全文获取类型
收费全文 | 378篇 |
免费 | 14篇 |
国内免费 | 6篇 |
专业分类
化学 | 283篇 |
力学 | 16篇 |
数学 | 37篇 |
物理学 | 62篇 |
出版年
2022年 | 16篇 |
2021年 | 16篇 |
2020年 | 12篇 |
2019年 | 14篇 |
2018年 | 16篇 |
2017年 | 10篇 |
2016年 | 16篇 |
2015年 | 9篇 |
2014年 | 20篇 |
2013年 | 53篇 |
2012年 | 16篇 |
2011年 | 19篇 |
2010年 | 6篇 |
2009年 | 9篇 |
2008年 | 13篇 |
2007年 | 13篇 |
2006年 | 13篇 |
2005年 | 6篇 |
2004年 | 6篇 |
2003年 | 2篇 |
2002年 | 7篇 |
2001年 | 8篇 |
1999年 | 9篇 |
1997年 | 5篇 |
1996年 | 2篇 |
1995年 | 3篇 |
1994年 | 4篇 |
1993年 | 2篇 |
1992年 | 2篇 |
1991年 | 4篇 |
1990年 | 8篇 |
1989年 | 10篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1986年 | 5篇 |
1984年 | 2篇 |
1983年 | 4篇 |
1982年 | 2篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1965年 | 1篇 |
1959年 | 2篇 |
1957年 | 2篇 |
1956年 | 1篇 |
1910年 | 1篇 |
排序方式: 共有398条查询结果,搜索用时 31 毫秒
91.
The geometries, electronic structures, thermochemical properties, polarizabilities, and hyperpolarizabilities of high capacity hydrogen storage media consisting of alkali metal such as Li or transition metal as Ti, that is, functionalized at the end of C and BN chains have been investigated theoretically using density functional theory (DFT). Fundamental aspects such as interaction energy, natural bond orbital (NBO), charge transfer, energy gap, and the projected density of states (PDOS) are elucidated to analyze the adsorption properties of H2 molecules. Our results revealed that H2 is introduced sequentially on the Ti-C7, Ti(B)-B4N3, and Ti(N)-B3N4 complexes and the H2 uptake capacity are found to be 10.89, 10.80, and 10.58 wt%, respectively. Moreover, two Ti atoms can be adsorbed concomitantly to the ends of C7, B4N3, and B3N4 chains where Ti sites can accommodate 16 H2 molecules, with 8 per Ti center, leading to a storage capacity of up to 26.40, 26.28, and 25.94 wt%, respectively. In addition, two binding mechanisms contribute to the adsorption of hydrogen molecules: polarization of the H2 under the electric field produced by the Ti–chain dipole and hybridization of the 3d orbitals of Ti with σ orbitals of H2. These lead to the hydrogen binding energies within the range of 0.22–0.56 eV/H2, open a prospect of a promising material system for hydrogen storage at ambient temperature. The large difference in charge transfer and interaction between the metal and chains is responsible for the large hyperpolarizability. Moreover, the C and BN chains can be stabilized effectively by C20 fullerene termination and store 8 H2 with an average binding energy of 0.22 eV/H2. The hydrogen desorption energies and temperatures indicate that the Ti-C7,Ti(B)-B4N3, Ti(N)-B3N4, Ti-C7-Ti, Ti(B)-B4N3-Ti(B), Ti(N)-B3N4-Ti(N), Ti-C7-C20, Ti(B)-B4N3-C20, and Ti(N)-B3N4-C20 complexes are easy to desorb H2 molecules. 相似文献
92.
K. A. Idriss H. Sedaira E. Y. Hashem M. S. Saleh S. A. Soliman 《Monatshefte für Chemie / Chemical Monthly》1996,127(1):29-42
Summary The electronic absorption spectra of 2-hydroxy-1,4-naphthoquinone (HNQ) in one- and two-component solvents are discussed. The visible absorption of this reagent has been shown to arise from a charge-transfer transition of the zwitterionic tautomer stabilized by a hydrogen bond donating solvent. The formation constant of the 1:1 hydrogen bonded complex with methanol, ethanol, andn-propanol has been determined at 25 °C from the spectral behaviour in mixed solvents. The tautomerization equilibrium ofHNQ in aqueous ethanol is demonstrated and characterized. The dependence of the absorbance maximum ofHNQ on the hydrogen bond donor ability of the solvent in the visible spectrum, as measured by theTaft-Kamlet parameter, has been established. Experimentally derived evidence is supplied to justify the recommendation ofHNQ as a novel probe for hydrogen bond donor ability in pure solvents and in mixed aqueous solvents.
