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991.
Jingen Wang Xihe Huang Changcang Huang Haiyan Song Duoping Yang 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(12):m583-m585
In the title complex, poly[triaquabis(dimethylformamide)di‐μ3‐oxalato‐μ2‐oxalato‐dilanthanum(III)], [La2(C2O4)3(C3H7NO)(H2O)3]n, both La ions are coordinated by nine O atoms, forming slightly distorted tricapped trigonal prisms. The two La ions, the terminal water O atom, and the O and N atoms of the dimethylformamide molecule reside on twofold rotation axes, giving the two La‐centered coordination geometries twofold or pseudo‐twofold symmetries. The two oxalate ligands, one of which rests on a center of inversion at the mid‐point of the C—C bond, adopt different bridging modes, connecting with the La ions to form two types of lanthanide oxalate chains, i.e. anionic {[La(C2O4)2(DMF)(H2O)2]n−}n (DMF is dimethylformamide) and cationic zigzag {[La(C2O4)(H2O)]n+}n, respectively. Each zigzag cationic chain is linked to four adjacent anionic chains via the bridging oxalate anions, and each anionic chain connects with four zigzag cationic chains, constructing a three‐dimensional neutral framework. 相似文献
992.
H. -H. Grapengeter Prof. Dr. Robert Kosfeld H. W. Offergeld 《Colloid and polymer science》1980,258(10):1104-1110
Zusammenfassung Es wurden die longitudinalen Protonen-RelaxationszeitenT
1 der beiden Systeme Hexamethyldisiloxan und Dekamethylcyclopentasiloxan im Temperaturbereich von 20 bis 300 K untersucht. Das Relaxationsverhalten des Hexamethyldisiloxans wird in diesem Temperaturbereich durch drei Bewegungsmechanismen bestimmt: die Methylgruppenrotation um die C3-Achse, die Segmentrotation um die Si-O-Bindung und einen noch nicht näher identifizierten Bewegungsprozeß, der zu einer sprunghaften Änderung der Spin-Gitter-Relaxationszeit führt. Diese Bewegungsmechanismen werden mit Ausnahme der Segmentrotation auch beim Dekamethylcyclopentasiloxan gefunden. Es wird eine Gegenüberstellung der experimentellen Daten der beiden hier untersuchten Systeme und eines polymeren Dimethylsiloxans gegeben. Dabei ist als interessantes Ergebnis hervorzuheben, daß die Methylgruppenrotation im Hexamethyldisiloxan die stärkste Behinderung erfährt.
Mit 4 Abbildungen und 1 Tabelle 相似文献
Summary In this experiment the longitudinal proton relaxation timesT 1 of hexamethyldisiloxane and decamethylcyclopen-tasiloxane were measured in the temperature range from about 20 K up to 300 K. Three motional mechanisms, the methyl group rotation about the C3-axis, the segmental rotation about the Si-O-bond and a motional process, not yet identified, which leads to a discontinuity ofT 1 have been found to be responsible for the spin-lattice relaxation of hexamethyldisiloxane in this temperature range. With the exception of segmental rotation these motional processes also determine the spin-lattice relaxation of decamethylcyclopentasiloxane. A comparison of experimental data of the two systems actually investigated and a polymeric dimethylsiloxane is presented, showing the methyl group rotation being most strongly hindered in the case of hexamethyldisiloxane.
Mit 4 Abbildungen und 1 Tabelle 相似文献
993.
Prof. Dr. Heinz Falk Karl Grubmayr Franz Neufingerl 《Monatshefte für Chemie / Chemical Monthly》1977,108(5):1185-1188
The photochemical (Z)?(E) isomerization of the exocyclic double bond is only possible in pyrromethenones (partial structure1) but not in2–4. From spectral investigations, chelation and methylation of certain positions within these structures it can be shown that it is most probably a proton transfer reaction which competes successfully with the isomerization. 相似文献
994.
Doz. Dr. Friedrich Höfler Ernst Brandstätter 《Monatshefte für Chemie / Chemical Monthly》1975,106(4):893-904
The reactions of Ge2(C6H5)6 with HCl and HBr lead in nearly quantitative yields to the 1,1,2,2-tetrahalo derivatives Cl2(C6H5)GeGe(C6H5)Cl2 (I) and Br2(C6H5)GeGe(C6H5)Br2 (II), resp. The Si?Ge bond of (C6H5)3SiGe(C6H5)3 is cleaved under the conditions of hydrohalogenation. The vibrational spectra of Ge2Br6, Ge2(C6H5)6, I, and II are reported. The influence of vibrational coupling on ν GeGe in these compounds is discussed in detail, including vibrational calculations. 相似文献
995.
