Showing papers by "Peter S. White published in 1994"

Scale Dependence and the Species-Area Relationship

Abstract: The complex relationship between species richness and area can be simplified by decomposing spatial scale into its components: grain, extent, and number of samples. We designed a 256 x 256-m study grid in the Oosting Natural Area in the Duke Forest, Orange County, North Carolina, such that the effects of these components can be disentangled. We found that grain, extent, and the number of samples all influenced the species-area relationship, although the effects of grain were dominant. We also found that species richness patterns were neither self-similar nor hierarchical. The degree to which diversity occurs in "hot spots" increases as a function of both grain and extent, but diversity hot spots tend to persist across a wide range of grains.

On the existence of ecological communities

Abstract: uncontroversial assumption that living organisms arepresent – because organisms live, consume, reproduce,and die; nutrients arrive by various means, and nutrientsleave by various means. Lindeman therefore defined theecosystem into existence. Although he certainly be-lieved that some processes regulated ecosystem struc-ture, his ideas of regulation did not impinge upon thedefinition of ecosystem. Ecosystems are not defined asintegrated, discrete entities.We suggest that community should be defined in aparallel manner as “the living organisms present withina space-time unit of any magnitude”. The statement“communities exist” is no longer a fruitless ontologicalproblem (as Keddy 1993 suggested) but a tautology. Byusing an operational definition, we can now ask ques-tions concerning the properties of communities, and theforces underlying variation in communities.Wilson (1991) asked whether “plant communitiesexist in a more meaningful sense, as integrated, discreteentities”. Keddy (1993) argued that this question isunscientific. Our operational definition transformsWilson’s question to “Are there any communities whichare integrated and discrete?” This reformulation shouldbe scientifically valid by Keddy’s standards, as long asprocedures for defining and detecting integration anddiscreteness can be identified.Wilson implies that the general usage of the termcommunity connotes a strong degree of integration.Keddy believes that it is possible (but scientificallyvacuous) to list criteria which distinguish whether asystem exists as a community; these criteria are notlisted, but presumably involve some sort of integrationbetween species/and or environment. Some ecologistsdo indeed believe that integration is the key to defini-tions of community. In attempting to establish an episte-mology of ecology, Scheiner (1993) considered com-munities as natural entities which are defined by ‘link-ing processes such as interactions among populations.Allen & Hoekstra (1990) recognized that there is muchKeywords: Biome; Community concept; Communityecology; Heterogeneity; Homogeneity.IntroductionWilson (1991) raised the question of whether plantcommunities exist. He included the existence of assem-bly rules, niche limitation, discreteness, discontinuity,and integratedness as potential criteria for the existenceof communities. In response, Keddy (1993) argued thatthough it may be possible to draft a list of criteria for theexistence of communities, the debate over existence ofcommunities is an ontological and epistemological gamewhich is peripheral (and, Keddy argued, even harmful)to scientific progress. We suggest that community ecolo-gists define community operationally, with as little con-ceptual baggage as possible, so that we can put thedebate about their existence behind us.An operational view of the communityDefinitionsLindeman (1942) defined the ecosystem as “thesystem composed of physical-chemical-biological proc-esses active within a space-time unit of any magnitude”.As such, it is a perfect operational definition. We beginby considering the ‘space-time unit’. The space aspectof this space-time unit could be completely arbitrary,such as a 0.1 hectare quadrat, a cubic meter of lakewater, or a political district such as a province or nation,or it could be somewhat less arbitrary, such as a lake, awatershed, or an island. The time aspect of the unitcould be measured arbitrarily as seconds or decades, orsomewhat less arbitrarily as days or years.Whatever space and time units are chosen, physical-chemical-biological processes will be present – with the

Conversion of nitriles to nitrenes via azavinylidenes in low-valent tungsten carbonyl complexes

