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251.
Grimme S 《Journal of computational chemistry》2003,24(13):1529-1537
Based on a partitioning of the total correlation energy into contributions from parallel‐ and antiparallel‐spin pairs of electrons, a modified third‐order Møller–Plesset (MP) perturbation theory is developed. The method, termed SCS–MP3 (SCS for spin‐component‐scaled) continues previous work on an improved version of MP2 (S. Grimme, J Chem Phys 2003, 118, 9095). A benchmark set of 32 isogyric reaction energies, 11 atomization energies, and 11 stretched geometries is used to assess to performance of the model in comparison to the standard quantum chemical approaches MP2, MP3, and QCISD(T). It is found, that the new method performs significantly better than usual MP2/MP3 and even outperforms the more costly QCISD method. Opposite to the usual MP series, the SCS third‐order correction uniformly improves the results. Dramatic enhancements are especially observed for the more difficult atomization energies, some of the stretched geometries, and reaction and ionization energies involving transition metal compounds where the method seems to be competitive or even superior to the widely used density functional approaches. Further tests performed for other complex systems (biradicals, C20 isomers, transition states) demonstrate that the SCS–MP3 model yields often results of QCISD(T) accuracy. The uniformity with which the new approach improves for very different correlation problems indicates significant robustness, and suggests it as a valuable quantum chemical method of general use. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 1529–1537, 2003 相似文献
252.
A master equation is used to study transitions between the stable limit cycle and stable focus in the two-variable bistable system. The distribution function of the mean first passage time between these attractors and the relative dispersion of the mean first return time from the stable focus to itself as a function of the intensity of fluctuations are calculated and discussed. A coherence resonance is observed for the return time from the focus to itself. 相似文献
253.
For the first time, charge transfer (CT) co-polymerization of styrene and methyl methacrylate, promoted by hydroquinone (HQ) at various feed compositions, has been achieved in a hydrophobic ionic liquid. The co-polymers have been characterized for thermal and molecular weight properties. The molecular weights obtained for the co-polymers made in ionic liquid were found to be slightly lower than the corresponding polymers synthesized in conventional solvent. The reactivity ratios of the co-polymers have been computed and compared with conventional CT polymerization. The reactivity ratios of Sty-MMA indicate that the co-polymerization has a tendency to alternate and to produce a higher styrene content in the co-polymers. The numerical values of the inverse of r1 and r2 indicate that both monomer radicals have a distinct cross propagation tendency. 相似文献
254.
The intergrowth of 1,10-decanedicarboxylic acid and urea give infinite hydrogen-bonded chains of the guest included in the hexagonal urea host. A deuterium high-resolution solid-state NMR study of the selectively deuterated intergrowth compound 1,10-decanedicarboxylic acid/hydrogenated urea at variable temperature in the range 90 < or = T < or = 300 K was performed on a single crystal. The analysis of the second moment as a function of temperature is shown to be compatible with the known phase transition occurring near T(c) = 203 K. Moreover, the spectra indicate that the orientational disorder is strong, and is compared to an axial uniform disorder. For this purpose, the general equation for the second moment of a system with uniform two-dimensional axial orientational disorder is given, and a method to take into account the non-uniform excitation of the pulse sequence is proposed. 相似文献
255.
Marti E. Kaisersberger E. Emmerich W.-D. 《Journal of Thermal Analysis and Calorimetry》2004,77(3):905-934
The definitions of the temperature resolution, the so-called resolution of DSC instruments given in literature are discussed.
A new definition of the resolution for DSC instruments is presented and outlined. The main characteristic introduced in this
new definition is a minimum between two caloric events as a prerequisite of an existing resolution. Possible candidates of
test substances have been evaluated. The oligomer n-hexatriacontane is revealing an interesting phenomenon, namely a lambda
transition which is in the peak temperature only 2 K lower than the melting temperature of 76°C. The substance was selected
as an ideal test substance for the quantification of the resolution of DSC instruments. The lambda transition is a second
order process which may reach under certain conditions the saturation of the occurring molecular dislocation within 0.2 K,
and after saturation the heat flow rate drops sharply down. Investigations concerning the main characteristics of n-hexatriacontane
in respect to the temperatures of transition (lambda transition and melting), to the involved enthalpies, and to the resolution
factors were performed as functions of the sample mass and the heating rate. The importance of relevant evaluation procedures
increasing the resolution factors of DSC curves are discussed and these procedures are integrated into the testing of the
resolution. The necessity for widening the experimental scope from instruments to evaluation procedures is forced by the existence
of instruments with built-in signal treatments based on electronic devices and software procedures. A comparison with literature
data is outlined for all of the mentioned characteristic values of n-hexatriacontane.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
256.
257.
制备了不同摩尔比的甲基丙烯酸-磷脂酰乙醇甲基丙烯酰胺共聚物,研究了这种共聚物在水面的表面压力(π)-分子面积(A)曲线、共聚物制成的MOS LB膜的电容(C)-电压(V)特性及此LB膜的相变温度。 相似文献
258.
259.
The high-pressure phase transition from ambient pressure α-LiBH4 to high-pressure β-LiBH4 was observed by Raman spectroscopy and X-ray diffraction between 0.8 and 1.1 GPa. The phase boundary between these two phases was mapped over a large range of temperatures using thermal conductivity studies and differential thermal analysis. The structure of the high-pressure phase could not be identified due to small number of experimentally observed reflections, but it was shown that it is different from previously reported theoretical predictions. 相似文献
260.
Exfoliated polystyrene (PS)/laponite nanocomposites were prepared successfully. The characteristic doo1 diffraction peak of organo-laponite disappeared in the XRD patterns of nanocomposites, indicating that the laponite layers were exfoliated and the ordered crystal structure of laponite was destroyed because of the styrene polymerization. TEM observations showed that the exfoliated laponite primary particles were dispersed randomly in the PS matrix with lateral dimensions from 1 nm to 10 rim. SEM results showed that the PS/laponite nanocomposite particles were almost monodispersed spheres with the size of about 120 rim. Because of the interaction between PS and laponite nanolayers, the nanocomposites exhibited higher thermal stability and glass transition temperature when compared to pure PS. 相似文献