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Acid System (acid + system)
Kinds of Acid System Selected AbstractsEnantioselective Addition of Boronates to Chromene Acetals Catalyzed by a Chiral Brųnsted Acid/Lewis Acid System,ANGEWANDTE CHEMIE, Issue 39 2010Philip N. Moquist Chirale ,,,-Dihydroxycarbonsäuren katalysieren die enantioselektive Addition von Alkenyl- und Arylboronaten an Chromenacetale. Die beste Carbonsäure ist das gezeigte, leicht zugängliche Tartarsäureamid. Zugabe von Lanthanoidtriflaten beschleunigt die Reaktion. Spektroskopische und kinetische Studien offenbaren einen Austauschprozess, der zu einem reaktiven Dioxoborolan-Intermediat führt, das eine enantioselektive Addition an das aus dem Chromenacetal gebildeten Pyrylium-Ion eingeht. [source] Partial Oxidation of 4- tert -Butyltoluene Catalyzed by Homogeneous Cobalt and Cerium Acetate Catalysts in the Br,/H2O2/Acetic Acid System: Insights into Selectivity and MechanismCHEMISTRY - A EUROPEAN JOURNAL, Issue 28 2007Abstract The partial oxidation of 4- tert -butyltoluene to 4- tert -butylbenzaldehyde by hydrogen peroxide in glacial acetic acid, catalyzed by bromide ions in combination with cobalt(II) acetate or cerium(III) acetate, has been studied in detail. Based on the observed differences in reaction rates and product distributions for the different catalysts, a reaction mechanism involving two independent pathways is proposed. After the initial formation of a benzylic radical species, either oxidation of this intermediate by the metal catalyst or reaction with bromine generated in situ occurs, depending on which catalyst is used. The first pathway leads to the exclusive formation of 4- tert -butylbenzaldehyde, whereas reaction of the radical intermediate with bromine leads to formation of the observed side products 4- tert -butylbenzyl bromide and its hydrolysis and solvolysis products 4- tert -butylbenzyl alcohol and 4- tert -butylbenzyl acetate, respectively. The cobalt(II) catalysts Co(OAc)2 and Co(acac)2 are able to quickly oxidize the radical intermediate, thereby largely preventing the bromination reaction (i.e., side-product formation) from occurring, and yield the aldehyde product with 75,80,% selectivity. In contrast, the cerium catalyst studied here exhibits an aldehyde selectivity of around 50,% due to the competing bromination reaction. Addition of extra hydrogen peroxide leads to an increased product yield of 72,% (cerium(III) acetate) or 58,% (cobalt(II) acetate). Product inhibition and the presence of increasing amounts of water in the reaction mixture do not play a role in the observed low incremental yields. [source] Hybrid Miniemulsion Polymerization of Acrylate/Oil and Acrylate/Fatty Acid SystemsMACROMOLECULAR REACTION ENGINEERING, Issue 3 2008Juchen Guo Abstract Acrylate,alkyd hybrid latex via miniemulsion polymerizations show promise as water-borne coating systems. However, poor homogeneity of the particles caused by the immiscibility of the alkyd in polyacrylate limits monomer conversion and film formation. To resolve this problem, the hybrid miniemulsion polymerization of acrylate in the presence of linoleic acid and sunflower seed oil was carried out. Products were characterized by solvent extraction, dynamic light scattering, gel permeation chromatography (GPC), differential scanning calorimeter (DSC), and transmission electron microscopy (TEM). The results provide clear evidence that substituting a fatty acid or natural oil with smaller molecular size (weight) for a conventional alkyd improves the grafting efficiency, and enhances the homogeneity of the hybrid polymer particles in water-borne latex systems. [source] Fully quantum mechanical energy optimization for protein,ligand structureJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 12 2004Yun Xiang Abstract We present a quantum mechanical approach to study protein,ligand binding structure with application to a Adipocyte lipid-binding protein complexed with Propanoic Acid. The present approach employs a recently develop molecular fractionation with a conjugate caps (MFCC) method to compute protein,ligand interaction energy and performs energy optimization using the quasi-Newton method. The MFCC method enables us to compute fully quantum mechanical ab initio protein,ligand interaction energy and its gradients that are used in energy minimization. This quantum optimization approach is applied to study the Adipocyte lipid-binding protein complexed with Propanoic Acid system, a complex system consisting of a 2057-atom protein and a 10-atom ligand. The MFCC calculation is carried out at the Hartree,Fock level with a 3-21G basis set. The quantum optimized structure of this complex is in good agreement with the experimental crystal structure. The quantum energy calculation is implemented in a parallel program that dramatically speeds up the MFCC calculation for the protein,ligand system. Similarly good agreement between MFCC optimized structure and the experimental structure is also obtained for the streptavidin,biotin complex. Due to heavy computational cost, the quantum energy minimization is carried out in a six-dimensional space that corresponds to the rigid-body protein,ligand interaction. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 1431,1437, 2004 [source] Light-emitting diode-compatible probes for indirect detection of anions in CEELECTROPHORESIS, Issue 19 2007Peter Balding Abstract A range of compounds were evaluated as probes for the indirect detection of inorganic ions using CE and light-emitting diodes (LEDs) as the light source. Emphasis was placed on examining probes likely to absorb strongly in the UV,Vis region near 350,430,nm as compounds, which absorb at longer wavelengths tend to be bulkier and adsorb onto the capillary wall. These probes should act as a replacement for the very effective but carcinogenic probe chromate. Two probes were identified and evaluated: p -nitrophenol and 4-hydroxy-3,5-dinitrobenzoic acid. The former showed the most potential with low-mobility anions, while the later had a moderate electrophoretic mobility and was more suitable for a wider mobility range of analytes. However, neither could match the efficiencies and LOD of chromate for the separation of the fast inorganic ions such as chloride, nitrate and sulphate. Nevertheless, application of the 4-hydroxy-3,5-dinitrobenzoic acid system to the determination of oxalate in Bayer liquors showed excellent sensitivity and selectivity. [source] Computational modelling of amino acid transfer interactions in the placentaEXPERIMENTAL PHYSIOLOGY, Issue 7 2010B. G. Sengers Amino acid transfer from mother to fetus via the placenta plays a critical role in normal development, and restricted transfer is associated with fetal growth restriction. Placental amino acid transfer involves the interaction of 15 or more transporters and 20 amino acids. This complexity means that knowing which transporters are present is not sufficient to predict how they operate together as a system. Therefore, in order to investigate how placental amino acid transfer occurs as a system, an integrated mathematical/computational modelling framework was developed to represent the simultaneous transport of multiple amino acids. The approach was based on a compartmental model, in which separate maternal, syncytiotrophoblast and fetal volumes were distinguished, and transporters were modelled on the maternal- and fetal-facing membranes of the syncytiotrophoblast using Michaelis,Menten-type kinetics. The model was tested in comparison with placental perfusion experiments studying serine,alanine exchange and found to correspond well. The results demonstrated how the different transporters can work together as an integrated system and allowed their relative importance to be assessed. Placental,fetal serine exchange was found to be most sensitive to basal membrane transporter characteristics, but a range of secondary, less intuitive effects were also revealed. While this work only addressed a relatively simple three amino acid system, it demonstrates the feasibility of the approach and could be extended to incorporate additional experimental parameters. Ultimately, this approach will allow physiological simulations of amino acid transfer. This will enhance our understanding of these complex systems and placental function in health and disease. [source] Facile Synthesis and Shape Evolution of Single-Crystal Cuprous OxideADVANCED MATERIALS, Issue 20 2009Xudong Liang Various morphologies of single-crystal Cu2O such as cube, octahedron, {110} truncated octahedron, and microrhombic dodecahedron with {110} surfaces are prepared in high yield through a facile solution-based one-step reduction method in an alkaline H2O/ethanol/oleic acid system in the presence of d-(+)-glucose. The formation mechanism of these microcrystals has been clarified as the synergic effect of oriented attachment and ripening mechanism. [source] Effect of different cooking methods on the antioxidant activity of some vegetables from PakistanINTERNATIONAL JOURNAL OF FOOD SCIENCE & TECHNOLOGY, Issue 3 2008Bushra Sultana Summary The effects of different cooking methods (boiling, frying and microwave cooking) on the antioxidant activity of some selected vegetables (peas, carrot, spinach, cabbage, cauliflower, yellow turnip and white turnip) were assessed by measuring the total phenolic contents (TPC), reducing power and percentage inhibition in linoleic acid system. TPC (gallic acid equivalents g/100 g of dry weight) and