Showing papers by "Peter S. White published in 2002"
TL;DR: This work proposes several strategies for utilizing external data (such as might be obtained using GIS) to aid in the completion of species lists, and demonstrates the potential of these approaches using simulation and case studies from Oklahoma.
Abstract: A substantial body of literature has accumulated on the topic of the estimation of species richness by extrapolation. However, most of these methods rely on an objective sampling of nature. This condition is difficult to meet and seldom achieved for large regions. Furthermore, scientists conducting biological surveys often already have preliminary but subjectively gathered species lists, and would like to assess the completeness of such lists, and/or to find a way to perfect them. We propose several strategies for utilizing external data (such as might be obtained using GIS) to aid in the completion of species lists. These include: (i) using existing species lists to develop predictive models; (ii) using the uniqueness of the environment as a guide to find underrepresented species; (iii) using spectral heterogeneity to locate environmentally heterogeneous regions; (iv) combining surveys with statistical model-building in an iterative manner. We demonstrate the potential of these approaches using simulation and case studies from Oklahoma. Copyright © 2002 John Wiley & Sons, Ltd.
342 citations
192 citations
TL;DR: Upon standing at room-temperature overnight, complex 3 converted quantitatively to another product which has been characterized as the C-C activation product, [Cp*(PMe3)Rh(Me)(CNSiPh3)]+BAr'4- (5).
Abstract: Addition of 1.0 equiv of Ph3SiH to [Cp*(PMe3)Rh(Me)(CH2Cl2)]+BAr‘4- (1) resulted in release of methane and quantitative formation of [Cp*(PMe3)Rh(SiPh3)(CH2Cl2)]+BAr‘4- (2). Subsequent addition of ...
128 citations
TL;DR: Several cationic palladium complexes containing bulky phosphinidine-imine, polyimine and imine-sulfide ligands have been proposed in the literature as discussed by the authors.
81 citations
TL;DR: The database content, the procedures used in its construction, the software made available to the user and a number of potential uses for the data are described.
Abstract: CRYSTMET is a database of critically evaluated crystallographic data for metals (including alloys, intermetallics and minerals) and associated bibliographic, chemical and physical information. Also included are simulated powder diffraction patterns for all of the entries. The database currently contains almost 70000 entries and covers the literature exhaustively from 1922 to the present. The database is available on CD-ROM with search/analysis software for use on personal computers. This software can be used with any database in the appropriate format; currently CRYSTMET and the ICSD databases are available. This paper describes the database content, the procedures used in its construction, the software made available to the user and a number of potential uses for the data.
74 citations
TL;DR: Homochiral 3D metal-organic open frameworks based on enantiopure atropisomeric dicarboxylic acid bridging ligands and dilanthanide secondary building units exhibit an unprecedented 4(9)6(6) topology, and possess ethoxy-protected BINOL functionalities that are pointing toward the open channels.
66 citations
TL;DR: The structure of Complex 3 contains two terminal hydrides and one bridging hydride ligand which exchange with a barrier of 9.1 kcal/mol as observed by (1)H NMR spectroscopy.
Abstract: Hydrogenolysis of [Cp*(PMe3)Rh(Me)(CH2Cl2)]+BAr‘4- (4, Ar‘ = 3,5-C6H3(CF3)2) in dichloromethane afforded the nonclassical polyhydride complex [Cp*(PMe3)Rh(H)(H2)]+BAr‘4- (1), which exhibits a single hydride resonance at all accessible temperatures in the 1H NMR spectrum. Exposure of solutions of 1 to D2 or T2 gas resulted in partial isotopic substitution in the hydride sites. Formulation of 1 as a hydride/dihydrogen complex was based upon T1 (T1(min) = 23 ms at 150 K, 500 MHz), JH-D (ca. 10 Hz), and JH-T (ca. 70 Hz) measurements. The barrier (ΔG⧧) to exchange of hydride with dihydrogen sites was determined to be less than ca. 5 kcal/mol. Protonation of Cp*(PMe3)Rh(H)2 (2) using H(OEt2)2BAr‘4 resulted in binuclear species [(Cp*(PMe3)Rh(H))2(μ-H)]+BAr‘4- (3), which is formed in a reaction involving 1 as an intermediate. Complex 3 contains two terminal hydrides and one bridging hydride ligand which exchange with a barrier of 9.1 kcal/mol as observed by 1H NMR spectroscopy. Additionally, the structures of 3 a...
