bluuebunny/crossref_metadata_2025 huggingface dataset:
Segmented block copolymers of poly(ethylene glycol) and poly(ethylene terephthalate)
Authors: García‐Gaitán B. , Pérez‐González M. Del P. , Martínez‐Richa A. , Luna‐Bárcenas G. , Nuño‐Donlucas S. M.
Published: July 2004

A new series of segmented copolymers were synthesized from poly(ethylene terephthalate) (PET) oligomers and poly(ethylene glycol) (PEG) by a two‐step solution polymerization reaction. PET oligomers were obtained by glycolysis depolymerization. Structural features were defined by infrared and nuclear magnetic resonance (NMR) spectroscopy. The copolymer composition was calculated via 1H NMR spectroscopy. The content of soft PEG segments was higher than that of hard PET segments. A single glass‐transition temperature was detected for all the synthesized segmented copolymers. This observation was found to be independent of the initial PET‐to‐PEG molar ratio. The molar masses of the copolymers were determined by gel permeation chromatography (GPC). © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4448–4457, 2004
Study of the conformational change of poly(N‐isopropylacrylamide)‐grafted chains in water with neutron reflection: Molecular weight dependence at high grafting density
Authors: Yim H. , Kent M. S. , Satija S. , Mendez S. , Balamurugan S. S. , Balamurugan S. , Lopez G. P.
Published: July 2004

The temperature‐dependent conformational change of poly(N‐isopropylacrylamide) (PNIPAM) brushes in D2O was investigated as a function of the molecular weight (M) at a high grafting density with neutron reflection. PNIPAM chains with three different M values were grafted at the same high surface density from a gold surface by atom transfer radical polymerization. A significant change in the segment concentration profile was observed for all three samples as the temperature passed through the lower critical solution temperature (∼30 °C), in contrast to previous results obtained for samples with much lower surface density. Somewhat surprisingly, the fractional change in the first moment of the segment concentration profile (〈z〉) from 20 to 41 °C was weaker with increasing M. This is contrary to the trend for systems involving only van der Waals (VDW) interactions, in which higher M chains experience larger conformational changes with change in solvent quality. Indeed, the M dependence of the first moment of the segment concentration profile for the grafted PNIPAM chains at 20 °C was much weaker than has been reported previously for dense brushes involving only VDW interactions under good solvent conditions. At 20 °C, the form of the segment concentration profile varied systematically with M. A single‐layer profile resulted for the highest M, but the profiles became more bilayer in character with decreasing M. At 41 °C, the profiles for all three samples were adequately described by a single dense layer with a smooth transition region to bulk D2O. The weak dependence of 〈z〉 on M at 20 °C and the trend from a bilayer profile at lower M to a single‐layer profile at higher M appear to be related. These results are interpreted in terms of concentration‐dependent segment–segment interactions that result in a weak attraction for high segment densities at 20 °C. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3302–3310, 2004
Quantum study on the electrocyclic reactions of CH2CHCHCHX (XH, F, Cl)
Authors: Deng Ping , Li Lai‐Cai , Tian An‐Min , Wong Ning‐Bew
Published: July 2004

The microscopic mechanisms of the electrocyclic reactions for cis‐1,3‐butadiene and its monofluoro‐, monochloroderivatives have been studied by density functional theory (DFT), using the B3LYP method and 6‐311++G** basis sets. We optimized the geometric configurations of reactants, transition states, and products; verified all the probable transition states through vibrational analysis; and calculated the relative single‐point energies at the QCISD(T)/6‐311++G**//B3LYP/6‐311++G**. The results show that the monofluoro‐, monochloroderivatives of cis‐1,3‐butadiene both have two conformers; the reactant favors the electrocyclic reaction when one outboard hydrogen atom of the CH2 groups is substituted by the fluorine or chlorine atom. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004
Quantum analogue of the Kolmogorov–Arnold–Moser transition in different quantum anharmonic oscillators
Authors: Chattaraj P. K. , Maiti B. , Sengupta S.
Published: July 2004

The results of numerical computations are presented for the Bohmian trajectories of the family of different one‐ and two‐dimensional anharmonic oscillators, which exhibit regular or chaotic motion in both classical and quantum domains, depending on the values of the parameters appearing in the respective Hamiltonians. Quantum signatures of the Kolmogorov–Arnold–Moser (KAM) transition from the regular to chaotic classical dynamics of these oscillators are studied using a quantum theory of motion (QTM) as developed by de Broglie and Bohm. A phase space distance function between two initially close Bohmian trajectories, the associated Kolmogorov–Sinai–Lyapunov (KSL) entropy, the phase space volume, the autocorrelation function, the associated power spectrum, and the nearest‐neighbor spacing distribution, clearly differentiate the quantum analogues of the corresponding regular and chaotic motions in the classical domain. These quantum anharmonic oscillators are known to be useful in several diverse branches of science. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004
Analysis of electron correlation in M2Te (MCu, Ag, Au) systems
Authors: Surong Qi‐Mu , Zhao Yongfang , Jing Xiaogong , Li Xinying , Su Wenhui
Published: July 2004

