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11.
Shaakov S Galili T Stavitski E Levanon H Lukas A Wasielewski MR 《Journal of the American Chemical Society》2003,125(21):6563-6572
We have synthesized a series of structurally related, covalently linked electron donor-acceptor triads having highly restricted conformations to study the effects of radical ion pair (RP) structure, energetics, and solvation on charge recombination. The chromophoric electron acceptor in these triads is a 4-aminonaphthalene-1,8-dicarboximide (6ANI), in which the 4-amine nitrogen atom is part of a piperazine ring. The second nitrogen atom of the piperazine ring is part of a para-substituted aniline donor, where the para substituents are X = H, OMe, and NMe(2). The imide group of 6ANI is linked to a naphthalene-1,8:4,5-bis(dicarboximide) (NI) electron acceptor across a phenyl spacer in a meta relationship. The triads undergo two-step photoinduced electron transfer to yield their respective XAn(*)(+)-6ANI-Ph-NI(*)(-) RP states, which undergo radical pair intersystem crossing followed by charge recombination to yield (3)NI. Time-resolved electron paramagnetic resonance experiments on the spin-polarized RPs and triplet states carried out in toluene and in E-7, a mixture of nematic liquid crystals (LCs), show that for all three triads, the XAn(*)(+)-6ANI-Ph-NI(*)(-) RPs are correlated radical pairs and directly yield values of the spin-spin exchange interaction, J, and the dipolar interaction, D. The values of J are all about -1 mT and show that the LC environment most likely enforces the chair conformation at the piperazine ring, for which the RP distance is larger than that for the corresponding boat conformation. The values of D yield effective RP distances that agree well with those calculated earlier from the spin distributions of the radical ions. Within the LC, changing the temperature shows that the CR mechanism can be changed significantly as the energy levels of the RPs change relative to that of the recombination triplet. 相似文献
12.
Abstract— The photooxidation of reduced lumiflavin in its acidic form, LfH 3 + , takes place in two consecutive steps. Upon illumination of LfH 3 + in its absorption band at 313 nm the semiquinone, LfH 3 + , is formed. Two LfH 2 + ions are consumed for every LfH 2 + formed. Illumination of the semiquinone in its absorption band at 495 nm causes further oxidation so that the oxidized LfH+ ion is formed. In this reaction one LfH 3 + ion is photolyzed for every LfH+ formed. In addition, a hydrogen atom is released in the photooxidation of LfH 2 + . Mechanisms for the two photoreactions are proposed. 相似文献
13.
The photoexcited triplet state of phenazine in toluene glasses at 35 K is investigated by light modulation-EPR spectroscopy. From the transient EPR spectra and the kinetics in the three canonical orientations (p = x, y, z) the rate parameters are determined. Thus, the depopulation rate constants kp, the anisotropic spin lattice relaxation rate constants Wp, and the ratios between the population constants Ap are calculated: kx = (2.2 ± 0.3) × 102 s?1, ky = (0.21 ± 0.04) × 102 s?1, kz = (0.06 ± 0.03) × 102 s?1, Wx = (8.6 ± 0.9) × 103 s?1, Wy = (11.0 ± 1.0) × 103 s?1, Wz = (14.0 ± 1.4) × 103 s?1, and Ax: Ay:Az ≈ 1:0.04:0.02. It is concluded therefore that the in-plane spin state |τx > is the active one. 相似文献
14.
In Computerized Tomography (CT), an image must be recovered from its sampled projections in the form of values of the Radon transform. In this work a method of recovering the image is based on the properties of the raised-cosine wavelet. This wavelet has a closed form which allows for certain precomputations and avoids convolution. The rate of convergence of the resulting algorithm to the image density function is found under suitable hypotheses. This algorithm is then tested on the standard Shepp–Logan 相似文献
15.
Tommy Dreyfus† Theodore Eisenberg† 《International Journal of Mathematical Education in Science & Technology》2013,44(3):489-497
There is a definite movement to remove many of the topics concerning polynomials from the school curriculum. The purpose of this paper is to point out why this movement is misguided and should be reversed. 相似文献
16.
Transgressive dune fields, which are active sand areas surrounded by vegetation, exist on many coasts. In some regions like in Fraser Island in Australia, small dunes shrink while large ones grow, although both experience the same climatic conditions. We propose a general mathematical model for the spatiotemporal dynamics of vegetation cover on sand dunes and focus on the dynamics of transgressive dunes. Among other possibilities, the model predicts growth parallel to the wind with shrinkage perpendicular to the wind, where, depending on geometry and size, a transgressive dune can initially grow although eventually shrink. The larger is the initial area the slower its stabilization process. The model’s predictions are supported by field observations from Fraser Island in Australia. 相似文献
17.
A. G. Shannon† 《International Journal of Mathematical Education in Science & Technology》2013,44(4):497-507
The semantic differential—one approach to attitude measurement— basically records a combination of a person's associations with a particular concept with a scaling procedure. This paper considers the implications of such a device for teachers of mathematics. 相似文献
18.
AbstractThe reactions of methyl radicals with MII-(ntp)(H2O)2 complexes were studied. The formation of an unstable intermediate (ntp)(H2O)MIII-CH3 is observed. This reaction is an equilibrium process, i.e. the MIII-C bond decomposes via homolysis. The (ntp)(H2O)MIII-CH3 complexes isomerize to a more stable form. The results compared to those obtained in analogous reactions of the MII-(nta)(H2O)2 complexes were shown to be similar with one exception – for the nta complexes no isomerization process is observed. 相似文献
19.
In this paper we consider an isotropic variant of the BMO‐type norm recently introduced (Bourgain, Brezis, and Mironescu, 2015). We prove that, when considering characteristic functions of sets, this norm is related to the perimeter. A byproduct of our analysis is a new characterization of the perimeter of sets in terms of this norm, independent of the theory of distributions.© 2016 Wiley Periodicals, Inc. 相似文献
20.
Israel Zilbermann Avraham Meshulam Haim Cohen Dan Meyerstein 《Supramolecular chemistry》2013,25(2):325-332
Abstract The mechanisms and kinetics of oxidation of ascorbate, AH?, by Ni(III)Li aq and by LiNi(III) (HPO4)2 ? complexes (L1 = meso-(5,12)-7,7,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; L2 = 1,8-dimethyl-1,3,6,8,10,13-hexaazacyclotetradecane) in neutral aqueous solutions have been investigated. The oxidation of ascorbate by the LiNi(III) (HPO4)2 ? and Ni(III)L1 aq proceeds via two consecutive reactions well separated in time. The products of the first reaction are the A.? radical anion and the corresponding Ni(II) complex. The oxidations by the LiNi(III)(HPO4)2 ? complexes proceed via the outer sphere mechanism, whereas the detailed mechanism of reaction of Ni(III)L1 aq cannot be determined. The rate of reaction decreases with the increase in the concentration of phosphate, thus indicating that LiNi(III)(HPO4)(H2O)+ and LiNi(III)OH2+ are stronger oxidizing agents than LiNi(III)(HPO4)? 2. The oxidation of ascorbate by Ni(III)L2 aq proceeds via three consecutive reactions which are well separated in time. Thus the results clearly point out that this process occurs via the inner sphere mechanism. The first transient observed is tentatively identified as L2(H2O)Ni(II)(A.?)2+, i.e., an unexpected complex of the ascorbate anion radical. Also in this process the last transient observed is the A.? anion radical. The stabilization of the ascorbyl radical in a transient complex might be of biological significance. 相似文献