全文获取类型
收费全文 | 1799篇 |
免费 | 26篇 |
国内免费 | 23篇 |
专业分类
化学 | 1477篇 |
晶体学 | 14篇 |
力学 | 15篇 |
数学 | 79篇 |
物理学 | 263篇 |
出版年
2022年 | 10篇 |
2021年 | 23篇 |
2020年 | 20篇 |
2019年 | 16篇 |
2018年 | 16篇 |
2017年 | 11篇 |
2016年 | 21篇 |
2015年 | 14篇 |
2014年 | 31篇 |
2013年 | 60篇 |
2012年 | 91篇 |
2011年 | 94篇 |
2010年 | 64篇 |
2009年 | 71篇 |
2008年 | 103篇 |
2007年 | 110篇 |
2006年 | 111篇 |
2005年 | 130篇 |
2004年 | 116篇 |
2003年 | 97篇 |
2002年 | 86篇 |
2001年 | 32篇 |
2000年 | 17篇 |
1999年 | 30篇 |
1998年 | 30篇 |
1997年 | 22篇 |
1996年 | 35篇 |
1995年 | 19篇 |
1994年 | 11篇 |
1993年 | 10篇 |
1992年 | 14篇 |
1991年 | 13篇 |
1989年 | 15篇 |
1988年 | 15篇 |
1987年 | 16篇 |
1986年 | 6篇 |
1985年 | 17篇 |
1984年 | 24篇 |
1983年 | 9篇 |
1982年 | 24篇 |
1981年 | 24篇 |
1980年 | 14篇 |
1979年 | 22篇 |
1978年 | 18篇 |
1977年 | 23篇 |
1976年 | 15篇 |
1975年 | 14篇 |
1974年 | 13篇 |
1973年 | 15篇 |
1971年 | 6篇 |
排序方式: 共有1848条查询结果,搜索用时 15 毫秒
91.
Geometry optimizations at the UHF/6-31G* and UMP2/6-31G* levels of theory were performed to find the transition state in the interconversion between norbornadiene (N) and quadricyclane (Q) radical cations. Two transition structures, TS+·1 and TS+·2, were obtained which have C1 and C2 symmetry, respectively. Vibrational analysis at the UHF and UMP2 levels of theory and IRC calculation showed that TS+·1 is the true transition state connecting N+· and Q+·, while TS+·2 is a second order saddle point. 相似文献
92.
Tsutomu Shiragami Katsunori Tanaka Yoshito Andou Shin-ichiro Tsunami Jin Matsumoto Hongxia Luo Yasuyuki Araki Osamu Ito Haruo Inoue Masahide Yasuda 《Journal of photochemistry and photobiology. A, Chemistry》2005,170(3):287-297
Tetraphenylporphyrinatoantimony(V) complexes, linked to boron-dipyrrin chromophores on axial ligands, were synthesized. The fluorescence spectra of 1a, 1b and 1c (3-[4-(N,N′-difluorobornyl-5-dipyrrinyl)phenyl]propoxo(methoxo)antimony(V) tetraphenylporphyrin bromide (1a); 6-[4-(N,N′-difluorobornyl-5-dipyrrinyl)phenyl]hexyloxo(methoxo)antimony(V) tetraphenylporphyrin bromide (1b); bis{3-[4-(N,N′-difluorobornyl-5-dipyrrinyl)phenyl]propoxo}antimony(V) tetraphenylporphyrin bromide (1c)) were analyzed under the excitations of N,N′-difluorobornyl-5-dipyrrinylphenyl (Bdpy) and tetraphenylporphyrinatoantimony(V) (Sb(TPP)) chromophores. Under the irradiation of Bdpy chromophore, the excitation energy was transferred from Bdpy chromophore to the Sb(TPP) moiety at 0.13–0.40 of the quantum yields, even in a polar solvent. On the other hand, the emission of Sb(TPP) chromophores was quenched by Bdpy chromophores at rate constants of 108–109 s−1, independent of on the solvent polarity. Under the excitation of the Bdpy chromophore of 1d (3-[4-(N,N′-difluorobornyl-5-dipyrrinyl)phenyl]propoxo(phenyloxo)antimony(V) tetraphenylporphyrin bromide) involving both the Bdpy and the phenoxy chromophores on the axial ligands, the excited singlet state of the Sb(TPP) chromophore generated by the energy transfer from the Bdpy chromophore was quenched by the phenoxy ligand via non-radiative processes involving electron transfer. However, rapid back electron-transfer may occur because no absorption of the anion radical of Sb(TPP) was observed by nanosecond laser photolysis. 相似文献
93.