The electron correlation in M2Te, (MCu, Ag, Au) systems are studied at the HF, MP2, and CCSD(T) levels with pseudopotential. The metallophilic attraction between heavy metal atoms is discussed by introducing a kind of simple model. Nearby the equilibrium point, the attraction is expressed as an angular force and estimated quantitatively by fitting the interaction energy as function of angle θ(MTeM). The behavior of electron correlation in the large distance as function of R(MM) is investigated by extracting the van der Waals coefficients of dispersion. An idea of the metallophilic contribution is given by carrying out a similar calculation on H2Te and being compared with M2Te. The trends of electron correlation and relativistic effects are discussed. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004
Separability between valence and conduction bands in transition metal clusters
Authors: Aprà Edoardo , Carter Emily A. , Fortunelli Alessandro
Published: July 2004

Simplified theories of transition metal electronic structure have been postulated for many decades. We test one such approximation, namely separate treatments of d (valence) and s/p (conduction) electrons in transition metal clusters, within a density functional theory (DFT) formalism. Two different basic approaches are considered: (a) an independent‐band approximation, in which the d‐ and s/p‐bands interact only via the ρ‐dependent components of the Kohn–Sham operator; and (b) a more realistic approximation, in which the lowest‐energy d‐ and s/p‐orbitals (separately derived) are allowed to interact through explicit off‐diagonal coupling matrix elements. The results are presented for the energy differences among three structural forms (icosahedral, cuboctahedral, and truncated decahedral) of 13‐atom Ni and Pt clusters. We demonstrate that an explicit decoupling of the d‐ and s/p‐bands does not produce accurate results for the clusters considered, not even for nickel, i.e., the transition metal for which d–s/p mixing should be at its minimum. By contrast, allowing the lowest‐energy orbitals of the two separate bands to interact improves the results considerably, and ensures a fair description of metal–metal bonding. This finding suggests that simplified models that exclude explicit d–s/p coupling should be employed with caution. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2004
Implementation of data‐dependent acquisitions in the study of melphalan DNA adducts by miniaturized liquid chromatography coupled to electrospray tandem mass spectrometry
Authors: Van den Driessche Bart , Van Dongen Walter , Lemière Filip , Esmans Eddy L.
Published: July 2004

The study of defects in DNA caused by xenobiotics, more particularly the study of DNA adducts, is an important field in cancer aetiology. The analysis of low abundance DNA adducts formed in vivo both in animals and humans requires the development and implementation of highly sensitive analytical methods. Since only a minute amount of DNA can be isolated (ca. 100 μg) it is evident that the amount of sample consumed per analysis should be as small as possible in order to gather as much analytical information as possible. In this article an example is given of how this problem can be solved by the implementation of data‐dependent acquisitions using capillary liquid chromatography coupled to electrospray tandem mass spectrometry. As a case study the alkylation of DNA by melphalan is presented. Slight modifications of the chromatographic conditions (mobile and stationary phases) can allow the automated analysis of other kinds of DNA adducts. Copyright © 2004 John Wiley & Sons, Ltd.
Simultaneous determination of t,t‐muconic, S‐phenylmercapturic and S‐benzylmercapturic acids in urine by a rapid and sensitive liquid chromatography/electrospray tandem mass spectrometry method
Authors: Barbieri Anna , Sabatini Laura , Accorsi Antonio , Roda Aldo , Violante Francesco Saverio
Published: July 2004

We describe a rapid and sensitive high‐performance liquid chromatography/electrospray tandem mass spectrometry (HPLC/ESI‐MS/MS) method for simultaneous determination of the most relevant metabolites of benzene and toluene, t,t‐muconic acid (t,t‐MA), S‐phenylmercapturic acid (S‐PMA), and S‐benzylmercapturic acid (S‐BMA). Urine samples were purified before analysis by solid‐phase microextraction (SPE) on SAX cartridges with 50 mg sorbent mass. The developed method fulfils all the standard requirements of precision and accuracy. Calibration curves were linear within the concentration range of the standards (0–80 μg/Lurine for t,t‐MA, and 0–25 μg/Lurine for S‐PMA and S‐BMA), and had correlation coefficients ≥0.997. Limits of detection were 6.0 μg/L for t,t‐MA, 0.3 μg/L for S‐PMA, and 0.4 μg/L for S‐BMA. The method was used to determine t,t‐MA, S‐PMA and S‐BMA levels in urine of 31 gasoline‐station workers, with personal monitoring data obtained from radial symmetry passive diffusive samplers. In the context of mean work‐shift exposures of 75.9 μg/m3 (range 9.4–220.2) for benzene and 331.9 μg/m3 (78.2–932.1) for toluene, metabolite concentrations in end‐of‐shift urine samples ranged from 23.5–275.3 μg/gcreatinine for t,t‐MA, non‐detectable to 0.9 μg/gcreatinine for S‐PMA, and 3.8–74.8 μg/gcreatinine for S‐BMA. No significant correlation was found between the environmental concentrations and urinary metabolites (p > 0.05 for all cases); the ratios of benzene metabolites could be influenced by exposure levels and co‐exposure to xylenes and toluene. The high throughput of this procedure should facilitate exploration of the metabolic effects of benzene‐related co‐exposure to toluene and alkylbenzenes in large populations of subjects exposed to gasoline. Copyright © 2004 John Wiley & Sons, Ltd.
Large Kr‐free subgraphs in Ks‐free graphs and some other Ramsey‐type problems
Authors: Sudakov Benny
Published: July 2004

