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Spectral Methods (spectral + methods)
Kinds of Spectral Methods Selected AbstractsA class of nonseparable and nonstationary spatial temporal covariance functionsENVIRONMETRICS, Issue 5 2008Montserrat Fuentes Abstract Spectral methods are powerful tools to study and model the dependency structure of spatial temporal processes. However, standard spectral approaches as well as geostatistical methods assume separability and stationarity of the covariance function; these can be very unrealistic assumptions in many settings. In this work, we introduce a general and flexible parametric class of spatial temporal covariance models, that allows for lack of stationarity and separability by using a spectral representation of the process. This new class of covariance models has a unique parameter that indicates the strength of the interaction between the spatial and temporal components; it has the separable covariance model as a particular case. We introduce an application with ambient ozone air pollution data provided by the U.S. Environmental Protection Agency (U.S. EPA). Copyright © 2007 John Wiley & Sons, Ltd. [source] Investigating the stimulus-dependent temporal dynamics of the BOLD signal using spectral methodsJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 3 2003Karsten Müller PhD Abstract Purpose To compare several spectral parameters using different durations of visual hemifield stimulation in order to explore the different temporal behavior of the blood oxygenation-level dependent (BOLD) signal in various brain regions. Materials and Methods Spectral methods were applied to three different groups of subjects with visual stimulation lasting 6, 12, and 30 seconds. Furthermore, diffusion weighting was applied in an interleaved way. The core of the data processing was the computation of the spectral density matrix using the multidimensional weighted covariance estimate. Spectral parameters of coherence and phase shift were computed. Results The correlation between signal changes and phase shifts was dependent on the duration of the visual stimulation. The shorter the duration of visual stimulation, the stronger the correlation between percentage signal change and phase shift. Conclusion The experiments with short and long stimuli differed mainly in the distribution of the activated voxels in the plane of percentage signal change and phase shift. It was revealed that the height of the signal change depends on the phase shift, whereas the diffusion weighting has no influence. J. Magn. Reson. Imaging 2003;17:375,382. © 2003 Wiley-Liss, Inc. [source] Novel Sesquiterpenoids from Siegesbeckia orientalisHELVETICA CHIMICA ACTA, Issue 1 2005Ying Xiang Five new sesquiterpenoids, namely, 8, -(angeloyloxy)-4,,6,,15-trihydroxy-14-oxoguaia-9,11(13)-dien-12-oic acid 12,6-lactone (1), 4,,6,,15-trihydroxy-8, -(isobutyryloxy)-14-oxoguaia-9,11(13)-dien-12-oic acid 12,6-lactone (2), 11,12,13 - trinorguai-6-ene-4,,10, -diol (3), (1(10)E,4E,8Z)-8-(angeloyloxy)-6,,15-dihydroxy-14-oxogermacra-(1(10),4,8,11(13)-tetraen-12-oic acid 12,6-lactone (9), and (1(10)E,4,)-8, -(angeloyloxy)-6,,14,15-trihydroxygermacra-1(10),11(13)-dien-12-oic acid 12,6-lactone (11), and three new artifacts, (1(10)E,4Z)-8, -(angeloyloxy)-9, -ethoxy-6,,15-dihydroxy-14-oxogermacra-1(10),4,11(13)-trien-12-oic acid 12,6-lactone (6), (1(10)E,4Z)-8, -(angeloyloxy)-9,,13-diethoxy-6,,15-dihydroxy-14-oxogermacra-1(10),4-dien-12-oic acid 12,6-lactone (7), and (1(10)E,4Z)-8, -(angeloyloxy)-9, -ethoxy-6,,15-dihydroxy-13-methoxy-14-oxogermacra-1(10),4-dien-12-oic acid 12,6-lactone (8), together with the three known sesquiterpenoids 4, 5, and 10, were isolated from the aerial parts of Sie esbeckia orientalis,L. Their structures were established by spectral methods, especially 1D- and 2D-NMR spectral methods. [source] Synthesis and characterization of dicyanate monomers containing methylene spacersJOURNAL OF APPLIED POLYMER SCIENCE, Issue 2 2008G. Anuradha Abstract The bisphenols containing methylene spacer were prepared by treating eugenol/2-allyl phenol with 2,6-dimethyl phenol/guiacol/o -cresol in the presence of AlCl3. All the bisphenols were converted to their respective cyanate esters by treating with CNBr. The structural confirmation was done by FTIR, 1H NMR, 13C NMR spectral methods, and elemental analysis. Thermal characterization was done by DSC and TGA. DSC transition shows that the Tg is in the range of 208,239°C. The Tg is highest for the cyanate ester Cy(b) with symmetric structure. The Tg of the cured network depends on the length and symmetry of the monomer, Tg being higher for shorter and the para-substituted monomers. The T10 values are in the range of 364,381°C. The char yield is in the range of 47,53%. From the char yield, the limiting oxygen index (LOI) value was determined, which is used to confirm the flame retardancy of the cyanate ester resins. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] Synthesis of a cellulose-grafted polymeric support and its application in the reductions of some carbonyl compoundsJOURNAL OF APPLIED POLYMER SCIENCE, Issue 1 2008Poonam K. Dhiman Abstract The reduction of carbonyl compounds by borohydride supported on a cellulose,anion exchange resin is known. The synthesis of a graft copolymer of cellulose and poly(4-vinyl pyridine) [CellO- g -poly(4-VP)] has been carried out with ceric ions as a redox initiator. A postgrafting treatment of CellO- g -poly(4-VP) with sodium borohydride has produced CellO- g -poly(4-VP) borane, a polymer-supported reducing agent. Optimum conditions pertaining to the maximum percentage of grafting have been evaluated as a function of the concentrations of the initiator, monomer, and nitric acid, amount of water, time, and temperature. The maximum percentage of grafting (585%) has been obtained with 0.927 mol/L of 4-vinyl pyridine and 0.018 mol/L of ceric ammonium nitrate in 120 min at 45°C. The polymeric support, CellO- g -poly(4-VP) borane, has been used for reduction reactions of different carbonyl compounds such as benzaldehyde, cyclohexanone, crotonaldehyde, acetone, and furfural. The graft copolymer has been characterized with IR and thermogravimetric analysis. The grafted cellulose has been found to be thermally stable. The reduced products have been characterized with IR and NMR spectral methods. The reagent has been reused for the reduction of a fresh carbonyl compound, and it has been observed that the polymeric reagent reduces the compounds successfully but with a little lower product yield. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source] Investigating the stimulus-dependent temporal dynamics of the BOLD signal using spectral methodsJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 3 2003Karsten Müller PhD Abstract Purpose To compare several spectral parameters using different durations of visual hemifield stimulation in order to explore the different temporal behavior of the blood oxygenation-level dependent (BOLD) signal in various brain regions. Materials and Methods Spectral methods were applied to three different groups of subjects with visual stimulation lasting 6, 12, and 30 seconds. Furthermore, diffusion weighting was applied in an interleaved way. The core of the data processing was the computation of the spectral density matrix using the multidimensional weighted covariance estimate. Spectral parameters of coherence and phase shift were computed. Results The correlation between signal changes and phase shifts was dependent on the duration of the visual stimulation. The shorter the duration of visual stimulation, the stronger the correlation between percentage signal change and phase shift. Conclusion The experiments with short and long stimuli differed mainly in the distribution of the activated voxels in the plane of percentage signal change and phase shift. It was revealed that the height of the signal change depends on the phase shift, whereas the diffusion weighting has no influence. J. Magn. Reson. Imaging 2003;17:375,382. © 2003 Wiley-Liss, Inc. [source] Structural determination of two new triterpenoids biotransformed from glycyrrhetinic acid by Mucor polymorphosporusMAGNETIC RESONANCE IN CHEMISTRY, Issue 2 2010Xiu Lan Xin Abstract Five hydroxylated derivatives of glycyrrhetinic acid by Mucor polymorphosporus were isolated. Among them, 6,, 7,-dihydroxyglycyrrhentic acid (2) and 27-hydroxyglycyrrhentic acid (3) are new compounds. Their chemical structures were identified by spectral methods including 2D-NMR. Copyright © 2009 John Wiley & Sons, Ltd. [source] Complete assignment of the 1H and 13C NMR spectra of garciniaphenone and keto-enol equilibrium statements for prenylated benzophenonesMAGNETIC RESONANCE IN CHEMISTRY, Issue 3 2008Priscilla B. M. C. Derogis Abstract This article reports the structural elucidation by IR, UV and MS spectroscopic data along with 1H and 13C NMR chemical shift assignments of two benzophenones isolated from the fruit pericarp of Garcinia brasiliensis Mart. (Clusiaceae): garciniaphenone, (1R,5S,7S)-3-benzoyl-4-hydroxy-6,6-dimethyl-5,7-di(3-methyl-2-butenyl)bicyclo[3.3.1]non-3-ene-2,9-dione, a novel triprenylated benzophenone; and 7- epi -clusianone, a tetraprenylated benzophenone that has already been extracted from another species of the same family. Furthermore, the keto-enol tautomeric equilibrium at solution-state was described for these compounds by 1D and 2D NMR spectral methods and one attempt to rationalize the different ratios between the noted tautomers was based on stereochemical features. Copyright © 2008 John Wiley & Sons, Ltd. [source] Structural determination of two new sesquiterpenes biotransformed from germacrone by Mucor alternataMAGNETIC RESONANCE IN CHEMISTRY, Issue 2 2008Niu Jia Abstract Eight transformed sesquiterpenes of germacrone by Mucor alternata were obtained. Their structures were characterized on the basis of spectral methods including 2D NMR. Among them, (1S, 4S, 5S, 10R)-isozedoarondiol (2) and (1R, 4S, 5S, 10R)-diepoxy-12-hydroxygermacrone (3) are new compounds. Copyright © 2007 John Wiley & Sons, Ltd. [source] Tryptamine derived amides with thiazole ring system from Thermoactinomyces strain TA66-2MAGNETIC RESONANCE IN CHEMISTRY, Issue 1 2008Korkmaz, la Akmemi Abstract A moderately thermophilic actinomycete strain, which was identified as Thermoactinomyces strain TA66-2, was isolated from hot-spring water. Fermentation, followed by solvent partition and chromatographic separations, resulted in the isolation of two new and two known molecules. The structures of the new compounds were elucidated as 2-(1-Propionylaminoethyl)thiazole-4-carboxylic acid [2-(1H-indol-3-yl)ethyl]amide and 2-(1-Acetylaminoethyl)thiazole-4-carboxylic acid [2-(1H-indol-3-yl)-ethyl]amide by using spectral methods (1D-, 2D-NMR and LC,ESI,MS). Copyright © 2007 John Wiley & Sons, Ltd. [source] On the angular momentum transport due to vertical convection in accretion discsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2010Geoffroy Lesur ABSTRACT The mechanism of angular momentum transport in accretion discs has long been debated. Although the magnetorotational instability appears to be a promising process, poorly ionized regions of accretion discs may not undergo this instability. In this Letter, we revisit the possibility of transporting angular momentum by turbulent thermal convection. Using high-resolution spectral methods, we show that strongly turbulent convection can drive outward angular momentum transport at a rate that is, under certain conditions, compatible with observations of discs. We find, however, that the angular momentum transport is always much weaker than the vertical heat transport. These results indicate that convection might be another way to explain global disc evolution, provided that a sufficiently unstable vertical temperature profile can be maintained. [source] A stabilized Hermite spectral method for second-order differential equations in unbounded domainsNUMERICAL METHODS FOR PARTIAL DIFFERENTIAL EQUATIONS, Issue 5 2007Heping Ma Abstract A stabilized Hermite spectral method, which uses the Hermite polynomials as trial functions, is presented for the heat equation and the generalized Burgers equation in unbounded domains. In order to overcome instability that may occur in direct Hermite spectral methods, a time-dependent scaling factor is employed in the Hermite expansions. The stability of the scheme is examined and optimal error estimates are derived. Numerical experiments are given to confirm the theoretical results.© 2007 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2007 [source] Synthesis and thermal degradation kinetics of cellulose estersPOLYMERS FOR ADVANCED TECHNOLOGIES, Issue 7 2003M. Sairam Abstract Polymers that are biodegradable currently achieve high interest in material science since they offer reductions of landfill space during waste management as well as new end-user benefits in various fields of applications. In this work, cellulose esters such as cellulose benzoate, cellulose succinate and cellulose cinnamate were prepared using dimethylaminopyridine along with dimethylaminopyridine-p-toluene sulfonic acid catalyst. Films of cellulose esters were cast from solution. Cellulose esters were characterized by spectral methods such as infrared, nuclear magnetic resonance, thermal method such as thermogravimetric analysis. Various methods of kinetic analysis were compared in the case of thermal degradation of the cellulose and cellulose esters. Copyright© 2003 John Wiley & Sons, Ltd. [source] Baroreflex Sensitivity: Measurement and Clinical ImplicationsANNALS OF NONINVASIVE ELECTROCARDIOLOGY, Issue 2 2008Maria Teresa La Rovere M.D. Alterations of the baroreceptor-heart rate reflex (baroreflex sensitivity, BRS) contribute to the reciprocal reduction of parasympathetic activity and increase of sympathetic activity that accompany the development and progression of cardiovascular diseases. Therefore, the measurement of the baroreflex is a source of valuable information in the clinical management of cardiac disease patients, particularly in risk stratification. This article briefly recalls the pathophysiological background of baroreflex control, and reviews the most relevant methods that have been developed so far for the measurement of BRS. They include three "classic" methods: (i) the use of vasoactive