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1.
Data obtained from the calibration of GPC columns of different permeabilities with standard polystyrenes are reported. For single columns the logarithm molecular weight–elution volume plot is linear for approximately one and one-half decades in molecular weight. GPC separations are such that the separability of two samples of similar molecular weight improves as their mean molecular weight decreases. Because of this the analysis of high molecular weight polymers can best be accomplished on a series of columns in which each column has a high permeability limit. The elution volume for columns in series is shown to be the sum of the elution volumes of the individual columns. As higher molecular weights are eluted a pronounced tailing effect is observed. 相似文献
2.
A study of the equilibrium of the molecular forms of rhodamine 19 in aqueous and ethanolic solution is carried out by determining the absorption and fluorescence characteristics of the zwitterionic and the cationic forms of the dye. The optical properties of rhodamine 19 are compared with those obtained for rhodamine 6G and also with those previously reported for rhodamine 3B and for the molecular forms of rhodamine B in water and ethanol. Different aspects of the molecular structure of the rhodamines and solvent effects are discussed, as well as their influence on the photophysical properties of the rhodamines. The aggregation of the molecular forms of rhodamine 19 is also studied in water and ethanol. 相似文献
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The symmetry properties of the lattice dynamics of nonrigid molecular crystals are discussed on mass-weighted translational coordinates and inertia moment-weighted rotational coordinates as well as molecular internal normal coordinates in molecular principal axes frame. An equivalent reducible representation of the wavevector group and the classification of molecular crystal normal modes are provided. A detailed illustrative example is presented. 相似文献
5.
David F. V. Lewis 《Journal of computational chemistry》1989,10(2):145-151
Results of molecular orbital (MO) calculations by the complete neglect of differential overlap (CNDO /2) method on 50 small molecules are reported. The summation of calculated atomic polarizabilities are equated with molecular polarizabilities, and these are compared with experimentally determined values. It is found that there is very good agreement between calculated and experimental molecular polarizability. This provides a reliable method for the determination of molecular polarizabilities for compounds for which experimental values are not known. The relationship between log P and polarizability is discussed and analyzed in terms of contributions from electronic components to the partitioning energy. 相似文献
6.
The predictions of the ellipsoidal Lorentz cavity model for different cavity axial ratios are compared with the predictions of a microscopic molecular crystal model for different molecular axial ratios. The models agree when the cavity depolarisation factor equals the crystal Lorentz factor. Numerical calculations show that the axial ratio of the cavity equivalent to the molecule is roughly equal to the molecular axial ratio up to ≈ 5. but increases rapidly thereafter. For molecular axial ratios of 9 and above there is no equivalent cavity. Implications of the results for the internal field in liquid crystals are considered. 相似文献
7.
本文根据三元多嵌段共聚反应的一般模型,运用母函数方法,严格推导出了型三元多嵌段共聚物的分子量分布及平均分子量的解析表达式.对几种具有特殊分布的预聚体进行了讨论. 相似文献
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In the framework of the Heitler-London method, a method for determining the allowed molecular multiplets is proposed. The method is based on the connection of the total molecular spin with the permutation symmetry of the coordinate wave function and on the isomorphism of the molecular point group with a certain subgroup of the electron permutation group. The method does not depend on the approximation in which the molecular ions are considered. 相似文献
9.
The electron transport properties of various molecular junctions based on the thiol-ended oligosilane are investigated through density functional theory combined with non-equilibrium Green's function formalism. Our calculations show that oligosilanes doped by the phenyl and-C10H6 groups demonstrate better rectifying effect and their rectification ratios are up to 15.41 and 65.13 for their molecular junctions. The current-voltage(I-V) curves of all the Au/ modified oligosilane/Au systems in this work are illustrated by frontier molecular orbitals, transmission spectra and density of states under zero bias. And their rectifying behaviors are analyzed through transmission spectra. 相似文献
10.
Moments of the molecular electron density can be related directly to several experimental observables, but formerly they have
only been accurately calculated through methods which lack consistency with standard quantum chemical methods. Here we report
analytical solutions to the basic molecular integrals required to compute the moments of the molecular electron charge density
over Gaussian basis functions. These are derived and cast into a practical closed form, suitable to interface with modern
codes for the calculation of the molecular electronic structure. Illustrative calculations for the hydrogen molecule, at both
the Hartree–Fock and the full configuration interaction levels of theory, are shown and discussed in connection with observables
linked directly to some of the calculated moments.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
11.
Yu. H. Krieger 《Journal of Structural Chemistry》1999,40(4):594-619
Structural instability of one-dimensional conducting molecular systems is suggested as a physical principle underlying the
operation of molecular electronic devices, and its potential application in such devices is analyzed. The theoretical basis
of this effect is discussed. The conditions and parameters determining its characteristics are analyzed from the viewpoint
of molecular electronics. Data on one-dimensional molecular structures which are of interest for the design of electronic
devices are given. Peculiarities of structural instability are discussed, and potential static and dynamic control over the
conductivity of one-dimensional systems is analyzed. Molecular structures are given as examples of hypothetical electronic
devices. The possibilities and prospects of this approach in the development of molecular electronics are discussed. 相似文献
12.
