Organized molecular systems as reaction media

Media are among the most important factors to be considered for organic synthesis in soft and non-polluting conditions satisfying the conditions for sustainable development. Organized Molecular Systems (OMS) are very useful when efforts are being made to apply the twelve green chemistry principles, more precisely to replace organic solvents, to carry out reactions in water, to employ catalysis and biocatalysis, to economize molecules (and, of course, of atoms) and to work with low-energy conditions. These OMS posses a number of advantages: solubilization of substances that are not normally soluble in the continuous phase of OMS, localization of reactants and products, and selective orientation and stabilization of the various entities in the various stages of the reaction. Rapid and selective reactions of preparative amounts of substrate can be carried out in such media, which are also very suitable for mechanistic studies. First, we performed organic photoreactions in microemulsions (macroscopically homogeneous and transparent media). Thus, we were able to confirm the interfacial localization of the processes by means of chemical internal sensors and infrared spectroscopy; to propose a formulation strategy for diminishing the number of substrates in the medium (molecular economy principle); and to use high interfacial concentration to carry out reactions in the liquid phase although they are generally only possible in the solid state. The most important scientific point was the demonstration of the generalization of the amphiphilicity concept, by using polar non-aqueous solvents. 1986 saw the end of a controversy concerning the use of formamide in place of water. With this type of solvent, we have been able to perform important reactions: the Wacker process, Diels-Alder reactions, and olefin amidations. Then we postulated the formation of aggregates without surfactants if differential solvations were operative. All organic reactions can be influenced by the spontaneous formation of aggregates. To finish, with such systems, it is possible to orientate the reactivity of competitive reactions (e.g. cyclization and polymerization) and help to protect the environment, for example in the synthesis of clean surfactants in clean conditions. With the extension of these observations and results to the use of rigid objects (similar to rigid micelles) we were able to obtain very high enantioselective excess in chiral processes.     

Surface-assisted laser desorption-ionization mass spectrometry of oligosaccharides using magnesium oxide nanoparticles as a matrix

We describe the use of magnesium oxide nanoparticles (MgO-NPs) as a new matrix for the analysis of oligosaccharides (maltoheptaose, palatinose, D-panose, N-acetyllactosamine and β-gentiobiose) by surface-assisted laser desorption-ionization time-of-flight mass spectrometry (SALDI-MS). The use of MgO-NPs results in mass spectra without background noise and no fragmented ions in the low-mass region. The MgO-NPs were characterized by FT-IR, scanning electron microcopy, transmission electron microscopy which reveal that the particles are well dispersed in solution. The system allows the detection of the analytes at <20 nM concentrations, with limits of detection of 4.0, 5.5, 6.0, 10.0 and 7.0 nM, respectively. The calibration plots are linear (R ² values between 0.9896 and 0.9985), and relative standard deviations are <10 %. We conclude that MgO-NPs-based SALDI-MS enables the efficient and sensitive analysis of oligosaccharides in the low-mass region.

Polypyridine block as molecular antenna in photoluminescence of macromolecular systems

Unique effects of π-conjugated polypyridine (PPy) blocks on fluorescence from the following macromolecular systems have been examined: (A) poly(2,2′-bipyridine-5,5′-diyl) (PBpy)-Ru(bpy)₂, (b) block-type copolymer of pyridine and selenophene, and (c) a system containing poly(6-hexylpyridine-2,5-diyl) (6HexPy) and poly(2-methylanthraquinone-1,4-diyl) (P2MeAQ). The PPy blocks in the systems serve as good molecular antennas and transfer the photoactivated energy to other molecular parts of the systems connected to the PPy blocks, thus causing emission of light from the other molecular parts.     

Reversible photoregulation of helical structures in short peptides under indoor lighting/dark conditions

[reaction: see text] A photochromic cross-linking agent with a spiropyran skeleton was developed for the reversible photoregulation of helical structures in short peptides. The helical contents of the cross-linked peptides could be regulated by ambient light and dark conditions at room temperature. This switching of the helical contents could be repeated several times without substantial loss of any activity.     

Surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) of low molecular weight organic compounds and synthetic polymers using zinc oxide (ZnO) nanoparticles

We have developed surface-assisted laser desorption/ionization mass spectrometry using zinc oxide (ZnO) nanoparticles with anisotropic shapes (ZnO-SALDI-MS). The mass spectra showed low background noises in the low m/z, i.e. less than 500 u region. Thus, we succeeded in SALDI ionization on low molecular weight organic compounds, such as verapamil hydrochloride, testosterone, and polypropylene glycol (PPG) (average molecular weight 400) without using a liquid matrix or buffers such as citric acids. In addition, we found that ZnO-SALDI has advantages in post-source decay (PSD) analysis and produced a simple mass spectrum for phospholipids. The ZnO-SALDI spectra for synthetic polymers of polyethylene glycol (PEG), polystyrene (PS) and polymethylmethacrylate (PMMA) showed the sensitivity and molecular weight distribution to be comparable to matrix-assisted laser desorption/ionization (MALDI) spectra with a 2,5-dihydroxybenzoic acid (DHB) matrix. ZnO-SALDI shows good performance for synthetic polymers as well as low molecular weight organic compounds.     

2',6'-Dimethylazobenzene as an efficient and thermo-stable photo-regulator for the photoregulation of DNA hybridization

The introduction of methyl groups into two ortho positions (2' and 6' positions) of the same benzene ring in an azobenzene remarkably raised both its photoregulation ability and the thermal stability of the cis-form.     

硅纳米结构对表面辅助激光解吸/电离质谱检测性能的提高

本文总结了多种构筑硅纳米结构的方法, 综述了近年来利用硅纳米结构提高表面辅助激光解吸/电离质谱(SALDI-MS)性能的研究工作, 展望了利用功能化的硅纳米结构表面进一步提高激光解吸/电离(LDI)效率的前景.     

Formation of molecular hydrogen in photocatalytic systems containing heteropolytungstates as electron carriers

It has been shown that the heteropolytungstates [P₂W₁₈O₆₂]^6– and [SiW₁₂O₄₀]^40– act as the carriers in electron transfer from the conduction band of photoexcited CdS to Pd/SiO₂ in the reaction of producing H₂ from H₂O. The quantum efficiencies of these reactions have been determined, as well as the variation of the efficiency with changes in the quantitative composition of the photocatalytic system.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 30, No. 4, pp. 231–235, July–August, 1994.     

A combined quantum chemical/molecular mechanical study of hydrogen-bonded systems

A computational approach, which involves the combination of the OPLS force field and molecular orbital MNDO , AM 1, and PM 3 methods, has been developed to describe the effects of a large, molecular mechanically simulated environment on the Hamiltonian of a quantum chemical system. To test the validity of the combined quantum mechanical/molecular mechanical (QM /MM ) potential, a systematic study of the structures and energies of neutral and charged hydrogen-bonded complexes has been carried out, including comparisons with pure semiempirical calculations and available experimental and ab initio data. It is shown that, in many cases, the hybrid QM /MM potential behaves better than do related MNDO /M , AM 1, and PM 3 methods. As a case in point, the draw-back of AM 1 favoring bifurcated H-bonded structures over single ones is not presented in the combined AM 1/OPLS scheme. Possible ways of improvement of the combined QM /MM potential are discussed. © 1992 John Wiley & Sons, Inc.     

