全文获取类型
收费全文 | 197085篇 |
免费 | 15635篇 |
国内免费 | 28516篇 |
专业分类
化学 | 152408篇 |
晶体学 | 4460篇 |
力学 | 5620篇 |
综合类 | 2211篇 |
数学 | 27231篇 |
物理学 | 49306篇 |
出版年
2024年 | 188篇 |
2023年 | 2229篇 |
2022年 | 3021篇 |
2021年 | 4867篇 |
2020年 | 5754篇 |
2019年 | 6427篇 |
2018年 | 4890篇 |
2017年 | 6790篇 |
2016年 | 6866篇 |
2015年 | 6122篇 |
2014年 | 8199篇 |
2013年 | 15671篇 |
2012年 | 12879篇 |
2011年 | 12256篇 |
2010年 | 10342篇 |
2009年 | 12898篇 |
2008年 | 13424篇 |
2007年 | 13939篇 |
2006年 | 12419篇 |
2005年 | 10489篇 |
2004年 | 9833篇 |
2003年 | 8201篇 |
2002年 | 6685篇 |
2001年 | 5320篇 |
2000年 | 5186篇 |
1999年 | 4393篇 |
1998年 | 3929篇 |
1997年 | 3161篇 |
1996年 | 2925篇 |
1995年 | 2921篇 |
1994年 | 2556篇 |
1993年 | 2130篇 |
1992年 | 2036篇 |
1991年 | 1435篇 |
1990年 | 1132篇 |
1989年 | 985篇 |
1988年 | 862篇 |
1987年 | 657篇 |
1986年 | 596篇 |
1985年 | 759篇 |
1984年 | 619篇 |
1983年 | 370篇 |
1982年 | 603篇 |
1981年 | 765篇 |
1980年 | 663篇 |
1979年 | 705篇 |
1978年 | 575篇 |
1977年 | 431篇 |
1976年 | 378篇 |
1973年 | 248篇 |
排序方式: 共有10000条查询结果,搜索用时 27 毫秒
31.
Chemical bonding in representative astrophysically relevant neutral,cation, and anion HCnH chains 下载免费PDF全文
Ioan Baldea 《中国物理 B》2022,31(12):123101-123101
Most existing studies assign a polyynic and cumulenic character of chemical bonding in carbon-based chains relying on values of the bond lengths. Building on our recent work, in this paper we add further evidence on the limitations of such an analysis and demonstrate the significant insight gained via natural bond analysis. Presently reported results include atomic charges, natural bond order and valence indices obtained from ab initio computations for representative members of the astrophysically relevant neutral and charged HC2k/2k+1H chain family. They unravel a series of counter-intuitive aspects and/or help naive intuition in properly understanding microscopic processes, e.g., electron removal from or electron attachment to a neutral chain. Demonstrating that the Wiberg indices adequately quantify the chemical bonding structure of the HC2k/2k+1H chains—while the often heavily advertised Mayer indices do not—represents an important message conveyed by the present study. 相似文献
32.
33.
Photocatalytic CO2 reduction to C1 fuels is considered to be an important way for alleviating increasingly serious energy crisis and environmental pollution. Due to the environment-friendly, simple preparation, easy formation of highly-stable metal-nitrogen(M-Nx) coordination bonds, and suitable band structure, polymeric carbon nitride-based single-atom catalysts(C3N4-based SACs) are expected to become a potential for CO2 reduction under visible-light irradiation. In this review, we summarize the recent advancement on C3N4-based SACs for photocatalytic CO2 reduction to C1 products, including the reaction mechanism for photocatalytic CO2 reduction to C1 products, the structure and synthesis methods of C3N4-based SACs and their applications toward photocatalytic CO2 reduction reaction(CO2RR) for C1 production. The current challenges and future opportunities of C3N4-based SACs for photoreduction of CO2 are also discussed. 相似文献
34.