In this paper we present three Ramsey‐type results, which we derive from a simple and yet powerful lemma, proved using probabilistic arguments. Let 3 ≤ r < s be fixed integers and let G be a graph on n vertices not containing a complete graph Ks on s vertices. More than 40 years ago Erdős and Rogers posed the problem of estimating the maximum size of a subset of G without a copy of the complete graph Kr. Our first result provides a new lower bound for this problem, which improves previous results of various researchers. It also allows us to solve some special cases of a closely related question posed by Erdős. For two graphs G and H, the Ramsey number R(G, H) is the minimum integer N such that any red‐blue coloring of the edges of the complete graph KN, contains either a red copy of G or a blue copy of H. The book with n pages is the graph Bn consisting of n triangles sharing one edge. Here we study the book‐complete graph Ramsey numbers and show that R(Bn, Kn) ≤ O(n3/log3/2n), improving the bound of Li and Rousseau. Finally, motivated by a question of Erdős, Hajnal, Simonovits, Sós, and Szemerédi, we obtain for all 0 < δ < 2/3 an estimate on the number of edges in a K4‐free graph of order n which has no independent set of size n1‐δ. © 2004 Wiley Periodicals, Inc. Random Struct. Alg., 2005
Corrigendum: The complexity of counting graph homomorphisms
Authors: Dyer Martin , Greenhill Catherine
Published: July 2004

We close a gap in the proof of Theorem 4.1 in our paper “The complexity of counting graph homomorphisms” [Random Structures Algorithms 17 (2000), 260–289]. © 2004 Wiley Periodicals, Inc. Random Struct. Alg., 2004
How adolescents construct their future: the effect of loneliness on future orientation
Authors: Seginer Rachel , Lilach Efrat
Published: July 2004

This study examined the effect of loneliness, gender, and two dimensions of prospective life domains on adolescent future orientation. Future orientation was studied in four prospective domains: social relations, marriage and family, higher education and work and career. These domains are described in terms of two dimensions: theme (relational vs. instrumental) and distance (near vs. distant future). Data collected from Israeli Jewish adolescents (11th graders) were analysed by repeated measures ANOVAs and ANCOVAs (covariate: depressive experiences) for seven future orientation variables: value, expectance, control (motivational variables), hopes, fears (cognitive representation variables), exploration, commitment (behavioural variables). As predicted, lonely adolescents scored lower than socially embedded adolescents on future orientation variables applied to the relational and near future domains and lonely boys scored lower than lonely girls. However, effects were found only on the three future orientation motivational variables and not on the cognitive representation and behavioural variables. Contrary to prediction controlling for the effect of depressive experiences did not reduce the effect of loneliness on the future orientation variables, but reduced the tendency of adolescents to score higher on all future orientation variables in the instrumental than in the relational prospective domains. The contribution of these findings to the understanding of adolescent loneliness and future orientation was discussed and directions for future research were suggested.
Pathways to self‐esteem in late adolescence: the role of parent and peer attachment, empathy, and social behaviours
Authors: Laible Deborah J. , Carlo Gustavo , Roesch Scott C.
Published: July 2004

The goal of this study was to examine both the direct and indirect relations of parent and peer attachment with self‐esteem and to examine the potential mediating roles of empathy and social behaviour. 246 college students ( years, ) completed self‐report measures of parent and peer attachment, empathy, social behaviour, and self‐esteem. Structural equation modelling revealed that parental attachment had mostly direct effects on self‐esteem. Among females, the links between peer attachment and self‐esteem, however, were entirely mediated by empathy and prosocial behaviour. The findings from this study suggest that although close supportive relationships with parents and peers are related to adolescent self‐esteem, these links are complex.
Urokinase plasminogen activator (uPA) is a positive regulator of outer root sheath keratinocyte proliferation
Authors: Cui Zhi‐hong , Yang Tian , Gao Qiang‐guo , Wang Yun
Published: July 2004

Urokinase plasminogen activator (uPA), a serine proteinase, is important in the development and epidermal wound healing, and seems to play a regulatory role in the proliferation of mouse epidermal keratinocytes (KC). In the present study, we found detectable uPA expression in outer root sheath (ORS) KC in the early anagen phase in mouse vibrissa follicles, but not in the late anagen or in the telogen and categen phases. uPA was also detected in ORS KC cultured from neonatal mice vibrissa. Specific exogenous inhibitors of uPA, amiloride and uPA antibody, significantly reduced the proliferation of ORS KC. Thus uPA is consistently elevated in the hyperproliferative hair follicle KC, and inhibition of the enzyme decreases hair follicle KC proliferation. We deduce that uPA is a very important mediator of the hair follicle cycle because its activity correlates with ORS KC proliferation.
New methodology for estimating biofuel consumption for cooking: Atmospheric emissions of black carbon and sulfur dioxide from India
Authors: Habib Gazala , Venkataraman Chandra , Shrivastava Manish , Banerjee Rangan , Stehr J. W. , Dickerson Russell R.
Published: July 2004