65 citations
TL;DR: In this article, the authors define functional stability and scale, the dynamic stability of forest ecosystems, and the persistence of Biodiversity as the product of an organism, a successional sere, and an ecological process.
Abstract: Even as ecologists documented the importance of disturbance and non-equilibrium in vegetation dynamics, they began to ask whether the dramatic changes at the local, patch scale, would contribute to an equilibrium at larger spatial scales. Heinselman (1973) hypothesized a ‘shifting mosaic’ for the dynamics of boreal forests in which the spatial location of recent burns and stands of various successional ages would shift, but each would continue to be present within the larger landscape at some relatively constant frequency. The ‘mosaic-cycle concept of ecosystems’ (Remmert 1991) also conjectured that an overall dynamic equilibrium would be maintained in spite of local dynamic changes. Recent studies of disturbance focused on functional resilience, biodiversity and landscape equilibrium (e.g. Romme 1982, Turner et al.1993, Peterson et al. 1998, Engelmark et al. 1999, Frehlich and Reich 1999, White et al. 1999, Walker et al. 1999, White and Jentsch 2001). We defi ne function as the product of an ecological unit (e.g. an organism, a successional sere) and an ecological process (e.g. photosynthesis, nitrogen fi xation). Thus, functional stability and Scale, the Dynamic Stability of Forest Ecosystems, and the Persistence of Biodiversity
57 citations
TL;DR: The present work demonstrates the potential of constructing chiral sensory materials based on well-defined enantiopure oligonaphthyls and can be effectively and enantioselectively quenched with trans-1,2-diaminocyclohexane with an enantiosity factor of 1.24.
Abstract: A series of well-defined enantiopure 1,1‘-binaphthyl-based oligomers linked through their 6,6‘-positions, ranging from quaternaphthol to decanaphthol, have been synthesized by Suzuki and Stille coupling reactions. These novel oligonaphthyls have been characterized by 1H and 13C{1H} NMR spectroscopy and high-resolution mass spectrometry. A combination of X-ray structural and CD studies suggests that these oligonaphthyls adopt zigzag but not helical conformations. As the chain length increases, the compounds show enhanced fluorescence. The fluorescence intensity of oligonaphthols is almost 2 orders of magnitude higher than that of 1,1‘-bi-2-naphthol and can be effectively and enantioselectively quenched with trans-1,2-diaminocyclohexane with an enantioselectivity factor of 1.24. The present work thus demonstrates the potential of constructing chiral sensory materials based on well-defined enantiopure oligonaphthols.
53 citations
TL;DR: In this article, a barrier of 9.4 kcal/mol for platinum migration around the η2-p-xylene ring was calculated for complex 6 at 183 K following variable-temperature NMR measurements.
47 citations
TL;DR: The Mn(III) meso-tetrakis(N-ethylpyridinium-2-yl)porphyrin, as a mixture of atropoisomers, has been shown to offer protection in oxidative stress injury ascribed to its high reactivity toward superoxide as a consequence of its favorable redox potential.
Abstract: Rotational (atropo-) isomers of Mn(III) meso-tetrakis(N-alkylpyridinium-2-yl)porphyrins and corresponding metal-free porphyrin ligands (where alkyl is methyl, ethyl, n-butyl, n-hexyl) and Zn(II) meso-tetrakis(N-methyl(ethyl,n-hexyl)pyridinium-2-yl)porphyrins were separated and isolated by reverse-phase HPLC. The identity of the rotational isomers of metal-free meso-tetrakis(N-methylpyridinium-2-yl)porphyrin was established by 1H NMR spectra and by the crystal structure of the fastest eluting fraction (Rf = 7.7%, Rw = 9.2%, P21/c, Z = 8, a = 14.2846(15) A, b = 22.2158(24) A, c = 29.369(3) A, β = 95.374(2)°) which, in accordance with 1H NMR interpretation, proved to be the αβαβ isomer. This result, together with elution intensity patterns, was used to identify the fractions of other Mn(III)-porphyrins, Zn(II)-porphyrins, and corresponding metal-free ligands in the series. All of the atropoisomers were inert toward isomerization which was not observable for 30 days at room temperature and reached only 50% in...
TL;DR: A chiral palladium catalyst [(S)-MeObiphep)Pd(NCAr)2(SbF6)2, (S)-4c], has been developed for a variety of asymmetric transformations and has activity comparable to that of the nitrile free Lewis acid catalyst for Diels-Alder, hetero-Diels- alder, and glyoxylate-ene reactions.
