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101.
102.
A theorem for end-charge quantization in quasi-one-dimensional stereoregular chains is formulated and proved. It is a direct analog of the well-known theorem for surface charges in physics. The theorem states the following: (1) Regardless of the end groups, in stereoregular oligomers with a centrosymmetric bulk, the end charges can only be a multiple of 12 and the longitudinal dipole moment per monomer p can only be a multiple of 12 times the unit length a in the limit of long chains. (2) In oligomers with a noncentrosymmetric bulk, the end charges can assume any value set by the nature of the bulk. Nonetheless, by modifying the end groups, one can only change the end charge by an integer and the dipole moment p by an integer multiple of the unit length a. (3) When the entire bulk part of the system is modified, the end charges may change in an arbitrary way; however, if upon such a modification the system remains centrosymmetric, the end charges can only change by multiples of 12 as a direct consequence of (1). The above statements imply that-in all cases-the end charges are uniquely determined, modulo an integer, by a property of the bulk alone. The theorem's origin is a robust topological phenomenon related to the Berry phase. The effects of the quantization are first demonstrated in toy LiF chains and then in a series of trans-polyacetylene oligomers with neutral and charge-transfer end groups. 相似文献
103.
Raffaele Battistuzzi 《Transition Metal Chemistry》1988,13(6):401-409
Summary 1-Phenyl-4,6-dimethylpyrimidine-2-thione (L) and its protonated cation 1-phenyl-4,6-dimethyl pyrimidinium-2-thione
, have been employed to prepare the following copper(I) complexes: CuXL (X=Cl, Br, I, ClO4 or BF4), (CuX)3L2 (X=Cl, Br, I or SCN), (CuX)2L5 (X= ClO4 or BF4) and the zwitterionic species CuXY(LH) X=Y=Cl, Br or I; X=Br; Y=Cl; X=I; Y=Br). Chemical analysis, conductivity, and near-and far-i.r. spectroscopic data are presented and the chemical relationships between them discussed in terms of postulated dinuclear or polynuclear species for the complexes. Metalligand vibrations suggest that the neutral ligand is N, S-bidentate in its copper(I) complexes as well as S-coordinat for the
cation in the zwitterionic compounds. Diagnostic i.r. bands frequencies of counterions and (Cu–X) modes indicate the coordinating character of Cl–, Br–, I–, SCN– and of ClO
4
–
, BF
4
–
(in CuXL) anions. For the chloro-complexes CuClL and (CuCl)3L2, salt-like species of the [CuL2][CuCl2] and [{Cu2L2Cl}n] [CuCl2]n type respectively, are proposed. The polarographic data for the perchlorate complexes have shown that in dimethylformamide (DMF) solution, the prevailing species are CuClO4L, CuClO4L2 and (CuClO4)2L5; their overall stability constants were determined. 相似文献
104.
The generation of a series of α-methoxymethoxy-substituted arylmethyllithiums was achieved by direct metalation of the corresponding arylmethyl methoxymethyl ethers. While the effect of substituents at the benzylic position is straightforward, substituents located on the aromatic ring promote the set up of a competition between lateral and aromatic metalation, strongly affected by the position and relative ortho directing properties of the new substituent. The proposed methodology allows a simple approach to the generation of a wide array of functionalized organolithium reagents. 相似文献
105.
The azimuthal distributions around the jet axis of leading pions produced in the jet fragmentation process in pp collisions are studied within the framework of the so-called generalized parton model. The observable leading-twist azimuthal asymmetries are estimated in kinematic configurations presently investigated at RHIC. It is shown how the main contributions coming from the Collins and Sivers effects can be disentangled. In addition, a test of the process dependence of the Sivers function is provided. 相似文献
106.
Sun Choi Inkyu Park Zhao Hao Hoi-Ying N. Holman Albert P. Pisano 《Applied Physics A: Materials Science & Processing》2012,107(2):421-428
We report a simple and effective method to develop long-term stable, top-down fabricated silicon nanowire (SiNW) pH sensors
along with systematic studies on the performance of the sensors. In this work, we fabricated the SiNW pH sensors based on
top-down fabrication processes. In order to improve the stability of the sensor performance, the sensors were coated with
a passivation layer (PECVD-based silicon nitride) for effective electrical insulation and ion-blocking. The stability, pH
sensitivity, and repeatability of the sensor response are critically analyzed with regard to the physics of sensing interface
between sample liquid and the sensor surface. Also, trade-off between the stability and pH sensitivity of the sensor response
is discussed. 相似文献
107.
Life is made of the intimate interaction of metabolism and genetics, both built around the chemistry of the most common elements of the Universe (hydrogen, oxygen, nitrogen, and carbon). The transmissible interaction of metabolic and genetic cycles results in the hypercycles of organization and de-organization of chemical information, of living and non-living. The origin-of-life quest has long been split into several attitudes exemplified by the aphorisms "genetics-first" or "metabolism-first". Recently, the opposition between these approaches has been solved by more unitary theoretical and experimental frames taking into account energetic, evolutionary, proto-metabolic and environmental aspects. Nevertheless, a unitary and simple chemical frame is still needed that could afford both the precursors of the synthetic pathways eventually leading to RNA and to the key components of the central metabolic cycles, possibly connected with the synthesis of fatty acids. In order to approach the problem of the origin of life it is therefore reasonable to start from the assumption that both metabolism and genetics had a common origin, shared a common chemical frame, and were embedded under physical-chemical conditions favourable for the onset of both. The singleness of such a prebiotically productive chemical process would partake of Darwinian advantages over more complex fragmentary chemical systems. The prebiotic chemistry of formamide affords in a single and simple physical-chemical frame nucleic bases, acyclonucleosides, nucleotides, biogenic carboxylic acids, sugars, amino sugars, amino acids and condensing agents. Thus, we suggest the possibility that formamide could have jointly provided the main components for the onset of both (pre)genetic and (pre)metabolic processes. As a note of caution, we discuss the fact that these observations only indicate possible solutions at the level of organic substrates, not at the systemic chemical level. 相似文献
108.
109.
Ethyl 2-diazo-4,4,4-trifluoro-3-oxobutanoate is a highly versatile intermediate for the synthesis of a wide range of trifluoromethyl heterocycles. With the use of rhodium(II) or copper(II) catalyzed carbene X-H insertion reactions as key steps, a diverse set of trifluoromethyl-oxazoles, -thiazoles, -imidazoles, -1,2,4-triazines, and -pyridines are available from the diazoketoester, either directly in a single step or with just one additional step. 相似文献
110.
Solid damping in micro electro mechanical systems 总被引:1,自引:1,他引:0
This paper focuses on the problem of the numerical evaluation of dissipation induced by thermoelastic coupling in microelectromechanical
systems. An ad hoc conceived, FE based, numerical procedure for the evaluation of the thermoelastic dissipation is proposed
and the numerical results are compared with analytical solutions. In order to introduce in the numerical response a dependence
on the size of the resonating devices, which is experimentally observed at very small dimensions, a new enhanced non-local
coupled thermoelastic model is proposed and the first results are discussed.
An erratum to this article can be found at 相似文献