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Branching Ratio (branching + ratio)
Selected AbstractsStudies on the growth and optical characterization of dysprosium gadolinium oxalate single crystalsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 2 2004A. Elizebeth Abstract Preparation and optical characterization of dysprosium gadolinium oxalate (DGO) single crystal is reported. The crystals were grown using silica gel technique, by the controlled reaction of rare earth nitrates with oxalic acid. Crystals were characterized using X-ray powder diffraction, optical absorption and fluorescence studies. Radiative transition probability, fluorescence branching ratio and radiative lifetime of Dy3+ in the crystal are evaluated by the parameterization of the absorption spectrum by the Judd-Ofelt theory. The recorded fluorescence spectrum showed two well resolved peaks at 480 nm and 571 nm and are assigned to the transitions from 4F9/2 , 6H15/2 and 6H13/2 of Dy3+. Stimulated emission crossection and optical gain of these transitions are also evaluated. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Kinetics of HCCl + NOx reactionsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 1 2002Randall E. Baren The kinetics of reactions of HCCl with NO and NO2 were investigated over the temperature ranges 298,572 k and 298,476 k, respectively, using laser-induced fluorescence spectroscopy to measure total rate constants and time-resolved infrared diode laser absorption spectroscopy to probe reaction products. Both reactions are fast, with k(HCCl + NO) = (2.75 ± 0.2) × 10,11 cm3 molecule,1 s,1 and k(HCCl + NO2) = (1.10 ± 0.2) × 10,10 cm3 molecule,1 s,1 at 296 K. Both rate constants displayed only a slight temperature dependence. Detection of products in the HCCl + NO reaction at 296 K indicates that HCNO + Cl is the major product with a branching ratio of , = 0.68 ± 0.06, and NCO + HCl is a minor channel with , = 0.24 ± 0.04. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 12,17, 2002 [source] Direct kinetics study of the temperature dependence of the CH2O branching channel for the CH3O2 + HO2 reactionINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 6 2001Matthew J. Elrod A direct kinetics study of the temperature dependence of the CH2O branching channel for the CH3O2 + HO2 reaction has been performed using the turbulent flow technique with high-pressure chemical ionization mass spectrometry for the detection of reactants and products. The temperature dependence of the CH2O-producing channel rate constant was investigated between 298 and 218 K at a pressure of 100 Torr, and the data were fitted to the following Arrhenius expression: 1.6 × 10,15 × exp[(1730 ± 130)/T] cm3 molecule,1 s,1. Using the Arrhenius expression for the overall rate of the CH3O2 + HO2 reaction and this result, the 298 K branching ratio for the CH2O producing channel is measured to be 0.11, and the branching ratio is calculated to increase to a value of 0.31 at 218 K, the lowest temperature accessed in this study. The results are compared to the analogous CH3O2 + CH3O2 reaction and the potential atmospheric ramifications of significant CH2O production from the CH3O2 + HO2 reaction are discussed. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 363,376, 2001 [source] Kinetic and mechanistic study of the reaction of O(1D) with CF2HBrINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 4 2001R. S. Strekowski A laser flash photolysis,resonance fluorescence technique has been employed to investigate the kinetics and mechanism of the reaction of electronically excited oxygen atoms, O(1D), with CF2HBr. Absolute rate coefficients (k1) for the deactivation of O(1D) by CF2HBr have been measured as a function of temperature over the range 211,425 K. The results are well described by the Arrhenius expression k1(T) = 1.72 × 10,10 exp(+72/T) cm3molecule,1 s,1; the accuracy of each reported rate coefficient is estimated to be ±15% (2,). The branching ratio for nonreactive quenching of O(1D) to the ground state, O(3P), is found to be 0.39 ± 0.06 independent of temperature, while the branching ratio for production of hydrogen atoms at 298 K is found to be 0.02,0.02+0.01. The above results are considered in conjunction with other published information to examine reactivity trends in O(1D) + CF2XY reactions (X,Y = H, F, Cl, Br). © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 262,270, 2001 [source] Time-dependent density functional theory calculations of X-ray absorptionINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4-5 2003J. J. Rehr Abstract There has been dramatic progress in recent years both