The dominance of biofuel combustion emissions in the Indian region, and the inherently large uncertainty in biofuel use estimates based on cooking energy surveys, prompted the current work, which develops a new methodology for estimating biofuel consumption for cooking. This is based on food consumption statistics, and the specific energy for food cooking. Estimated biofuel consumption in India was 379 (247–584) Tg yr−1. New information on the user population of different biofuels was compiled at a state level, to derive the biofuel mix, which varied regionally and was 74:16:10%, respectively, of fuelwood, dung cake and crop waste, at a national level. Importantly, the uncertainty in biofuel use from quantitative error assessment using the new methodology is around 50%, giving a narrower bound than in previous works. From this new activity data and currently used black carbon emission factors, the black carbon (BC) emissions from biofuel combustion were estimated as 220 (65–760) Gg yr−1. The largest BC emissions were from fuelwood (75%), with lower contributions from dung cake (16%) and crop waste (9%). The uncertainty of 245% in the BC emissions estimate is now governed by the large spread in BC emission factors from biofuel combustion (122%), implying the need for reducing this uncertainty through measurements. Emission factors of SO2from combustion of biofuels widely used in India were measured, and ranged 0.03–0.08 g kg−1from combustion of two wood species, 0.05–0.20 g kg−1from 10 crop waste types, and 0.88 g kg−1from dung cake, significantly lower than currently used emission factors for wood and crop waste. Estimated SO2emissions from biofuels of 75 (36–160) Gg yr−1were about a factor of 3 lower than that in recent studies, with a large contribution from dung cake (73%), followed by fuelwood (21%) and crop waste (6%).
From rift to drift: Mantle melting during continental breakup
Authors: Nielsen Thomas K. , Hopper John R.
Published: July 2004

Volcanic rifted margins show a temporal evolution in igneous crustal thickness and thus provide additional insights into mantle dynamics compared to the steady state situation at mid‐ocean ridges. Although details between different provinces vary, volcanic rifted margins generally show a short‐lived pulse of extreme magmatism that quickly abates to a steady state mid‐ocean ridge. The generation of thick igneous crust at volcanic rifted margins requires either melting of hot mantle material to higher degrees than observed at mid‐ocean ridges or melting of larger amounts of mantle material than would be the case for plate‐driven upwelling. To assess under what conditions buoyantly driven upwelling or small‐scale convection at rifting plate boundaries is important, a fluid dynamical model with non‐Newtonian viscosity that includes the feedback from melting on the physical properties of the mantle is developed. To generate a pulse of high magmatic production requires a viscosity and density structure that also leads to excessive fluctuations in magmatic productivity or a sustained high productivity that continues long after breakup. A viscosity increase due to dehydration caused by melting effectively suppresses buoyant upwelling above the depth to the dry solidus, thereby restricting shallow flow to plate‐driven upwelling. While this stabilizes the time dependence and forces the productivity to values consistent with mid‐ocean ridge accretion, it does so at the expense of eliminating the breakup instability. Models that assume an abrupt change in prerift lithospheric thickness suffer from the same deficits. However, including a sublithospheric hot layer leads to a model that can predict the temporal evolution of igneous crustal thickness observed in refraction seismic data from the southeast Greenland volcanic rifted margin.
Observations of low‐frequency magnetic oscillations in the Martian magnetosheath, magnetic pileup region, and tail
Authors: Espley J. R. , Cloutier P. A. , Brain D. A. , Crider D. H. , Acuña M. H.
Published: July 2004

Observations are presented of low‐frequency magnetic oscillations in the Martian magnetosheath, magnetic pileup region, and tail as observed by the Magnetometer/Electron Reflectometer experiment on board Mars Global Surveyor. Within the dayside magnetosheath the oscillations are found to be predominantly compressional, elliptically polarized waves with wave vectors that have large angles relative to the mean field and dominant frequencies that are significantly below the local proton gyrofrequency. On the basis of these observations we identify these oscillations as mirror mode instabilities. In the nightside magnetosheath the oscillations are predominantly transverse and elliptical, and they propagate at smaller angles relative to the mean field at frequencies within a factor of 2–10 less than the local proton gyrofrequency. These characteristics lead us to associate these oscillations with ion/ion‐resonant instabilities that arise from counterstreaming plasma populations such as the solar wind and pickup ions of planetary origin. Within the magnetic pileup region and tail the oscillations have considerably smaller amplitudes and are linearly polarized, obliquely propagating, ultralow‐frequency oscillations which may be a mix of multiple wave modes.
Seasonal current variability on the New Jersey inner shelf
Authors: Kohut Josh T. , Glenn Scott M. , Chant Robert J.
Published: July 2004

The well‐sampled ocean off the coast of New Jersey provides a data‐rich environment in which to study ocean current variability over the inner shelf. Using a year‐long HF radar data set, complemented with in situ and meteorological observations, the annual‐ and seasonal‐scale variabilities are examined. The hydrographic variability of the inner shelf off New Jersey is largely bimodal between summer stratification and winter mixing. An annual oceanographic and atmospheric data set was separated into these two regimes. The influence of stratification is evident through a relatively steady current response strongly correlated with the wind during the stratified season and a more variable response less correlated with the wind during the mixed season. When the water column is mixed, the influence of the local topography on the surface current variability is dependent on the slope, with a tendency for the variability to be more aligned with steeper topography.
Retrieved tropospheric and stratospheric BrO columns over Lauder, New Zealand
Authors: Schofield R. , Kreher K. , Connor B. J. , Johnston P. V. , Thomas A. , Shooter D. , Chipperfield M. P. , Rodgers C. D. , Mount G. H.
Published: July 2004

