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141.
Han Joong Koh Suk Jin Kang Dennis N. Kevill 《Phosphorus, sulfur, and silicon and the related elements》2013,188(7):1404-1415
Kinetic and product studies of the solvolyses of acyclic phosphorochloridates are extended to two cyclic diesters, 2-chloro-1,3,2-dioxaphospholane-2-oxide (1) and 2-chloro-5,5-dimethyl-1,3,2-dioxaphosphorinane-2-oxide (2). Slightly faster solvolyses are observed for 1 than for the acyclic dimethyl phosphorochloridate (3), and 2 solvolyzes somewhat slower than 3. An extended Grunwald–Winstein equation treatment shows similar sensitivities to changes in solvent nucleophilicity and solvent ionizing power for 1, 2, and 3, and a concerted SN2 attack is proposed in each case. Product studies for the solvolyses of 2 in aqueous alcohols are presented. 相似文献
142.
Martin Pomerantz Jiping Wang Seeyearl Seong Kyle P. Starkey Long Nguyen Dennis S. Marynick 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-4):507-508
Abstract Poly(benzo[1,2-b:4,5-b′]dithiophene-4,8-diyl vinylene) (1) has been prepared by the pyrolysis of the precursor polymer 2 and studied. Quantum mechanical calculations on the aromatic and quinoid monomers, oligomers and polymers indicate that 1 is a planar aromatic polymer. 相似文献
143.
Elise A. Dennis Alexander W. Gundlach–Graham Christie G. Enke Steven J. Ray Anthony J. Carado Charles J. Barinaga David W. Koppenaal Gary M. Hieftje 《Journal of the American Society for Mass Spectrometry》2013,24(5):690-700
Resolution in time–of–flight mass spectrometry (TOFMS) is ordinarily limited by the initial energy and space distributions within an instrument’s acceleration region and by the length of the field–free flight zone. With gaseous ion sources, these distributions lead to systematic flight–time errors that cannot be simultaneously corrected with conventional static–field ion–focusing devices (i.e., an ion mirror). It is known that initial energy and space distributions produce non–linearly correlated errors in both ion velocity and exit time from the acceleration region. Here we reinvestigate an old acceleration technique, constant–momentum acceleration (CMA), to decouple the effects of initial energy and space distributions. In CMA, only initial ion energies (and not their positions) affect the velocity ions gain. Therefore, with CMA, the spatial distribution within the acceleration region can be manipulated without creating ion–velocity error. The velocity differences caused by a spread in initial ion energy can be corrected with an ion mirror. We discuss here the use of CMA and independent focusing of energy and space distributions for both distance–of–flight mass spectrometry (DOFMS) and TOFMS. Performance characteristics of our CMA–DOFMS and CMA–TOFMS instrument, fitted with a glow–discharge ionization source, are described. In CMA–DOFMS, resolving powers (FWHM) of greater than 1000 are achieved for atomic ions with a flight length of 285 mm. In CMA–TOFMS, only ions over a narrow range of m/z values can be energy–focused; however, the technique offers improved resolution for these focused ions, with resolving powers of greater than 2000 for a separation distance of 350 mm. 相似文献
144.
Anne Techen Sylvia Czapla Kristian Möllnitz Dennis Budach Pablo Wessig Michael U. Kumke 《Helvetica chimica acta》2013,96(11):2046-2067
Fluorescence probes consisting of well‐established fluorophores in combination with rigid molecular rods based on spirane‐type structures were investigated with respect to their fluorescence properties under different solvent conditions. The attachment of the dyes was accomplished by 1,3‐dipolar cycloaddition between alkynes and azides (‘click’ reaction) and is a prime example for a novel class of sensor constructs. Especially, the attachment of two (different) fluorophores on opposite sides of the molecular rods paves the way to new sensor systems with less bulky (compared to the conventional DNA‐ or protein‐based concepts), nevertheless rigid spacer constructs, e.g., for FRET‐based sensing applications. A detailed photophysical characterization was performed in MeOH (and in basic H2O/MeOH mixtures) for i) rod constructs containing carboxyfluorescein, ii) rod constructs containing carboxyrhodamine, iii) rod constructs containing both carboxyfluorescein and carboxyrhodamine, and iv) rod constructs containing both pyrene and perylene parts. For each dye (pair), two rod lengths with different numbers of spirane units were synthesized and investigated. The rod constructs were characterized in ensemble as well as single‐molecule fluorescence experiments with respect to i) specific rod? dye and ii) dye? dye interactions. In addition to MeOH and MeOH/NaOH, the rod constructs were also investigated in micellar systems, which were chosen as a simplified model for membranes. 相似文献
145.
A Stereoselective aldol reaction between bis(tert-butyldimethylsilyloxy) acetone and an isoxazoline-derived azaenolate is a key step in the construction of a C-ring model of sesbanimide A. 相似文献
146.
147.