Abstract: Azavinylidene complexes of the type Tp[prime](CO)[sub 2]W=N=CRNu are available from the reaction of [Tp[prime](CO)[sub 3]W(N=CR)][BF[sub 4]] with nucleophiles (Tp[prime] = hydridotris(3,5-dimethylpyrazolyl)borate; R = Me, Nu = H (1a); R = Ph, Nu = Et (1d); R = Me, Nu = MeO (1e)). Related azavinylidene complexes Tp[prime](CO)[sub 2]W=N=CHR (R = Me (1a); R = Et (1b); R = CH[sub 2]Ph (1c)) form via insertion of nitrile into the W-H bond of Tp[prime](CO)[sub 3]WH when the metal reagent is photolyzed in the presence of nitriles. Donation of the lone pair of electrons from the azavinylidene nitrogen to the tungsten center, compatible with electron counting guidelines, is reflected in the low IR stretching frequencies of the carbonyl ligands and their downfield carbon resonance in [sup 15]C NMR. NMR spectra and an X-ray structure of Tp[prime](CO)[sub 2]W=N=CHCH[sub 3]Ph (1c) indicate that the azavinylidene ligand is nearly linear ([sup 3]J[sub WH] = 5.6 Hz, [sup 2]J[sub WC] = 27 Hz, and [alpha](W-N-C) = 166.6(7)[degree]). Complex 1c crystallized in the monoclinic space group P2[sub 1]/n with unit cell dimensions of a = 12.163(1), b = 17.207(2), and c = 13.312(1) [angstrom] and [beta] = 102.103(7)[degree], with Z = 4. 32 refs., 2 figs., 5 tabs.

Nitrene Transfer to Trimethylphosphine from Cationic Tungsten Tosylnitrene Complexes [Tp'(CO)2W(NTs)][X]

Abstract: Reaction of Tp{prime}(CO){sub 3}WH(Tp{prime}=hydridotris(3.5-dimethyl-1-pyrazolyl)borate) with tosyl azide in THF at reflux affords the amido complex Tp{prime}(CO){sub 2}W(N(H)Ts) (1). Crystals of 1 are monoclinic, space group P2{sub 1}/n, with cell parameters a = 12.4855(11) A, b=20.5976(16){Angstrom}, c= 12.7311(11){Angstrom}, {beta}=96.510(10){degrees}, V=3253.0(5) {Angstrom}{sup 3}, and D{sub calcd}=1.591 g cm{sup {minus}3} for Z = 4. Least-squares refinement led to final R and R{sub w} values of 0.038 and 0.046, respectively. Complex 1 undergoes oxidation by silver triflate or iodine to give the imido complexes [Tp{prime}-(CO){sub 2}W(NTs)]X (X=OTf, 2a; X=I{sub 3},2b). The single crystal X-ray structure of 2b is also reported (orthorhombic, space group Pcab, a= 15.1764(16) {Angstrom}, b = 16.8890(14) {Angstrom}, c=26l909(3) {Angstrom}, V=6897.2(13) {Angstrom}{sup 3}, and D{sub calcd}= 2.094 g cm{sup {minus}3} for Z=8; R=0.044, R{sub 2}=0.052). The cationic nitrene complex 2 reacts with LiBH{sub 4} to regenerate the amido compound 1. Addition of trimethylphosphine to solutions of 2 leads to the formation of three species the phosphinimine TsN=PMe3 (3) and two tungsten complexes, [Tp{prime}(CO){sub 2}W(PMe{sub 3}){sub 2}][X](X = OTf, 4a; X=I{sub 3}, 4b) and [Tp{prime}(CO)(PMe{sub 3})W(NTs)][X] (X=OTf, 5a; X=I{sub 3}, 5b). Complex 5b undergoes substitution by iodide to produce the tungsten monocarbonyl Tp{prime}(CO)IW(NTs) (6).

Calixresorcinarenes as Ligands: Synthesis and Characterization of Transition-Metal Cavitand Complexes

Abstract: A partially-unlinked cavitand, derived from the condensation of acetaldehyde and resorcinol and having three ethylene bridges, functions as a monodentate ligand toward transition metal ions. Treatment of the cavitand with MCl 2 and a bidendate ligand (L) in the presence of NaOH led to isolation of metal complexes with the formula (cavitand) 2 ML (M=Co, Cu, Zn) or (cavitand) 2 ML 2 (M=Ni). The square-planar complexes of Cu(II) and Co(II), the tetrahedral complex of Zn(II), and the octahedral complex of Ni(II) were characterized spectroscopically and in one case by X-ray crystallography