Aptamers are single‐stranded nucleic acid molecules selected in vitro to bind to a variety of target molecules. Aptamers bound to proteins are emerging as a new class of molecules that rival commonly used antibodies in both therapeutic and diagnostic applications. With the increasing application of aptamers as molecular probes for protein recognition, it is important to understand the molecular mechanism of aptamer–protein interaction. Recently, we developed a method of using atomic force microscopy (AFM) to study the single‐molecule rupture force of aptamer/protein complexes. In this work, we investigate further the unbinding dynamics of aptamer/protein complexes and their dissociation‐energy landscape by AFM. The dependence of single‐molecule force on the AFM loading rate was plotted for three aptamer/protein complexes and their dissociation rate constants, and other parameters characterizing their dissociation pathways were obtained. Furthermore, the single‐molecule force spectra of three aptamer/protein complexes were compared to those of the corresponding antibody/protein complexes in the same loading‐rate range. The results revealed two activation barriers and one intermediate state in the unbinding process of aptamer/protein complexes, which is different from the energy landscape of antibody/protein complexes. The results provide new information for the study of aptamer–protein interaction at the molecular level. 相似文献
996.
997.
The electronic structure of the tetrahedral molecule VCL4 is investigated within the CNDO-MO approximations. The metal and ligand valence orbitals, 3d, 4s, 4p; and 3s, 3p; respectively, have been systematically varied in an attempt to minimize the total energy; “optimum” V 4s(χ4 = 1.10) and 4p(d 3 p 2) orbitals have been established, but V 3d(d n ) and Cl(-δ) valence orbitals are only seen to favor lower energy for expanded orbitals. Since determining the one-electron molecular orbital level which is occupied by the vanadium lone electron is a major aspect of this investigation, all calculations have been performed in triplicate: calculations assuming the unpaired electron occupies the 3a 1, 2 e and 4t 2 molecular orbital (ground state electronic configurations2 A 1,2 E, and2 T 2, respectively). The Hartree-Fock equations have been solved by Roothaan's SCF method for open shells, but off-diagonal multipliers between filled and partly filled molecular orbitals of the same symmetry have been neglected. As a qualitative estimate of the error introduced by this simplification, the pertinent overlap integrals between the eigenfunctions from calculations for the three possible configurations,2 A 1,2 E, and2 T 2, are investigated as functions of the component 3d(d n ) and Cl(-δ) valence orbitals. The overlap integrals from the relevant2 A 1 and2 T 2 calculations are reasonably small, but the neglect of off-diagonal multipliers in calculations on the2 E state is found to be a poor approximation. An ordering of the non-filled molecular orbitals in VCl4 of 4t 2 < 3a 1 < 2e < 5t 2 seems most consistent with the numerous calculations. This suggested ground state electronic configuration of2 T 2 introduces new aspects to the consideration of a (dynamic) Jahn-Teller effect in VCl4. Experimental data pertinent to the electronic structure of VCl4 has been briefly summarized, but unfortunately it is inadequate to confirm or deny the present calculations. 相似文献
998.
Dr. Bernhard Reiser 《Theoretical chemistry accounts》1976,41(4):287-299
The success of perturbation calculations of second order for the NFE (“Nearly Free Electron”) metals and that of the two-parameter model of Pettifor for the transition elements show that the lattice-stability of the metals has simple physical reasons. Using the results of Harrison, Heine and Weaire, Deegan, and Pettifor, a model is developed which allows to explain the stability of the three metal lattices in terms of differences in the potentials. Only those potential differences are considered which are caused by the different packing of the lattices. With the aid of the virial theorem the band structure energy is connected with the potential bandstructure energy. The sequence of stability is predicted to be body centered cubic (bcc), hexagonal close packed (hcp), face centered cubic (fcc) with increasing valence electron concentration. The ranges of stability can be expressed in simple numbers. This simple model holds in principle for NFE as well as for transition metals because it contains no assumptions restricted to only one of these metal types. Deviations of the observed lattice stability from the model can be understood from the approximations involved. 相似文献
999.
Dr. J. Dembczyński W. Ertmer U. Johann P. Stinner 《Zeitschrift für Physik A Hadrons and Nuclei》1980,294(4):313-317
The hyperfine structure of the metastable atomic states (3d 74s)5 F 2,3,4,5 and (3d 7 4s)3 F 2,3,4 of57Fe has been measured using theABMR- LIRF method (atomic beam magnetic resonance detected by laser induced resonance fluorescence). From these measurements the following hfs constantsA of the magnetic dipole interaction have been obtained (corrected for second order effects):A(5 F 2)=55.994(7) MHzA(5 F 3)=69.632(5) MHzA(5 F 4)=78.435(4) MHzA(5 F 5)=87.246(3) MHzA(3 F 2)=143.328(4) MHzA(3 F 3)=50.602(10) MHzA(3 F 4)=13.456(5) MHz 相似文献
1000.
Prof. Dr. Edmund Hlawka 《Monatshefte für Mathematik》1980,89(1):19-44
A new proof of a theorem of W. Neiss is given that the rays of the Quadratwurzelschnecke are uniformly distributed mod 2 . The exact order of the discrepancy of the sequence is determined. Multidimensional generalizations of this sequence are also considered. It is shown that the Quadratwurzelschnecke is something like a roulette.
Mit 2 Abbildungen
Herrn Prof. R. M. Redheffer in Freundschaft zum 60. Geburtstag gewidmet 相似文献
Mit 2 Abbildungen
Herrn Prof. R. M. Redheffer in Freundschaft zum 60. Geburtstag gewidmet 相似文献