Spectroscopic measurements of BrO using direct sun and zenith‐sky viewing geometries are combined in an optimal estimation retrieval algorithm to obtain tropospheric and stratospheric columns of BrO. Seventy‐two twilight periods are investigated over Lauder, New Zealand (45.0°S, 169.7°E), between March 2001 and April 2003. A direct comparison between tropospheric and stratospheric columns retrieved at 80°, 84°, and 87° solar zenith angles (SZAs) from the spectroscopic measurements and those calculated by the three‐dimensional chemical transport model SLIMCAT shows good agreement. The stratospheric Bry loading of 21 pptv from the SLIMCAT calculations is consistent with the ground‐based measurements. The seasonal and diurnal variation of the stratospheric BrO columns evident from the ground‐based measurement retrievals is well described by the SLIMCAT model. The tropospheric column retrievals illustrate a high variability with a mean value of 0.2 pptv if the troposphere is assumed to be well mixed. An upper limit of 0.9 pptv is established for the ubiquitous BrO tropospheric column at 80° under cloud free conditions.
Photomosaics of the cathodoluminescence of 60 sections of meteorites and lunar samples
Authors: Akridge D. G. , Akridge J. M. C. , Batchelor J. D. , Benoit P. H. , Brewer J. , DeHart J. M. , Keck B. D. , Jie Lu , Meier A. , Penrose M. , Schneider D. M. , Sears D. W. G. , Symes S. J. K. , Yanhong Zhang
Published: July 2004

Cathodoluminescence (CL) petrography provides a means of observing petrographic and compositional properties of geological samples not readily observable by other techniques. We report the low‐magnification CL images of 60 sections of extraterrestrial materials. The images we report include ordinary chondrites (including type 3 ordinary chondrites and gas‐rich regolith breccias), enstatite chondrites, CO chondrites and a CM chondrite, eucrites and a howardite, lunar highland regolith breccias, and lunar soils. The CL images show how primitive materials respond to parent body metamorphism, how the metamorphic history of EL chondrites differs from that of EH chondrites, how dark matrix and light clasts of regolith breccias relate to each other, how metamorphism affects eucrites, the texture of lunar regolith breccias and the distribution of crystallized lunar spherules (“lunar chondrules”), and how regolith working affects the mineral properties of lunar soils. More particularly, we argue that such images are a rich source of new information on the nature and history of these materials and that our efforts to date are a small fraction of what can be done.
Nitrogen 15–enriched Precambrian kerogen and hydrothermal systems
Authors: Jia Yiefei , Kerrich Robert
Published: July 2004

Nitrogen isotope data are reported for kerogen from Precambrian black shales at prehnite‐pumpellyite to greenschist facies and for micas from postmetamorphic hydrothermal quartz vein systems. Nitrogen in micas was acquired from N‐bearing aqueous metamorphic hydrothermal fluids generated, from breakdown of sedimentary kerogen and dehydration of K‐silicates, at deeper crustal levels. The 2.7 Ga kerogens and hydrothermal K‐micas yield δ15N values of 15–24‰, compared to existing data of 2–6‰ in Phanerozoic counterparts. Paleoproterozoic equivalents have intermediate δ15N of 7–12‰, implying a secular decrease in δ15N of shale kerogen. The 15N‐enriched nitrogen in Archean shales and hydrothermal vein systems cannot be caused either by N isotopic shifts accompanying metamorphism or Rayleigh fractionation because premetamorphic and postmetamorphic samples from the same terrane are both enriched and lack covariation of δ15N with N, C/N ratios, or metamorphic grade. The magnitude of shifts during progressive metamorphism of sedimentary rocks in previous studies is constrained at <1‰ to greenschist and about 3‰ to amphibolite facies. Furthermore, 15N‐enriched values cannot stem from long‐term preferential diffusional loss of 14N as samples were selected from terranes where 40Ar/39Ar ages are within a few million years of concordant U‐Pb ages; nitrogen is structurally bound in micas, whereas Ar is not. It is possible that the 15N‐enriched values stem from a different N cycle in the Archean, with large biologically mediated fractionations, yet the magnitude of the fractionations observed exceeds any presently known, and chemoautotrophic communities tend to depleted values. Earlier results on Archean cherts show a range from −6 to 30‰. Given the temporal association of chert‐banded iron formation with mantle plumes, the range is consistent with mixing between mantle N2 of −5‰ and the enriched marine reservoir identified in this study. We attribute the secular trend from Archean kerogen of 15–24‰ to Phanerozoic kerogen averaging 3–4‰ to a secondary atmosphere derived from CI‐chondrite‐like material and comets with δ15N of 30–42‰. Shifts of δ15N to its present values of 0‰ can be accounted for by a combination of sequestration of atmospheric N2 into sedimentary rocks and mantle degassing of −5‰ mantle N2.
The transport pathways and sources of PM10 pollution in Beijing during spring 2001, 2002 and 2003
Authors: Wang Y. Q. , Zhang X. Y. , Arimoto R. , Cao J. J. , Shen Z. X.
Published: July 2004

PM10 concentration data for Beijing were combined with trajectory clustering and potential source contribution function (PSCF) methods to identify the principal transport pathways and sources for high‐concentration aerosol events in the spring. Three major pathways exist: the main one passes over Kazakhstan, through desert and semi‐arid regions in southwestern Mongolia, and then through Inner Mongolia and onwards to Beijing. Four major potential sources were identified, these are: (1) border areas between Kazakhstan and China, (2) desert and semi‐desert regions in western Mongolia, (3) the Northern High Dust Desert and (4) the Loess Plateau of China. The pathways and sources for pollution aerosol also are associated with Asian dust emissions; this suggests a significant contribution of Asian dust to PM10 pollution at Beijing.
Analysis of a mid‐latitude E‐region LQP event observed during the Coqui 2 Campaign
Authors: Urbina Julio , Kudeki Erhan , Franke Steven J. , Zhou Qihou
Published: July 2004

