Organometallic complexes for nonlinear optics: Part 31. Cubic hyperpolarizabilities of ferrocenyl-linked gold and ruthenium complexes

The complexes [Fe{η-C₅H₄---(E)---CH=CH---4-C₆H₄CCX}₂] [X=SiMe₃ (1), H (2), Au(PCy₃) (3), Au(PPh₃) (4), Au(PMe₃) (5), RuCl(dppm)₂ (7), RuCl(dppe)₂ (8)] and [Fe{η-C₅H₄---(E)---CH=CH---4-C₆H₄CH=CRuCl(dppm)₂}₂](PF₆)₂ (6) have been prepared and the identities of 1 and 7 confirmed by single-crystal X-ray structural studies. Complexes 1–8 exhibit reversible oxidation waves in their cyclic voltammograms attributed to the Fe^II/III couple of the ferrocenyl groups, 6–8 also showing reversible (7, 8) or non-reversible (6) processes attributed to Ru-centered oxidation. Cubic nonlinearities at 800 nm by the Z-scan method are low for 1–5; in contrast, complexes 6 and 7 exhibit large negative γ_real and large γ_imag values. A factor of 4 difference in γ and two-photon absorption cross-section σ₂ values for 6 and 7 suggest that they have potential as protically switchable NLO materials.     

Application of Response Surface Methodology for Optimization of Cadmium Ion Removal from an Aqueous Solution by Eggshell Powder

The removal of cadmium(Cd) from synthetic solutions by batch adsorption process was performed using eggshell powder, which is mainly composed of calcite(CaCO₃). In order to optimize the adsorption process, a response surface methodology(RSM) based on Central Composite Design(CCD) was applied. Developed model for Cd remo-val yields(R,%) response was statistically validated by variance analysis(ANOVA) which showed a high determination coefficient value(R²=0.9889). According to Minitab software, the optimal conditions were found at temperature of 44℃, eggshell adsorbent dose of 2.98 g, initial Cd concentration of 36.74 mg/L and initial pH of 7. Under these conditions, the Cd removal yield was 98.76%. The deviation value of 1.24% confirms the validity of the model for the adsorption process optimization. The adsorption isotherm has been described by a Freundlich model. In addition, the predominant sorption mechanisms are the chemisorptions or precipitation(non-reversible) and ion exchange(reversible).     

Monitoring the structural alterations induced in β-lactoglobulin during ultrafiltration: learning from chemical and thermal denaturation phenomena

This work aims to identify of non-reversible structural changes induced in β-lactoglobulin by permeation through porous ultrafiltration membranes. The evaluation of these structural changes is performed using a fluorescence methodology, which combines the use of three different, complementary, fluorescence techniques: steady-state fluorescence, picosecond time-resolved fluorescence and steady-state fluorescence anisotropy. The identification of the nature of the structural changes induced upon permeation is possible through comparison of the fluorescence responses obtained for β-lactoglobulin solutions collected after permeation (permeates and retentates) with those induced by chemical (addition of Guanidine hydrochloride, GndHCl) and thermal denaturation of β-lactoglobulin.

The fluorescence approach used allowed to identify irreversible losses of structural integrity of β-lactoglobulin in the permeates, while β-lactoglobulin retentates seemed to be unaffected by the ultrafiltration process.

The mechanisms that regulate the structural alterations of β-lactoglobulin and the magnitude of these alterations depend on the protein to membrane pore size ratio, λ, being more substantial at higher λ (severe pore constriction). Under these conditions (permeation with a 10 kDa membrane) the structural changes induced in the proteins are dictated by the high shear stress at the membrane pore walls. The increase of the membrane cut-off (30 kDa membrane) induces a decrease in the magnitude of the shear stress and the effect of protein–membrane chemical interactions becomes noticeable.     

The complexity of catalysis: origins of enantio- and diastereocontrol in sulfur ylide mediated epoxidation reactions

The reaction of chiral sulfur ylides with aldehydes and ketones has emerged as a useful asymmetric process for the synthesis of epoxides. Processes employing either catalytic or stoichiometric amounts of sulfides have been developed. Although a large number of chiral sulfur ylides have been tested in the epoxidation process, only a few have delivered high diastereo- and enantio- selectivity. This review examines the factors that influence stereocontrol (steric hindrance of the sulfide, ylide conformation, ylide face selectivity, reversibility of betaine formation, solvent, and metal salts). This analysis leads to the conclusion that high reversibility in betaine formation leads to high diastereoselectivity but low enantioselectivity, and non-reversible betaine formation leads to low diastereoselectivity and high enantioselectivity (provided that other criteria are met). To achieve both high diastereoselectivity and high enantioselectivity simultaneously, requires non-reversible formation of the anti-betaine and reversible formation of the syn-betaine. Thus, factors that influence the degree of reversibility in betaine formation are critically important since with subtle changes in reaction conditions (solvent, temperature, metal ions) both high enantio- and diastereoselectivity can often be achieved.     

