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  • 1
    Language: English
    In: Angewandte Chemie International Edition, Oct 5, 2015, Vol.54(41), p.12171(5)
    Description: To purchase or authenticate to the full-text of this article, please visit this link: http://onlinelibrary.wiley.com/doi/10.1002/anie.201507113/abstract Byline: Ronja Missong, Janine George, Andreas Houben, Markus Hoelzel, Richard Dronskowski Keywords: bonding theory; density-functional theory; guanidine; neutron diffraction; solid-state structures; strontium Abstract Strontium guanidinate, SrC(NH).sub.3, the first compound with a doubly deprotonated guanidine unit, was synthesized from strontium and guanidine in liquid ammonia and characterized by X-ray and neutron diffraction, IR spectroscopy, and density-functional theory including harmonic phonon calculations. The compound crystallizes in the hexagonal space group P6.sub.3/m, constitutes the nitrogen analogue of strontium carbonate, SrCO.sub.3, and its structure follows a layered motif between Sr.sub.2+ ions and complex anions of the type C(NH).sub.3.sub.2-; the anions adopt the peculiar trinacria shape. A comparison of theoretical phonons with experimental IR bands as well as quantum-chemical bonding analyses yield a first insight into bonding and packing of the formerly unknown anion in the crystal. Author Affiliation: Chair of Solid-State and Quantum Chemistry, Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52056 Aachen (Germany) http://www.ssc.rwth-aachen.de Heinz Maier-Leibnitz Zentrum (MLZ), TU Munich, Lichtenbergstrasse 1, 85748 Garching (Germany)
    Keywords: Carbonates – Chemical Properties ; Carbonates – Analysis ; Strontium Compounds – Chemical Properties ; Strontium Compounds – Analysis
    ISSN: 1433-7851
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  • 2
    Language: English
    In: Science of the Total Environment, 01 September 2018, Vol.634, pp.305-315
    Description: The leaching of P from the upper 20 cm of forest topsoils influences nutrient (re-)cycling and the redistribution of available phosphate and organic P forms. However, the effective leaching of colloids and associated P forms from forest topsoils was so far sparsely investigated. We demonstrated through irrigation experiments with undisturbed mesocosm soil columns, that significant proportions of P leached from acidic forest topsoils were associated with natural colloids. These colloids had a maximum size of 400 nm. By means of Field-flow fractionation the leached soil colloids could be separated into three size fractions. The size and composition was comparable to colloids present in acidic forest streams known from literature. The composition of leached colloids of the three size classes was dominated by organic carbon. Furthermore, these colloids contained large concentrations of P which amounted between 12 and 91% of the totally leached P depending on the type of the forest soil. The fraction of other elements leached with colloids ranged between 1% and 25% (Fe: 1–25%; C : 3–17%; Al: 〈4%; Si, Ca, Mn: all 〈2%). The proportion of colloid–associated P decreased with increasing total P leaching. Leaching of total and colloid-associated P from the forest surface soil did not increase with increasing bulk soil P concentrations and were also not related to tree species. The present study highlighted that colloid-facilitated P leaching can be of higher relevance for the P leaching from forest surface soils than dissolved P and should not be neglected in soil water flux studies.
    Keywords: Colloids ; Forest Soil ; Leaching ; Mesocosm ; Nanoparticles ; Phosphorus ; Environmental Sciences ; Biology ; Public Health
    ISSN: 0048-9697
    E-ISSN: 1879-1026
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  • 3
    Language: English
    In: Angewandte Chemie International Edition, 05 October 2015, Vol.54(41), pp.12171-12175
    Description: Strontium guanidinate, SrC(NH), the first compound with a doubly deprotonated guanidine unit, was synthesized from strontium and guanidine in liquid ammonia and characterized by X‐ray and neutron diffraction, IR spectroscopy, and density‐functional theory including harmonic phonon calculations. The compound crystallizes in the hexagonal space group 6/, constitutes the nitrogen analogue of strontium carbonate, SrCO, and its structure follows a layered motif between Sr ions and complex anions of the type C(NH); the anions adopt the peculiar trinacria shape. A comparison of theoretical phonons with experimental IR bands as well as quantum‐chemical bonding analyses yield a first insight into bonding and packing of the formerly unknown anion in the crystal. : Strontium guanidinate, SrC(NH), the first compound with a doubly deprotonated guanidine unit, was synthesized, and its properties investigated using X‐ray and neutron powder diffraction as well as IR spectroscopy. Combined with quantum‐theoretical calculations, this allows a qualitative and quantitative discussion of some first insights into the structure of the anionic guanidine unit.