Wen‐Jing Shi Dr. Mohamed E. El‐Khouly Dr. Kei Ohkubo Prof. Shunichi Fukuzumi Prof. Dennis K. P. Ng 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(34):11332-11341
An efficient functional mimic of the photosynthetic antenna‐reaction center has been designed and synthesized. The model contains a near‐infrared‐absorbing aza‐boron‐dipyrromethene (ADP) that is connected to a monostyryl boron‐dipyrromethene (BDP) by a click reaction and to a fullerene (C60) using the Prato reaction. The intramolecular photoinduced energy and electron‐transfer processes of this triad as well as the corresponding dyads BDP‐ADP and ADP‐C60 have been studied with steady‐state and time‐resolved absorption and fluorescence spectroscopic methods in benzonitrile. Upon excitation, the BDP moiety of the triad is significantly quenched due to energy transfer to the ADP core, which subsequently transfers an electron to the fullerene unit. Cyclic and differential pulse voltammetric studies have revealed the redox states of the components, which allow estimation of the energies of the charge‐separated states. Such calculations show that electron transfer from the singlet excited ADP (1ADP*) to C60 yielding ADP.+‐C60.? is energetically favorable. By using femtosecond laser flash photolysis, concrete evidence has been obtained for the occurrence of energy transfer from 1BDP* to ADP in the dyad BDP‐ADP and electron transfer from 1ADP* to C60 in the dyad ADP‐C60. Sequential energy and electron transfer have also been clearly observed in the triad BDP‐ADP‐C60. By monitoring the rise of ADP emission, it has been found that the rate of energy transfer is fast (≈1011 s?1). The dynamics of electron transfer through 1ADP* has also been studied by monitoring the formation of C60 radical anion at 1000 nm. A fast charge‐separation process from 1ADP* to C60 has been detected, which gives the relatively long‐lived BDP‐ADP.+C60.? with a lifetime of 1.47 ns. As shown by nanosecond transient absorption measurements, the charge‐separated state decays slowly to populate mainly the triplet state of ADP before returning to the ground state. These findings show that the dyads BDP‐ADP and ADP‐C60, and the triad BDP‐ADP‐C60 are interesting artificial analogues that can mimic the antenna and reaction center of the natural photosynthetic systems. 相似文献
148.
Dennis Cao Dr. Michal Juríček Zachary J. Brown Dr. Andrew C.‐H. Sue Dr. Zhichang Liu Juying Lei Anthea K. Blackburn Dr. Sergio Grunder Dr. Amy A. Sarjeant Prof. Ali Coskun Prof. Cheng Wang Prof. Omar K. Farha Prof. Joseph T. Hupp Prof. J. Fraser Stoddart 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(26):8457-8465
We report the synthesis of two [2]catenane‐containing struts that are composed of a tetracationic cyclophane (TC4+) encircling a 1,5‐dioxynaphthalene (DNP)‐based crown ether, which bears two terphenylene arms. The TC4+ rings comprise either 1) two bipyridinium (BIPY2+) units or 2) a BIPY2+ and a diazapyrenium (DAP2+) unit. These degenerate and nondegenerate catenanes were reacted in the presence of Cu(NO3)2?2.5 H2O to yield Cu‐paddlewheel‐based MOF‐1050 and MOF‐1051. The solid‐state structures of these MOFs reveal that the metal clusters serve to join the heptaphenylene struts into grid‐like 2D networks. These 2D sheets are then held together by infinite donor–acceptor stacks involving the [2]catenanes to produce interpenetrated 3D architectures. As a consequence of the planar chirality associated with both the DNP and hydroquinone (HQ) units present in the crown ether, each catenane can exist as four stereoisomers. In the case of the nondegenerate (bistable) catenane, the situation is further complicated by the presence of translational isomers. Upon crystallization, however, only two of the four possible stereoisomers—namely, the enantiomeric RR and SS forms—are observed in the crystals. An additional element of co‐conformational selectivity is present in MOF‐1051 as a consequence of the substitution of one of the BIPY2+ units by a DAP2+ unit: only the translational isomer in which the DAP2+ unit is encircled by the crown ether is observed. The overall topologies of MOF‐1050 and MOF‐1051, and the selective formation of stereoisomers and translational isomers during the kinetically driven crystallization, provide evidence that weak noncovalent bonding interactions play a significant role in the assembly of these extended (super)structures. 相似文献
149.
The mixed moments for the Askey–Wilson polynomials are found using a bootstrapping method and connection coefficients. A similar bootstrapping idea on generating functions gives a new Askey–Wilson generating function. Modified generating functions of orthogonal polynomials are shown to generate polynomials satisfying recurrences of known degree greater than three. An important special case of this hierarchy is a polynomial which satisfies a four term recurrence, and its combinatorics is studied. 相似文献
150.
Tomá? Sára Thomas C Schwarz Dennis Kurzbach Christoph H Wunderlich Christoph Kreutz Robert Konrat 《ChemistryOpen》2014,3(3):115-123
Protein–protein interactions are of utmost importance to an understanding of biological phenomena since non-covalent and therefore reversible couplings between basic proteins leads to the formation of complex regulatory and adaptive molecular systems. Such systems are capable of maintaining their integrity and respond to external stimuli, processes intimately related to living organisms. These interactions, however, span a wide range of dissociation constants, from sub-nanomolar affinities in tight complexes to high-micromolar or even millimolar affinities in weak, transiently formed protein complexes. Herein, we demonstrate how novel NMR and EPR techniques can be used for the characterization of weak protein–protein (ligand) complexes. Applications to intrinsically disordered proteins and transiently formed protein complexes illustrate the potential of these novel techniques to study hitherto unobserved (and unobservable) higher-order structures of proteins. 相似文献