Preparation of N(SeCl)2+X–(X = SbCl6or FeCl4), F3CCSeNSeCCF3+SbCl6–, F3CCSeNSeCCF3, F3CCSeNSeCCF3and F3CCSeSeC(CF3)C(CF3)SeSeCCF3. Electron diffraction study of F3CCSeSeCCF3and crystal structure of the eight-membered heterocycle F3CCSeSeC(CF3)C(CF3)SeSeCCF3

Abstract: The salts N(SeCl)2+SbCl6–1 and N(SeCl)2+FeCl4–2 were synthesized by reaction of SeCl3+ X–(X = SbCl6 or FeCl) with N(SiMe3)3; 1 was also formed by reaction of Se2NCl3 with SbCl5. Reaction of 1 with SnCl2 and F3CCCCF3 led to the formation of F3C[graphic omitted]CF3+ SbCl6–3. In this reaction the Se2N + cation is a likely intermediate because SnCl2 seems to be essential for chloride abstraction in the first reaction step to generate Se2N+in situ which then adds F3CCCCF3 to yield 3. Compound 3 is a useful building block to generate selenium compounds such as F3C[graphic omitted]CF34, F3C[graphic omitted]CF35 and F3C[graphic omitted]CF36. The heterocycle 5 was shown by electron diffraction to have an approximately planar four-membered ring structure. The structure of compound 6 was determined by X-ray crystallography: orthorhombic, space group Pbca, a= 10.1920(21), b= 13.0615(20) and c= 22.050(5)A. In order to rationalize the structures of 5 and the cation F3C[graphic omitted]CF3+, ab initio calculations were made on model compounds in which the CF3 groups were replaced by a fluorine atom (i.e.F[graphic omitted]F for 5and F[graphic omitted]F+ for the cation in 3). In addition, mass spectrometric experiments were performed in order to examine the structures and stabilities of the unligated cation F3C[graphic omitted]CF3+ as well as its neutral counterpart. The existence of the neutral radical 4 was established by means of neutralization–reionization mass spectrometry.

Solution and solid-state structural chemistry of tungsten pyrazolylborate π-allyl complexes ; 2D 183W and 1H NOESY NMR studies

Abstract: 2D 1 H NOESY and inverse detected 183 W NMR measurements for the pseudo-octahedral η 3 -allyl complexes W(CO) 2 (Tp')(η 3 -CHCDPh) (1) and W(CO) 2 (Tp')(η 3 -CH 2 CHCHCH 3 ) (2), where Tp'=hydridotris(3,5-dimethylpyrazolyl)borate are reported. The observed solution structure for 1 differs from that found in the solid-state via x-ray diffraction (isomers can arise due to allyl rotation and/or coordination of either allyl face). Crystal data for 1: WC 26 H 31 BN 6 O 3 , space group P2 1/c , a=9.775(1) A, b=17.360(2) A, c=16.144(5) A, β=102.26(2) o , V=2677(1) A 3 , Z=4

X-RAY CRYSTAL STRUCTURE OF A MONOMERIC TRIS(ARSINO)GALLANE, [(Me3Si)2As]3Ga

Abstract: The solid-state structure of [(Me3Si)2As]3Ga (1) has been established by single-crystal X-ray analysis. Triclinic crystals of 1 belong to the space group P1, with a = 10.7529(23), b = 10.7899(23), c = 17.55(6) A, α = 88.077(23)°, β = 84.537(23)°, γ = 60.282(16)° for Z = 2. Refinement of atomic parameters converged at R = 0.058 (Rw = 0.064) for 2378 observed reflections with I>2.5σ(I). The monomeric molecule adopts a trigonal planar configuration with Ga-As = 2.4171(23), 2.4250(22) and 2.4213(24) A, and As-Ga-As (av.) = 120.00(1)°. Compound 1 is only the second example of a monomeric tris(arsino)gallane to be structurally characterized in this manner.

[(SeCl)2][FeCl4]: Synthesis, Quantumchemical Calculations, Vibrational Data and Single‐Crystal X‐ray Structure

Abstract: The tetrachloroferrate salt of the [(SeCl)2N]+ cation, [(SeCl)2N][FeCl4] (1), has been prepared and structurally characterized. The [(SeCl)2N]+ cation exists in a C2v “u”- shaped geometry with the following structural parameters: d(Se-Cl) = 2.16 A, d(SeN) = 1.70 A,· (ClSeN) = 108° and· (SeNSe) = 147°. The harmonic vibrational frequencies for two isomers (“u” and “s” isomer) have been computed by ab initio methods and are compared with the experimental IR and Raman spectra.