聚对苯乙烯磺酸钠添加剂的分子量对水煤浆浆体性质的影响 总被引:3,自引:0,他引:3
选用了变质程度不同的八种煤和三种不同分子量的聚对苯乙烯磺酸钠(PSS)添加剂,详细考查了该添加剂的分子量对水煤浆浆体性质的影响。结果发现,在考查PSS相对分子量的范围内(质均分子量为5.34×104~33.39×104),八种煤的水煤浆成浆性随着分子量的增大而增加,水煤浆成浆性与PSS添加剂的平均分子量的关系可归因于添加剂在煤粒上的吸附,分子量小的PSS在煤粒上的吸附量大于分子量大的PSS;PSS分子量的增加有利于水煤浆的流变性由胀性向假塑性转变;PSS分子量的增加使水煤浆的静态稳定性得到显著的改善。 相似文献
13.
The dissociation of field ionized doubly charged organic molecular ions into doubly charged fragment ions and neutral fragments is discussed. The kinetic energy released during the dissociation of the singly and doubly charged molecular ions rules out the possibility of a direct correlation between their mechanisms of formation. Further, the pressure as well as the temperature dependence revealed that the singly charged molecular ions are formed by direct ionization of the neutral molecules, while the doubly charged molecular ions are formed through a second ionization process of the adsorbed molecular ions on the field anode surface. 相似文献
14.
Using density functional theory(DFT) combined with nonequilibrium Green's function investigates the electron-transport properties of several molecular junctions based on the PBTDT-CH=NH molecule, which is modified by one to four alkyl groups forming PBTDT-(CH_2)_nCH=NH. The electronic structures of the isolated molecules(thiol-ended PBTDT-(CH2)_nCH=N) have been investigated before the electron-transport calculations are performed. The asymmetric current-voltage characteristics have been obtained for the molecular junctions. Rectifying performance of Au/S-PBTDT-CH=N-S/Au molecular junction can be regulated by introducing alkyl chain. The N3 molecular junction exhibits the best rectifying effect. Its maximum rectifying ratio is 878, which is 80 times more than that of the molecular junction based on the original N molecular junction. The current-voltage(I-V) curves of all the sandwich systems in this work are illustrated by transmission spectra and molecular projection density analysis. 相似文献
15.
The viscoelastic properties of dendrimers of generation 1-4 are studied using nonequilibrium molecular dynamics. Flow properties of dendrimer melts under shear are compared to systems composed of linear chain polymers of the same molecular weight, and the influence of molecular architecture is discussed. Rheological material properties, such as the shear viscosity and normal stress coefficients, are calculated and compared for both systems. We also calculate and compare the microscopic properties of both linear chain and dendrimer molecules, such as their molecular alignment, order parameters and rotational velocities. We find that the highly symmetric shape of dendrimers and their highly constrained geometry allows for substantial differences in their material properties compared to traditional linear polymers of equivalent molecular weight. 相似文献
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An ab initio method for calculation on many-electron molecular systems with the approximation of the inactive part of a molecule by frozen molecular fragment is presented. In the following method the SCF calculations are performed in two series. First the molecular orbitals resulting from the first SCF calculation (modest basis set) are localized. In the second SCF run, the basis set is extended for the active part of the molecule, while molecular orbitals of the inactive part, selected from the localized set, are kept frozen. The results are in good agreement with the extended basis set calculation. 相似文献
18.
Rescattering electrons offer great potential as probes of molecular properties on ultrafast timescales. The most famous example is molecular tomography, in which high harmonic spectra of oriented molecules are mapped to "tomographic images" of the relevant molecular orbitals. The accuracy of such reconstructions can be greatly affected by the distortion of scattering wave functions from their asymptotic forms due to interactions with the parent ion. We investigate the validity of the commonly used plane wave approximation in molecular tomography, showing how such distortions affect the resulting orbital reconstructions. 相似文献
19.
Karl F. Blom 《Journal of mass spectrometry : JMS》1988,23(3):194-203
The basic principles of an approach to mass spectral elemental analysis based on the moments of the low-resolution isotopic pattern are presented. The moment analyses are faster and more general than conventional methods of isotopic pattern analysis. The procedure is applied to the analysis of compounds which exhibit ‘pure’ molecular ions and the analysis of compounds which produce significant fragment ions at masses overlapping the molecular ion. It appears as though reasonable selectivity can be achieved for low relative molecular mass compounds (relative molecular mass < 400 a) using standard instrumentation and data acquisition softy are. Preliminary studies of instrumental factors affecting the measurement of moments are discussed. 相似文献
20.
Lev S. Zarkhin Leonid I. Manevitch Nikolai S. Enikolopian 《Macromolecular Symposia》1989,26(1):431-450
Elementary events in polymer fracture at the molecular level are the breakdowns of single polymeric chains. Breakdown of a stretched macromolecule is accompanied by its mechanical degradation. Comparison of experimental data on the molecular products of thermal and mechanical degradation points to significant distinctions between these two processes. Generation of highly excited (“hot”) molecular products of mechanical degradation by the growing crack cannot be explained in terms of the kinetic concept of polymer fracture. Elaborate theoretical analysis of polymeric chain breakdown and generation of “hot” molecular products in subpicosecond time range is made by numerical molecular dynamic simulation with a computer. The mechanisms of primary and secondary chain breakdowns are established. The principal initiators of breakdowns are localized nonlinear modes of the lattice soliton type. Generation of highly excited (“hot”) molecular products occurs at the initial stage of mechanical degradation (during≈1 picosecond after the first breakdown). 相似文献