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61.
Pietro Traldi Donata Favretto Silvia Catinella Olga Bortolini 《Journal of mass spectrometry : JMS》1993,28(7):745-751
The influence of different instrumental parameters on the mass displacements observed in an ion trap and a quadrupole mass filter was investigated. In ion trap measurements it was observed that isolated ions (obtained by both ‘apex’ and ‘two-step’ isolation methods) show the same mass displacements determined in the usual mass spectra; isobaric ions of different structure irradiated with supplementary a.c. fields are ejected at different a.c. voltages, and different cooling times do not affect the mass value. In the quadrupole mass filter, differences in mass were found among isobaric ions of different structure, although to a smaller extent than those observed in the ion trap. All the data obtained are in agreement with the hypothesis of ion–r.f. field interactions originating from the polarizability of the ion. 相似文献
62.
Alessandro B. Tossi Helmut Görner Dietrich Schulte-Frohlinde 《Photochemistry and photobiology》1989,50(5):585-597
The reactions of polyuridylic acid [poly(U)] with Ru(bpy)3(3+) [Ru(III)] and SO4.-, following UV and visible light irradiation of Ru(bpy)3(2+) [Ru(II)] in the presence of S2O8(2-), were studied in an argon-saturated aqueous solution using time-resolved absorption and conductivity methods. The kinetics of the Ru(III) conversion to Ru(II) in the presence of poly(U) was monitored spectroscopically either in the absence of SO4.- [rapid mixing with Ru(III)] or in its presence (after laser flash excitation, lambda exc = 353 nm). The conversion of Ru(III) to Ru(II) is complete at a [nucleotide]/[sensitizer] (N/S) ratio greater than or equal to 10 (rate constant k = 12 s-1) for rapid mixing and at N/S greater than or equal to 6 (k = 15 s-1 at N/S = 10) after laser pulsing. Conductivity measurements following the laser pulse revealed a fast conductivity increase (risetime less than 10 micros), due to the formation of charged species and protons. A slower increase in the 0.1-0.5 s range was observed for poly(U) but it is considerably smaller for poly(dU) and absent in uracil containing monounits. The slow increase is unaffected by pH changes in the 3.5-7 range, markedly reduced in the 7-9 range and is replaced by a slight decrease in conductivity in buffered solutions. An explanation is that poly(U)-bound excited Ru(II) reacts with S2O8(2-) forming Ru(III) and SO4.- as oxidizing species both of which react with poly(U) bases. The resulting base radicals react with Ru(III) or the ligands in the ruthenium complex, producing protons which give rise to the slow conductivity increase (k = 15 s-1 at N/S = 10). The formation of single-strand breaks and the ensuing release of condensed counterions does not appear to contribute significantly to the slow conductivity signal. At N/S less than 10 the observed rate and extent of Ru(III)--Ru(II) conversion and of the slow proton production vary markedly with the N/S ratio. 相似文献
63.
Alconchel S Sapiña F Martínez E 《Dalton transactions (Cambridge, England : 2003)》2004,(16):2463-2468
The molybdenum bimetallic interstitial carbides Fe(3)Mo(3)C and Co(3)Mo(3)C have been synthesized by temperature-programmed reaction (TPR) between the molybdenum bimetallic interstitial nitrides Fe(3)Mo(3)N and Co(3)Mo(3)N and a flowing mixture of CH(4) and H(2) diluted in Ar. These compounds have been characterized by X-ray diffraction, laser Raman spectroscopy, elemental analysis, energy dispersive analysis of X rays, thermal analysis (in air) and scanning electron microscopy (field emission). Their structures have been refined from X-ray powder diffraction data. These carbides crystallize in the cubic system, space group Fd3m[a= 11.11376(6) and 11.0697(3)[Angstrom] for Fe and Co compounds, respectively]. 相似文献
64.