Patrick Imrie Jianyong Jin 《Journal of polymer science. Part A, Polymer chemistry》2022,60(2):149-174
4D printing is an exciting branch of additive manufacturing. It relies on established 3D printing techniques to fabricate objects in much the same way. However, structures which fall into the 4D printed category have the ability to change with time, hence the “extra dimension.” The common perception of 4D printed objects is that of macroscopic single-material structures limited to point-to-point shape change only, in response to either heat or water. However, in the area of polymer 4D printing, recent advancements challenge this understanding. A host of new polymeric materials have been designed which display a variety of wonderful effects brought about by unconventional stimuli, and advanced additive manufacturing techniques have been developed to accommodate them. As a result, the horizons of polymer 4D printing have been broadened beyond what was initially thought possible. In this review, we showcase the many studies which evolve the very definition of polymer 4D printing, and reveal emerging areas of research integral to its advancement. 相似文献
35.
Peng Zhou Yang Yi Dr. Yuan-Zhao Hua Dr. Shi-Kun Jia Prof. Dr. Min-Can Wang 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(2):e202103688
The application of dinuclear zinc catalysts in a dearomatization reaction has been developed. Catalytic asymmetric dearomatization [3+2] annulations of 2-nitrobenzofurans or 2-nitrobenzothiophenes with CF3-containing N-unprotected isatin-derived azomethine ylides catalyzed by dinuclear zinc catalysts are realized with excellent diastereomer ratios (dr) of >20 : 1 and enantiomeric excess (ee) of up to 99 %. This protocol provides a practical, straightforward access to structurally diverse pyrrolidinyl spirooxindoles containing a 2,3-fused-dihydrobenzofuran (or dihydrobenzothiphene) moiety, and four contiguous stereocenters. Reactions can be performed on a gram scale. The absolute configuration of products is confirmed by X-ray single crystal structure analysis, and a possible mechanism is proposed. 相似文献
36.
Reshma Jose Dr. Srinivasu Kancharlapalli Dr. Tapan K. Ghanty Prof. Sourav Pal Prof. Gopalan Rajaraman 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(18):e202104526
The coordinatively unsaturated chromium(II)-based Cr3[(Cr4Cl)3(BTT)8]2 (Cr−BTT; BTT3−=1,3,5-benzenetristetrazolate) metal–organic framework (MOF) has been shown to exhibit exceptional selectivity towards adsorption of O2 over N2/H2. Using periodic density functional theory (DFT) calculations, we attempted to decipher the origin of this puzzling selectivity. By computing and analyzing the magnetic exchange coupling, binding energies, the partial density of states (pDOS), and adsorption isotherms for the pristine and gas-bound MOFs [(Cr4(X)4Cl)3(BTT)8]3− (X=O2, N2, and H2), we unequivocally established the role of spin states and spin coupling in controlling the gas selectivity. The computed geometries and gas adsorption isotherms are consistent with the earlier experiments. The binding of O2 to the MOF follows an electron-transfer mechanism resulting in a CrIII superoxo species (O2.−) with a very strong antiferromagnetic coupling between the two centers, whereas N2/H2 are found to weakly interact with the metal center and hence only slightly perturb the associated coupling constants. Although the gas-bound and unbound MOFs have an S=0 ground state (GS), the nature of spin the configurations and the associated magnetic exchanges are dramatically different. The binding energy and the number of oxygen molecules that can favorably bind to the Cr center were found to vary with respect to the spin state, with a significant energy margin (47.6 kJ mol−1). This study offers a hitherto unknown strategy of using spin state/spin couplings to control gas adsorption selectivity in MOFs. 相似文献
37.