    Keywords: Bonding Theory ; Density‐Functional Theory ; Guanidine ; Neutron Diffraction ; Solid‐State Structures ; Strontium
    ISSN: 1433-7851
    E-ISSN: 1521-3773
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  • 4
    Language: English
    In: International Review of Financial Analysis, May 2014, Vol.33, pp.33-38
    Description: Forecasting Value-at-Risk (VaR) for financial portfolios is a crucial task in applied financial risk management. In this paper, we compare VaR forecasts based on different models for return interdependencies: volatility spillover (Engle & Kroner, 1995), dynamic conditional correlations (Engle, 2002, 2009) and (elliptical) copulas (Embrechts et al., 2002). Moreover, competing models for marginal return distributions are applied. In particular, we apply extreme value theory (EVT) models to GARCH-filtered residuals to capture excess returns. Drawing on a sample of daily data covering both calm and turbulent market phases, we analyze portfolios consisting of German Stocks, national indices and FX-rates. VaR forecasts are evaluated using statistical backtesting and Basel II criteria. The extensive empirical application favors the elliptical copula approach combined with extreme value theory (EVT) models for individual returns. 99% VaR forecasts from the EVT-GARCH-copula model clearly outperform estimates from alternative models accounting for dynamic conditional correlations and volatility spillover for all asset classes in times of financial crisis.
    Keywords: Financial Crisis ; Portfolio Value-At-Risk ; Dynamic Conditional Correlations ; Elliptical Copulas ; Extreme Value Theory ; Business
    ISSN: 1057-5219
    E-ISSN: 1873-8079
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  • 5
    Language: English
    In: International review of financial analysis, 2014, pp. 33-38
    ISSN: 10575219
    Source: Deutsche Zentralbibliothek für Wirtschaftswissenschaften
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  • 6
    Language: English
    In: Journal of risk, 2014, Vol.17(2), pp. 51-91
    Description: Modeling multivariate return distributions via copula functions is a common approach in financial risk management. However, evidence of the impact of choosing a particular copula function on different portfolio compositions is still lacking, as empirical studies typically analyze only a single portfolio strategy (eg, equally weighted portfolios) at a time. We evaluate copula models for three different portfolio strategies in a twenty-asset/daily return framework with respect to the accuracy of value-at-risk forecasts. From this risk management perspective, (dynamic) t copulas turn out to be a sensible choice for different portfolio strategies, especially when the trading strategy does not exclude highly volatile assets.
    Keywords: Portfolio-Management ; Risikomanagement ; Multivariate Verteilung ; Risikomaß ; Kapitalmarktrendite ; Aktienindex ; Deutschland
    ISSN: 14651211
    E-ISSN: 17552842
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  • 7
    Language: English
    In: Ecological Indicators, 2011, Vol.11(5), pp.1084-1092
    Description: Regime shifts in ecosystems whose patterns and properties may be very complex and thus manifold have profound implications for sustainability. Detecting structural breaks in natural processes, however, turns out to be an ambitious task because the lack of well defined target values and reference periods renders application of standard statistical (process or quality) control methods all but impossible. We develop an iterative procedure combining econometric, time series and quantile methods that produce a graphic display referred to as a “shiftogram,” which indicates potential shifts within univariate components of an ecosystem of interest by characterizing their specific and often fairly complex properties. The shiftogram approach can be routinely applied as a scanning device to any (univariate) time series. We provide a search algorithm that iteratively looks for the best value of some quality-of-fit criterion for a time series where the break point is not known beforehand. The approach is demonstrated by the application to univariate examples of fish recruitment, a climate change phenomenon and a canonical variable bundling the effect of different biodiversity indices. Analysis of ecosystem level shifts (i.e. regime shifts) can then be conducted by applying the shiftogram method to multiple component variables and examining correspondence among their resulting shift point and shift types. Alternatively we illustrate how regime shifts can be examined directly by applying the shiftogram approach to multivariate time series data after reduction to a univariate case through canonical data reduction techniques.
