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21.
The surface-enhanced Raman scatterings of dimethyl sulfide, diethyl sulfide and dimethyl disulfide have been investigated in silver sol. The dimethyl disulfide molecule decomposes on silver to the corresponding mercaptide implying facile cleavage of its S---S bond. The C---S bond in dialkyl monosulfide appears not to cleave on silver. For diethyl sulfide, the C2 conformation seems to be favorable on silver than other conformations. 相似文献
22.
23.
In the paper, we describe a polynomial time algorithm that, for every input graph, either outputs the minimum bisection of the graph or halts without output. More importantly, we show that the algorithm chooses the former course with high probability for many natural classes of graphs. In particular, for every fixedd≧3, all sufficiently largen and allb=o(n 1?1/[(d+1)/2]), the algorithm finds the minimum bisection for almost alld-regular labelled simple graphs with 2n nodes and bisection widthb. For example, the algorithm succeeds for almost all 5-regular graphs with 2n nodes and bisection widtho(n 2/3). The algorithm differs from other graph bisection heuristics (as well as from many heuristics for other NP-complete problems) in several respects. Most notably:
- the algorithm provides exactly the minimum bisection for almost all input graphs with the specified form, instead of only an approximation of the minimum bisection,
- whenever the algorithm produces a bisection, it is guaranteed to be optimal (i.e., the algorithm also produces a proof that the bisection it outputs is an optimal bisection),
- the algorithm works well both theoretically and experimentally,
- the algorithm employs global methods such as network flow instead of local operations such as 2-changes, and
- the algorithm works well for graphs with small bisections (as opposed to graphs with large bisections, for which arbitrary bisections are nearly optimal).
24.
含氰基侧基聚芳醚砜的合成与表征 总被引:1,自引:0,他引:1
汉2,6-二卤苯甲腈、4,4′-二氯二苯砜、4,4′-二羟基二苯砜、间苯二酚、对苯二酚、酚酞及双酚A为主要原料,合成了含不同结构单元和不同氰基含量的含氰基侧基的聚芳醚砜,并采用IR、^13CNMR等分析手段对其结构进行了表征,同时研究了结构单元和氰基含量对聚芳醚砜性能的影响。 相似文献
25.
Marsella MJ Yoon K Estassi S Tham FS Borchardt DB Bui BH Schreiner PR 《The Journal of organic chemistry》2005,70(5):1881-1884
Photochemical cyclization of compound 1, a homoenediyne (-CCC=CCH2CC-) bearing two ethynylanthracene chromophores, yields two isomeric dihydrocyclopent[a]indene ring systems, spiro-fused to the 9-position of a 9,10-dihydroanthracene moiety. Evidence of a photochemically initiated diradical cyclization pathway is proposed on the basis of (i) hydrogen abstraction from reaction with 1,4-cyclohexadiene (1,4-CHD) and (ii) the observation of 1,4-addition of benzene (solvent). The reaction was further analyzed by a complete density functional theory (DFT) study, using an unrestricted approach (UBLYP) with a 6-31G* basis set for the open-shell triplet states of the reactants, products, and diradical intermediates to model the photochemical nature of observed transformation. A mechanism detailing the observed cyclization/addition reaction is proposed. 相似文献
26.
Emilio Aicart Huu Van Tra Lina Andreoli-Ball Donald Patterson 《Journal of solution chemistry》1994,23(11):1183-1201
Expansion coefficients , isothermal compressibilities, thermal pressure coefficients and heat capacities have been measured at 25°C for the cyclohexane+trans-decalin system. An S-shaped composition dependence, positivelnegative for highllow cyclohexane compositions is found for C
p
E
dV
E
/dT and the thermal expansion contribution to C
p
E
namely VT. The thermal motion contribution to C
p
E
, namely C
v
is close to zero. The positive excursion of these mixing quantities at high cyclohexane content is anomalous. Correspondingly, the mixing quantity-VT deviates strongly in this region from the predicted equality with H
E
. The literature and this work show that all these excess quantities behave similarly for cyclohexane mixed with cyclooctane, methylcyclohexane and some highly branched alkanes. The unusual composition dependence of the thermodynamic quantities is consistent with order occurring when any large alkane molecule of globular shape is added to cyclohexane. This is speculatively associated with an interference by the globular alkane with the relatively free rotation of cyclohexane molecules. 相似文献
27.