Arecibo ISR observations and VHF coherent backscatter data collected with a radar situated in southern Puerto Rico are examined to understand the causes of quasi‐periodic variations in VHF returns from lower E‐region heights (∼93 km). It is found that Bragg scale density irregularities causing the observed VHF backscatter are formed and propagated horizontally in a density perturbation field localized at the upper boundary of a tidal ion layer. The density perturbation has a 7 km horizontal wavelength, propagates in southwest direction, and it is not aligned with the geomagnetic field. Thus the production mechanism of the observed density perturbation cannot be the gradient‐drift instability often invoked to account for the generation of mid‐latitude sporadic E radar echoes. Nevertheless, secondary gradient‐drift instability operating on the gradients of the southwest propagating perturbation remains the most likely cause of Bragg scale electron density waves responsible for the VHF returns.
Fluvial mixing during floods
Authors: Manson J. R. , Wallis S. G.
Published: July 2004

Mixing in fluvial systems, as characterised by longitudinal dispersion, may be thought intuitively to scale with the river discharge or flow rate. Here, this tenet is demonstrated theoretically to be well‐founded during normal flows but to be perfidious during flood conditions. The results suggest that during floods longitudinal mixing rates in rivers can be several orders of magnitude larger than during normal flows and can actually decrease with further increases in flow magnitude. The very large longitudinal dispersion coefficients found during extreme flows cannot be predicted using existing empirical equations; neither can the rapid decrease with increasing flow rate. This is not surprising because these equations do not cater for the extremely strong transverse velocity shear that exists in these flows that contain zones of quite different velocity and depth. The conclusions are important because it appears that the great majority of observations of fluvial mixing have been made during normal flows when a scaling with flow rate occurs. During floods, however, this behaviour switches and longitudinal dispersion scales inversely and non‐linearly with flow rate.
The uncertainty of climatological values
Authors: Pavia Edgar G.
Published: July 2004

The uncertainty of climatological values is calculated with a method using the average absolute deviation from the mean, or absolute error. This method is related to the mean of the magnitude of the anomalies and its applicability is readily verified with a proposed approximate numerical technique, in contrast with the standard error method whose applicability is cumbersome to verify. Different data sets were tested with this technique showing that the method applies in a wide variety of geophysical cases, including non‐Gaussian cases. When it applies, the absolute error method provides additional information on the mean, such as the error due to the sample size.
Electrostatic instabilities induced by large‐amplitude left‐hand polarized waves
Authors: Gomberoff L. , Hoyos J. , Brinca A. L. , Ferrer R.
Published: July 2004

We study the effect of a large‐amplitude left‐hand polarized wave on ion acoustic instabilities. We show that the presence of a finite amplitude left‐hand polarized wave produces electrostatic instabilities above a threshold amplitude. These instabilities occur when the phase velocities of the counter‐streaming ion acoustic waves become equal, due to the action of the nonlinear wave. They do not exist in the absence of large‐amplitude waves. We examine their growth rates and threshold amplitude behavior as a function of the beam speed, temperature, and large‐amplitude wave frequency.
Structures of protein domains that create or recognize histone modifications
Authors: Bottomley Matthew J
Published: July 2004

DNA is packed together with histone proteins in cell nuclei to form a compact structure called chromatin. Chromatin represents a scaffold for many genetic events and shows varying degrees of condensation, including a relatively open form (euchromatin) and a highly condensed form (heterochromatin). Enzymes such as histone acetyltransferases (HATs) and methylases covalently label the amino‐termini of histones, thereby creating a ‘histone code’ of modifications that is interpreted by the recruitment of other proteins through recognition domains. Ultimately, this network of interacting proteins is thought to control the degree of chromatin condensation so that DNA is available when it is required for genomic processes. Reviewed here are the structures of HAT and SET domains, which mediate the acetylation and methylation of histones, respectively, and bromodomains and chromodomains, which recognize the modified histones. How these structures have increased our understanding of DNA regulation is also discussed.
Contracted basis Lanczos methods for computing numerically exact rovibrational levels of methane
Authors: Wang Xiao-Gang , Carrington Tucker
Published: July 2004

We present a numerically exact calculation of rovibrational levels of a five-atom molecule. Two contracted basis Lanczos strategies are proposed. The first and preferred strategy is a two-stage contraction. Products of eigenfunctions of a four-dimensional (4D) stretch problem and eigenfunctions of 5D bend-rotation problems, one for each K, are used as basis functions for computing eigenfunctions and eigenvalues (for each K) of the Hamiltonian without the Coriolis coupling term, denoted H0. Finally, energy levels of the full Hamiltonian are calculated in a basis of the eigenfunctions of H0. The second strategy is a one-stage contraction in which energy levels of the full Hamiltonian are computed in the product contracted basis (without first computing eigenfunctions of H0). The two-stage contraction strategy, albeit more complicated, has the crucial advantage that it is trivial to parallelize the calculation so that the CPU and memory costs are independent of J. For the one-stage contraction strategy the CPU and memory costs of the difficult part of the calculation scale linearly with J. We use the polar coordinates associated with orthogonal Radau vectors and spherical harmonic type rovibrational basis functions. A parity-adapted rovibrational basis suitable for a five-atom molecule is proposed and employed to obtain bend-rotation eigenfunctions in the first step of both contraction methods. The effectiveness of the two methods is demonstrated by calculating a large number of converged J=1 rovibrational levels of methane using a global potential energy surface.
Ab initio calculations on low-lying electronic states of TeO2 and Franck-Condon simulation of the (1)1B2←X̃ 1A1 TeO2 absorption spectrum including anharmonicity
Authors: Lee Edmond P. F. , Mok Daniel K. W. , Chau Foo-tim , Dyke John M.
Published: July 2004

