Showing papers by "Peter S. White published in 1990"

Light regimes beneath closed canopies and tree-fall gaps in temperate and tropical forests

Abstract: Light regimes beneath closed canopies and tree-fall gaps are compared for five temperate and tropical forests using fish-eye photography of intact forest canopies and a model for calculating light penetration through idealized gaps. Beneath intact canopies, analyses of canopy photographs indicate that sunflecks potentially contribute 37–68% of seasonal total photosynthetically active radiation. In all of the forests, potential sunfleck duration is brief (4–6 min), but the frequency distributions of potential sunfleck duration vary because of differences in canopy geometry and recent disturbance history. Analysis of the photographs reveals that incidence angles for photosynthetically active radiation beneath closed canopies are not generally vertical for any of the forests, but there was considerable variation both among and within sites in the contribution of overhead versus low-angle lighting. Calculations of light penetration through idealized single-tree gaps in old growth Douglas-fir – hemlock forests...

Preparations and crystal structures of Se2I4(AsF6)2.SO2 and Se2I4(Sb2F11)2 and electronic structure of the eclipsed diselenium tetraiodide dication, containing two SeI2 radical cations joined via a .pi.*-.pi.* bond and a delocalized 4p.pi.-5p.pi. bond. Implications for bonding in S2O42-, Se82+, and S82+

Abstract: Se 2 I 4 (AsF 6 ) 2 •SO 2 and Se 2 I 4 (Sb 2 F 11 ) 2 were prepared quantitatively by reacting stoichiometric amounts of Se, I 2 , and AsF 5 and by reacting Se and I 2 Sb 2 F 11 in liquid SO 2 , respectively. The 77 Se NMR spectra (−70°C) of the salts in SO 2 solution indicated the presence fo Se 4 2+ , SeI 3 + . Crystal data for Se 2 I 4 (AsF 6 ) 2 •SO 2 : monoclinic, space groupe P2 1 /c. Crystal data for Se 2 I 4 (Sb 2 F 11 ) 2 : triclinic, space group P1

Experimental evidence for comparable strength of phosphorus-sulfur and phosphorus-nitrogen p.pi.-bonding within the heteronaphthalenic framework: spectroscopic and structural studies of the 1,3,2-benzazathiaphospholium system as a cation and as neutral halides

Abstract: The new heteronaphthalenic framework 1,3,2-benzazathiaphospholium has been prepared as a free cation 1[NSP] and as the neutral chloride 2[NSPCl] and bromide 2[NSPBr] derivatives. The spectroscopic and structural (crystal data for C 6 H 5 AlCl 4 NPS: monoclinic, P2 1 /c, a=6.4773 (6) A, b=23.5434 (21) A, c=8.5101 (6) A, β=98.048 (7) o , Z=4) features of the cation are consistent with a hybrid of the corresponding dithia 1[S 2 P] and diamino 1[N 2 P] derivatives. In particular, the P-N and P-S bond lengths are very similar to those of the parent cations and represent experimental evidence for P-N and P-S pπ-bonding of comparable strength, in agreement with the theoretical π-bond energies. The neutral halides, 2-chloro-1,2,3-benzazathiaphosphole (crystal data for C 6 H 5 ClNPS: monoclinic, P2 1 /c, a=8.5607 (10) A, b=10.6948 (10) A, c=8.5994 (13) A, β=95.516 (11) o , Z=4) and 2-bromo-1,2,3-benzazathiaphosphole (crystal data for C 6 H 5 BrNPS: monoclinic, P2 1 /c, a=8.5345 (12) A, b=10.6990 (14) A, c=8.8213 (7) A, β=94.705 (8) o , Z=4) are isostructural and adopt dimeric structures, with long P-Z bonds

Molecular mechanics calculations on template-directed steroid chlorinations : are transition states rigidified by the geometric trajectory requirements for effective energy transfer ?