Das Absorptionsmaximum von 2-Hydroxy-1,4-naphthochinon im sichtbaren Bereich als neues Maß für die Wasserstoffbrückenbindungsbildungsfähigkeit von Lösungsmitteln und Lösungsmittelgemischen
Zusammenfassung Die Absorptionsspektren von 2-Hydroxy-1,4-naphthochion (HNQ) in Ein- und Zweikomponentensystemen werden diskutiert. Die Absorption im sichtbaren Bereich stammt von einemcharge-transfer — Übergang des zwitterionischen Tautomers, das über Wasserstoffbrückenbindungen zum Lösungsmittel stabilisiert wird. Die Bildungskonstanten der binären Komplexe mit Methanol, Ethanol undn-Propanol wurden aus spektroskopischen Daten in Lösungsmittelgemischen bei 25 °C emittelt. Das Tautomeriegleichgewicht vonHNQ in wäßrigem Ethanol wird iskutiert. Die Abhängigkeit des Absorptionsmaximums vonHNQ vond der Wasserstoffbrückenbindungsbildungsfähigkeit des Lösungsmittels wird mittels desT aft-Kamlet — Parameters beschrieben. Experimentelle Ergebnisse ermutigen zur Verwendung vonHNQ als neue Testsubstanzfür die Wasserstoffbrückenbindungsbildungsfähigkeit von reinen und gemischten Lösungsmitteln.相似文献
93.
Samia M. Rida Farid S. G. Soliman El-Sayed A. M. Badawey E. El-Ghazzawi O. Kader Thomas Kappe 《Journal of heterocyclic chemistry》1988,25(4):1087-1093
The synthesis of some substituted 3-hydroxy-1-oxo-1H,5H-pyrido[1,2-a]benzimidazole-4-carbonitriles and 4-ethyl carboxylates 3 and their 0- and N-dialkyl derivatives 5,6 is described. 3-Ethoxy-5-ethyl-2-phenyl-1H,5H-pyrido[1,2-a]benzimidazol-1-one 7 was obtained during the course of ethylating the parent ester 3t with triethyl phosphate. Chlorination of 3 with phosphorus oxychloride afforded the corresponding 1,3-dichloropyrido[1,2-a]benzimidazoles 8 which were converted to a variety of azido, amino, morpholino and methoxy derivatives of the system. The synthesis of the indolopyridobenzimidazole 15 is also described. Two compounds exhibited in vitro antibacterial activity. Many compounds were screened for antileukemic, antimicrobial, herbicidal and plant antifungal potencies but were inactive. 相似文献
94.
Y. M. Issa H. M. Abdel Fattah M. M. Omar A. A. Soliman 《Monatshefte für Chemie / Chemical Monthly》1995,126(2):163-171
Summary The stability constants of complexes of trivalent Y, La, Ce, Pr, Sm, Gd, Dy, Ho, Er, and Yb ions with somethio-Schiff bases have been determined potentiometrically using theCalvin-Bjerrum titration technique as modified byIrving andRossotti at 25 °C and an ionic strength of 0.1M (NaCl) in 70% (v/v) aqueous ethanol. 1:1 and 1:2 complexes are formed and evidenced by conductometric studies. ThepK
a
values of the SH group were correlated with theHammet constants of substituents. The values of the stability constants are correlated with the atomic numbers of the lanthanides and with the sum of the ionization constants of the ligands.
Komplexierungsverhalten von Lanthaniden mit einigenthio-Schiff schen Basen
Zusammenfassung Die Stabilitätskonstanten von Komplexen trivalenter Y-, La-, Ce-, Pr-, Sm-, Gd-, Dy-, Ho-, Er- und Yb-Ionen mit einigenthio-Schiffschen Basen wurden potentiometrisch unter Verwendung der vonIrving undRossotti modifiziertenCalvin-Bjerrum-Titrationstechnik bei 25 °C undI=0.1M (NaCl) in 70% (v/v) wäßrigem Ethanol bestimmt. Wie konduktometrisch gezeigt werden konnte, treten 1:1- und 1:2-Komplexe auf. DiepK a -Werte der SH-Gruppen wurden mit denHammetschen Substituentenkonstanten korreliert, die Stabilitätskonstanten mit den Kernladungszahlen der Lanthaniden und mit der Summe der Ionisationskonstanten der Liganden.相似文献
95.
