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Resonance Effects (resonance + effects)
Selected AbstractsAre the Hirshfeld and Mulliken population analysis schemes consistent with chemical intuition?INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 9 2009Soumen Saha Abstract In the present article, we report a comparative study between the Hirshfeld and Mulliken population analysis schemes (abbreviated as HPA and MPA, respectively). Trends of atomic charges derived from these two population analysis schemes are compared with those expected from other commonly used chemical concepts like electronegativity, inductive effects, and resonance effects. Although previous studies on intramolecular reactivity sequences demonstrated that HPA generates reliable and non-negative (and thus physically more realistic) condensed Fukui function (FF) values, the present study reveals problems with the HPA charge partitioning technique. Specifically, HPA fails to reproduce reliable intermolecular and intramolecular charge trends in several systems. Reasons for the success and failure of HPA are discussed and a method for improving the Hirshfeld charge partitioning is proposed. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009 [source] Analysis and characterization of package resonanceINTERNATIONAL JOURNAL OF RF AND MICROWAVE COMPUTER-AIDED ENGINEERING, Issue 1 2005El-Badawy El-Sharawy Abstract This article presents FDTD analysis and prediction of the overall effects of the resonance in ceramic and plastic packages. Practical structures that result in strong resonance effects are investigated, such as coupled cross lines with a via transition and multiple via transitions on a single line. A detailed study of the mutual coupling, including resonance effects, is also performed and presented. The results of the analysis indicate that the package performance can be very sensitive to the resonance effects. A broadband equivalent-circuit model for multiple transition discontinuities (two via holes) on a single line has been developed and presented. Good agreement between the frequency response of the FDTD and the circuit models has been achieved over a broad band of frequencies. The analysis presented in this article provides insight about the limits at which coupling effects on the overall package performance can be neglected. The equivalent-circuit model presented in this article can greatly simplify the analysis and simulation of a complex package, including several transition discontinuities, assuming that the coupling is small. © 2004 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2005. [source] A theoretical density functional study of association of Zn2+ with oxazolidine and its thio derivatives in the gas phaseJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8 2010Zaki S. Safi Abstract We have performed density functional theory (DFT) calculations in order to study the gas-phase interaction of oxo- and thio-oxazolidine derivatives with Zn2+. The calculations were performed at B3LYP/6-311+(2df,2p) level of theory. It has been found, in all cases, that the direct association of Zn2+ with the carbonyl and thiocarbonyl groups takes place at the heteroatom attached to position 2 irrespective of its nature. This preference has been attributed to the resonance effects caused by the nearest heteroatoms (oxygen and nitrogen). The most stable complexes correspond to structures with Zn2+ bridging between the heteroatom at position 2 or 4 of the 4- or 2-enol (or the 4- or 2-enethiol) tautomer and the dehydrogenated ring nitrogen atom, N3. Zn2+ association has a clear catalytic effect on the tautomerization processes which connect the oxo,thione forms with the enol,enethiol tautomers. Hence, although the enol,enethiol tautomers of oxazolidine and its thio derivatives should not be observed in the gas phase, the corresponding Zn2+ complexes are the most stable species and should be accessible, because the tautomerization barriers are smaller than the Zn2+ binding energies. Copyright © 2010 John Wiley & Sons, Ltd. [source] A paradigm for the mechanisms and products of spontaneous polymerizationsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 24 2009H. K. Hall Jr. Abstract In spontaneous vinyl and ring-opening copolymerizations, polar and resonance effects on the intermediates from bond-forming initiation offer a continuous spectrum of reactivities and polymer structures. In bond-forming initiation, an electron-rich donor monomer forms a bond to an acceptor monomer. The donor monomer may be a vinyl monomer with O, N, or aryl substituent or it may be an aza- or oxacycle. The acceptor monomer may be a vinyl monomer carrying CN, COOR, or SO2R substituent or it may be a cyclic anhydride or maleimide. Beyond this, the donor may have a ,-like strained single bond, whereas the acceptor may be an electrophilic quinodimethane. Lewis acids may be used to enhance the electrophilicity of acceptor monomers. Reaction rates and polymer composition are determined by systematically varying the stability of the first intermediate, designated P (for polymethylene). The nature of the intermediate will vary from a highly reactive trans biradical, which initiates chain alternating copolymerization, to a cis/gauche zwitterion, which can initiate chain ionic homopolymerization, to an extremely stabilized zwitterion, which cannot add monomer, but builds up in concentration and terminates by combination, forming alternating copolymer. This model embraces the existing literature for a wide variety of monomers and possesses predictive power. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2009 [source] ,Green earths': vibrational and elemental characterization of glauconites, celadonites and historical pigmentsJOURNAL OF RAMAN SPECTROSCOPY, Issue 8 2008Francesca Ospitali Abstract ,Green earths' are employed since antiquity as pigments in the creation of artworks. The minerals responsible for the colour belong to four groups: (1) the clayey micas celadonite and glauconite, undoubtedly the most common; (2) smectites; (3) chlorites; (4) serpentines. Whereas there have been several studies on clayey materials, mineralogical analyses in the field of cultural heritage are mainly limited to the identification of the green earth without specific characterization of the mineralogical species. This work shows a preliminary characterization by the multi-techniques approach of some raw minerals (glauconite, celadonite and ferroceladonite). Vibrational analyses have been correlated with elemental analyses, thanks to the hyphenated instrumentation of scanning electron microscopy with EDS and Raman structural and chemical analyser (SEM-EDS-SCA) probes, which permitted collection of EDS and Raman spectra on the same microscopic area. Micro-Raman and Fourier transform infrared attenuated total reflectance (FTIR-ATR) spectroscopies were able to distinguish between celadonite and glauconite. The use of different lasers revealed resonance effects in the Raman spectra. In addition to pure minerals, archaeological samples and commercial green earths were also analysed, thereby enabling a more precise classification of the green pigments in heterogeneous samples such as wall paintings. Some commercially available green earths were found to contain organic dyes. Copyright © 2008 John Wiley & Sons, Ltd. [source] Effects of parasitic resonance on a vertical transition in multilayer printed circuit boardsMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 9 2010Sung Tae Choi Abstract This article reports on parasitic resonance effects of a ground plane with/without a ground opening underneath a signal via with an open stub on the performance of a vertical transition in a multilayer printed circuit board. An equivalent circuit model that accounts for the parasitic resonance due to the open stub inductance and the parasitic capacitance is developed and verified with the measured and simulated results. The fabricated transition exhibits a return loss better than about 10 dB and an insertion loss less than 1.9 dB up to 21.5 GHz. A good agreement is observed between measured and simulated results. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 2098,2100, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25404 [source] Estimation of Carboxylic Acid Ester Hydrolysis Rate ConstantsMOLECULAR INFORMATICS, Issue 9-10 2003H. Hilal Abstract SPARC chemical reactivity models were extended to calculate hydrolysis rate constants for carboxylic acid esters from molecular structure. The energy differences between the initial state and the transition state for a molecule of interest are factored into internal and external mechanistic perturbation components. The internal perturbations quantify the interactions of the appended perturber (P) with the reaction center (C). These internal perturbations are factored into SPARC's mechanistic components of electrostatic and resonance effects. External perturbations quantify the solute-solvent interactions and are factored into H-bonding, field stabilization and steric effects. These models have been tested using 1471 measured hydrolysis rate constants in water and mixed-solvent systems at different temperatures. The aggregate RMS deviation of the calculated versus observed values was 0.374,M,1s,1; close to the intralaboratory experimental error. [source] QSAR of Progestogens: Use of a Priori and Computed Molecular Descriptors and Molecular GraphicsMOLECULAR INFORMATICS, Issue 4 2003Rudolf Kiralj Abstract Quantitative Structure-Activity Relationship (QSAR) study of two sets of oral progestogens was carried out by using Principal Component Analysis (PCA), Hierarchical Cluster Analysis (HCA) and Partial Least Squares (PLS). A priori, computed (at DFT 6-31G** level) and molecular graphics and modeling descriptors were employed. Molecular graphics and modeling studies of crystal structures of complexes progesterone receptor (PR)-progesterone, Fab,-progesterone and PR-metribolone have been performed. QSAR of progestogens is a three-dimensional phenomenon (over 96% of information is explained by the first three Principal Components), which can be, although it exhibits significant non-linearity, treated well with linear methods such as