reducing power of the methanolic extracts of raw-, microwaved-, boiled- and fried vegetables ranged 0.333,2.97, 0.52,2.68, 0.48,2.08, 1.00,2.02 and 0.391,2.24, 0.822,1.10, 0.547,1.16, 0.910,4.07, respectively. The level of inhibition of peroxidation ranged 71.4,89.0, 66.4,87.3, 73.2,89.2 and 77.4,91.3%, respectively. Antioxidant activity of the vegetables examined was appreciably affected because of varying cooking treatments. TPC of vegetables, generally, decreased by boiling, frying and microwave cooking. There was a significant (P < 0.05) increase in reducing power as a result of frying. However, boiling and microwave cooking did not affect reducing power. Inhibition of peroxidation increased by boiling and frying, whereas, in contrast it was decreased by microwave cooking. The results of the present investigation showed that all the cooking methods affected the antioxidant properties of the vegetables; however, microwave treatment exhibited more deleterious effects when compared with those of other treatments. Thus an appropriate method might be sought for the processing of such vegetables to retain their antioxidant components at maximum level. [source] Establishment of a quantitative structure,activity relationship model for evaluating and predicting the protective potentials of phenolic antioxidants on lipid peroxidationJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 3 2003Zhiyong Cheng Abstract Antioxidant activities of phenolic compounds have been extensively explored, but the determinant factors underlying their mechanisms of action remain to be elucidated. In the present work, a series of phenolic compounds (hydroxylated connamic, benzoic acid, and polyphenol) were studied for their protection against lipid peroxidation (LPO) in two model experiments, pre-emulsified linoleic acid system and phosphate buffered linolenic acid system. The mechanisms of action as well as activity determinants were investigated by computational chemistry and multiple-linear regression analysis. Upon elucidating the LPO inhibition properties and the relationship between their structural natures and antioxidant activities (SAR), a fairly satisfactory multidescriptor quantitative SAR model was derived, which extended our understanding of LPO inhibition mechanisms and should be valuable in assessing or predicting the anti-LPO activity of phenolic antioxidants. © 2003 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 92:475,484, 2003 [source] Antioxidant activity of hydrolysates derived from porcine plasmaJOURNAL OF THE SCIENCE OF FOOD AND AGRICULTURE, Issue 11 2009Xueming Xu Abstract BACKGROUND: In China alone, more than 400 million pigs are slaughtered each year to provide meat. Porcine blood is rich in proteins but is usually discarded, which can cause environmental contamination. Recovering porcine blood and converting it to high-value products is therefore economically and environmentally desirable. However, very little information on antioxidant peptides from porcine blood by-products is currently available. In this study the antioxidant properties of porcine plasma hydrolysates PPE and PPA prepared with pepsin and papain respectively were investigated. RESULTS: Both PPE and PPA showed excellent antioxidant activity in a linoleic acid system (AL) compared with ,-tocopherol (VE) at the same concentration (P < 0.01). Their activities were respectively 3.33 and 1.83 times stronger than that of VE at a concentration of 10 µg mL,1 and 5.4 and 5.6 times stronger at 100 µg mL,1. The 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical-scavenging activity (DRSA) reached 48.4 and 43.1% for PPE and PPA respectively at 500 µg mL,1. The ferrous ion-chelating power (FICP) of PPE at 100 µg mL,1 was about 1.5 times stronger than that of 10 µmol L,1 ethylene diamine tetraacetic acid (EDTA) in a 50 µmol L,1 Fe2+ system, whereas the FICP of PPA at 100 µg mL,1 was 61% that of 10 µmol L,1 EDTA. Furthermore, PPE was separated on Resource 15RPC and Superdex peptide 10/300GL columns, and the antioxidant activity of the peptides and its relationship to their polarity and molecular weight (MW) were analysed. The hydrolysate was divided into four groups (R1,R4) with hydrophobicities ranging from weak to strong by Resource 15RPC, while it was divided into three groups (S1, MW 7,12 kDa; S2, MW 3,7 kDa; S3, MW 1,3 kDa) by Superdex peptide 10/300GL. CONCLUSION: The results showed that AL was significantly and positively correlated with the relative amounts of R1, S2 and S3 and that DRSA was dependent on R3 and S1. The fractions of PPE were not responsible for FICP. Copyright © 2009 Society of Chemical Industry [source] Two new structures in the glycine,oxalic acid systemACTA CRYSTALLOGRAPHICA SECTION C, Issue 6 2010Nikolay