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排序方式: 共有196条查询结果,搜索用时 31 毫秒
31.
32.
π–π and CH···N interactions are vital in biological systems. In this study, stacking and hydrogen-bonded interactions in pyrazine
and triazine dimers were investigated by density functional theory combined with symmetry-adapted perturbation theory (DFT-SAPT)
and counterpoise (CP)-corrected supermolecular MP2, SCS-MP2, B3LYP-D and CCSD(T) calculations. All interaction energies were
computed using the optimized structures at the CP-corrected SCS/aug-cc-pVDZ level, which gave 1–2 kJ/mol lower interaction
energies than the ones computed at the MP2 level. For both dimers, doubly hydrogen-bonded and cross-(displaced) stacked orientations
were found to be the lowest energy ones. The reference CCSD(T) calculations favored the former structure in both dimer systems,
whereas MP2 and SCS-MP2 located the latter as the lowest energy isomer. In particular, the former was found to be lower in
energy than the latter by 2.28 and 1.01 kJ/mol at the CCSD(T)/aug-cc-pVDZ level for pyrazine and triazine, respectively. B3LYP-D
produced interaction energies in agreement with the CCSD(T) at the equilibrium geometries, but it overestimates them at the
short range and underestimates at the long intermonomer separations. Furthermore, it tends to give smaller equilibrium distances
compared to the CCSD(T). DFT-SAPT method was in a good agreement with the reference CCSD(T) calculations. This suggests that
DFT-SAPT can be employed to compute the full potential energy surface of these dimers. Moreover, DFT-SAPT calculations showed
that the electrostatic and dispersion contributions are the most important energy components stabilizing these dimers. The
present study aims to show which theoretical method is the most promising one for the investigation of intermolecular interactions
dominated by π–π and CH···N. Therefore, the findings obtained in this study can be used to unravel the structures of nucleic
acid bases and other systems stabilized by π–π and CH···N interactions. 相似文献
33.
Li MR Adem U McMitchell SR Xu Z Thomas CI Warren JE Giap DV Niu H Wan X Palgrave RG Schiffmann F Cora F Slater B Burnett TL Cain MG Abakumov AM van Tendeloo G Thomas MF Rosseinsky MJ Claridge JB 《Journal of the American Chemical Society》2012,134(8):3737-3747
Combining long-range magnetic order with polarity in the same structure is a prerequisite for the design of (magnetoelectric) multiferroic materials. There are now several demonstrated strategies to achieve this goal, but retaining magnetic order above room temperature remains a difficult target. Iron oxides in the +3 oxidation state have high magnetic ordering temperatures due to the size of the coupled moments. Here we prepare and characterize ScFeO(3) (SFO), which under pressure and in strain-stabilized thin films adopts a polar variant of the corundum structure, one of the archetypal binary oxide structures. Polar corundum ScFeO(3) has a weak ferromagnetic ground state below 356 K-this is in contrast to the purely antiferromagnetic ground state adopted by the well-studied ferroelectric BiFeO(3). 相似文献
34.
Deprotonation of a cyclotriphosphazene with a tert-butylamino group in the side chain results in ring expansion to a very stable, planar cyclohexaphosphazene derivative that still contains eight P-Cl bonds suitable for forming macromolecular structures. 相似文献
35.
36.
Alejandro Adem 《Mathematische Annalen》1992,292(1):319-327
Partially supported by an NSF grant 相似文献
37.
Plant-derived antioxidant compounds have the potential to prevent cell damage caused by free radicals. As a holoparasitic plant, Cuscuta campestris Yuncker is being valorized for treatment of liver injury and cancer prevention in traditional medicine. The main purpose of this present study is to elucidate the antioxidant- and anticancer-associated contents of C. campestris by spectroscopic and chromatographic methods. Diethyl ether, ethyl acetate, methanol, n-butanol, and water were used as extraction solvents to reach a wide range of secondary metabolites synthesized by this plant. Antioxidant potentials of these extracts were characterized by (2,2-diphenyl-1-picrylhydrazyl)-free radical scavenging activity. Their anticancer activities were evaluated on SNU-398 hepatocellular carcinoma cells and controlled on the normal adult human dermal fibroblasts (hDFs) cells. Their phenolic compounds were analyzed by high-performance liquid chromatography with diode array detector to illuminate the responsible anticancer agent(s). The ethyl acetate extract revealed the most significant antioxidant effect. Methanol and ethyl acetate extracts were found to be cytotoxic on the SNU-398 cell lines with CC50 values of 18.7 and 19.6?µg?mL?1, respectively, whereas these extracts were not cytotoxic to the adult hDFs cells. Due to their abundance in the extracts, isorhamnetin, kaempferol, and quercetin may have caused this anticancer activity. Methanol extract has the highest concentration of phenolic compounds. Based on chromatographic analyses, we propose that these anticancer effects were positively correlated with plant phenolic compounds. The results showed that this plant is worth further study for its therapeutic uses. 相似文献
38.