Reagent controlled addition of chiral sulfur ylides to chiral aldehydes

The degree of reagent and substrate control in the reaction of chiral sulfur ylides with chiral aldehydes has been investigated. Specifically, the reactions of the two enantiomers of the chiral benzyl sulfonium salt 1 with glyceraldehyde acetonide were studied in detail. Of the two new stereogenic centers created, it was found that the C1 stereochemistry was largely controlled by the reagent, whereas control at the C2 center was dependent on the aldehyde used. In one case, the trans isomer was produced via reversible formation of the intermediate betaine, whereas in the alternative case, the C2 center was under Felkin Anh/Cornforth control through non-reversible formation of the betaine. Thus, the aldehyde stereocenter influenced the degree of reversibility in betaine formation, which impacted on the stereocontrol at the C2 position.     

Pyrene fluorescence quenching and triplet-state formation in the presence of DABCO (1,4-Diazabicyclo[2-2-2]octane): A laser photolysis study

In this paper, we present a study of fluorescence quenching of pyrene by DABCO in cyclohexane solutions. The absorption spectra of pyrene singlet state ¹S-ⁿS have been measured in the presence and in the absence of DABCO; the spectra are practically identical. The de-excitation of singlet pyrene by DABCO seems to be a non-reversible process leading to the triplet state, the probability of formation of this triplet state being near unity in the quenching process.     

Determination of carbon monoxide in ambient air using piezoelectric crystal sorption detection

A new piezoelectric quartz crystal (PQC) sorption detector was developed to monitor carbon monoxide (CO) at sub-ppm level in ambient air. Out of the 28 coating materials studied, the palladium(II) acetamide complex with a 1:10 mole ratio of Pd(II) to acetamide was found to be the best. The detection is based on a non-reversible gas/coating interaction with sensitivity depending on gas flowrate. For 5-15 min exposure at a flowrate of 50 ml/min, the working ranges were found to vary from 0.7 to 40 ppm (total exposure from 8 to 160 μg CO) and detection limits (S/N=2) from 0.7 to 2 ppm CO (total exposure to 8 μg CO). The repeatability at 10 ppm CO was 11.8% (R.S.D. for n=3). The sensor lifetime was found dependent on exposure up to 160 μg CO or not exceeding 1000 Hz accumulative shift of frequency to avoid saturation of active sites at the crystal surface. No interference to CO detection was found for H₂, H₂S, SO₂, NO₂, CO₂, HCHO, gasoline and water vapors at concentrations much higher than ambient air. Compared to existing CO monitor, the PQC detector developed has advantages of adequate selectivity, high sensitivity, fast response and a much lower detection limit for detecting CO at sub-ppm levels. However, it is limited by the total exposure to a maximum of 160 μg CO that restricts its application to intermittent monitoring of low CO concentration. The present work has demonstrated the advantages of using strong non-reversible interaction to enhance PQC sensitivity, as the total exposure can be adjusted easily by a suitable control of the gas flowrate.     

Polarographic Study of Lead-Tropolone Complexation

Abstract

A.C., D. C. and derivative pulse polarography of lead-tropolone complexes has been studied. It has been shown that the application of usual treatments for complexation in d. c. polarography are invalid in this system, where the irreversible wave in presence of the ligand is a composite step. It seems generally advisable to apply derivative pulse or a.c. polarography before using non-reversible d.c. steps for evaluation of K and p.     

Base-catalyzed redox reactions of α-hydroxyalkylazaaromatic N-oxides

The (α-hydroxybenzyl)pyridine 1-oxides on heating with aqueous sodium hydroxide yielded the corresponding benzoylpyridines in redox reactions. When the phenyl groups of the foregoing compounds was replaced by hydrogen or methyl the redox reactions occurred less readily. A kinetic isotope effect of 3 was found for 4-(α-deuterio-α-hydroxybenzyl)pyridine 1-oxide on reaction with sodium hydroxide. The removal of the α-deuterio group is non-reversible. A mechanism consistent with these observations is given. 6-Acetoxyphenanthridine 5-oxide yielded with concentrated hydrochloric acid a redox product, 6-formylphenanthridine hydrochloride, whereas under the same conditions neither (α-hydroxybenzyl)pyridine 1-oxides nor 2-hydroxy-methylquinoline 1-oxide yielded a redox product.     

Modulated DSC as a tool for polyethylene structure characterization

The Modulated Differential Scanning Calorimeter (MDSC) technique, using TA Q1000 instrument, has been applied as a tool to study the reversible and non-reversible heat flow characteristics of a wide range of polyethylenes. It was found that the heat flow characteristic is dependent upon the heating rates and modulation period used in the test. By using a set of standard test conditions, MDSC was found to be useful in studying the effect of previous thermal processing conditions, additive effects, and also the density, MI, type of comonomer, and molecular architecture.     