    Keywords: Structural Break Analysis ; Regime Shifts ; Climate Change ; Population Dynamics ; Shiftogram ; Environmental Sciences
    ISSN: 1470-160X
    E-ISSN: 1872-7034
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  • 8
    Language: German
    In: Angewandte Chemie, 05 October 2015, Vol.127(41), pp.12339-12343
    Description: Strontiumguanidinat, SrC(NH), die erste Verbindung mit zweifach deprotonierter Guanidineinheit, wurde aus Strontium und Guanidin in flüssigem Ammoniak synthetisiert und mittels Röntgen‐ und Neutronenpulverdiffraktion, IR‐Spektroskopie und Dichtefunktionaltheorie inklusive harmonischer Phononenrechnungen charakterisiert. Die in der hexagonalen Raumgruppe P6 /m kristallisierende Verbindung ist das Stickstoffanalogon des Strontiumcarbonats, SrCO, und dem Motiv nach schichtartig aus Sr‐Ionen und komplexen Anionen des Typs C(NH) aufgebaut; letztere nehmen die eigentümliche Trinacriagestalt an. Der Vergleich theoretischer Phononenrechnungen mit experimentellen IR‐Banden sowie quantenchemische Bindungsanalysen erlauben einen ersten Einblick in die Bindungs‐ und Packungsverhältnisse des bisher unbekannten Anions im Kristall. SrC(NH) wurde synthetisiert und seine Eigenschaften mittels Röntgen‐ und Neutronenpulverdiffraktion sowie IR‐Spektroskopie untersucht. Im Zusammenspiel mit quantentheoretischen Rechnungen werden erste Erkenntnisse zur Struktur der anionischen Guanidineinheit sowohl qualitativ als auch quantitativ diskutiert.
    Keywords: Bindungstheorie ; Dichtefunktionaltheorie ; Festkörperstrukturen ; Guanidin ; Neutronenbeugung ; Strontium
    ISSN: 0044-8249
    E-ISSN: 1521-3757
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  • 9
    Language: English
    In: Journal of Plant Nutrition and Soil Science, April 2016, Vol.179(2), pp.159-167
    Description: Nanoparticles and colloids affect the storage and hence the availability of P in forest ecosystems. We investigated the fine colloids present in forest soils and their association with inorganic and organic P. To differentiate between the different P forms, we performed liquid‐state P‐nucelar magnetic resonance (P‐NMR) measurements on forest bulk soil extracts, on colloid extracts and on the electrolyte phase of their soil suspensions. The P‐NMR spectra indicated that soil nanoparticles and colloids were more enriched with organic than with inorganic P forms compared to the electrolyte phase. The P concentration was enriched in the colloidal fraction in comparison to the bulk soil and the phosphate diesters were more dominant in the colloidal fraction when compared to the bulk soil. The colloidal P‐diester to P‐monoester ratios were 2 to 3 times higher in the colloidal fraction than in the bulk soil. In contrast, relatively large percentages of inorganic P were found in the electrolyte phase. Supplementary (not shown) Data are available at the JuSER Server (juser.fz‐juelich.de, reference number: FZJ‐2016‐01739) .
    Keywords: Colloids ; Nanoparticles ; Phosphorus ; P‐Diesters ; P‐Nmr
    ISSN: 1436-8730
    E-ISSN: 1522-2624
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  • 10
    Language: English
    In: International Review of Financial Analysis, May, 2014, Vol.33, p.33(6)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.irfa.2013.07.006 Byline: T. Berger, M. Missong Abstract: Forecasting Value-at-Risk (VaR) for financial portfolios is a crucial task in applied financial risk management. In this paper, we compare VaR forecasts based on different models for return interdependencies: volatility spillover (Engle & Kroner, 1995), dynamic conditional correlations (Engle, 2002, 2009) and (elliptical) copulas (Embrechts et al., 2002). Moreover, competing models for marginal return distributions are applied. In particular, we apply extreme value theory (EVT) models to GARCH-filtered residuals to capture excess returns. Drawing on a sample of daily data covering both calm and turbulent market phases, we analyze portfolios consisting of German Stocks, national indices and FX-rates. VaR forecasts are evaluated using statistical backtesting and Basel II criteria. The extensive empirical application favors the elliptical copula approach combined with extreme value theory (EVT) models for individual returns. 99% VaR forecasts from the EVT-GARCH-copula model clearly outperform estimates from alternative models accounting for dynamic conditional correlations and volatility spillover for all asset classes in times of financial crisis.
    Keywords: Financial Crises -- Models
    ISSN: 1057-5219
    Source: Cengage Learning, Inc.
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