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101.
Guha PM Phan H Kinyon JS Brotherton WS Sreenath K Simmons JT Wang Z Clark RJ Dalal NS Shatruk M Zhu L 《Inorganic chemistry》2012,51(6):3465-3477
Copper(II) acetate mediated coupling reactions between 2,6-bis(azidomethyl)pyridine or 2-picolylazide and two terminal alkynes afford 1,2,3-triazolyl-containing ligands L(1)-L(6). These ligands contain various nitrogen-based Lewis basic sites including two different pyridyls, two nitrogen atoms on a 1,2,3-triazolyl ring, and the azido group. A rich structural diversity, which includes mononuclear and dinuclear complexes as well as one-dimensional polymers, was observed in the copper(II) complexes of L(1)-L(6). The preference of copper(II) to two common bidentate 1,2,3-triazolyl-containing coordination sites was investigated using isothermal titration calorimetry and, using zinc(II) as a surrogate, in (1)H NMR titration experiments. The magnetic interactions between the copper(II) centers in three dinuclear complexes were analyzed via temperature-dependent magnetic susceptibility measurements and high-frequency electron paramagnetic resonance spectroscopy. The observed magnetic superexchange is strongly dependent on the orientation of magnetic orbitals of the copper(II) ions and can be completely turned off if these orbitals are arranged orthogonal to each other. This work demonstrates the versatility of 1,2,3-triazolyl-containing polyaza ligands in forming metal coordination complexes of a rich structural diversity and interesting magnetic properties. 相似文献
102.
Zheng W Singh K Wang Z Wright JT van Tol J Dalal NS Meulenberg RW Strouse GF 《Journal of the American Chemical Society》2012,134(12):5577-5585
Herein we report doping of ZnSe by Cr ions leads to formation of small ZnCr(2)Se(4) spinel inclusions within the cubic sphalerite lattice of a 2.8 nm CrZnSe quantum dot (QD). The Cr ion incorporates as a pair of Cr(III) ions occupying edge-sharing tetragonal distorted octahedral sites generated by formation of three Zn ion vacancies in the sphalerite lattice in order to charge compensate the QD. The site is analogous to the formation of a subunit of the ZnCr(2)Se(4) spinel phase known to form as inclusions during peritectoid crystal growth in the ternary CrZnSe solid-state compound. The oxidation state and site symmetry of the Cr ion is confirmed by X-ray absorption near edge spectroscopy (XANES), crystal field absorption spectroscopy, and electron paramagnetic resonance (EPR). Incorporation as the Cr(III) oxidation state is consistent with the thermodynamic preference for Cr to occupy an octahedral site within a II-VI semiconductor lattice with a half-filled t(2g) d-level. The measured crystal field splitting energy for the CrZnSe QD is 2.08 eV (2.07 eV form XANES), consistent with a spinel inclusion. Further evidence of a spinel inclusion is provided by analysis of the magnetic data, where antiferromagnetic (AFM) exchange, a Curie-Weiss (C-W) temperature of θ = -125 K, and a nearest-neighbor exchange coupling constant of J(NN) = -12.5 K are observed. The formation of stable spinel inclusions in a QD has not been previously reported. 相似文献
103.
Abstract It is well established that crystals of charge transfer (CT) complexes formed between aromatic donors and acceptors are built up of linear stacks of closely packed molecules. There are two main types of CT crystal structure, (i) a parallel arrangement of linear stacks made up of the individual component molecules, AAAA…DDDD… and (ii) a parallel arrangement of linear stacks made up of alternating molecules, ADADA… In each case it is to be expected that the intermolecular forces between the face to face packed molecules in each stack are far greater than those forces between stacks. A considerable anisotropy in physical properties which are a strong function of intermolecular interactions is a natural result of these crystal structures. Such anisotropy has been reported in the few crystals of the ADAD structure which have been examined, for example, in triplet exciton motion in biphenxl2 and anthracene tetracyanobenzene (TCNB) complexes3,4 and in charge carrier mobilities μ in anthracene5 and phenthrene-pyromellitic-acid-dianhydride6 as well as anthracene-trinitrobenzene7. Carrier mobilities which were reported were all low, < 0.1 cm2/V sec., and though the major interesting questions were posed, such as what is the effect of different donors on μ?; or are there separate conduction and valence bands for acceptors and donors?; the dearth of mobility data on a range of materials leaves these questions unanswered. We have recently measured the mobilities of holes in several TCNB complexes as a first step towards understanding carrier transport in such materials. These are a particularly interesting series of materials in that triplet motion is said to be primarily two dimensional in anthracene TCNB3, and unidimensional in biphenyl TCNB2. Since the transport of charge, like triplet exciton motion, is dominated by exchange interactions, and since CT triplet 相似文献
104.