Rosselli S Ramminger AD Wagner T Lieser G Höger S 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(15):3481-3491
Shape-persistent macrocycles based on the phenyl-ethynyl-butadienyl backbone containing two extraannular hydroxyl groups were prepared by the oxidative coupling of the appropriate phenylethynyl oligomers. Carbodiimide-directed coupling with independently synthesized polystyrene carboxylic acid oligomers led to ABA coil-ring-coil block copolymers in which the central macrocycle serves as rigid and the polystyrene oligomers as flexible elements. Depending on the size of the coil blocks, these structures aggregate in cyclohexane into supramolecular hollow cylindrical brushes in which the rigid core is surrounded by the flexible matrix. However, in the solid state it is not possible to identify a morphology in which isolated channels based on aggregated macrocycles are embedded in a matrix of polystyrene. Detailed X-ray and electron diffraction studies on samples prepared from a solution in cyclohexane under equilibrium conditions show that the material adopts a lamellar morphology in the solid state in which columns of macrocycles are aggregated into layers which are separated by polystyrene. 相似文献
65.
Patricia M. Nassar Rose M.Z.Georgetto Naal Silvia H. de Pauli João B.S. Bonilha Laura T. Okano Frank H. Quina 《Journal of colloid and interface science》1997,190(2):461
In aqueous solution, the interaction between sodium dodecyl sulfate (SDS) and poly(ethylene glycol) (PEG) results in the formation of small aggregates or clusters of SDS attached to the PEG polymer chain. Selectivity coefficients for exchange of two monovalent (N-methyl-4-cyanopyridinium cation and Tl+) and two divalent (methylviologen cation and Cu2+) counterions at the surface of SDS–PEG clusters, determined employing photophysical techniques, are similar, but not identical, to those for exchange at the surface of SDS micelles in the absence of PEG. The principal factor affecting ion exchange selectivity in SDS–PEG clusters does not appear to be aggregate size or surface charge density but rather the presence of poly(oxyethylene) subunits at the aggregate surface. 相似文献
66.
Silvia Gimnez-Santamarina Salvador Cardona-Serra Juan M. Clemente-Juan Alejandro Gaita-Ario Eugenio Coronado 《Chemical science》2020,11(39):10718
Molecular spin qubits are chemical nanoobjects with promising applications that are so far hampered by the rapid loss of quantum information, a process known as decoherence. A strategy to improve this situation involves employing so-called Clock Transitions (CTs), which arise at anticrossings between spin energy levels. At CTs, the spin states are protected from magnetic noise and present an enhanced quantum coherence. Unfortunately, these optimal points are intrinsically hard to control since their transition energy cannot be tuned by an external magnetic field; moreover, their resilience towards geometric distortions has not yet been analyzed. Here we employ a python-based computational tool for the systematic theoretical analysis and chemical optimization of CTs. We compare three relevant case studies with increasingly complex ground states. First, we start with vanadium(iv)-based spin qubits, where the avoided crossings are controlled by hyperfine interaction and find that these S = 1/2 systems are very promising, in particular in the case of vanadyl complexes in an L-band pulsed EPR setup. Second, we proceed with a study of the effect of symmetry distortions in a holmium polyoxotungstate of formula [Ho(W5O18)2]9− where CTs had already been experimentally demonstrated. Here we determine the relative importance of the different structural distortions that causes the anticrossings. Third, we study the most complicated case, a polyoxopalladate cube [HoPd12(AsPh)8O32]5− which presents an unusually rich ground spin multiplet. This system allows us to find uniquely favorable CTs that could nevertheless be accessible with standard pulsed EPR equipment (X-band or Q-band) after a suitable chemical distortion to break the perfect cubic symmetry. Since anticrossings and CTs constitute a rich source of physical phenomena in very different kinds of quantum systems, the generalization of this study is expected to have impact not only in molecular spin science but also in other related fields such as molecular photophysics and photochemistry.We employ a python computational tool to compare 3 relevant case studies with increasingly complex ground states: vanadyl complexes, Ho(iii) square antiprisms and Ho(iii) cubic structures. 相似文献
67.
