全文获取类型
收费全文 | 10379篇 |
免费 | 419篇 |
国内免费 | 61篇 |
专业分类
化学 | 7858篇 |
晶体学 | 81篇 |
力学 | 227篇 |
数学 | 1294篇 |
物理学 | 1399篇 |
出版年
2023年 | 70篇 |
2022年 | 78篇 |
2021年 | 127篇 |
2020年 | 206篇 |
2019年 | 167篇 |
2018年 | 129篇 |
2017年 | 115篇 |
2016年 | 268篇 |
2015年 | 250篇 |
2014年 | 284篇 |
2013年 | 511篇 |
2012年 | 743篇 |
2011年 | 916篇 |
2010年 | 435篇 |
2009年 | 285篇 |
2008年 | 670篇 |
2007年 | 728篇 |
2006年 | 767篇 |
2005年 | 749篇 |
2004年 | 591篇 |
2003年 | 502篇 |
2002年 | 434篇 |
2001年 | 111篇 |
2000年 | 105篇 |
1999年 | 100篇 |
1998年 | 92篇 |
1997年 | 99篇 |
1996年 | 120篇 |
1995年 | 84篇 |
1994年 | 88篇 |
1993年 | 73篇 |
1992年 | 65篇 |
1991年 | 64篇 |
1990年 | 49篇 |
1989年 | 34篇 |
1988年 | 46篇 |
1987年 | 33篇 |
1986年 | 23篇 |
1985年 | 81篇 |
1984年 | 50篇 |
1983年 | 34篇 |
1982年 | 58篇 |
1981年 | 58篇 |
1980年 | 47篇 |
1979年 | 48篇 |
1978年 | 43篇 |
1977年 | 40篇 |
1976年 | 35篇 |
1975年 | 35篇 |
1974年 | 22篇 |
排序方式: 共有10000条查询结果,搜索用时 406 毫秒
181.
Joseph T. Repine Douglas S. Johnson Andrew D. White Michael A. Stier Trent Rankin Samarendra N. Maiti 《Tetrahedron letters》2007,48(31):5539-5541
A series of regioisomerically monofluorinated 1-(naphthalen-1-yl)piperazines is described. 相似文献
182.
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
183.
[reaction: see text] Glucuronyl iodide 1 has been studied in detail as a "disarmed" glycosyl donor. In a model reaction, using N-iodosuccinimide (NIS) as a promoter and 2-phenylethanol as acceptor, best results were obtained using NIS with I(2), followed by trimethylsilyltrifluoromethanesulfonate (TMSOTf). When a series of primary and secondary alcohols was glycosylated using these conditions, yields of 60-83% of beta-glucuronides were obtained. Various "nonheavy" metal salts also effectively catalyzed the model reaction but led to significant amounts of alpha-product with less reactive secondary alcohols. 相似文献
184.
Beevers AP Fettes KJ Sabbagh G Murad FK Arnold JR Cosstick R Fisher J 《Organic & biomolecular chemistry》2004,2(1):114-119
High-resolution NMR spectroscopy has been used to establish the conformational consequences of the introduction of a single 3[prime or minute]-S-phosphorothiolate link in the DNA strand of a DNA : RNA hybrid. These systems are of interest as potential antisense therapeutic agents. Previous studies on similarly modified dinucleotides have shown that the conformation of the sugar to which the sulfur is attached shifts to the north (C(3[prime or minute])-endo/C(2[prime or minute])-exo). Comparisons made between NOESY cross-peak intensities, and coupling constants from PE-COSY spectra, for both non-modified and modified duplexes confirm that this conformational shift is also present in the double helical oligonucleotide system. In addition it is noted that in both the dinucleotides and the modified duplex, the conformation of the sugar ring 3[prime or minute] to the site of modification is also shifted to the north. That this pattern is observed in the small monomeric system as well as the larger double helix is suggestive of some pre-ordering of the sequences. The conclusion is supported by consideration of the (1)H chemical shifts of the heterocyclic bases near the site of the modification. The enhanced stability that these conformational changes should bring was confirmed by UV thermal melting studies. Subsequently a series of singly and doubly 3[prime or minute]-S-phosphorothiolate-modified duplexes were investigated by UV. The results are indicative of an additive effect of the modification with thermodynamic benefit being derived from alternate spacing of two modified linkers. 相似文献
185.