in calculations and in the interpretation of X-ray absorption spectra (XAS). Often an independent-electron approximation with final state potentials is adequate. However, for soft X-rays (i.e., energies less than about 1 keV) local field effects can be important. Such local fields arise from the dynamic screening of both the external X-ray field and the coupling to the core hole created in the absorption process. These effects require a theory that goes beyond the independent-electron approximation. We developed an efficient approach for treating such effects in molecules and solids based on a generalization of time-dependent density functional theory (TDDFT), with a local approximation for the screening response. The approach has been implemented in our self-consistent, real-space Green's function code FEFF8 in terms of screened dipole transition matrix elements. Typical results are discussed for the XAS of the N4,5 edges of solid Xe and for the L2,3 edges of 3d transition metals. Our approach accounts for the deviations of the L3/L2 intensity branching ratio from the 2:1 value of the independent electron approximation. For the N4,5 edges of Xe, the approach also accounts for the observed fine structure. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003 [source] Theory of Ca L2,3 -edge XAS using a novel multichannel multiple-scattering methodJOURNAL OF SYNCHROTRON RADIATION, Issue 1 2005Peter Krüger A new method for calculating X-ray absorption spectroscopy (XAS) at the L2,3 edges of Ca and transition metals is presented. It is based on the multichannel multiple-scattering theory by Natoli et al. [Phys. Rev. B, (1990), 42, 1944,1968] combined with the eigen-channel R-matrix formalism. Atomic multiplet-like effects, owing to the Coulomb interaction of photoelectrons and the 2p hole, are taken into account through a configuration interaction ansatz for the final-state wavefunction. The various multiplet states lead to a set of channels for the photoelectron wavefunction, which is calculated in multiple-scattering theory. The method is applied to Ca, an important element for biological applications of XAS. An L:L branching ratio of 3:4 is found, in good agreement with experiment but in contrast to the statistical value 2:1 obtained in all one-electron approaches. By using a linear mixture between statically screened (,90%) and unscreened (,10%) core-hole potential, the line shape, too, agrees well with the experimental one. [source] Intrinsic Oxygen Use Kinetics of Transformed Plant Root CultureBIOTECHNOLOGY PROGRESS, Issue 3 2001Patrick T. Asplund Root meristem oxygen uptake, root tip extension rate, and specific growth rate are assessed as a function of dissolved oxygen level for three transformed root cultures. The influence of hydrodynamic boundary layer was considered for all measurements to permit correlation of oxygen-dependent kinetics with the concentration of oxygen at the surface of the root meristem. Oxygen uptake rate is shown to be saturated at ambient conditions, and a saturation level of approximately 300 ,mole O2/(cm3 tissue·hr) was observed for all three of these morphologically diverse root types. In nearly all cases, the observation of a minimum oxygen pressure, below which respiration, extension, or root growth would not occur, could be accounted for as a boundary layer mass transfer resistance. The critical oxygen pressure below which respiration declines is below saturated ambient oxygen conditions. In contrast, critical oxygen pressures for root tip extension were much higher; extension was nearly linear for the two thicker root types (Hyoscyamus muticus, henbain; Solanum tuberosum, potato) above ambient oxygen levels. The performance of the thinnest root, Brassica juncea (Indian mustard) was consistent with reduced internal limitations for oxygen transport. Extension rates did not correlate with biomass accumulation. The fastest growing henbain culture (, = 0.44 day,1) displayed the slowest extension rate (0.16 mm/hr), and the slowest growing mustard culture (, = 0.22 day,1) had the fastest tip extension rate (0.3 mm/hr). This apparent paradox is explained in terms of root branching patterns, where the root branching ratio is shown to be dependent upon the oxygen-limited mersitem extension rate. The implications of these observations on the performance of root culture in bioreactors is discussed. [source] Zwitterionic States in Gas-Phase Polypeptide Ions Revealed by 157-nm Ultra-Violet PhotodissociationCHEMISTRY - A EUROPEAN JOURNAL, Issue 30 2006Frank Kjeldsen Dr. Abstract A new method of detecting the presence of deprotonation and determining its position