Simulation of temperature-modulated DSC of transitions represented by abrupt changes in heat capacity

Computer simulations have been applied to elucidate the response of a sample to temperaturemodulated differential scanning calorimetry (tm-DSC) during transitions. Two cases have been simulated; a latent heat without supercooling (represented by an abrupt heat capacity pulse with perfect reversibility) and a latent heat with perfect supercooling or large hysteresis (an abrupt heat capacity change without reversibility, i.e. the change in heat capacity is seen on heating, but not on cooling). Because the simulation was applied to these well-characterized phenomena, the results are useful to reveal actual sample thermal responses during transitions. The non-reversible component was observed in both cases and has no distinct difference. Higher harmonics due to non-linearity of the transitions were also observed. Furthermore, by inspecting thermal response of the sample and the essential feature of tm-DSC, a new method of data analysis has been devised.     

Anodic dissolution of the mercury electrode by the CH3O− ions formed during the reduction of organic compounds in methanol

Anodic dissolution of the HMDE from CH₃O^? ions formed during the electrochemical reduction of organic compounds in methanol arises only if the electroreduction is non-reversible. When, however, the organic substances undergo a reversible reduction, the CH₃O^? ions formed during the reduction are neutralized by the protons released during reoxidation. The anodic dissolution of the mercury electrode is attributed to the formation of insoluble Hg₂(OCH₃)₂, as with many other ions which depolarize the mercury electrode in aqueous solution with the formation of insoluble salts.     

ON THE PHOTOCHEMISTRY OF THE DINUCLEOTIDE TpC*

Abstract— In an aqueous buffered solution of TpC monochromatic ultraviolet light lowers the absorption peak to approximately half its value, characterized by a photostationary state. The quantum yield for this photochemical decomposition is independent of the wavelength between 2400 and 3000 Å.At pH 7 the quantum yield amounts to 0.006, at pH 2 to 0.002, respectively. At neutral pH about 85 per cent of the photochemical transformation produced are reversible in the dark, at pH 2 only 40 per cent. Three photoproducts, being non-reversible in the dark, were isolated electrophoretically and characterized by their absorption spectra. These three compounds have an absorption peak around 2680 Å, one compound has a second absorption maximum at 3120 Å (pH 7).     

Photochemical Metal-Hydride Hydrogen Atom Transfer Mediated Radical Hydrofunctionalization of Dienes and Allenes

The metal-hydride hydrogen atom transfer (MHAT) method is widely recognized as a powerful technique for functionalizing alkenes. However, its application in the functionalization of industrial feedstock dienes and allenes is relatively rare due to challenges related to chemo-selectivity. In recent studies, Co-porphines have been identified as highly efficient catalysts under photoirradiation for MHAT, demonstrating an exceptional level of chemoselectivity for the functionalization of dienes and allenes, while disregarding other simple olefins present. This novel method enables the selective reductive coupling of pyridines with dienes and the allylation of aldehydes by the combination with Ti catalysis. Mechanistic studies and density functional theory (DFT) calculations support the idea that the non-reversible transfer of hydrogen atoms from cobalt hydride to dienes and allenes, leading to the generation of allyl radicals, is the key step in the catalytic cycle.     

Isothermal Crystallisation of PCL Studied by Temperature Modulated Dynamic Mechanical and TMDSC Analysis

Temperature modulated dynamic mechanical analysis (TMDMA) was performed in the same way as temperature modulated DSC (TMDSC) measurements. As in TMDSC TMDMA allows the investigation of reversible and non-reversible phenomena during crystallisation of polymers. The advantage of TMDMA compared to TMDSC is the high sensitivity for small and slow changes in crystallinity, e.g. during re-crystallisation. The combination of TMDMA and TMDSC yields new information about local processes at the surface of polymer crystallites. It is shown that during and after isothermal crystallisation the surface of the individual crystallites is in equilibrium with the surrounding melt. This revised version was published online in August 2006 with corrections to the Cover Date.     

Comparison of reversible melting behaviour of poly(3-hydroxybutyrate) using quasi-isothermal and other modulated temperature differential scanning calorimetry techniques

Melting behaviour of poly(3-hydroxybutyrate) (PHB) has been investigated by conventional DSC and each of several methods of modulated temperature differential scanning calorimetry (mT-DSC) such as heat-cool, iso-scan, step-scan and quasi-isothermal (QI). Thermal properties were investigated after fast and slow cooling crystallisation treatments. Multiple melting peak behaviour was observed for all methods except conventional melting with an average heating rate. Comparison of the mT-DSC data revealed that PHB underwent reversing melting including several reversible events and some non-reversible contributions under the modulation conditions. The main melting of PHB was irreversible, as were crystallisation and annealing, where the crystals can approach equilibrium. The various fusion enthalpy values were measured and they confirmed significant melt-recrystallisation of PHB with different melting conditions. Only the QI method revealed a true reversible contribution.     