A. Tumanov Glycinium semi-oxalate-II, C2H6NO2+·C2HO4,, (A), and diglycinium oxalate methanol disolvate, 2C2H6NO2+·C2O42,·2CH3OH, (B), are new examples in the glycine,oxalic acid family. (A) is a new polymorph of the known glycinium semi-oxalate salt, (C). Compounds (A) and (C) have a similar packing of the semi-oxalate monoanions with respect to the glycinium cations, but in (A) the two glycinium cations and the two semi-oxalate anions in the asymmetric unit are non-equivalent, and the binding of the glycinium cations to each other is radically different. Based on this difference, one can expect that, although the two forms grow concomitantly from the same batch, a transformation between (A) and (C) in the solid state should be difficult. In (B), two glycinium cations and an oxalate anion, which sits across a centre of inversion, are linked via strong short O,H...O hydrogen bonds to form the main structural fragment, similar to that in diglycinium oxalate, (D). Methanol solvent molecules are embedded between the glycinium cations of neighbouring fragments. These fragments form a three-dimensional network via N,H...O hydrogen bonds. Salts (B) and (D) can be obtained from the same solution by, respectively, slow or rapid antisolvent crystallization. [source] Utilization of spent petrochemical sulfuric acid in the production of wet-process phosphoric acidJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 12 2005Andrzej Chojnacki Abstract The possibility of the application of spent sulfuric acid from the petrochemical industry in wet-process phosphoric acid technology was investigated. The effect of organic impurities in sulfuric acid from benzol acidic refining on the solubility of calcium sulfate hydrates is discussed. Solubility isotherms and regression equations for CaSO4,H3PO4,H2SO4,H2O (pure and containing impurities) systems are presented. Phase transition temperatures between dihydrate and hemihydrate calcium sulfate were determined. The difference between pure and polluted sulfuric acid systems observed is negligible over the range of typical wet-process phosphoric acid technology parameters. It is concluded that the application of spent sulfuric acid from benzol acidic refining does not have a negative influence on the crystallization process of dihydrate calcium sulfate and therefore can be applied in wet-process phosphoric acid technology. Copyright © 2005 Society of Chemical Industry [source] Comparison of some representative density functional theory and wave function theory methods for the studies of amino acidsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 4 2009Wenbo Yu Abstract Energies of different conformers of 22 amino acid molecules and their protonated and deprotonated species were calculated by some density functional theory (DFT; SVWN, B3LYP, B3PW91, MPWB1K, BHandHLYP) and wave function theory (WFT; HF, MP2) methods with the 6-311++G(d,p) basis set to obtain the relative conformer energies, vertical electron detachment energies, deprotonation energies, and proton affinities. Taking the CCSD/6-311++G(d,p) results as the references, the performances of the tested DFT and WFT methods for amino acids with various intramolecular hydrogen bonds were determined. The BHandHLYP method was the best overall performer among the tested DFT methods, and its accuracy was even better than that of the more expensive MP2 method. The computational dependencies of the five DFT methods and the HF and MP2 methods on the basis sets were further examined with the 6-31G(d,p), 6-311++G(d,p), aug-cc-pVDZ, 6-311++G(2df,p), and aug-cc-pVTZ basis sets. The differences between the small and large basis set results have decreased quickly for the hybrid generalized gradient approximation (GGA) methods. The basis set convergence of the MP2 results has been, however, very slow. Considering both the cost and the accuracy, the BHandHLYP functional with the 6-311++G(d,p) basis set is the best choice for the amino acid systems that are rich in hydrogen bonds. © 2008 Wiley Periodicals, Inc. J Comput Chem 2009 [source] Transferable intermolecular potentials for carboxylic acids and their phase behaviorAICHE JOURNAL, Issue 2 2010Amir Vahid Abstract Transferable step potentials are characterized for 39 carboxylic acids. The reference potential is treated with discontinuous molecular dynamics, including detailed molecular structure. Thermodynamic perturbation theory is used to interpret the simulation results and to provide an efficient basis for molecular modeling and characterization of the attractive forces. Four steps are used for representation of the attractive forces with only the first and last steps varied independently. The two middle steps are interpolated such that each site type is characterized by