Samia M. Rida Farid S. G. Soliman El-Sayed A. M. Badawey Thomas Kappe 《Journal of heterocyclic chemistry》1988,25(6):1725-1728
The synthesis of some 3-substituted and 2,3-disubstituted-1-oxo-1H,5H-pyrido[1,2-a]benzimidazole-4-carbo-nitriles 5,6 by fusing 1H-benzimidazole-2-acetonitrile 1 with some β-keto esters 2,4 in the presence of ammonium acetate or with ethyl β-aminocrotonate 3 is described. The tricyclic compounds were converted to their N-5 methyl of N-5 ethyl derivatives 8,9. Vilsmeir-Haack formylation of 3-methyl-1-oxo-1H,5H-pyrido[1,2-a]-benzimidazole-4-carbonitrile 5a afforded its 2-formyl derivative 10. Chlorination of 5 and 6 with phosphorus oxychloride yielded the respective 1-chloropyrido[1,2-a]benzimidazole-4-carbonitriles 11,12 which were utilized to prepare the 1-azido, 1-amino, 1-piperidino and 1-methoxy derivatives of the ring system. Compound 11a exhibited strong in vitro activity against S. aureus. Four compounds were screened against P-388 lymphocytic leukemia in mice but were inactive. 相似文献
96.
This review article summarizes the structural features of complexes of salicylidene Schiff bases containing, in addition to the phenolic-OH and the azomethine (–RC=N–) groups, a thiole group, and/or a sulfur atom participating in coordination. Structural aspects of metal complexes of
salicylidene-2-aminothiophenol, salicylidene-3-aminothiophenol, salicylidenethiosemicarbazone, salicylidenedithiocarbazates,
salicylidenedithiocarbazates, salicylideneaminopropyleneaminocyclopentenedithiocarboxylates, salicylideneimidazoles, and salicylidene-thiosalicylidene-1,3-propanediamine
are reported. 相似文献
97.
Some basic cardiovascular drugs containing secondary or tertiary amino groups are determined spectrophotometrically. The method is simple and sensitive; it is based on the interaction of the drugs, as n-electron donors, with 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) as a pi-acceptor. The highly coloured radical anion exhibits maximum absorption at 460 nm. The drugs determined are pindolol, dipyridamole, hydralazine hydrochloride, quinidine sulphate, prenylamine lactate and tolazoline hydrochloride. Beer's law is obeyed for these drugs. The procedure is sensitive enough to permit unit dose assay of the individual drugs in their pharmaceutical formulations. The assay results are in accord with the pharmacopoeial assay results. 相似文献
98.
Said Ahmed Soliman Ghozlan Ismail Abdelshafy Abdelhamid Mohamed Hilmy Elnagdi Hatem Moustafa Gaber 《Journal of heterocyclic chemistry》2005,42(6):1185-1189
3‐Diethylaminoacrylonitrile ( 1 ) reacts with hydrazonyl halides ( 2a‐d ) to yield 1,3‐disubstituted pyrazole‐4‐carbonitriles 5a‐d. The acetyl 1‐p‐chlorophenylpyrazole‐4‐carbonitrile ( 5a ) condensed with hydrazine hydrate to yield the bishydrazone 10 and with dimethylformamide dimethylacetal to yield 1‐aryl‐3‐(3‐dimethylamino)acryloyl pyrazole‐4‐carbonitrile ( 11 ). This enamine reacts with hydrazine hydrate to yield the pyrazolylpyrazole ( 12 ) and with naphthoquinone to yield the 3‐naphthofuranoyl pyrazole 13. The pyra‐zolyl pyridine derivative 14 was obtained upon treatment of 11 with acetylacetone in the presence of ammonium acetate. Compound 11 was coupled with p‐chlorobenzene diazonium chloride to yield the hydrazone 16 that was coupled further with p‐chlorobenzenediazonium chloride to yield the formazane 18. 相似文献
99.
100.
Umar Ndagi Ndumiso N. Mhlongo Mahmoud E. Soliman 《Applied biochemistry and biotechnology》2018,185(3):655-675
UM-164, a potent Src/p38 inhibitor, is a promising lead compound for developing the first targeted therapeutic strategy against triple-negative breast cancer (TNBC). However, lack of understanding of conformational features of UM-164 in complex with Src serves a challenge in the rational design of novel Src dual inhibitors. Herein, we provide an in-depth insight into conformational features of Src-UM-164 using different computational approaches. This involved molecular dynamics (MD) simulation, principal component analysis (PCA), thermodynamics calculations, dynamic cross-correlation (DCCM) analysis, and hydrogen bond formation. Findings from this study revealed that (1) the binding of UM-164 to Src induces a more stable and compact conformation; (2) the binding of UM-164 results in increased correlation among the active site residue; (3) the presence of multiple phenyl rings and fluorinated phenyl group in UM-164 contributes to the steric effect; (4) a relatively high-binding free energy estimated for the Src-UM-164 system is affirmative of its experimental potency; (5) hydrophobic packing contributes significantly to the drug binding in Src-UM-164; and (6) observed increase in H-bond distance of interacting residue atoms and Dasatinib compared to UM-164. Findings from this study can serve as a baseline in the design of novel Src inhibitors with dual inhibitory properties. 相似文献