOM). The studied compounds were exopolysaccharides (EPS), specifically exudates of roots (polygalacturonic acid) and of soil bacteria (Phytagel). Potentiometric acid–base titrations were performed and fitting of the obtained results indicated the presence of two main classes of acidic sites, defined by their pK _a values, for both EPS but of a different nature when comparing the two compounds. The two studied exopolysaccharides presented different acidic/basic site ratios: 0.15 for Phytagel and 0.76 for polygalacturonic acid. Spectroscopic techniques (¹³C/¹¹³Cd NMR, FTIR) distinguished different Cd surroundings for each of the studied EPS, which is in agreement with the titration results. Furthermore, these analyses indicated the presence of –COOH and –OH groups in various proportions for each exopolysaccharide, which should be linked to their reactivity towards cadmium. Cadmium titrations (voltammetric measurements) also differentiated different binding sites for each compound and allowed the determination of the strength of the Cd-binding site of the EPS. Fitting of the results of such voltammetric measurements was performed using PROSECE (Programme d’Optimisation et de Speciation Chimique dans l’Environnement), a software coupling chemical speciation calculation and binding parameter optimization. The fitting, taking into account the Cd²⁺/H⁺ competition towards exopolysaccharides, confirmed the acid-base titrations and spectroscopic analyses by revealing two classes of binding sites: (i) one defined as a strong complexant regarding its Cd²⁺–EPS association (logK = 9–10.4) and with basic functionality regarding H⁺–EPS association (pK _a = 11.3–11.7), and (ii) one defined as a weak complexant (logK = 7.1–8.2) and with acidic functionality (pK _a = 3.7–4.0). Therefore the combination of spectroscopic analyses, voltammetry, and fitting allowed the precise characterization of the binding sites of the studied exopolysaccharides, mimicking the main SOM components. Furthermore, the binding parameters obtained by fitting can be used in biogeochemical models to better define the role of key SOM compounds like exudates of roots and of soil bacteria on trace metal transport or assimilation.     

Structure of hybrid organic–inorganic sols for the preparation of hydrothermally stable membranes

A procedure for the preparation of hybrid sols for the synthesis of organic–inorganic microporous materials and thin film membranes is reported. We describe silane reactivity and sol structure for acid-catalysed colloidal sols from mixtures of either tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTES), or bis(triethoxysilyl)ethane (BTESE) and MTES. Early-stage hydrolysis and condensation rates of the individual silane precursors were followed with ²⁹Si liquid NMR and structural characteristics of more developed sols were studied with Dynamic Light Scattering. Condensation was found to proceed at more or less similar rates for the different precursors. Homogeneously mixed hybrid colloids can therefore be formed from precursor mixtures. The conditions of preparation under which clear sols with low viscosity could be formed from BTESE/MTES were determined. These sols were synthesised at moderate water/silane and acid/silane ratios and could be applied for the coating of defect-free microporous membranes for molecular separations under hydrothermal conditions.     

Novel Nafion®/ORMOSIL hybrids via in situ sol-gel reactions: 2. Probe of ORMOSIL phase nanostructure by 29Si solid state NMR spectroscopy

Perfluorosulfonic Acid (PFSA)]/[Organically-Modified Silicon Oxide (ORMOSIL)] hybrids were formulated via sol-gel reactions of mixtures of tetraethoxysilane (TEOS) and diethoxydimethylsilane (DEDMS) in the nanophase-separated morphology of a PFSA membrane (Nafion^®). The molecular structures of the ORMOSIL phases were investigated by ²⁹Si solid state NMR spectroscopy and the spectra compared with those of corresponding bulk ORMOSILs. PFSA-in situ ORMOSIL composition can be tailored by manipulating TEOS/DEDMS source solution composition and quantified by ²⁹Si solid state NMR spectroscopy. Copolymerization of TEOS and DEDMS monomers, rather than block formation, occurs within the PFSA as well as in the bulk. As DEDMS feed concentration increases, ORMOSIL nanostructures are more hydrophobic and more flexible. Dimethylsiloxane rings are dominant in PFSAs in which hydrolysis + condensation of pure DEDMS occurs.     

Structure of lipid A from the marine gram-negative bacterium <Emphasis Type="Italic">Pseudoalteromonas nigrifaciens</Emphasis> IAM 13010<Superscript>T</Superscript>

The structure of lipid A from the marine γ-proteobacterium Pseudoalteromonas nigrifaciens IAM 13010^T that was prepared by hydrolysis of the corresponding lipopolysaccharide by acetic acid (1%) was determined by chemical analysis, ¹³C NMR spectroscopy, and MALDI/TOF and LSIMS mass spectrometry. It was shown that lipid A is a β-1,6-bonded disaccharide of glucosamine that is substituted by two phosphoric acids (in the C1 and C4′ positions), two (R)-3-hydroxyalkanoic (normal and branched) acids with ester bonding (at the C3 and C3′-positions), and (R)-3-hydroxydodecanoic and (R)-3-dodecanoyloxydodecanoic acids (both with amide bonding at C2′ and C2, respectively). It was hypothesized that this type of structure is typical of lipid A from bacteria of the genus Pseudoalteromonas in general. __________ Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 425–429, September–October, 2007.     