Ab initio calculations have been carried out on low-lying singlet and triplet states of TeO2 at different levels of theory with basis sets of up to the augmented-polarized valence-quintuple-ζ quality. Equilibrium geometrical parameters, harmonic vibrational frequencies, and relative electronic energies of the X̃ 1A1, 1B1, 1B2, 1A2, 3A1, 3B1, 3B2, and 3A2 states of TeO2 have been calculated. Potential energy functions (PEFs) of the X̃ 1A1 and the (1)1B2 states were computed at the complete-active-space self-consistent-field multireference configuration interaction level, with a basis set of augmented-polarized valence-quadruple-ζ quality. Franck-Condon factors (FCFs) for the electronic transition between the X̃ 1A1 and (1)1B2 states of TeO2 were calculated with the above-mentioned ab initio PEFs. The (1)1B2←X̃ 1A1 absorption spectrum of TeO2 was simulated employing the computed FCFs, which include Duschinsky rotation and anharmonicity, and compared with the recently published laser-induced fluorescence (LIF) spectrum of Hullah and Brown [J. Mol. Spectrosc. 200, 261 (2000)]. The ab initio results and spectral simulation reported here confirm the upper electronic state involved in the LIF spectrum to be the (1)1B2 state of TeO2 and also confirm the vibrational assignments of Hullah and Brown. However, our simulated spectrum suggests that the reported LIF spectrum from 345 to 406 nm represents only a portion of the full (1)1B2←X̃ 1A1 absorption spectrum of TeO2, which extends from ca. 406 to 300 nm. Another dye other than the two used by Hullah and Brown is required to cover the 345–300 nm region of the LIF band. Ab initio calculations show strong configuration mixing of the (1)1B2 electronic surface with higher 1B2 states in a region of large TeO bond length (⩾2.0 Å) and OTeO bond angle (⩾135.0°).
Direct potential fit analysis of the X 1Σ+ ground state of CO
Authors: Coxon John A. , Hajigeorgiou Photos G.
Published: July 2004

A collection of 21 559 highly precise spectroscopic line positions from pure rotational and vibration-rotational spectra for seven isotopomers of carbon monoxide in the X 1Σ+ ground electronic state has been employed in direct least-squares fits of the rovibrational Hamiltonian operator obtained from Watson’s work [J. Mol. Spectrosc. 80, 411 (1980)] and that obtained by Herman and Ogilvie [Adv. Chem. Phys 103, 187 (1998)]. Fully analytical models are used for the various functions, including the Born-Oppenheimer internuclear potential function, and an account is taken of breakdown of the Born-Oppenheimer approximation. The resulting representations are more compact than currently available traditional Ukl/Δkl extended Dunham descriptions, and they generate quantum-mechanical eigenvalues that reproduce reliably the spectroscopic line positions to within the associated measurement uncertainties. Rayleigh-Schrödinger perturbation theory has been used to calculate highly accurate rotational and centrifugal distortion constants Bυ–Oυ for nine isotopomers of carbon monoxide. These constants are just as successful at reconstructing the observed spectroscopic information as the quantum-mechanical eigenvalues of the fitted Hamiltonian operators.
Ab initio potential energy surfaces of the ion-molecule reaction: C2H2+O+
Authors: Fukuzawa Kaori , Matsushita Toshio , Morokuma Keiji
Published: July 2004

High level ab initio calculations using complete active space self-consistent field and multi reference single and double excitation configuration interaction methods with cc-pVDZ (correlation consistent polarized valence double ζ) and cc-pVTZ (triple ζ) basis sets have been performed to elucidate the reaction mechanism of the ion-molecule reaction, C2H2(1Σg+)+O+(4S), for which collision experiment has been performed by Chiu et al. [J. Chem. Phys. 109, 5300 (1998)]. The minor low-energy process leading to the weak spin-forbidden product C2H2+(2Πu)+O(1D) has been studied previously and will not be discussed here. The major pathways to form charge-transfer (CT) products, C2H2+(2Πu)+O(3P) (CT1) and C2H2+(4A2)+O(3P) (CT2), and the covalently bound intermediates are investigated. The approach of the oxygen atom cation to acetylene goes over an energy barrier TS1 of 29 kcal/mol (relative to the reactant) and adiabatically leads the CT2 product or a weakly bound intermediate Int1 between CT2 products. This transition state TS1 is caused by the avoided crossing between the reactant and CT2 electronic states. As the C-O distance becomes shorter beyond the above intermediate, the C1 reaction pathway is energetically more favorable than the Cs pathway and goes over the second transition state TS2 of a relative energy of 39 kcal/mol. Although this TS connects diabatically to the covalent intermediate Int2, there are many states that interact adiabatically with this diabatic state and these lead to the other charge-transfer product CT1 via either of several nonadiabatic transitions. These findings are consistent with the experiment, in which charge transfer and chemical reaction products are detected above 35 and 39 kcal/mol collision energies, respectively.
Comparison in fractal dimension between those obtained from structure factor and viscoelasticity of gel networks of 1,3:2,4-bis-O-(p-methylbenzylidene)-D-sorbitol in polystyrene melt at gel point
Authors: Takenaka Mikihito , Kobayashi Toshiaki , Saijo Kenji , Tanaka Hirokazu , Iwase Naoki , Hashimoto Takeji , Takahashi Masaoki
Published: July 2004