Synthesis and SpectroscOpic Charcterization of Di-and Triphenyltin Derivatives of Dithiocarbomatic Acid

A series of triphenyltin dithiocarbamates Ph₃SnS₂CNR₂, where NR₂=NMe₂,-NEt₂, N(pr-n)₂,-N(CH₂CH₂OH)₂, and diphenyltin derivatives of dithiocarbamatic acid Ph₂Sn(S₂CNR₂)₂,where NR₂=NMe₂,-NEt₂,-N(Pr-n)₂,N(Bu-n)₂, N(CH₂CH₂OH),were synthesiced by the action of the correspending phenyltin (Ⅳ) chlorides with dithiocarbamates in dichloromethane. Their structure were characterized by elemental analysis, IR, HNMR and MS.     

高分辨NMR研究金属盐对水溶性铑膦配合物分子结构的影响

用高分辨NMR研究了NaCl、NiSO₄、CuSO₄、Fe₂(SO₄)₃和Cr₂(SO₄)₃对水溶性铑膦配合物HRh(CO)(TPPTS)₃[TPPTS;P(m-C₆H₄SO₃Na)₃]分子结构的影响.³¹P(¹H)和¹HNMR谱显示,于室温下在HRh(CO)(TPPTS)₃中加入的NaCl或NiSO₄对配合物的特征³¹P(¹H)和¹HNMR谱峰无明显影响;当加入CuSO₄后,配合物的Rh-H质子峰强度弱化明显,进而消失,且原配合物的特征磷谱峰强度减弱,新生成的磷物种谱峰逐渐成为磷谱的主要物种.当加入Fe₂(SO₄)₃或Cr₂(SO₄)₃后,三价金属离子的强顺磁性使NMR灵敏度下降,谱峰宽化,该2种盐均易与水溶性铑膦配合物产生强烈的相互作用,易使配合物特征谱峰消失.实验结果表明,上述金属盐对配合物结构破坏性大小的顺序为;Fe₂(SO₄)₃>Cr₂(SO₄)₃>CuSO₄》NiSO₄～NaCl.     

溶解有机质对光照自然水体生物膜体系中H2O2生成的影响

通过模拟实验研究了生物膜胞外聚合物(EPS)和乙二胺四乙酸(EDTA) 2种典型溶解有机质(DOM)成分对自然水体生物膜体系中过氧化氢(H₂O₂)生成特征的影响, 并研究了体系初始pH值、 DOM浓度、 溶解氧(DO)等因素的影响. 结果表明, DOM的存在对自然水体生物膜体系中H₂O₂的生成有明显影响. 光照能促使EPS产生H₂O₂, 而EPS的存在对生物膜产生H₂O₂的直接影响不显著, EPS与生物膜共存体系中的H₂O₂由二者共同产生; EDTA本身不产生H₂O₂, 且对H₂O₂分解影响很小, 但会显著抑制生物膜产生H₂O₂, 且浓度越高抑制作用越明显. 体系pH值、 DOM浓度和DO均能不同程度影响EPS产生H₂O₂及EDTA抑制生物膜产生H₂O₂的作用.     

Mg改性对Co/γ-Al2O3-TiO2催化燃烧丙烷性能影响

采用分步浸渍法制备了系列Mg改性的Co/γ-Al_2O_3-TiO_2催化剂,通过X射线衍射(XRD)、紫外可见漫反射光谱(DR-UV-vis)、N_2吸附-脱附(BET)、X射线光电子能谱(XPS)和H_2程序升温还原(H_2-TPR)等技术对催化剂进行表征,并考察了其对丙烷燃烧的催化性能。结果表明,Co在原始γ-Al_2O_3-TiO_2载体和Mg改性MgO/γ-Al_2O_3-TiO_2载体上均以Co_3O_4的形式存在;Mg掺入后与Al_2O_3作用形成MgAl_2O_4尖晶石,改善了载体的织构性质,提升了Co_3O_4在催化剂载体表面的暴露数量和分散程度。此外,MgAl_2O_4与Co_3O_4相互作用提升了Co_3O_4颗粒表面Co~(3+)/Co~(2+)和O_(ads)/O_(latt)的比例,并削弱了Co-O键键能,从而提升了其对丙烷的催化燃烧活性。当Mg负载量为15%(质量分数)时,在Co/MgO(15%)/γ-Al_2O_3-TiO_2催化剂上进行丙烷燃烧,丙烷90%转化率的温度比无Mg掺杂的Co/γ-Al_2O_3-TiO_2催化剂的降低了45℃,并且连续反应40 h其活性保持稳定。