Fluorierung von Cyanurchlorid und Tieftemperatur‐Kristallstruktur von [(ClCN)3F]+ [AsF6]−

Abstract: Durch Tieftemperaturfluorierung von Cyanurchlorid, (ClCN)3, mit F2/AsF5 in SO2F2-Losung wurde [(ClCN)3F]+ [AsF6]− (1) in nahezu quantitativer Ausbeute dargestellt, aus SO2 bei Raumtemperatur umkristallisert und durch eine Tieftemperatur-Rontgenstrukturanalyse (−170°C) identifiziert: R 3c, trigonal, a = b = 10,4246(23) A, c = 5,1850(24) A, V = 1 429,1(4) A3, Z = 6, RF = 0,056, Rw = 0,076 (fur wesentliche Reflexionen), RF = 0,088, Rw = 0,079 (fur alle Reflexe). Die Fluorierung von festem (ClCN)3 mit [NF4]+ [Sb2F11]− im Molverhaltnis 1:2 lieferte NF3, CClF3, SbF3, N2 und Spuren an CF4. Eine qualitative Reihenfolge bezuglich der FPDE-Werte von NF4+ und (XCN)3F+ (X = H, F, Cl) wurde ab initio berechnet. Fluorination of Cyanuric Chloride and Low-Temperature Crystal Structure of [(ClCN)3F]+[AsF6]− The low-temperature fluorination of cyanuric chloride, (ClCN)3, with F2/AsF5 in SO2F2 solution yielded the salt [(ClCN)3F]+ [AsF6]− (1) essentially in quantitative yield. Compound 1 was identified by a low-temperature single crystal X-ray structure determination: R 3c, trigonal, a = b = 10.4246(23) A, c = 15.1850(24) A, V = 1429.1(4) A 3, Z = 6, RF = 0.056, Rw = 0.076 (for significant reflections), RF = 0.088, Rw = 0.079 (for all reflections). Fluorination of neat (ClCN)3 with [NF4]+ [Sb2F11]− yielded NF3, CClF3, SbF3, N2 and traces of CF4. A qualitative scale for the oxidizing strength of the oxidative fluorinators NF4+ and (XCN)3F+ (X = H, F, Cl) has been computed ab initio.

On the Synthesis of Mixed-Metal and Mixed-Pnicogen 13-15 Ring-Systems.

Abstract: : Several pathways toward the synthesis of mixed-metal and mixed-pnicogen 13-15 ring systems have been explored. Equilibration reactions have proven successful toward the preparation of the latter compounds while most attempts to prepare mixed-metal compounds have resulted in unexpected and novel 13-15 species. These reactions and their products are discussed.

Synthesis and Characterization of a Neutral Six-coordinate Tris-adduct of an Indium(III) Trihalide. X-ray Crystal Structure of InCl3(THF)3

Abstract: : The reaction of InCl3 with excess THF afforded the adduct InCl3(THF)3 in a good yield (77.1% based on InCl3), and the X-ray crystal structure shows it to be a hexa-coordinated tris-adduct of indium(III) chloride. The crystal structure consists of discrete monomeric units with no abnormally short intermolecular separations. The geometry about the indium atom approximates to octahedral with the three chlorine atoms (mean In-Cl = 2.422 A) and the oxygen atoms of each THF molecule (mean In-O = 2.254 A) residing in the meridional conformation. Crystals of InCl3(THF)3 belong to the monoclinic space group P 2 sub 1/c with unit cell parameters a = 8.1863(2)A, b = 12.5141(2) A, c =16.815(4) A, Beta = 93.84(4) deg, V = 1718.8(7) A3 for Z = 4. Full-matrix least-squares refinement based on 1603 reflections with I > 2.5(sigma) (I) converged at R sub w = 0.062 (R sub w = 0.084). IR spectroscopic measurements gave absorption bands near 1027 and 857/cm, which are consistent with coordinated THF molecules. Variable temperature NMR studies showed that the tris-adduct was stable to disproportionation in solution between 20-60 deg C. Indium compound, Six coordinate synthesis, Crystal structure.