Theoretical study on the reaction mechanism of (CH<Subscript>3</Subscript>)<Subscript>3</Subscript>CO<Superscript>.</Superscript> radical with NO

The reaction mechanism of (CH₃)₃CO^. radical with NO is theoretically investigated at the B3LYP/6-31G* level. The results show that the reaction is multi-channel in the single state and triplet state. The potential energy surfaces of reaction paths in the single state are lower than that in the triple state. The balance reaction: (CH₃)₃CONO⇔(CH₃)₃CO^.+NO, whose potential energy surface is the lowest in all the reaction paths, makes the probability of measuring (CH₃)₃CO^. radical increase. So NO may be considered as a stabilizing reagent for the (CH₃)₃CO^. radical.     

Structure of Galactomannans from <Emphasis Type="Italic">Gleditsia delavayi</Emphasis> and <Emphasis Type="Italic">G. aquatica</Emphasis> by <Superscript>1</Superscript>H and <Superscript>13</Superscript>C NMR Spectroscopy

The structure of galactomannans isolated from seeds of G. delavayi and G. aquatica was studied by ¹H and ¹³C NMR spectroscopy. It was found that the galactomannans consisted mainly of β-1-4-bound mannopyranoses, a part of which was substituted on the C-6 hydroxyl by terminal units of α-galactopyranose.__________Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 12–13, January–February, 2005.     

Preparation and Characterization of GdTaO<Subscript>4</Subscript>:Eu<Superscript>3+</Superscript> Sol-Gel Luminescence Thin Films

High quality GdTaO₄:Eu³⁺ luminescence films have been successfully prepared through a modified sol-gel process. The films were prepared using inorganic materials as raw materials, and the thermal decomposition and UV assisted technique were introduced to improve the quality of the film and reduce the period for forming the thick film. Results of structural studies by atomic force microscopy (AFM) and X-ray diffraction (XRD) showed that the surface was smooth and the structure was monoclinic with the average grain size of about 55 nm. The emission and excitation spectra of the film were investigated. Related to the transition ⁵ D₀ →⁷ F₁ and ⁵ D₀→⁷ F₂ of Eu³⁺ ions, the main luminescence peaks were observed at 591 and 611 nm respectively, and the luminescence peak at 345 nm was detected simultaneously related to the TaO₄³⁻ emission. Transmission spectrum and decay curve of the luminescence are also presented in this paper.     

Structural studies of bacterial cellulose through the solid-phase nitration and acetylation by CP/MAS <Superscript>13</Superscript>C NMR spectroscopy

The solid-phase nitration and acetylation processes of bacterial cellulose have been investigated mainly by CP/MAS ¹³C NMR spectroscopy to clarify the features of these reactions in relation to the characterization of the disordered component included in the microfibrils. CP/MAS ¹³C NMR spectra of bacterial and Valonia cellulose samples are markedly changed as the nitration progresses, in a similar way to the case of cotton linters previously reported; and the relative reactivity of the OH groups in the glucose residues is found to decrease in the order of O(6)H>O(2)H>O(3)H. Moreover, the nitration rate and mode greatly depend on the concentration of nitric acid in the reaction media. At dilute and medium concentrations, the O(6)H groups in the crystalline and disordered components are subjected to nitration at nearly the same rate, indicating that these two components are distributed almost at random in the entire region of each microfibril. The preferential penetration of nitric acid into each microfibril also occurs prior to nitration at the medium concentration, resulting in an increase in the mole fraction of the disordered component. In contrast, all OH groups undergo nitration very rapidly at the higher concentration, although nitration levels off to a certain extent for O(3)H groups. In solid-phase acetylation, no regio-selective reactivity is observed among the three kinds of OH groups, which may be due to the characteristic reaction that proceeds in a very thin layer between the acetylated and nonacetylated regions in each microfibril. The almost random distribution of the disordered component in the entire region of the microfibrils is also confirmed in this solid-phase acetylation. On the basis of these results, the mechanism of the solid-phase reactions and the microfibril structure are discussed.     

Local structure and photoluminescent characteristics of Eu<Superscript>3+</Superscript> in ZnO–SiO<Subscript>2</Subscript> glasses

The photoluminescence properties of xZnO–(100−x)SiO₂ (x = 0, 5, 10, 20) containing 1% Eu₂O₃ prepared by a sol–gel method were systematically investigated. The results indicated that the relative proportion of f–f transitions to charge transfer (CT) absorption decreased with the increase of ZnO concentration. The intensity of ⁵D₀–⁷F_J transitions of Eu³⁺ ions was enhanced with the increase of ZnO content due to local structure changes and decreased quantities of Eu³⁺ ions clusters. The results of fluorescence line narrow (FLN) spectra indicated that Eu³⁺ ions occupied one site in SiO₂ glass and two sites in ZnO–SiO₂ glasses. The second-order crystal field parameters were calculated. B₂₀ and B₂₂ for site 1 increased with excitation energy, while ones hardly changed for site 2.