Symmetry-adapted-cluster/symmetry-adapted-cluster configuration interaction methodology extended to giant molecular systems: ring molecular crystals

The symmetry adapted cluster (SAC)/symmetry adapted cluster configuration interaction (SAC-CI) methodology for the ground, excited, ionized, and electron-attached states of molecules was extended to giant molecular systems. The size extensivity of energy and the size intensivity of excitation energy are very important for doing quantitative chemical studies of giant molecular systems and are designed to be satisfied in the present giant SAC/SAC-CI method. The first extension was made to giant molecular crystals composed of the same molecular species. The reference wave function was defined by introducing monomer-localized canonical molecular orbitals (ml-CMO's), which were obtained from the Hartree-Fock orbitals of a tetramer or a larger oligomer within the electrostatic field of the other part of the crystal. In the SAC/SAC-CI calculations, all the necessary integrals were obtained after the integral transformation with the ml-CMO's of the neighboring dimer. Only singles and doubles excitations within each neighboring dimer were considered as linked operators, and perturbation selection was done to choose only important operators. Almost all the important unlinked terms generated from the selected linked operators were included: the unlinked terms are important for keeping size extensivity and size intensivity. Some test calculations were carried out for the ring crystals of up to 10 000-mer, confirming the size extensivity and size intensivity of the calculated results and the efficiency of the giant method in comparison with the standard method available in GAUSSIAN 03. Then, the method was applied to the ring crystals of ethylene and water 50-mers, and formaldehyde 50-, 100-, and 500-mers. The potential energy curves of the ground state and the polarization and electron-transfer-type excited states were calculated for the intermonomer distances of 2.8-100 A. Several interesting behaviors were reported, showing the potentiality of the present giant SAC/SAC-CI method for molecular engineering.     

Cndo/2 conformational calculation for conjugated and non-conjugated molecular systems

In order to check the effectiveness of the well known CNDO/2 method for predicting molecular conformations, twenty-six molecular systems, involving B, C, N, O, F, Si, S and Cl atoms, were studied. If these molecules are classified into three groups, according to simple rules, the CNDO/2 method is found to fail systematically in one of them. In this group the twisted bond is expected to be delocalized. It is concluded that the CNDO/2 method is not useful in predicting conformations of molecules in which the twisted bond is delocalized.     

Solvation of xyloglucan in water/alcohol systems by molecular dynamics simulation

Xyloglucan in water solution turns into a gel with addition of alcohol such as methanol and ethanol. In regard to this phenomenon, we investigated the adhesive property of alcohol to xyloglucan and proposed the mechanism of the gelation by molecular dynamics (MD) simulation of a xyloglucan in water, water/methanol, and water/ethanol solution for 10 ns. The alcohol molecules showed its adhesive property to the xyloglucan and made the swelling-shrinking motion of the xyloglucan slow. Alcohol molecules solvated to the xyloglucan mainly in hydrophobic way so as to fill the void of water hydration shell, resulting in reformation of the hydrogen-bond network of water molecules around the solute. We also found that alcohol molecules have strong tendency to hydrogen-bond on xylose O3 in xyloglucan. According to these results, we proposed the gelation mechanism of xyloglucan in water/alcohol solution.     

Use of Pd/Sepiolite systems as hydrogenation catalysts I. Hydrogenolysis of N-blocked amino acids and dipeptides with molecular hydrogen

In this work we have assessed the use of sepiolites from Vallecas (Madrid, Spain) as supports of Pd catalysts applicable to the liquid-phase unblocking of amino acids and dipeptides protected by a benzyloxycarbonyl group (Z) with gaseous hydrogen. The sepiolites used have been compared with other supports employed by our team. We have also discussed the influence of the type of amino acid or dipeptide, the catalytic support and the precursor Pd salt on the unblocking rate.

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(, ) Pd , , (Z), . , . , Pd .

     

Surface-assisted laser desorption/ionization-mass spectrometry using TiO2-coated steel targets for the analysis of small molecules

Matrix-assisted laser desorption/ionization-mass spectrometry (MALDI–MS) measurements in the low-molecular-mass region, ranging from 0 to 1000 Daltons are very often difficult to perform because of signal interferences originating from matrix ions. In order to overcome this problem, a stainless steel target was coated with a homogeneous titanium dioxide layer. The layer obtained was further investigated for its ability to desorb small molecules, e.g., amino acids, sugars, poly(ethylene glycol) (PEG) 200, or extracts from Cynara scolymus leaves. The stability of the layer was determined by repeated measurements on the same target location, which was monitored by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) before and after surface-assisted laser desorption/ionization (SALDI) analysis. In addition, this titanium dioxide layer was compared with an already published method with titanium dioxide nanopowder as inorganic matrix. As a result of this work, the titanium dioxide layer produced minimal background interference, enabling simple interpretation of the detected mass spectra. Furthermore, the TiO₂ coating provides a target that can be reused many times for SALDI–MS measurements.     