drugs, particularly the ,-adrenoreceptor agonist phenylephrine, (ii) the Valsalva maneuver, which produces a natural challenge for the baroreceptors by voluntarily increasing intrathoracic and abdominal pressure through straining, and (iii) the neck chamber technique, which allows a selective activation/deactivation of carotid baroreceptors by application of a negative/positive pressure to the neck region. Two more recent methods based on the analysis of spontaneous oscillations of systolic arterial pressure and RR interval are also reviewed: (i) the sequence method, which analyzes the relationship between increasing/decreasing ramps of blood pressure and related increasing/decreasing changes in RR interval through linear regression, and (ii) spectral methods, which assess the relationship (in terms of gain) between specific oscillatory components of the two signals. The limitations of the coherence criterion for the computation of spectral BRS are discussed, and recent proposals for overcoming them are presented. Most relevant clinical applications of BRS measurement are finally reviewed with particular reference to patients with myocardial infarction and heart failure. [source] Structural and Magnetic Resolution of a Two-Step Full Spin-Crossover Transition in a Dinuclear Iron(II) Pyridyl-Bridged CompoundCHEMISTRY - A EUROPEAN JOURNAL, Issue 32 2006Jarrod J. M. Amoore Abstract A dinuclear iron(II) complex containing the new pyridyl bridging ligand, 2,5-di(2,,2,,-dipyridylamino)pyridine (ddpp) has been synthesised and characterised by single-crystal X-ray diffraction, magnetic susceptibility and Mössbauer spectral methods. This compound, [Fe2(ddpp)2(NCS)4],4,CH2Cl2, undergoes a two-step full spin crossover. Structural analysis at each of the three plateau temperatures has revealed a dinuclear molecule with spin states HS,HS, HS,LS and LS,LS (HS: high spin, LS: low spin) for the two iron(II) centres. This is the first time that resolution of the metal centres in a HS,LS ordered state has been achieved in a two-step dinuclear iron(II) spin-crossover compound. Thermogravimetric data show that the dichloromethane solvate molecules can be removed in two distinct steps at 120,°C and 200,°C. The partially de-solvated clathrate, [Fe2(ddpp)2(NCS)4],CH2Cl2, undergoes a one-step transition with an increased transition temperature with respect to the as synthesised material. Structural characterisation of this material reveals subtle changes to the coordination geometries at each of the iron(II) centres and striking changes to the local environment of the dinuclear complex. The fully de-solvated material remains high spin over all temperatures. Interestingly, the solvent can be re-introduced into the monosolvated solid to achieve complete conversion back to the original two-step crossover material, [Fe2(ddpp)2(NCS)4],4,CH2Cl2. [source] Synthesis and Neurotrophic Activities of N-p -Tolyl/phenylsulfonyl L -Amino Acid Thiolester DerivativesCHINESE JOURNAL OF CHEMISTRY, Issue 3 2008Zhan-Nan YANG Abstract Various N-p- tolyl/phenylsulfonyl L -amino acid thiolester derivatives were designed and synthesized according to combination of functional groups. The synthesized compounds were identified by spectral methods (1H NMR, 13C NMR, MS and HRMS). Preliminary bioassays indicated that the compounds 4a, 4b, 4d, 5c, 5d, 5g, 6b and 6d showed remarkable activities and the compounds 4c, 5b and 6c showed moderate activities to inhibit anoxic damage of PC12 cells at 10 µg/mL, but the compounds 4d and 6d only showed moderate protective activities against PC12 cells at 5 µg/mL. Preliminary bioassays also indicated that the compounds 4c, 5a, 5c, 5d, 5e and 6b showed moderate activities to induce PC12-differentiation at 10 µg/mL. [source] Multiscale resolution of shortwave-longwave interactionCOMMUNICATIONS ON PURE & APPLIED MATHEMATICS, Issue 1 2009Chris Stucchio In the study of time-dependent waves, it is computationally expensive to solve a problem in which high frequencies (shortwaves, with wavenumber k = kmax) and low frequencies (longwaves, near k = kmin) mix. Consider a problem in which low frequencies scatter off a sharp impurity. The impurity generates high frequencies that propagate and spread throughout the computational domain, while the domain must be large enough to contain several longwaves. Conventional spectral methods have a computational cost that is proportional to O(kmax/kmin log(kmax/kmin)). We present here a multiscale algorithm (implemented for the Schrödinger equation but generally applicable) that solves the problem with cost (in space and time) O(kmaxL log(kmax/kmin) log(kmaxL)). Here, L is the width of the region in which the algorithm resolves all frequencies and is independent of kmin. © 2008 Wiley Periodicals, Inc. [source] | |||||||||||||||||||