three parameters: the diameter, ,, the depth of the inner well, ,1, and the depth of the outer well, ,4. The depths of the attractive wells are optimized to fit experimental vapor pressure and liquid density data. Generally, the vapor pressure is correlated to an overall 43% average absolute deviation (% AAD) and the liquid density to 5% AAD. The deviations tend to be largest for the higher molecular weight acids. These deviations are larger than the errors previously encountered in characterizing organic compounds, but carboxylic acids present exceptional challenges owing to their peculiar dimerization behavior. Simultaneous correlation of vapor pressure, vapor compressibility factor, and phase equilibria of water + carboxylic acids place several constraints on the nature of the potential model, with the parameters of the present model representing a reasonable tradeoff. In other words, our model represents minimal deviations for vapor pressure, vapor compressibility factor, and phase equilibria of all acids simultaneously while varying the parameters ,, ,1, ,4, ,CC(dimerizing site bonding energy), ,AD(acceptor-donor bonding energy), and KHB(hydrogen bonding volume) for the acid O and OH site types. The present model is characterized by one acceptor and one dimerizing site on the carbonyl oxygen and one acceptor and one donor site on the hydroxyl oxygen. The acceptor and donor are capable of interacting with water while the dimerizing site is not. With this model, the saturated vapor compressibility factor of acids with seven or fewer carbons is near 0.5 while higher carbon ratios lead to a compressibility factor approaching 1.0. To compensate for the high vapor pressure deviations of the transferable potential model, a correction is introduced to customize the molecule-molecule self interaction energy. This adaptation results in deviations of 3.1% for vapor pressure of the pure acid database. To validate the behavior of the model for carboxylic acids in mixtures, 33 binary solutions were considered. Acids in this database ranged from formic to hexadecanoic. The average absolute deviation in bubble pressure for aqueous acid systems is 4.4%, 10.5% for acid + acid systems, and 4.7% for acid + n-alkane systems without a customized interaction correction. When applying the correction, deviations were 2.4% for aqueous systems, 2% for acid systems, and 2.8% for acid + n-alkane systems. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] Energetic, Low-Melting Salts of Simple HeterocyclesPROPELLANTS, EXPLOSIVES, PYROTECHNICS, Issue 4 2003Gregory Drake The synthesis of three new families of heterocyclic-based salts was undertaken and accomplished. Three triazole systems, 1H-1,2,4-triazole, 4-amino-1,2,4-triazole, and 1H-1,2,3-triazole, were used as proton bases with nitric (HNO3), perchloric (HClO4), and dinitramidic (HN(NO2)2) acid systems. In all cases, stable salts were recovered and fully characterized by vibrational spectra (IR, Raman), multinuclear NMR spectroscopy, material balance, density measurements, and elemental analyses, as well as DSC, TGA and initial safety testing (impact). Many of these salts have melting points well below 100,°C, yet high decomposition onsets, defining them as new, highly energetic members of the well known class of materials identified as ionic liquids. Additionally, the single crystal X-ray diffraction study of 1,2,4-triazolium perchlorate was investigated, revealing the expected structure. [source] New Developments in the Pharmacotherapy of Alcohol DependenceTHE AMERICAN JOURNAL ON ADDICTIONS, Issue 2001Hugh Myrick M.D. Neuroscientific underpinnings and pharmacotherapeutic treatments of sub-stance use disorders are rapidly developing areas of study. In particular, there have been exciting new developments in our understanding of the involvement of excitatory amino acid neurotransmitter systems and the opiate and serotonin systems in the pathophysiology of alcohol withdrawal, alcohol dependence, and in subtypes of individuals with alcoholism. In this article, new developments in the pharmacotherapy of alcohol dependence will be reviewed. In particular, the use of anticonvulsants in alcohol withdrawal and protracted abstinence syndromes will be discussed. New data on opiate antagonists and acamprosate, an agent that exerts actions through excitatory amino acid systems in relapse prevention, will be reviewed. Finally, there will be a review of new data concerning the use of serotonin reuptake inhibitors in subtypes of alcoholism and the use of combination pharmacotherapy. [source] Hydrogen-bonding interactions in (3,4-dimethoxyphenyl)acetic acid monohydrateACTA CRYSTALLOGRAPHICA SECTION C, Issue 7 2008Barbara Hachu The crystal structure of the title compound, C10H12O4·H2O, consists