全文获取类型
收费全文 | 20221篇 |
免费 | 1738篇 |
国内免费 | 1106篇 |
专业分类
化学 | 14320篇 |
晶体学 | 214篇 |
力学 | 813篇 |
综合类 | 110篇 |
数学 | 2081篇 |
物理学 | 5527篇 |
出版年
2024年 | 36篇 |
2023年 | 240篇 |
2022年 | 510篇 |
2021年 | 565篇 |
2020年 | 537篇 |
2019年 | 537篇 |
2018年 | 454篇 |
2017年 | 436篇 |
2016年 | 791篇 |
2015年 | 730篇 |
2014年 | 966篇 |
2013年 | 1406篇 |
2012年 | 1743篇 |
2011年 | 1817篇 |
2010年 | 1159篇 |
2009年 | 1000篇 |
2008年 | 1331篇 |
2007年 | 1191篇 |
2006年 | 1051篇 |
2005年 | 932篇 |
2004年 | 819篇 |
2003年 | 699篇 |
2002年 | 726篇 |
2001年 | 509篇 |
2000年 | 359篇 |
1999年 | 350篇 |
1998年 | 211篇 |
1997年 | 192篇 |
1996年 | 228篇 |
1995年 | 185篇 |
1994年 | 177篇 |
1993年 | 154篇 |
1992年 | 139篇 |
1991年 | 129篇 |
1990年 | 101篇 |
1989年 | 89篇 |
1988年 | 58篇 |
1987年 | 45篇 |
1986年 | 46篇 |
1985年 | 60篇 |
1984年 | 31篇 |
1983年 | 26篇 |
1982年 | 27篇 |
1981年 | 26篇 |
1980年 | 19篇 |
1979年 | 19篇 |
1978年 | 21篇 |
1977年 | 15篇 |
1976年 | 18篇 |
1957年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
932.
933.
934.
Alexander Popkov Jiří Hanusek Jiří Čermák Vratislav Langer Robert Jirásko Michal Holčapek Milan Nádvorník 《Journal of Radioanalytical and Nuclear Chemistry》2010,285(3):621-626
In this communication the evaluation of eleven new metallocomplex alanine synthons bearing C2-symmetric benzyl groups with electron-donating and electron-withdrawing substituents is described. α-Methylated glycine synthons (alanine complexes) were evaluated alongside alanine synthons in order to obtain a deeper understanding of the relationship between their structures and stereochemistry of monoalkylated products and to choose several candidates for their further tests for stereospecific preparation of 6-[18F]FDOPA. Glycine-derived analogues of the complexes 3–5 are the best candidates for the development of a 6-[18F]FDOPA preparation procedure. In the model epimerisation reaction they demonstrated the best performance, much better compared to the previously described compound 2. Complexes 3, 5 and 8 are the best in asymmetric preparation of β-13C monolabelled α-aminoisobutyric acid. They have to be tested in the preparation of α-methyl amino acids like 6-[18F]-α-methylDOPA and 2-[18F]-α-methyltyrosine. 相似文献
935.
Park K West A Raheja E Sellner B Lischka H Windus TL Hase WL 《The Journal of chemical physics》2010,133(18):184306
Electronic structure calculations at the CASSCF and UB3LYP levels of theory with the aug-cc-pVDZ basis set were used to characterize structures, vibrational frequencies, and energies for stationary points on the ground state triplet and singlet O(2)+C(2)H(4) potential energy surfaces (PESs). Spin-orbit couplings between the PESs were calculated using state averaged CASSCF wave functions. More accurate energies were obtained for the CASSCF structures with the MRMP2/aug-cc-pVDZ method. An important and necessary aspect of the calculations was the need to use different CASSCF active spaces for the different reaction paths on the investigated PESs. The CASSCF calculations focused on O(2)+C(2)H(4) addition to form the C(2)H(4)O(2) biradical on the triplet and singlet surfaces, and isomerization reaction paths ensuing from this biradical. The triplet and singlet C(2)H(4)O(2) biradicals are very similar in structure, primarily differing in their C-C-O-O dihedral angles. The MRMP2 values for the O(2)+C(2)H(4)→C(2)H(4)O(2) barrier to form the biradical are 33.8 and 6.1 kcal/mol, respectively, for the triplet and singlet surfaces. On the singlet surface, C(2)H(4)O(2) isomerizes to dioxetane and ethane-peroxide with MRMP2 barriers of 7.8 and 21.3 kcal/mol. A more exhaustive search of reaction paths was made for the singlet surface using the UB3LYP/aug-cc-pVDZ theory. The triplet and singlet surfaces cross between the structures for the O(2)+C(2)H(4) addition transition states and the biradical intermediates. Trapping in the triplet biradical intermediate, following (3)O(2)+C(2)H(4) addition, is expected to enhance triplet→singlet intersystem crossing. 相似文献
936.