Design, synthesis, and binding affinities of pyrrolinone-based somatostatin mimetics 总被引:1,自引:0,他引:1
Smith AB Charnley AK Mesaros EF Kikuchi O Wang W Benowitz A Chu CL Feng JJ Chen KH Lin A Cheng FC Taylor L Hirschmann R 《Organic letters》2005,7(3):399-402
[structure: see text] Tetrapyrrolinone somatostatin (SRIF) mimetics (cf. 1), based on a heterochiral (D,L-mixed) pyrrolinone scaffold, were designed, synthesized, and evaluated for biological activity. The iterative synthetic sequence, incorporating the requisite functionalized coded and noncoded amino acid side chains, comprised a longest linear synthetic sequence of 23 steps. Binding affinities at two somatostatin receptor subtypes (hsst 4 and 5) reveal micromolar activity, demonstrating that the d,l-mixed pyrrolinone scaffold can be employed to generate functional mimetics of peptide beta-turns. 相似文献
94.
Tsuda O Ishihara M Koga Y Fujiwara S Setsuhara Y Sato N 《The journal of physical chemistry. B》2005,109(11):4917-4922
Three-dimensional plasma enhanced chemical vapor deposition (CVD) of hydrogenated amorphous carbon (a-C:H) has been demonstrated using a new type high-density volumetric plasma source with multiple low-inductance antenna system. The plasma density in the volume of phi 200 mm x 100 mm is 5.1 x 10(10) cm(-3) within +/-5% in the lateral directions and 5.2 x 10(10)cm(-3) within +/-10% in the axial direction for argon plasma under the pressure of 0.1 Pa and the total power as low as 400 W. The uniformity of the thickness and refractive index is within +/-3.5% and +/-1%, respectively, for the a-C:H films deposited on the substrates placed on the six side walls, the top of the phi 60 mm x 80 mm hexagonal substrate holder in the pure toluene plasma under the pressure is as low as 0.04 Pa, and the total power is as low as 300 W. It is also found that precisely controlled ion bombardment by pulse biasing led to the explicit observation in Raman and IR spectra of the transition from polymer-like structure to diamond-like structure accompanied by dehydrogenation due to ion bombardment. Moreover, it is also concluded that the pulse biasing technique is effective for stress reduction without a significant degradation of hardness. The stress of 0.6 GPa and the hardness of 15 GPa have been obtained for 2.0 microm thick films deposited with the optimized deposition conditions. The films are durable for the tribology test with a high load of 20 N up to more than 20,000 cycles, showing the specific wear rate and the friction coefficient were 1.2 x 10(-7) mm3/Nm and 0.04, respectively. 相似文献
95.
Sandanayaka AS Watanabe N Ikeshita K Araki Y Kihara N Furusho Y Ito O Takata T 《The journal of physical chemistry. B》2005,109(7):2516-2525
Three rotaxanes, with axles with two zinc porphyrins (ZnPs) at both ends penetrating into a necklace pending a C60 moiety, were synthesized with varying interlocked structures and axle lengths. The intra-rotaxane photoinduced electron transfer processes between the spatially positioned C60 and ZnP in rotaxanes were investigated. Charge-separated (CS) states (ZnP*+, C60*-)rotaxane are formed via the excited singlet state of ZnP (1ZnP*) to the C60 moiety in solvents such as benzonitrile, THF, and toluene. The rate constants and quantum yields of charge separation via 1ZnP decrease with axle length, but they are insensitive to solvent polarity. When the axle becomes long, charge separation takes place via the excited triplet state of ZnP (3ZnP*). The lifetime of the CS state increases with axle length from 180 to 650 ns at room temperature. The small activation energies of charge recombination were evaluated by temperature dependence of electron-transfer rate constants, probably reflecting through-space electron transfer in the rotaxane structures. 相似文献
96.