High effectiveness of oligothienylenevinylene as molecular wires in Zn-porphyrin and C60 connected systems

Photoinduced energy transfer and electron transfer processes have been found between the excited singlet state of Zn-porphyrin and C(60) via an oligothienylenevinylene bridge depending on the length of the oligothiophene and solvent polarity.     

Cationic-modified cyclodextrin nanosphere/anionic polymer as flocculation/sorption systems

Simultaneous removal of dissolved and colloidal substances has been a challenging task. The cationic-modified beta-cyclodextrin nanospheres synthesized in this work, in conjunction with a water-soluble polyacrylamide-based anionic polymer, potentially provide a novel approach to address the problem. The cyclodextrin was rendered cationic using (2,3-epoxypropyl)trimethylammonium chloride as a reagent. The cationicity of the modified cyclodextrin and the reaction between cyclodextrin and the reagent were characterized by electrophoresis measurement, polyelectrolyte titration, and NMR. As a dual-component flocculation system, the cationic cyclodextrin/anionic polymer significantly induced clay flocculation, lowering the relative turbidity of the clay suspension over a wide pH range. Meanwhile, as a nanospherical absorbent, the modified cyclodextrins exhibited strong affinity toward aromatic compounds via inclusion complex formation in the hydrophobic cavities, which was monitored by UV spectroscopy. These systems facilitated the simultaneous removal of dissolved and colloidal substances, which was unachievable previously. In addition, the interaction between anionic polymers and the clay particles pretreated with cationic cyclodextrin was investigated in order to reveal the flocculation mechanism.     

Dipole moments of symmetrical molecular systems

It is shown that quantization of nuclear motion causes the intrinsic dipole moment of a molecular system to depart from the classical representation, e.g., it is different from zero for symmetrical molecules. A formula is derived for the mean dipole moment ¯p as a function of temperature with allowance for the internal motion of the nuclei, which is functionally related to the dipole moment. Calculations are performed for ammonia with allowance for the inversion splitting, which is due to tunneling between two equivalent equilibrium configurations having their dipole moments in opposite directions. The temperature coefficient of ¯p may be positive or negative, in accordance with the relation between the tunneling frequency and the temperature; the formula usually employed is valid only in the limiting case of low frequencies and high temperatures. A deduction is given for the criterion for instability of the maximally symmetrical configuration with respect to odd nuclear displacements (dipole distortions); this is based on a simple model system having an inversion center, a totally symmetric ground state, and a triply degenerate odd excited state of the T_1u type. The experimental consequences of the results are discussed, as well as the concept of symmetry for a molecular system in which the maximally symmetrical configuration is unstable.     

Linear pi-conjugated systems derivatized with C60-fullerene as molecular heterojunctions for organic photovoltaics

This tutorial review covers recent contributions in the area of linear pi-conjugated systems bound to fullerenes in view of their application as active materials in photovoltaic devices. The first part discusses the concepts of double-cable polymer and molecular hetero-junction and presents several examples of chemically or electrochemically synthesized C60-derivatized conjugated polymers. The second and main part of the article concerns the various classes of C60-derivatized pi-conjugated oligomers designed in view of their utilization in single-component photovoltaic devices. Thus, C60-containing pi-conjugated systems such as oligoarylenevinylenes, oligoaryleneethynylenes and oligothiophenes are discussed on the basis of the relationships between molecular structure, photophysical properties and performances of the derived photovoltaic devices. A brief last section presents some recent examples of surface-attached molecular hetero-junctions based on self-assembled monolayers and discusses possible routes for future research.