Abdu Y. Bunch Xueqin Wang Umit S. Ozkan 《Journal of molecular catalysis. A, Chemical》2007,270(1-2):264-272
Effect of H2S on the catalytic performance of the reduced and sulfided Ni–Mo/Al2O3 catalysts in hydrodeoxygenation of benzofuran is studied. The steady-state reaction experiments showed a decrease in activity for both reduced and sulfided catalysts when H2S was introduced into the feed. The reaction conversion of benzofuran over the reduced catalyst still remained superior to that of the sulfided catalyst in the presence of H2S, however, at high reaction temperatures, the product distribution over the pre-reduced catalyst is similar to the sulfided catalyst. The studies with temperature-programmed desorption (TPD), temperature-programmed reaction (TPRxn) and X-ray photoelectron spectroscopy (XPS) techniques showed a partial sulfidation of the reduced catalysts when exposed to H2S under reaction conditions, however, the catalyst does not go through a complete conversion to a molybdenum sulfide phase. Instead oxygen–sulfur exchange on the surface leaves behind oxisulfide species, with catalytic activity closely resembling that of the reduced catalysts. The effect of H2S on the reaction performance is mainly coming from the competitive adsorption between H2S and benzofuran and the formation of SH groups with decomposition of H2S at high temperatures. 相似文献
39.
İlke Taşçıoğlu Adem Tataroğlu Akif Özbay Şemsettin Altındal 《Radiation Physics and Chemistry》2010,79(4):457-461
The effect of γ-ray exposure on the metal–insulator–semiconductor (MIS) structures has been investigated using the electrical characteristics at room temperature. The MIS structures are irradiated with 60Co γ-ray source. The energy distribution of interface states was determined from the forward bias I–V characteristics by taking into account the bias dependence of the effective barrier height and ideality factor. The value of series resistance decreases with increasing dose. Experimental results confirmed that γ-ray irradiation have a significant effect on electrical characteristics of MIS structures. 相似文献
40.
Serap Beli Simon J. Coles Derya Davarc David B. Davies Michael B. Hursthouse Adem Kl 《Polyhedron》2007,26(18):5283-5292
Reaction of cyclophosphazene, N3P3Cl6 (1), with the sodium derivative of the fluorinated diol, 2,2,3,3,4,4-hexafluoropentane-1,5-diol (2), in THF solution at room temperature afforded five isolated products, whose structures have been characterised by X-ray crystallography and 1H, 19F and 31P NMR spectroscopy: the mono-spiro compound, N3P3Cl4(OCH2CF2CF2CF2CH2O) (3), its ansa isomer (4), a di-spiro derivative N3P3Cl2(OCH2CF2CF2CF2CH2O)2 (5), its spiro-ansa (6) isomer and the tri-spiro compound N3P3(OCH2CF2CF2CF2CH2O)3 (7). Quantitative 31P NMR measurements of the soluble portion of the reaction mixture show that in the reaction of (1) with the sodium derivative of the fluorinated pentanediol (2) there is a small preference for spiro compounds compared to ansa compounds (ratio ca. 1.3:1), similar to the analogous reaction of (1) with the sodium derivative of the fluorinated butanediol where there is a slightly greater proportion of spiro compared to ansa compounds (ratio ca. 1.5:1). The relative proportions of spiro and ansa compounds is likely to depend on the fine balance in stabilities of the different medium-sized rings in the fluorinated pentanediol (spiro, 8- and ansa, 10-membered rings) compared to the fluorinated butanediol (spiro, 7- and ansa, nine-membered rings) derivatives of cyclophosphazene. 相似文献