TL;DR: In this article, poisoning experiments with (R)- and (S)-BINAM indicate that while the chiral cavity can differentiate the poison chiral poisons, it is the CHI ligand which controls the enantioselectivity of the ene product.
Journal Article•
TL;DR: The Pt(II) Lewis acids P2Pt(OTf)2 (2; P2 = dppe (a), R-BINAP (b)), which catalyze the Diels−Alder reaction of acryloyl-N-oxazolidinone (4) with cyclopentadiene (HCp), were generated in situ by activation of P 2Pt precursors 1 with 2 equiv of triflic acid (HOTf), and catalysts 2 and catalytically relevant Lewis acid−Lewis base complexes
TL;DR: Aryloxy carbyne complexes are the favored products with stronger nucleophiles, while weaker ones result in a mixture of aryloxides and η2-ketenyl coupled complexes as discussed by the authors.
01 Jan 2002
TL;DR: The Fasti as discussed by the authors provides a valuable reference point for thinking about the problem of second editions in Ovidian poetry, and the Fasti provides a reading in terms of Augustan ideology, whether with or against the grain.
Abstract: Ovid is the only one who testifies to having suppressed poems and to having reissued books in new formats. The poems of the Amores were all he cared to acknowledge from his twenties, but perhaps not all that he produced. The contraction of data has at least the advantage of shifting attention from his life to his poems and to the spirit in which they address the Augustan milieu. When Ovid published the letters comprised in the Tristia , he suppressed the names of the recipients because he feared that they might feel compromised to be associated with him. Of the major Ovidian works, the Fasti most openly invites a reading in terms of Augustan ideology, whether with or against the grain. The Fasti also provides a valuable reference point for thinking about the problem of second editions in Ovid. Keywords: Amores ; Augustan milieu; Fasti ; Ovid; Tristia
TL;DR: In this article, photolysis has been used to release the elaborated alkynes from the metal complex of Tp(CO)(NHR)W{CH3C⋮CCH3} (NH2R = ((S)-(−)-α-methylbenzylamine), by fractional crystallization.
16 Sep 2002
TL;DR: Piaget's theory of cognitive development serves as a basis for a comparative analysis of middle school students' understanding of classification in the physical and digital library and implications for the design of educational science digital libraries are discussed.
Abstract: Piaget's theory of cognitive development serves as a basis for a comparative analysis of middle school students' understanding of classification in the physical and digital library. Attention is also given to student comprehension of scientific classification. Results of this pilot study show that although participants had good comprehension of classification principles in the physical environment, with which they are more familiar, their understanding diminishes in the digital environment and when addressing scientific classification. Results are compared to an earlier study and implications for the design of educational science digital libraries are discussed.
TL;DR: In this article, the X-ray cell data and the ab initioz coordinates were then used in the production of reliable calculated powder patterns for CoTe2, CrSe2, HfS2 and ZrTe2 with no coordinates in CRYSTMET.
Abstract: There are 39 CRYSTMET® entries in the hexagonal space group P-3m1 (164) reporting both distinct pure phase compounds and atomic coordinates. Having the same Wyckoff positions in the same space group as the C6 structure type, all are isopointal with it. The range of observed c/a values extends from about 0.65 to 1.83. Three types are distinguished: Layered materials with CdI2 type, the CeCd2 type which is a slight distortion of the hexagonal AlB2 type, and the intermediate EuGe2 type made of the materials AuTe2, BaSi2, EuGe2, and SrGe2. Ab initio modeling of the 26 entries with CdI2 and EuGe2 type and atomic coordinates reproduces convincingly both their c/a axial ratios and z coordinates. For CoO2 and SiTe2, both c/a and z deviate to a degree from the reported values, indicating that those materials should be reexamined for superstructures, stoichiometry, etc. Ab initio modeling of the 11 cell-and-type entries with CdI2 type and no coordinates in CRYSTMET reproduced convincingly their reported axial ratios. The X-ray cell data and the ab initioz coordinates were then used in the production of reliable calculated powder patterns for CoTe2, CrSe2, HfS2, HfSe2, HfTe2, NbTe2, SnSe2, VS2, VTe2, ZrS2, and ZrTe2. All 11 patterns have been inserted in the intense diffraction line search system of CRYSTMET operated under the Materials Toolkit. Comparison of calculated patterns for SnSe2 and ZrTe2 with experimental entries in the PDF exposes the complementarity of calculated and experimental powder patterns and suggests that JCPDS pattern #15-223 should be reinterpreted in terms of the CdI2 structure type. The CeCd2⇔AlB2 type transformation is modeled and discussed on YCd2 using both ab initio methods and a hard-sphere model. For z<0.45, the ab initio solution is identical with that from the hard-sphere model while a quantum regime is predicted in the small region 0.45
TL;DR: XAS is among the major applications at synchrotron radiation facilities and many existing beamlines perform the measurement of XAFS (X-ray absorption fine structure) spectra, with determination of which models best fit the measured data.