We investigated time evolution of shear moduli in the physical gelation process of 1,3:2,4-bis-O-(p-methylbenzylidene)-D-sorbitol in polystyrene melt. At the gel point, storage and loss shear moduli, G′ and G″, were described by the power law of frequency ω, G′∼G″∼ωn, with the critical exponent n being nearly equal to 2/3, in agreement with the value predicted by the percolation theory. We also investigated the structure factor over two decades in length scale at gel point by using ultra-small-angle X-ray scattering, and small-angle X-ray scattering. We found the power-law behavior in low-q region, indicating that the gel network forms the self-similar structure with mass-fractal dimension. Comparison between the exponent of mass-fractal dimension from structure factor and that from viscoelasticity indicates that hydrodynamic interactions are completely screened out and the excluded volume effects are dominant in the gel. The gel strength was found to increase with the decrease in the lower limit length scale of fractality.
Channel switching effect in photodissociating N2O+ ion at 312.5 nm
Authors: Xu Haifeng , Guo Ying , Li Qifeng , Shi Yong , Liu Shilin , Ma Xingxiao
Published: July 2004

A experimental observation is presented on the N2O+ photodissociation process, which exhibits a complete channel switching effect in a narrow energy range. The N2O+ ions, prepared at the X 2Π (000) state by (3+1) multiphoton ionization of neutral N2O molecules at 360.6 nm, were excited to different vibrational levels in the A 2Σ+ state in a wavelength range of 275–328 nm. Based on the estimates of total released kinetic energies from the time-of-flight mass spectrum, it was found that the dissociation pathway of N2O+(A 2Σ+), NO+(X 1Σ+)+N(4S) with lower dissociation limit, changes abruptly and completely to NO+(X 1Σ+)+N(2D) with higher dissociation limit, in a excitation energy range of merely 250 cm−1 at λ∼312.5 nm. This phenomenon was explained by competition between the two dissociation pathways across the special excitation energy region.
Quasielastic and inelastic neutron scattering on hydrated calcium silicate pastes
Authors: Faraone Antonio , Fratini Emiliano , Baglioni Piero , Chen Sow-Hsin
Published: July 2004

Using the inverse geometry spectrometer QENS at the Intense Pulsed Neutron Source of the Argonne National Laboratory, we collected quasielastic and inelastic neutron scattering spectra of hydrated tricalcium and dicalcium silicate, the main components of ordinary Portland cement. Data were obtained at different curing time, from a few hours to several months. Both the quasielastic and inelastic spectra have been analyzed at the same time according to the relaxing cage model, which is a model developed to describe the dynamics of water at supercooled temperatures. Short-time and long-time dynamics of hydration water in hydrated cement pastes as a function of the curing time have been simultaneously obtained. The results confirm the findings reported in previous experiments showing that it is possible to fit consistently the quasielastic and inelastic spectra giving insights on the effect of the curing time on the short-time vibrational dynamics of hydration water.
Theoretical investigation of electromechanical effects for graphyne carbon nanotubes
Authors: Coluci V. R. , Galvão D. S. , Baughman R. H.
Published: July 2004

We present a theoretical study of the electronic and mechanical properties of graphyne-based nanotubes (GNTs). These semiconducting nanotubes result from the elongation of one-third of the covalent interconnections of graphite-based nanotubes by the introduction of yne groups. The effect of charge injection on the dimensions of GNTs was investigated using tight-binding calculations. Low amounts of electron injection are predicted to cause qualitatively different responses for armchair and zigzag graphyne nanotubes. Although the behavior is qualitatively similar to the usual carbon nanotubes, the charge-induced strains are predicted to be smaller for the GNTs than for ordinary single walled carbon nanotubes.
Modulated hydrogen beam study of adsorption-induced desorption of deuterium from Si(100)-3×1:D surfaces
Authors: Rahman F. , Kuroda M. , Kiyonaga T. , Khanom F. , Tsurumaki H. , Inanaga S. , Namiki A.
Published: July 2004

We have studied the kinetic mechanism of the adsorption-induced-desorption (AID) reaction, H+D/Si(100)→D2. Using a modulated atomic hydrogen beam, two different types of AID reaction are revealed: one is the fast AID reaction occurring only at the beam on-cycles and the other the slow AID reaction occurring even at the beam off-cycles. Both the fast and slow AID reactions show the different dependence on surface temperature Ts, suggesting that their kinetic mechanisms are different. The fast AID reaction overwhelms the slow one in the desorption yield for 300 K⩽Ts⩽650 K. It proceeds along a first-order kinetics with respect to the incident H flux. Based on the experimental results, both two AID reactions are suggested to occur only on the 3×1 dihydride phase accumulated during surface exposure to H atoms. Possible mechanisms for the AID reactions are discussed.