Abstract: Molecular mechanics calculations have been performed on the transition states for hydrogen abstraction by chlorine atoms that are attached to the iodines of various iodoaryl templates, esterified at C-3α of cholestanol The calculations successfully account for the experimental observations in cases in which the template selectively directs chlorination to C-9, to C-14, or to C-17; they also agree with the findings in one case in which C-9 and C-14 are chlorinated equally

Cycloaddition reactions of dichlorodithionitronium hexafluoroarsenate(V) with alkynes and alkenes; the preparation and X-ray crystal structure of 1,3-dichloro-1,3,2-dithiazolidinium hexafluoroarsenate(V)

Abstract: The dichlorodithionitronium cation, ClSNSCl+, undergoes symmetry-allowed, concerted, cycloaddition reactions with alkynes and alkenes in liquid sulphur dioxide. With ethyne and propyne it gives cor...

Structure of tris(phenylthio)phosphine

Abstract: C 18 H 15 PS 3 cristallise dans R3 avec a=12,6760 et c=18,8921 A, Z=6; affinement jusqu'a R=0,031. La structure consiste en unites moleculaires discretes. Le centre phosphine a la geometrie pyramidale et la symetrie C 3 . Le compose contient un exemple rare d'une liaison simple P III -S veritable avec une longueur de liaison de 2,122 A. La liaison C-S est de 1,785 A

Synthesis of a biologically active isomer of frontalin and some new products from acylative decarboxylation of 3-methyltricarballylic acid

Abstract: 2-Acetonyl-2-methylsuccinic anhydride, 3, the major product from base-catalysed reaction of 3-methyltricarballylic acid with acetic anhydride, was converted by simple manipulation of functional groups into 1,5-dimethyl-2,7-dioxabicyclo[3.2.1]octane. This bicyclic ketal (6) is isomeric with frontalin, 7, an aggregation pheromone for Dendroctonus bark beetles. It elicited electroantennogram responses in D. ponderosae and D. rufipennis. Besides 3 and the known compounds, 4 and 5, two new minor products, 8 and 9, were identified from base-catalysed acylative decarboxylation of 3-methyltricarballylic acid. Keywords: pheromone analogue, Dendroctonus, bark beetles.

The Preparation and X-Ray Crystal Structure of Te(N3)3SbF6 Containing the Triazidotellurium(IV) Cation, Te(N3)+ 3.

Abstract: The crystal structure of Te(N3)3SbF6 containing the first binary tellurium–nitrogen cation, triazidotellurium(IV) (Te(N3)3+), has been determined by X-ray crystallography. Single crystals of Te(N3)...

Synthesis of a Biologically Active Isomer of Frontalin and Some New Products from Acylative Decarboxylation of 3-Methyltricarballylic Acid.

Abstract: 2-Acetonyl-2-methylsuccinic anhydride, 3, the major product from base-catalysed reaction of 3-methyltricarballylic acid with acetic anhydride, was converted by simple manipulation of functional groups into 1,5-dimethyl-2,7-dioxabicyclo[3.2.1]octane. This bicyclic ketal (6) is isomeric with frontalin, 7, an aggregation pheromone for Dendroctonus bark beetles. It elicited electroantennogram responses in D. ponderosae and D. rufipennis. Besides 3 and the known compounds, 4 and 5, two new minor products, 8 and 9, were identified from base-catalysed acylative decarboxylation of 3-methyltricarballylic acid. Keywords: pheromone analogue, Dendroctonus, bark beetles.

Cyclopentadienyl Oxides of Molybdenum.

Abstract: An improved preparation of CpMoCl 2 (O) (Cp=η-C 5 H 5 ) from [MoCl 3 (O)] n and Bu 3 n Sn(C 5 H 5 ) is described. Reduction of CpMoCl 2 (O) with zinc powder in tetrahydrofuran (THF) gave diamagnetic {[CpMoCl] 4 (μ-O) 6 }(ZnCl(THF)) 2 , formally containing two Mo(IV) and two Mo(V) centers. Reoxidation of {[CpMoCl] 4 (μ-O) 6 }(ZnCl(THF)) 2 with O 2 gave green [CpMoCl(O)] 2 (μ-O) (containing two Mo(V) centers), colorless [ZnCl(THF)(μ-Cl)] ∞