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991.
Energized molecules are the essential actors in chemical transformations in solution. As the rearrangement of bonds requires a movement of nuclei, vibrational energy is often the driving force for a reaction. Vibrational energy can be redistributed within the "hot" molecule, or relaxation can occur when molecules interact. Both processes govern the rates, pathways, and quantum yields of chemical transformations in solution. Unfortunately, energy transfer and the breaking, formation, and rearrangement of bonds take place on ultrafast timescales. This Review highlights experimental approaches for the direct, ultrafast measurement of photoinduced femtochemistry and energy flow in solution. In the first part of this Review, we summarize recent experiments on intra- and intermolecular energy transfer. The second part discusses photoinduced decomposition of large organic peroxides, which are used as initiators in free radical polymerization. The mechanisms and timescales of their decarboxylation determine the initial steps of polymerization and the microstructure of the polymer product. 相似文献
992.
Othman A. Farghaly 《Microchemical Journal》2003,75(2):119-131
Tap water samples (Assiut city, lie in the middle north of upper Egypt, approx. 370 km from Cairo, January-March, 2002) were taken from the eight sampling sites of different locations at Assiut city. The samples are analyzed to determine the total content of cadmium, copper, lead and zinc by differential pulse anodic stripping voltammetry (DPASV) while nickel and cobalt are determined by a new simple differential pulse adsorptive stripping voltammetry (DPAdSV), using dimethylglyoxime (DMG) as the complexing agent. This method uses sodium sulfite as the supporting electrolyte, which facilitates the removal of oxygen interference without the traditional necessity of purging with inert gas. The effect of various parameters was studied using DPASV (for Cd, Pb, Cu and Zn) and AdSV (for Ni and Co) methods. Subsequently, under the so found experimental conditions, the stability of calibration curves and the detection limits (μg/l) have been determined. The data achieved (for all metals utility) are comparable to those measured by the graphite furnace atomic absorption spectrophotometric (GF-AAS) method. The effects of the interferences between these metal ions have been investigated. Moreover, the effect of storage was discussed and the obtained results were compared favorably with standard official methods. Statistical analysis of the database exhibits applicability and the accuracy of the techniques. The results obtained from the two techniques (Voltammetry and GF-AAS) are in very good agreements in the most tap water samples. 相似文献
993.
994.
Gerhard Buchbauer Waltraud Pernold Dorothea Rassl Brigitte Blach 《Monatshefte für Chemie / Chemical Monthly》1981,112(4):517-527
The synthesis of some new potentially, nitrogenefree spasmolytics is described. Formally the benzene nucleus in some mandelic acid esters, known by their mild spasmolytic activity, is substituted for the isocamphane bicyclus by a) SeO2 oxidation of 1-(3,3-Dimethyl-2-exo-norbornyl)-ethanone (5) to the corresponding ketoaldehyde6, b) oxidation of6 to the isocamphane analogous mandelic acid4 and c) esterification of derivatives of4 with isoamylalcohol and benzylalcohol. Attemps to dehydrate the methylester7 to camphenylideneacetic acid methylester (12) by various methods failed, probably because of the ring strain.
Teil der Diplomarbeit vonB. Blach, Universität Wien 1981. 相似文献
995.
T. Kimura Y. Takahashi T. Kamiyama M. Fujisawa 《Journal of Thermal Analysis and Calorimetry》2007,88(2):587-595
Excess enthalpies (H
E)
of 17 binary mixtures of o- and m-isomers of dichlorobenzene, difluorobenzene, methoxymethylbenzene,
dimethylbenzene, dimethoxybenzene, aminofluorobenzene, fluoronitrobenzene,
diethylbenzene, chlorofluorobenzene, fluoroiodobenzene, bromofluorobenzene,
chloromethylbenzene, fluoromethylbenzene, bromomethylbenzene, iodomethylbenzene,
fluoromethoxybenzene, dibromobenzene at 298.15 K were measured. All excess
enthalpies measured were very small, and those of o-+m-isomers of aminofluorobenzene, dibromobenzene and
iodomethylbenzene were negative but 14 other binary mixtures of isomers were
positive over the whole range of mole fractions. H
E
of o-+m-isomers
of dimethoxybenzene showed the largest enthalpic instability and those of
aminofluorobenzene showed the largest enthalpic stability. There was a correlation
between dipole–dipole interaction, dipole–induced dipole interaction
or entropies of vaporization and excess partial molar enthalpies at infinite
dilution. 相似文献
996.