PLS. Progestogen activity depends primarily on double bond contents and resonance effects which define the skeletal conformation, and also on substituent characteristics (size, conformational and electronic properties). Sterical relationships between a substituent at C6(sp2) or C6(sp3)-, and sulfur atom from Met 801 residue of PR are important for progesterone binding to the protein and can be quantified. Basically the same was observed for substituents at ,-C10 with respect to residue Met759. [source] Amino and cyano N atoms in competitive situations: which is the best hydrogen-bond acceptor?ACTA CRYSTALLOGRAPHICA SECTION B, Issue 6 2001A crystallographic database investigation The relative hydrogen-bond acceptor abilities of amino and cyano N atoms have been investigated using data retrieved from the Cambridge Structural Database and via ab initio molecular orbital calculations. Surveys of the CSD for hydrogen bonds between HX (X = N, O) donors, N,T,C,N (push,pull nitriles) and N,(Csp3)n,C,N molecular fragments show that the hydrogen bonds are more abundant on the nitrile than on the amino nitrogen. In the push,pull family, in which T is a transmitter of resonance effects, the hydrogen-bonding ability of the cyano nitrogen is increased by conjugative interactions between the lone pair of the amino substituent and the C,N group: a clear example of resonance-assisted hydrogen bonding. The strength of the hydrogen-bonds on the cyano nitrogen in this family follows the experimental order of hydrogen-bond basicity, as observed in solution through the pKHB scale. The number of hydrogen bonds established on the amino nitrogen is greater for aliphatic aminonitriles N,(Csp3)n,C,N, but remains low. This behaviour reflects the greater sensitivity of the amino nitrogen to steric hindrance and the electron-withdrawing inductive effect compared with the cyano nitrogen. Ab initio molecular orbital calculations (B3LYP/6-31+G** level) of electrostatic potentials on the molecular surface around each nitrogen confirm the experimental observations. [source] Comparative QSAR Studies on Toxicity of Phenol Derivatives Using Quantum Topological Molecular Similarity IndicesCHEMICAL BIOLOGY & DRUG DESIGN, Issue 5 2010Bahram Hemmateenejad Quantitative structure activity relationship (QSAR) analyses using a novel type of electronic descriptors called quantum topological molecular similarity (QTMS) indices were operated to describe and compare the mechanisms of toxicity of phenols toward five different strains (i.e., Tetrahymena pyriformis, L1210 Leukemia, Pseudomonas putida, Raja japonica and Cucumis sativus). The appropriate QSAR models for the toxicity data were obtained separately employing partial least squares (PLS) regression combined with genetic algorithms (GA), as a variable selection method. The resulting QSAR models were used to identify molecular fragments of phenol derivatives whose electronic properties contribute significantly to the observed toxicities. Using this information, it was feasible to discriminate between the mechanisms of action of phenol toxicity to the studied strains. It was found that toxicities of phenols to all strains, except with L1210 Leukemia, are significantly affected by electronic features of the phenolic hydroxyl group (C-O-H). Meanwhile, the resulting models can describe the inductive and resonance effects of substituents on various toxicities. [source] Ab Initio Group Contribution Method for Activation Energies of Hydrogen Abstraction ReactionsCHEMPHYSCHEM, Issue 1 2006Mark Saeys Prof. Abstract The group contribution method for activation energies is applied to hydrogen abstraction reactions. To this end an ab initio database was constructed, which consisted of activation energies calculated with the ab initio CBS-QB3 method for a limited set of well-chosen homologous reactions. CBS-QB3 is shown to predict reaction rate coefficients within a factor of 2,4 and Arrhenius activation energies within 3,5 kJ,mol,1of experimental data. Activation energies in the set of homologous reactions vary over 156 kJ,mol,1with the structure of the abstracting radical and over 94 kJ,mol,1with the structure of the abstracted hydrocarbon. The parameters required for the group contribution method, the so-called standard activation group additivity values, were determined from this database. To test the accuracy of the group contribution method, a large set of 88 additional activation energies were calculated from first principles and compared with the predictions from the group contribution method. It was found that the group contribution method yields accurate activation energies for hydrogen-transfer reactions between hydrogen molecules, alkylic hydrocarbons, and vinylic hydrocarbons, with the largest deviations being less than 6 kJ,mol,1. For reactions between allylic and propargylic hydrocarbons, the transition state is believed to be stabilized by resonance effects, thus requiring the introduction of an appropriate correction term to obtain a reliable prediction of the activation energy for this subclass of hydrogen abstraction reactions. [source] | |||||||||||||