Jacob de Kok†‡ Silvia E. Braslavsky‡ Carel J. P. Spruit† 《Photochemistry and photobiology》1981,34(6):705-710
Abstract— C-phycocyanin from Synechococcus sp. ( Anacystis nidulans ) shows photoreversible absorption changes when dissolved in buffer containing 75% ethylene glycol (vol/vol). Irradiation with red light (638 nm) causes a 7.5% decrease in absorbance around the absorption maximum (620 m), while the absorbance around 500 nm increases. Subsequent irradiation with green light (500 nm) partially reverses this change. Final photoreversibility at around 620 nm amounts to ca. 2.5% of the maximum absorbance. These reactions are ascribed to two interconvertible species PCr and PCg , the former with a higher absorbance in the red. the latter in the green. The rate of dark reversion from PCg to PCr is strongly enhanced by ferricyanide. It is proposed that with this reagent, dark reversion occurs via an oxidized form of PCg . Furthermore, ferricyanide in the presence of ethylene glycol is capable of reversibly oxidizing part of the chromophores of C-phycocyanin, presumably to a radical. In the absence of ethylene glycol, however, ferricyanide causes total irreversible bleaching of the pigment in the dark. The induced photoreversibility of C-phycocyanin is ascribed to the perturbing action on the protein structure by ethylene glycol in high concentrations. This solvent proved the most suitable perturbant of several compounds tested. 相似文献
68.
Abe K Akagi T Anthony PL Antonov R Arnold RG Averett T Band HR Bauer JM Borel H Bosted PE Breton V Button-Shafer J Chen JP Chupp TE Clendenin J Comptour C Coulter KP Court G Crabb D Daoudi M Day D Dietrich FS Dunne J Dutz H Erbacher R Fellbaum J Feltham A Fonvieille H Frlez E Garvey D Gearhart R Gomez J Grenier P Griffioen KA Hoibraten S Hughes EW Hyde-Wright C Johnson JR Kawall D Klein A Kuhn SE Kuriki M Lindgren R Liu TJ Lombard-Nelsen RM Marroncle J Maruyama T Maruyama XK McCarthy J Meyer W 《Physical review letters》1995,74(3):346-350
69.
Adams MR Aïd S Anthony PL Averill DA Baker MD Baller BR Banerjee A Bhatti AA Bratzler U Braun HM Breidung H Busza W Carroll TJ Clark HL Conrad JM Davisson R Derado I Dhawan SK Dietrich FS Dougherty W Dreyer T Eckardt V Ecker U Erdmann M Faller F Fang GY Figiel J Finlay RW Gebauer HJ Geesaman DF Griffioen KA Guo RS Haas J Halliwell C Hantke D Hicks KH Hughes VW Jackson HE Jancso G Jansen DM Jin Z Kaufman S Kennedy RD Kinney ER Kirk T Kobrak HG Kotwal AV Kunori S Lancaster S Lord JJ Lubatti HJ 《Physical review letters》1995,74(9):1525-1529
70.
van den Brand JF Ent R Anthony PL Arnold RG Arrington J Beise EJ Belz JE Bosted PE Bulten H Chapman MS Coulter KP Dietrich FS Epstein M Filippone BW Gao H Gearhart RA Geesaman DF Hansen J Holt RJ Jackson HE Jones CE Keppel CE Kinney ER Kuhn S Lee K Lorenzon W Lung A Makins NC Margaziotis DJ McKeown RD Milner RG Mueller B Napolitano J Nelson J O'Neill TG Papavassiliou V Petratos GG Potterveld DH Rock SE Spengos M Szalata ZM Tao LH van Bibber K Wasson DA White JL Zeidman B 《Physical review D: Particles and fields》1995,52(9):4868-4871