of (3,4-dimethoxyphenyl)acetic acid and water molecules linked by O,H...O hydrogen bonds to form cyclic structures with graph-set motifs R12(5) and R44(12). These hydrogen-bond patterns result in a three-dimensional network with graph-set motifs R44(20) and R44(22), and the formation of larger macrocycles, respectively. The C,C bond lengths and the endocyclic angles of the benzene ring show a noticeable asymmetry, which is connected with the charge-transfer interaction of the carboxyl or methoxy groups and the benzene ring. The title compound is one of the simple carboxylic acid systems that form hydrates. Thus, the significance of this study lies in the analysis of the interactions in this structure and the aggregations occurring via hydrogen bonds in two crystalline forms of (3,4-dimethoxyphenyl)acetic acid, namely the present hydrate and the anhydrous form [Chopra, Choudhury & Guru Row (2003). Acta Cryst. E59, o433,o434]. The correlation between the IR spectrum of this compound and its structural data are also discussed. [source] Evaluation of heat and oxidative damage during storage of processed tomato products.JOURNAL OF THE SCIENCE OF FOOD AND AGRICULTURE, Issue 9 2003Abstract Tomato products (pulp, puree and paste) submitted to accelerated aging (30, 40 and 50 °C for 3 months) were studied to evaluate variations in the kinetics of the degradation of antioxidants and antioxidant activity. The carotenoids lycopene and ,-carotene, ascorbic acid, rutin and total phenolics were analysed. The antioxidant activity was measured using (a) the xanthine oxidase (XOD)/xanthine system, which generates superoxide radicals and hydrogen peroxide, and (b) the linoleic acid/CuSO4 system, which promotes lipid peroxidation. The ascorbic acid content decreased even at 30 °C, following pseudo-first-order kinetics, with an activation energy of 105 200 J mol,1 for tomato pulp and 23 600 J mol,1 for tomato paste. The lower the initial ascorbic acid content, the higher was the degradation rate. Variations in phenolic compounds occurred at 40 °C and higher, following pseudo-zero order kinetics. The antioxidant activity of the hydrophilic fraction of the tomato products depended on both antioxidant degradation and the Maillard reaction and could not be described by a kinetic model. The ,-carotene content decreased even at 30 °C, whereas the lycopene content was stable in all samples. The antioxidant activity of the lipophilic fraction of the tomato products decreased following pseudo-first order kinetics, with an activation energy of 22 200 J mol,1 for tomato pulp and 20 200 J mol,1 for tomato paste. It is concluded that significant ,oxidative damage' can occur in tomato products during their commercial shelf-life. Copyright © 2003 Society of Chemical Industry [source] Bis(di-2-pyridylamine-,2N2,N2,)(nitrato-,2O,O,)nickel(II) nitrateACTA CRYSTALLOGRAPHICA SECTION C, Issue 7 2009Juraj, ernįk In the ionic title compound, [Ni(NO3)(C10H9N3)2]NO3, the central NiII atom exhibits cis -NiN4O2 octahedral coordination with three chelating ligands, viz. one nitrate anion and two di-2-pyridylamine (dpya) molecules. A second nitrate group acts as a counter-ion. The complex cations and the nitrate anions are also linked by N,H...O hydrogen bonds. The compound was prepared in two different reproducible ways: direct synthesis from Ni(NO3)2 and dpya yielded systematically twinned crystals (the twinning law is discussed), while single crystals were obtained unexpectedly from the Ni(NO3)2/dpya/maleic acid/NaOH system. [source] Effect of residual vanadyl ions on the spectroscopic analysis of humic acids: a multivariate approachEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 3 2008E. H. Novotny Summary In a study of the vanadyl (VO2+)-humic acids system, the residual vanadyl ion suppressed fluorescence and specific electron paramagnetic resonance (EPR) and NMR signals. In the case of NMR, the proton rotating frame relaxation times (T1,H) indicate that this suppression is due to an inefficient H-C cross polarization, which is a consequence of a shortening of T1,H. Principal components analysis (PCA) facilitated the isolation of the effect of the VO2+ ion and indicated that the organic free radical signal was due to at least two paramagnetic centres and that the VO2+ ion preferentially suppressed the species whose electronic density is delocalized over O atoms (greater g -factor). Additionally, the newly obtained variables (principal components , PC) indicated that, as the result of the more intense tillage a relative increase occurred in the accumulation of: (i) recalcitrant structures; (ii) lignin and long-chain alkyl structures; and (iii) organic free radicals with smaller g -factors. [source] | ||||||||||||||||||||||||||||