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91.
Jones MA Britz DA Morton JJ Khlobystov AN Porfyrakis K Ardavan A Briggs GA 《Physical chemistry chemical physics : PCCP》2006,8(17):2083-2088
The endohedral fullerene epoxide N@C60O was synthesised, isolated by High Performance Liquid Chromatography (HPLC), and characterised by Electron Spin Resonance (ESR). This nitrogen radical displays predominantly axial symmetry characteristics as expected for a monoadduct, evidenced by a zero-field splitting D parameter of 6.6 MHz and an E parameter of 0.5 MHz in powder at 77 K. Photo- and thermally-activated silencing of the nitrogen radical were observed, the latter showing the evolution of a new spin signal during heating at 100 degrees C. We suggest that loss of nitrogen spin is due to coupling with a radical formed by opening of the epoxide ring. This implies that the reaction of C60O with C60 in the solid state proceeds via a radical, rather than ionic, intermediate. 相似文献
92.
[structure: see text]. A one-pot, diastereoselective synthesis of a cis,cis-tris[60]fullerene adduct of 6,8,15,17-tetraphenylheptacene has been demonstrated starting from [60]fullerene and 5,7,9,14,16,18-hexahydro-6,8,15,17-tetraphenylheptacene. 相似文献
93.
Previously published differential pulse methods for the determination of certain progestogens (laevonorgestrel and norethisterone) are not applicable to combined low-dosage oral contraceptives because interference from excipients in the tablets completely eliminates the polarographic response. An ultrafiltration device allows rapid prior extraction of the interfering substances before the polarographic determination in 50% () methanol—phosphate buffer (pH 6.0). Recoveries of 100 ± 1% were obtained by the recommended method, and data for a range of formulations are in excellent agreement with expected values. Electrode characteristics of the progestogens and interfering substances are reported, based on studies employing cyclic voltammetry at a hanging mercury drop electrode, a.c. polarography and normal pulse polarography. Competitive adsorption processes seem to occur when the progestogen and excipients are simultaneously present. 相似文献
94.
Caroline R.S. Briggs David O’Hagan Judith A.K. Howard 《Journal of fluorine chemistry》2003,119(1):9-13
The conformation of 2-fluoro-N-(2-fluoroethyl)-propionamide 4 in the solid state indicates the influence of both a β-fluorine-amide gauche effect and an α-fluoroamide effect. The structure reveals the influence of two recently observed stereoelectronic effects associated with the CF bond, which has resulted in the successful prediction of the solid state conformation of amide 4. A gauche relationship (−69.9°) was observed for atoms N(1)C(4)C(5)F and a syn planar (2.0°) relationship was observed for N(1)C(3)C(2)F. The paper demonstrates the predictive power of using the CF bond as a tool in influencing the conformation of amides and peptides. 相似文献
95.
Single crystal X-ray diffraction studies and a theoretical analysis indicate a preferred conformation for O-β-fluoroethyl esters, where the CF and CO(CO) bonds are gauche rather than anti to each other. The OCCF dihedral angles for three compounds and five independent structures indicate a range of only 63.4-69.6°. Evaluation of a rotational energy profile around this bond in a model system (β-fluoroethyl acetate) predicted a similar dihedral angle and the gauche conformation to be the minimum on the rotational energy profile. High level ab initio calculations measured the gauche conformer to be 0.95 kcal mol−1 lower in energy than the anti conformer and application of a solvation model further increased this differential to 1.6 kcal mol−1, consistent with a previous solution state (NMR) evaluation of this system. 相似文献
96.