A New Diterpene Acid from the Flowers of Heteropappus altaicus 总被引:2,自引:0,他引:2
QingHuaLIU JunShanYANG YangLU QiTaiZHENG LiMAO 《中国化学快报》2005,16(7):921-924
A new compound, named heteraltaic acid was isolated from the flowers of Heteropappus altaicus (willd) Novopokr. The compound was elucidated as (5R, 6S, 8aS) -5-[2-(3-furyl) ethyl-5, 6, 8a-trimethyl-4a, 5, 6, 7, 8, 8a -hexahydro-l-naphthalenecarboxylic acid] by the combination of 1D and 2D NMR techniques (HSQC, HMBC) and X-ray analysis. 相似文献
28.
Synthesis, characterization and crystal structures of two polyphilic mesogenic compounds 总被引:1,自引:0,他引:1
Philippe Kromm Michel Cotrait Jean Claude Rouillon Philippe Barois Huu Tinh Nguyen 《Liquid crystals》1996,21(1):121-131
Three homologous series of semi-perfluorinated liquid crystals: 4-(2,2,3,3,4,4,4-heptafluoro-butyloxycarbonyl)phenyl, 4-(2,2,3,3,4,4,5,5-octafluoropentyloxycarbonyl)phenyl and 4-(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyloxycarbonyl)phenyl 4-[(n-perfluoroalkyl)alkyloxy]benzoates have been synthesized. Their mesomorphic properties were studied by DSC, microscopic observation and X-ray diffraction. Their layer spacings are smaller than the molecular length (L). The ratio dSA/L is about 0.7. The crystal structure of two derivatives of the first family have also been investigated. In both structures the molecules pack in smectic C-like sheets where neighbouring molecules are antiparallel, interacting through dipolar and van der Waals forces. The interactions between contiguous sheets, through the ends of perfluorinated chains are very weak. The X-ray diffraction results on the mesophases and on the crystalline structures of two compounds of the first family are compared. From this, we propose a model of the smectic phases with a zig-zag structure. 相似文献
29.
The possible geometries for cyclobutadiene and the electronic spectra corresponding to those geometries were calculated by a modified Pariser-Parr method with inclusion of doubly-excited as well as singly-excited configurations in calculating the -energy. The bond lengths of the most stable forms agreed well with previous calculations where the empirical parameters used in the -energy calculation were different from those used here, and where only singly-excited configurations were included. With these additional refinements, the rectangular singlet is calculated to be more stable than the square triplet. It is noted that the geometry predicted to have the lowest energy is a function of the approximations employed.
This research was supported in part by Research Grant GP 4290 from the National Science Foundation. 相似文献
Zusammenfassung Mögliche Geometrien und die zugehörigen Spektren von Cyclobutadien wurden nach Pariser-Parr unter Einschlu\ der einfach und zweifach angeregten Konfigurationen berechnet. Die BindungslÄngen der stabilsten Form stimmen gut mit früheren Rechnungen mit anderen gewÄhlten Parametern überein, bei denen nur die einfach angeregten Konfigurationen berücksichtigt worden waren. Und zwar ergibt sich, da\ eine rechteckige Form (Singulett) stabiler als die quadratische (Triplett) ist. Allerdings hÄngt diese Geometrie von den gewÄhlten Parametern ab.
Résumé Les geometries possibles du butadiéne et les spectres électroniques correspondants ont été calculés par une méthode de Pariser-Parr motifiée avec interaction de configurations avec les états mono et diexcités des électrons . Les longueurs de liaison des formes les plus stables sont en accord avec celles obtenues précédemment avec des paramètres empiriques différents dans un calcul ne tenant compte que des configurations monoexcitées. Dans notre calcul amélioré le singulet rectangulaire est plus stable que le triplet carré. La géométrie la plus stable dépend de l'approximation utilisée.
This research was supported in part by Research Grant GP 4290 from the National Science Foundation. 相似文献
30.
In order to enhance the
electrogenerative leaching rate of sphalerite reasonably, the basic principle
of generative process was applied to the simultaneous leaching of sphalerite
in the presence of MnO2. It was confirmed by experimental
results that both mental ions and electric energy could be obtained in the
simultaneous electrogenerative leaching process. The leaching cell had the
open circuit potential about 1.0 V, about 2000 C of electric quantity was
obtained, the optimal leaching ratio of Zn2+ and
Mn2+ was 48.5, 39.6% respectively, after electrogenerative
leaching for 11 h. 相似文献