Celestino Angeli Stefano Borini Renzo Cimiraglia 《Theoretical chemistry accounts》2004,111(2-6):352-357
n–electron valence state perturbation theory (NEVPT) is a form of multireference perturbation theory where all the zero-order wave functions are of multireference nature, being generated as eigenfunctions of a two–electron model Hamiltonian. The absence of intruder states makes NEVPT an interesting choice for the calculation of electronically excited states. Test calculations have been performed on several valence and Rydberg transitions for the formaldehyde and acetone molecules; the results are in good accordance with the best calculations and with the existing experimental data.Contribution to the Jacopo Tomasi Honorary Issue 相似文献
997.
The title compound glycin·3,5-dihydroxybenzoic acid (GDB) has been synthesized and characterized by single-crystal X-ray analysis. It crystallizes in orthorhombic, space group Pca21 with a = 25.513(10), b = 4.668(2), c = 9.150(4)(A), Z = 4, V = 1089.8(8)(A)3, C9H13NO7, Mr = 247.20, Dc = 1.507 g/m3, μ(MoKα) = 0.132 mm-1, F(000) = 520, R1 = 0.0322 and wR2 = 0.0611 for 885 observed reflections (I > 2σ(I)). The title compound is a 1:1 molecular complex of glycin and 3,5-dihydroxybenzoic acid, which is linked by O-H…O and N-H…O hydrogen bonds to form a three-dimensional framework. Test of SHG (second harmonic generation) indicates that NLO generation of the crystal is higher than that of KDP. 相似文献
998.
Nikhil M. Kriplani David P. Nackashi Christian J. Amsinck Neil H. Di Spigna Michael B. Steer Paul D. Franzon Ramon L. Rick Gemma C. Solomon Jeffrey R. Reimers 《Chemical physics》2006
Two efficient, physically based models for the real-time simulation of molecular device characteristics of single molecules are developed. These models assume that through-molecule tunnelling creates a steady-state Lorentzian distribution of excess electron density on the molecule and provides for smooth transitions for the electronic degrees of freedom between the tunnelling, molecular-excitation, and charge-hopping transport regimes. They are implemented in the fREEDA™ transient circuit simulator to allow for the full integration of nanoscopic molecular devices in standard packages that simulate entire devices including CMOS circuitry. Methods are presented to estimate the parameters used in the models via either direct experimental measurement or density-functional calculations. The models require 6–8 orders of magnitude less computer time than do full a priori simulations of the properties of molecular components. Consequently, molecular components can be efficiently implemented in circuit simulators. The molecular-component models are tested by comparison with experimental results reported for 1,4-benzenedithiol. 相似文献
999.
The structure of the double electric layer (DEL) on a liquid dropping Sn-Ga electrode containing 8 at. % of Sn is studied in acetonitrile (AN) solutions of electrolytes. It is shown that the transition from aqueous to AN solutions of electrolytes makes it possible to expand the region of investigation of parameters of DEL on an Sn-Ga electrode from negative charges to a zero charge and to small positive charges. As follows from the obtained data, throughout the entire interval of charges studied, an excess, as compared with an Hg electrode, chemisorption interaction (Sn-Ga)-AN is absent. It is established that the distance of the closest approach of AN dipoles to an ionic core increases upon going from Hg to Sn-Ga to Ga. The Ga, Bi-Ga, and Sn-Ga electrodes, which possess close values of the “electrochemical work function,” are used as an example to show that the metal-solvent chemisorption interaction increases with decreasing distance of the closest approach of its dipoles to the ionic core of the metal. The effect of this factor becomes stronger as the donor number of the solvent increases. 相似文献
1000.
Godehard Sutmann 《Journal of Electroanalytical Chemistry》1998,450(2):1946
We have performed molecular dynamics computer simulations of water in homogeneous external electric fields which were varied in a wide range of field strengths. The dielectric response is found to be linear up to fields E0≈0.01 V/Å from where dielectric saturation effects become important. At fields of E0≈3 V/Å a phase transition into an ordered, ice-like structure is observed, which is stabilized through hydrogen-bonds. With an increasing external electric field, the frequency spectrum of the water dynamics shows a remarkable red shift of the intramolecular modes and a blue shift of the librational motions, where the frequency varies quadratically with the field strength. A simple analytical model is discussed which reproduces the observed behavior. 相似文献