John R. Briggs Christopher Crocker Walter S. McDonald Bernard L. Shaw 《Journal of organometallic chemistry》1979,181(1):213-221
Five complexes of type cis-[PtCl2(PR3)Q] (PR3 =PMe3, PMe2Ph, PEt3; Q = CH2 CHOCOCH3 or CH2=CHCH2OCOCH3) have been prepared. The crystal structure of cis-[PtCl2[PME2Ph)(CH2=CHOCOCH3)] is described. Crystals of cis-[PtCl2(PME2Ph)(CH2-CHOCOCH3)] are triclinic, with a 8.441(4), b 13.660(5), c 7.697(3) Å, a 101.61(3)°, β 111.85(3)° γ 95.22(3)°, pP1, Z = 2. The structure was determined from 2011 reflections I σ 3σ (I) and refined to R = 0.037. The CH3COO grouping is syn to the cis-PMe2Ph ligand, with bond lengths of PtCl (trans to P) 2.367(3), PtCl (trans to olefin) 2.314(3), PtP 2.264(2), and PtC of 2.147(12) and 2.168(11) Å. The complexes cis-[PtCl2- (PR3)Q] were studied by variable temperature 1H and 31P NMR spectroscopy. Spectra of the vinyl acetate complexes were temperature dependent as a result of rotation about the platinum—olefin bond. The rotation was “frozen out” at ca. 240 K; for cis-[PtCl2(PME2Ph)(CH2=CHOCOCH3] ΔG≠ (rotation) 15.0 ± 0.2 kcal mol-1. NMR parameters for the rotamers are reported. NMR studies of the interaction between chloro-bridged complexes of type [Pt2Cl2(PR3)2] (PR3 = P-N-Pr3 or PMe2Ph) and vinyl acetate shows that even at low temperatures (213 K) equilibrium favours the bridged complex and the proportion of trans-[PtCl2(PR3)CH2=CHOCOCH3)] is very small e.g. 2%. The allyl acetate complexes cis-[PtCl2(PR3)(CH2=CHCH2OCOCH3)] showed only one rotamer over the range 333–213 K. Reversible dissociation of cis-[PtCl2(PMe2Ph)- (CH2=CHCH2OCOCH3)] to [Pt2Cl4(PMe2Ph)2] + allyl acetate was studied at ambient temperature. At low temperatures e.g. 213–190 K addition of allyl acetate to a CDCl3 solution of [Pt2Cl2(P-n-Pr3)2] reversibly gave some olefin complex trans-[PtCl2(P-n-Pr3)(CH2=CHCH2OCOCH3)] and some O-bonded complex trans-[PtCl2(P-n-Pr3)(CH2=CHCH2OCOCH3)]. 相似文献
97.
Steven J. Britz Erich Schrott Susanne Widell Winslow R. Briggs 《Photochemistry and photobiology》1979,29(2):359-365
Abstract— In the presence of methylene blue, red light causes the reduction of a h-type cytochrome in particulate fractions from corn coleoptiles. Two types of difference spectra for the cytochromes in these fractions are presented: (a) red light-minus-dark in the presence of methylene blue, and (b) dithionite-reduced-minus-oxidized. Comparison of these spectra shows that photoexcited methylene blue selectively reduces a b-type cytochrome which constitutes at most only 30% of the total dithionite-reducible cytochrome present in the most active fractions. The photoreducible cytochrome has an alpha band at room temperature near 557 nm. Bleaching of methylene blue precedes cytochrome reduction under appropriate conditions, suggesting that the photoreduced dye is donating an electron to the cytochrome. This electron transfer does not involve a flavin, at least as judged by the absence of light-induced spectral changes attributable to flavins. Preliminary kinetic studies suggest that EDTA provides the pool of electrons for the reaction. The cytochrome cannot be assigned exclusively either to mitochondria or to endoplasmic reticulum, as judged by its sedimentation properties. These results and the current literature are discussed in the context of the hypothesis that this b-type cytochrome may be involved in the photoreception mechanism for blue and uv light in vivo. 相似文献
98.
Gill Barequet Amy J. Briggs Matthew T. Dickerson Michael T. Goodrich 《Computational Geometry》1998,11(3-4):125-141
An offset-polygon annulus region is defined in terms of a polygon P and a distance δ > 0 (offset of P). In this paper we solve several containment problems for polygon annulus regions with respect to an input point set. Optimization criteria include both maximizing the number of points contained in a fixed size annulus and minimizing the size of the annulus needed to contain all points. We address the following variants of the problem: placement of an annulus of a convex polygon as well as of a simple polygon; placement by translation only, or by translation and rotation; off-line and on-line versions of the corresponding decision problems; and decision as well as optimization versions of the problems. We present efficient algorithms in each case. 相似文献
99.
100.
Dynamics of single InGaN quantum dots 总被引:1,自引:0,他引:1
R. A. Taylor J. W. Robinson J. H. Rice A. Jarjour J. D. Smith R. A. Oliver G. A. D. Briggs M. J. Kappers C. J. Humphreys Y. Arakawa 《Physica E: Low-dimensional Systems and Nanostructures》2004,21(2-4):285
Decay dynamics for single InGaN quantum dots are presented using time-resolved photoluminescence. The recombination is shown to be characterized by a single exponential decay, in contrast to the non-exponential recombination dynamics seen in the 2D wetting layer. The lifetimes of single dots in the temperature range 4–60 K decrease with increasing temperature. Different dots show similar lifetimes of 2 ns. 相似文献