Hyeong Jin Yun Hyunjoo Lee Ji Bong Joo Nam Dong Kim Jongheop Yi 《Electrochemistry communications》2010,12(6):769-772
We describe a method tuning the band-gap energy (Eg) of visible light sensitive TiO2-xCx nanoparticle. Eg tends to become smaller with the increase in the amount of carbon dopant in TiO2-xCx nanoparticle due to the increase in excess electrons. Photo-catalytic oxidative activity, however, did not depend on only the value of Eg, but also the energy level of valence band. TiO1.96C0.04 nanoparticle having Eg of 2.6 eV showed outstanding performance in oxidative decomposition of phenol under the irradiation of visible light. 相似文献
937.
Zhi‐An He Guang‐Zhou Lu Lin‐Lin Zhu Yong‐Xue Wang Fu‐Lin Yan Ji‐Xia Zhang 《中国化学会会志》2010,57(2):256-259
Two new ent‐kaurane diterpenoids, taihangexcisoidesin C (1) and its acetonide, taihangexcisoidesin D (2), along with 9 known compounds were isolated from the leaves of Isodon excisoides. The structures of the new compounds were elucidated using 1D and 2D NMR spectroscopy analysis. Compounds 3‐6 and 8‐9 were tested for their antibacterial activity against Escherichia coli, Staphylococcus aureus, S. epidermidis and S. saprophyticus. Compound 6 showed inhibitory effects on Staphylococcus aureus, S. epidermidis and S. saprophyticus MIC values 32, 16 and 16 μg/mL, respectively. All of the compounds didn't show effects on Escherichia coli (MIC > 10 mg/mL). 相似文献
938.
We report a diagnostic method for Anaplasma phagocytophilum (A. phagocytophilum) infection in cattle using a nested PCR and microchip electrophoresis (ME). A. phagocytophilum causes human granulocytic anaplasmosis and granulocytic ehrlichiosis, which are emerging tick‐borne zoonotic diseases. Nested PCR was used to amplify genomic DNA samples extracted from cattle blood. The amplified PCR products were analyzed under a sieving gel matrix of 0.7% poly(ethyleneoxide) (Mr=8 000 000) in a conventional glass microchip. In the ME assay, A. phagocytophilum was analyzed within 35 s with a relative standard deviation of 1.30% (n=5) using a programmed field strength gradient (PFSG) as follows: 615.3 V/cm for 0–24 s, 66.7 V/cm for 24–34 s, 615.3 V/cm for 34–100 s. The ME‐PFSG assay was clinically validated by comparing the 16S rRNA gene levels obtained by this method with those measured using conventional slab gel electrophoresis performed with ten cattle blood samples suspected of A. phagocytophilum infection. In contrast to slab gel electrophoresis, the proposed ME‐PFSG methodology had increased sensitivity (200–450 pg/μL), a faster analysis time (<35 s), and required a smaller sample volume (~162 fL). 相似文献
939.
The cationic Ir(III) acetone complex (POCOP)Ir(H)(2)(acetone)(+) (POCOP = 2,6-bis(di-tert-butylphosphinito)phenyl) was shown to catalyze the reduction of a variety of tertiary amides to amines using diethylsilane as reductant. Mechanistic studies established that a minor species generated in the reaction, the neutral silyl trihydride Ir(V) complex (POCOP)IrH(3)(SiEt(2)H), was the catalytically active species. High concentrations of this species could be conveniently generated by treatment of readily available (POCOP)IrHCl with tert-butoxide in the presence of Et(2)SiH(2) under H(2). Thus, using this mixture in the presence of a trialkylammonium salt, a wide array of tertiary amides, including extremely bulky substrates, are rapidly and quantitatively reduced to tertiary amines under mild conditions with low catalyst loading. A detailed mechanistic study has been carried out and intermediates identified. In brief, (POCOP)IrH(3)(SiEt(2)H) reduces the amide to the hemiaminal silyl ether that, in the presence of a trialkylammonium salt, is ionized to the iminium ion, which is then reduced to the tertiary amine by Et(2)SiH(2). Good functional group compatibility is demonstrated, and a high catalyst stability has provided turnover numbers as high as 10,000. 相似文献
940.
The collisionless photodissociation dynamics of isobutene (i-C(4)H(8)) at 193 nm via photofragment translational spectroscopy are reported. Two major photodissociation channels were identified: H + C(4)H(7) and CH(3) + CH(3)CCH(2). Translational energy distributions indicate that both channels result from statistical decay on the ground state surface. Although the CH(3) loss channel lies 13 kcal mol(-1) higher in energy, the CH(3):H branching ratio was found to be 1.7 (5), in reasonable agreement with RRKM calculations. 相似文献