Abstract: The X-ray absorption spectrometry (XAS) technique has been widely used to determine the local structure of materials that are poorly suited to study by ordinary diffraction methods, such as fine particles or amorphous matter. XAS is among the major applications at synchrotron radiation facilities and many existing beamlines perform the measurement of XAFS (X-ray absorption fine structure) spectra. XAFS spectra can also be measured with conventional X-ray sources. Measurement of XAFS spectra is relatively straightforward, but real difficulties arise in the analysis and interpretation of the data. Contrary to single-crystal diffraction techniques, the structure is not obtained directly from the measured XAFS data. Model structures must be assumed and the corresponding simulated XAFS spectra must be calculated, with determination of which models best fit the measured data. Model building is a most important part of XAFS analysis, but creation of three-dimensional structures from crystal-chemical considerations can be a very time-consuming task. Utilization of a database of crystal structures and of its built-in structure analysis and display tools can considerably reduce the time and effort required by this task. As XAS is often used to study metals, a database of alloys and intermetallic compounds, such as CRYSTMET, incorporating an array of powerful tools, is then very useful.
TL;DR: In this paper, a linear regression on a plot of x coordinates optimized ab initio at experimental cell data versus X-ray coordinates for all 48 C16-type compositions for pure-phase compounds with coordinates in the CRYSTMET database gave y = 0998x, R2 = 088.
Abstract: Searchable calculated powder patterns of crystalline substances are commonly used nowadays to complement experimental patterns for the identification of materials by diffraction methods Reliable such patterns are often calculable from the known cell, type and chemistry data on types with fixed Wyckoff positions, like NaCl, but not on type C16 of AB2 compounds, with CuAl2 as its representative material, because of the variable x coordinate of its B site Ab initio cell and coordinate relaxation for four key compounds with C16 type, all started from the same c/a = 1 and x = 1/6 configuration, converged on the correct axial ratios and fractional coordinates A linear regression on a plot of x coordinates optimized ab initio at experimental cell data versus X-ray coordinates for all 48 C16-type compositions for pure-phase compounds with coordinates in the CRYSTMET database gave y = 0998x, R2 = 088 Credible calculated powder patterns were then produced for AlHf2, AlZr2, AuTl2, BCr2, GaHf2, IrZr2 and TlTh2, all with cell, type and pure-phase composition information but no coordinates in this database, using their X-ray cell data and ab initio coordinates Those patterns, now inserted in the search system of the database, will help users identify those phases from powder diffraction angle and intensity data The corresponding entries in a new `X-ray cell-and-type, ab initio coordinates' category of full structure entries may be helpful for other applications, such as the simulation of EXAFS spectra or of backscattered electron patterns
Journal Article•
TL;DR: The X-ray absorption spectrometry (XAS) technique has been widely used to determine the local structure of materials that are poorly suited to study by ordinary diffraction methods, such as fine particles or amorphous matter as mentioned in this paper.
Abstract: The X-ray absorption spectrometry (XAS) technique has been widely used to determine the local structure of materials that are poorly suited to study by ordinary diffraction methods, such as fine particles or amorphous matter. XAS is among the major applications at synchrotron radiation facilities and many existing beamlines perform the measurement of XAFS (X-ray absorption fine structure) spectra. XAFS spectra can also be measured with conventional X-ray sources. Measurement of XAFS spectra is relatively straightforward, but real difficulties arise in the analysis and interpretation of the data. Contrary to single-crystal diffraction techniques, the structure is not obtained directly from the measured XAFS data. Model structures must be assumed and the corresponding simulated XAFS spectra must be calculated, with determination of which models best fit the measured data. Model building is a most important part of XAFS analysis, but creation of three-dimensional structures from crystal-chemical considerations can be a very time-consuming task. Utilization of a database of crystal structures and of its built-in structure analysis and display tools can considerably reduce the time and effort required by this task. As XAS is often used to study metals, a database of alloys and intermetallic compounds, such as CRYSTMET, incorporating an array of powerful tools, is then very useful.