Dong Gun Oh Prof. Hristiyan A. Aleksandrov Haneul Kim Dr. Iskra Z. Koleva Dr. Konstantin Khivantsev Prof. Georgi N. Vayssilov Prof. Ja Hun Kwak 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(49):e202200684
Pd-based catalysts are the most widely used for CO oxidation because of their outstanding catalytic activity and thermal stability. However, fundamental understanding of the detailed catalytic processes occurring on Pd-based catalysts under realistic conditions is still lacking. In this study, we investigated CO oxidation on metallic Pd clusters supported on Al2O3 and SiO2. High-angle annular dark-field scanning transmission electron microscopy revealed the formation of similar-sized Pd clusters on Al2O3 and SiO2. In contrast, CO chemisorption analysis indicated a gradual change in the dispersion of Pd (from 0.79 to 0.2) on Pd/Al2O3 and a marginal change in the dispersion (from 0.4 to 0.24) on Pd/SiO2 as the Pd loading increased from 0.27 to 5.5 wt %; these changes were attributed to differences in the metal-support interactions. Diffuse reflectance infrared Fourier-transform spectroscopy revealed that fewer a-top CO species were present in Pd supported on Al2O3 than those in Pd supported on SiO2, which is related to the morphological differences in the metallic Pd clusters on these two supports. Despite the different dispersion profiles and surface characteristics of Pd, O2 titration demonstrated that linearly bound CO (with an infrared signal at 2090 cm−1) reacted first with oxygen in the case of CO-saturated Pd on Al2O3 and SiO2, which suggests that a-top CO on the terrace site plays an important role in CO oxidation. The experimental observations were corroborated by periodic density functional calculations, which confirmed that CO oxidation on the (111) terrace sites is most plausible, both kinetically and thermodynamically, compared to that on the edge or corner sites. This study will deepen the fundamental understanding of the effect of Pd clusters on CO oxidation under reaction conditions. 相似文献
38.
Kento Nishikibe Keisuke Nishikawa Momochika Kumagai Matsumi Doe Yoshiki Morimoto 《化学:亚洲杂志》2022,17(1):e202101137
There are marine cytotoxic bromotriterpenoids, named the thyrsiferol family that are structurally characterized by some tetrahydropyran (THP) and tetrahydrofuran (THF) rings. The thyrsiferol family belongs to natural products that are often difficult to determine their stereostructures even by the current, highly advanced spectroscopic methods, especially in acyclic systems including stereogenic tetrasubstituted carbon centers. In such cases, it is effective to predict and synthesize the possible stereostructures. Herein, to elucidate ambiguous stereostructures and unassigned absolute configurations of aplysiol B, laurenmariannol, and saiyacenol A, members of the thyrsiferol family, we carried out their asymmetric chemical syntheses featuring 6-exo and 5-exo oxacyclizations of epoxy alcohol precursors and 6-endo bromoetherification of a bishomoallylic alcohol. In this paper, we report total assignments of their stereostructures through their asymmetric chemical syntheses and also their preliminary cytotoxic activities against some tumor cells. These results could not have been achieved without depending on asymmetric total synthesis. 相似文献
39.
Prof. Dr. Peter Hess 《Chemphyschem》2022,23(6):e202100880
In the periodic table the position of each atom follows the ‘aufbau’ principle of the individual electron shells. The resulting intrinsic periodicity of atomic properties determines the overall behavior of atoms in two-dimensional (2D) bonding and structure formation. Insight into the type and strength of bonding is the key in the discovery of innovative 2D materials. The primary features of 2D bonding and the ensuing monolayer structures of the main-group II–VI elements result from the number of valence electrons and the change of atom size, which determine the type of hybridization. The results reveal the tight connection between strength of bonding and bond length in 2D networks. The predictive power of the periodic table reveals general rules of bonding, the bonding-structure relationship, and allows an assessment of published data of 2D materials. 相似文献
40.
《Mendeleev Communications》2022,32(1):123-125
The synthesis, structure and properties of copper(II) perchlorate complexes with antipyrine (AP), [Cu(AP)4(H2O)](ClO4)2 and [Cu(AP)5](ClO4)2, are described and compared with those of alternative compounds containing different AP ligands. 相似文献