Zhao L Zhu G Zhang D Di Y Chen Y Terasaki O Qiu S 《The journal of physical chemistry. B》2005,109(2):764-768
Using 1,2-bis(trimethoxysilyl)ethane as organosiloxane precursor and a triblock copolymer surfactant, Pluronic F127, as template, a highly ordered mesoporous organosilica with large cagelike pores has been successfully synthesized. Its structure was resolved to be 3-D cubic Fmm by high-resolution transmission electron microscopy. The intergrowth of hexagonal close-packed and cubic close-packed phases was observed in this material. The effect of different siliceous precursors on the mesostructure was also investigated. 相似文献
97.
Microemulsion-based synthesis of titanium phosphate nanotubes via amine extraction system 总被引:2,自引:0,他引:2
Yin Z Sakamoto Y Yu J Sun S Terasaki O Xu R 《Journal of the American Chemical Society》2004,126(29):8882-8883
The first titanium phosphate nanotubes with alternating interlayer spacings have been successfully prepared and characterized. The synthesis is accomplished in a reverse microemulsion formed in an amine extraction system. TEM data from samples made after different times of reaction suggest a scrolling-formation mechanism. 相似文献
98.
Mishima O 《The Journal of chemical physics》2004,121(7):3161-3164
Emulsified high-density amorphous ice, made by pressure-induced amorphization of emulsified ice Ih, was decompressed at about 160 K. The onset of an endothermic event was observed around 0.4 GPa during the decompression. This is consistent with existence of the glass transition to a liquid state, implying the close relationship between melting and amorphization. 相似文献
99.
Ohsuna T Slater B Gao F Yu J Sakamoto Y Zhu G Terasaki O Vaughan DE Qiu S Catlow CR 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(20):5031-5040
High-resolution electron microscopy (HREM) has been used to image the surface structure of nano- and micrometer-sized synthetic crystals of zeolite-Linde-L (LTL). Columnar holes and rotational, nano-sized, wheel-like defects were observed within the crystals, where the hole has a minimum size equal to that of the rotational defect. Predictions of surface structure from atomistic computer simulation concur with the observations from HREM and provide insight into the crystal growth mechanism of perfect and defective LTL. Analysis of the energetics of the formation of rotational defect structures reveals that the driving force for defect creation is thermodynamic and furthermore, the rotational defects could be created in high concentrations. Formation of a columnar hole is found to be slightly energetically unfavourable and therefore we speculate that the incidence of both rotational and nano-sized vacancy defects is strongly dependent on kinetic factors and reaction conditions. The morphology of nano- and microcrystalline LTL is contradistinct and we use insights from simulation to propose an explanation of the disparity in crystal shape. 相似文献
100.
Shigeru Shimada Osamu Yamazaki Yohichi Suzuki 《Journal of organometallic chemistry》2004,689(19):3012-3023
Hypervalent organobismuth compounds, 6-tert-butyl-5,6,7,12-tetrahydrodibenz[c,f][1,5]azabismocines, with 13 different substituents on the bismuth atom including halogens, alkyl, alkenyl, alkynyl, aryl, or phenylthio groups have been synthesized. A key compound, 12-chloro-6-tert-butyl-5,6,7,12-tetrahydrodibenz[c,f][1,5]azabismocine, which is a precursor for other azabismocines, has been synthesized by two different procedures; one is based on Akiba’s method using 2-bromobenzylbromide as one of the starting materials and the other is a newly developed one using a cheaper starting material, 2-chlorobenzyl chloride. The structures of 12 new bismuth compounds were determined by X-ray diffraction. The eight-membered tetrahydroazabismocine ring has proved to be highly flexible and the hypervalent Bi-N bond distances vary ranging from 2.568(3) to 2.896(5) Å, depending on the electronic nature of the substituents on the bismuth atom. The Bi-N bond distances have good linear relationship against Hammett’s σm constants. 相似文献