in gas-phase polypeptide cations is described. The method involves 157-nm ultra-violet photodissociation (UVPD) and is based on monitoring the losses of CO2 (44 Da) from electronically excited deprotonated carboxylic groups relative to competing COOH losses (45 Da) from neutral carboxylic groups. Loss of CO2 is a strong indication of the presence of a zwitterionic [(+),,,(,),,,(+)] salt bridge in the gas-phase polypeptide cation. This method provides a tool for studying, for example, the nature of binding within polypeptide clusters. Collision-activated dissociation (CAD) of decarboxylated cations localizes the position of deprotonation. Fragment abundances can be used for the semiquantitative assessment of the branching ratio of deprotonation among different acidic sites, however, the mechanism of the fragment formation should be taken into account. Cations of Trp-cage proteins exist preferentially as zwitterions, with the deprotonation position divided between the Asp9 residue and the C terminus in the ratio 3:2. The majority of dications of the same molecule are not zwitterions. Furthermore, 157-nm UVPD produces abundant radical cations M.+ from protonated molecules through the loss of a hydrogen atom. This method of producing M.+ ions is general and can be applied to any gas-phase peptide cation. The abundance of the molecular radical cations M.+ produced is sufficient for further tandem mass spectrometry (MS/MS), which, in the cases studied, yielded side-chain loss of a basic amino acid as the most abundant fragmentation channel together with some backbone cleavages. [source] Kinetics of the CH3O2 + HO2 reaction: A temperature and pressure dependence study using chemical ionization mass spectrometryINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 10 2007M. Teresa Raventós-Duran A temperature and pressure kinetic study for the CH3O2 + HO2 reaction has been performed using the turbulent flow technique with a chemical ionization mass spectrometry detection system. An Arrhenius expression was obtained for the overall rate coefficient of CH3O2 + HO2 reaction: k(T) = (3.82+2.79,1.61) × 10,13 exp[(,781 ± 127)/T] cm,3 molecule,1 s,1. A direct quantification of the branching ratios for the O3 and OH product channels, at pressures between 75 and 200 Torr and temperatures between 298 and 205 K, was also investigated. The atmospheric implications of considering the upper limit rate coefficients for the O3 and OH branching channels are observed with a significant reduction of the concentration of CH3OOH, which leads to a lower amount of methyl peroxy radical. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 571,579, 2007 [source] Gas-phase radical,radical recombination reactions of nitroxides with substituted phenyl radicalsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 4 2004J. L. Heidbrink Fourier-transform ion cyclotron resonance mass spectrometry has been used to examine gas-phase reactions of four different nitroxide free radicals with eight positively charged pyridyl and phenyl radicals (some containing a Cl, F, or CF3 substituent). All the radicals reacted rapidly (near collision rate) with nitroxides by radical,radical recombination. However, some of the radicals were also able to abstract a hydrogen atom from the nitroxide. The results establish that the efficiency (kreaction/kcollision) of hydrogen atom abstraction varies with the electrophilicity of the radical, and hence is attributable to polar effects (a lowering of the transition-state energy by an increase in its polar character). The efficiency of the recombination reaction is not sensitive to substituents, presumably due to a very low reaction barrier. Even so, after radical,radical recombination has occurred, the nitroxide adduct was found to fragment in different ways depending on the structure of the radical. For example, a cationic fragment was eliminated from the adducts of the more electrophilic radicals via oxygen anion abstraction by the radical (i.e., the nitroxide adduct cleaves heterolytically), whereas adducts of the less electrophilic radicals predominantly fragmented via homolytic cleavage (oxygen atom abstraction). Therefore, differences in the product branching ratios were found to be attributable to polar factors. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 216,229 2004 [source] Transition metals as electron traps.JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 10 2009Abstract Transition metal cations Co2+, Ni2+ and Zn2+ form 1 : 1 : 1 ternary complexes with 2,2,-bipyridine (bpy) and peptides in aqueous methanol solutions that have been studied for tripeptides GGG and GGL. Electrospray ionization of these solutions produced singly charged [Metal(bpy)(peptide , H)]+ and doubly charged [Metal(bpy)(peptide)]2+ ions (Metal = metal ion) that underwent charge reduction by glancing collisions with Cs atoms at 50 and 100 keV collision energies. Electron transfer to [Metal(bpy)(peptide)]2+ ions was less than 4.2 eV exoergic and formed abundant fractions of non-dissociated charge-reduced intermediates. Charge-reduced [Metal(bpy)(peptide)]+ ions dissociated by the loss of a hydrogen atom, ammonia, water and ligands that depended on the metal ion. The Ni and Co complexes mainly dissociated by the elimination of ammonia, water, and the peptide ligand. The Zn complex dissociated by the elimination of ammonia and bpy. A sequence-specific fragment was observed only for the Co complex. Electron transfer to [Metal(bpy)(peptide , H)]+ was 0.6,1.6 eV exoergic and formed intermediate radicals that were detected as stable anions after a second electron transfer from Cs. [Metal(bpy)(peptide , H)] neutrals and their anions dissociated by the loss of bpy and peptide ligands with branching ratios that depended on the metal ion. Optimized structures for several spin states, electron transfer and dissociation energies were addressed by combined density functional theory and Møller,Plesset perturbational calculations to aid interpretation of experimental data. The experimentally observed ligand loss and backbone cleavage in charge-reduced [Metal(bpy)(peptide)]+ complexes correlated with the dissociation energies at the present level of theory. The ligand loss in +CR, spectra showed overlap of dissociations in charge-reduced [Metal(bpy)(peptide , H)] complexes and their anionic counterparts which complicated spectra interpretation and correlation with calculated dissociation energies. Copyright © 2009 John Wiley & Sons, Ltd. [source] Entropy considerations in kinetic method experimentsJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 9 2004Chrys WesdemiotisArticle first published online: 7 SEP 200 Abstract In extended kinetic method experiments, relative binding enthalpies (,affinities') and relative entropies are obtained based on unimolecular dissociation kinetics. A series of ion-bound dimers AXBi is formed, in which the sample (A) and structurally similar reference molecules (Bi) are bridged by a central cation or anion (X). The branching ratios of the AXBi set to AX and BiX are determined at different internal energies, usually by subjecting AXBi to collisionally activated dissociation at various collision energies. The dependence of the natural logarithm of the branching ratios on the corresponding BiX bond enthalpies (X affinities of Bi) is evaluated as a function of internal energy to thereby deduce the AX bond enthalpy (X affinity of A) as well as an apparent relative entropy of the competitive dissociation channels, ,(,Sapp). Experiments with proton- and Na+ -bound dimers show that this approach can yield accurate binding enthalpies. In contrast, the derived ,(,Sapp) values do not correlate with the corresponding thermodynamic entropy differences between the channels leading to AX and BiX, even after scaling. The observed trends are reconciled by the transition state switching model. According to this model, the kinetics of barrierless dissociations, such as those encountered in kinetic method studies, are dominated by a family of tight transition states (,entropy bottlenecks') lying lower in energy than the corresponding dissociation thresholds. In general, the relative energies of these tight transition states approximately match those of the dissociation products, but their relative entropies tend to be much smaller, as observed experimentally. Copyright © 2004 John Wiley & Sons, Ltd. [source] Optical spectra of Tm3+ -doped YAl3(BO3)4 single crystalsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 3 2007E. Cavalli Abstract Flux grown YAl3(BO3)4 (YAB) crystals doped with Tm3+ have been characterised by optical spectroscopy techniques. The absorption and emission spectra in the UV-VIS-NIR region have been measured at 10 and 298 K. The visible emission decay profiles have been measured at room temperature. The low temperature spectra have been used to obtain the energy level scheme of the optically active ion in YAB. The Judd-Ofelt parametrization scheme has been applied to the analysis of the room temperature absorption spectra in order to evaluate the intensity parameters, the branching ratios and the radiative lifetimes of the emitting states. These have been compared with the experimental values. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | |||||||||||||