Evan M. Benbow Naresh S. Dalal Susan E. Latturner 《Journal of solid state chemistry》2009,182(11):3055-3062
Binary eutectic mixtures of early lanthanide metals and late transition metals have been explored as media for crystal growth of new intermetallic phases. A large family of R6T13−xAlxMy phases (R=La or Nd; T=Fe or Mn; M=main group elements) with the La6Co11Ga3 structure type can be crystallized from La/Ni and Nd/Fe eutectics. The tetragonal structure of these compounds features slabs of transition metal atoms capped with mixed T/Al sites and separated by layers of lanthanide ions. The growth of large crystals of the lanthanum analogs allows for the study of the anisotropic magnetic properties of the transition metal slabs. For La6Fe13−xAlxMy analogs, these order antiferromagnetically with TN strongly dependent on the Fe/Al ratio on the mixed sites. Growth of Mn analogs is reported for the first time; the transition metal slabs in La6(Mn/Ni)10Al3 phases order ferromagnetically with a TC of 200 K. 相似文献
105.
106.
Dark energy models which alter the relative scaling behavior of dark energy and matter could provide a natural solution to the cosmic coincidence problem-why the densities of dark energy and dark matter are comparable today. A generalized class of dark energy models is introduced which allows noncanonical scaling of the ratio of dark matter and dark energy with the Robertson-Walker scale factor a(t). We show that determining whether there is a coincidence problem, and the extent of cosmic coincidence, can be addressed by several forthcoming experiments. 相似文献
107.
S.R. Dalal 《Stochastic Processes and their Applications》1979,9(1):99-107
A class of random processes with invariant sample paths, that is, processes which yield (with probability one) probability distributions that are invariant under a given transformation group of interest, are introduced and their properties are studied. These processes, named Dirichlet Invariant processes, are closely related to the Dirichlet processes of Ferguson. These processes can be used as priors for Bayesian analysis of some nonparametric problems. As an application Bayes and Minimax estimates of an arbitrary distribution, symmetric about a known point, are obtained. 相似文献
108.
On the basis of electric field effects on the EPR spectra of Cr5+ centers in KD2AsO4 and KD2PO4, and of a symmetry analysis, we show that the spectra should be interpreted in terms of a four-site exchange process rather than two-site processes as others have done. Using this procedure, we obtain new and more extensive data on the dynamic processes in these crystals show that above Tc the changes in EPR line shape reveal only one type of dynamic process, a polarization fluctuation. 相似文献
109.
Rakvin B Maltar-Strmecki N Ramsey CM Dalal NS 《The Journal of chemical physics》2004,120(14):6665-6673
With the view of understanding the low frequency (40-50 cm(-1)) motional processes in L-alanine around 4 K, we have carried out heat capacity (CP) and electron spin echo (ESE) measurements on L-alanine and L-alanine-d7. The obtained CP data show the so-called boson peak (seen as a maximum in CP/T3 versus T plots) in the low temperature region (1.8-20 K). The phase memory time, T(M), and spin lattice relaxation time, T1, of the spin probe, the so-called first stable alanine radical (SAR1), *CHCH3COOH, have been measured between 4 and 105 K. The obtained relaxation rate 1/T1 shows an anomalous increase which coincides with the emergence of a boson peak in the low temperature region (4-20 K). Together, the ESE and the CP data confirm the existence of a thermally activated dynamic orientational disorder in the lattices of both compounds below 20 K. The results help explain the discrepancy between the CP data from powders and single crystals of alanine, as well as the proanomalous relaxation mechanisms for SAR1 in these lattices, and they also provide a mechanism for the spin-lattice relaxation process for SAR1 at cryogenic temperatures. 相似文献
110.
V. F. Dalal M. H. Litt S. E. Rickert 《Journal of polymer science. Part A, Polymer chemistry》1985,23(10):2659-2668
Poly(bismethylene hydroquinone) (PBHQ) has been synthesized. Fourier transform infrared, 13C solid-state CP/MAS NMR and elemental analysis provide strong evidence of two methylene bridges per hydroquinone molecule. The polymer was (1) air oxidized using ammonia and (2) chemically oxidized using bromine/KOH. The initial structure and oxidation process was studied by comparison of the unoxidized and oxidized polymers using Fourier transform infrared, elemental analysis, and 13C solid-state CP/MAS NMR. 相似文献