Ogrin D van Poppel LH Bott SG Barron AR 《Dalton transactions (Cambridge, England : 2003)》2004,(21):3689-3694
The reaction of AlMe(3) and [((t)Bu)(2)Al(micro-OPh)](2) with pyrazine (pyz), 4,4'-bipyridine (4-4'-bipy), 1,2-bis(4-pyridyl)ethane (bpetha) and 1,2-bis(4-pyridyl)ethylene (bpethe) yields (Me(3)Al)(2)(micro-pyz)(1), (Me(3)Al)(2)(micro-4,4'-bipy)(2), (Me(3)Al)(2)(micro-bpetha)(3), (Me(3)Al)(2)(micro-bipethe)(4), Al((t)Bu)(2)(OPh)(pyz)(5), [((t)Bu)(2)Al(OPh)](2)(micro-4,4-bipy)(6a), [((t)Bu)(2)Al(OPh)](2)(micro-bpetha)(7a), [((t)Bu)(2)Al(OPh)](2)(micro-bipethe)(8a). Compounds 1-4, 6a and 7a have been confirmed by X-ray crystallography. In solution compounds 1-4 undergo a rapid ligand-dissociation equilibrium resulting in a time-average spectrum in the (1)H NMR. In contrast, the solution equilibria for compounds 5-8a are sufficiently slow such that the mono-aluminium compounds may be observed by (1)H NMR spectroscopy: Al((t)Bu)(2)(OPh)(4,4-bipy)(6b), Al((t)Bu)(2)(OPh)(bpetha)(7b) and Al((t)Bu)(2)(OPh)(bpethe)(8b). The inability to isolate [((t)Bu)(2)Al(OPh)](2)(micro-pyz) and the relative stability of each complex is discussed with respect to the steric interactions across the bridging ligand (L) and the electronic effect on one Lewis acid-base interaction by the second Lewis acid-base interaction on the same ligand. 相似文献
186.
Carmalt CJ Newport AC O'Neill SA Parkin IP White AJ Williams DJ 《Inorganic chemistry》2005,44(3):615-619
The mono(guanidinato) complex [Ti(NMe2)2Cl{i-PrNC[N(SiMe3)2]N-i-Pr}] (1) was prepared by reaction of [Ti(NMe2)2Cl2] with 1 or 2 equiv of the lithium guanidinate salt [Li{i-PrNC[N(SiMe3)2]N-i-Pr}]. Compound 1 has been characterized by X-ray crystallography. Treatment of TiCl4 with 2 equiv of [Li{i-PrNC[N(SiMe3)2]N-i-Pr}] resulted in the formation of dark red crystals. X-ray crystallography showed that these crystals consist of a 70:30 mixture of two bis(guanidinato) complexes, namely, [TiCl2{i-PrNC[N(SiMe3)2]N-i-Pr}{i-PrNC(N=CMe2)N-i-Pr}] (2) and [TiCl2{i-PrNC[N(SiMe3)2]N-i-Pr}{i-PrNC[N(H)-i-Pr]N-i-Pr}] (3). Both compounds 2 and 3 possess a transformed guanidinate ligand. Low-pressure chemical vapor deposition of either compound 1 or [TiCl2{i-PrNC(NMe2)N-i-Pr}] (4) at 600 degrees C results in thin films of titanium carbonitride. 相似文献
187.
Horvath JD Koritnik A Kamakoti P Sholl DS Gellman AJ 《Journal of the American Chemical Society》2004,126(45):14988-14994
Kinked-stepped, high Miller index surfaces of metal crystals are chiral and, therefore, exhibit enantiospecific properties. Previous temperature-programmed desorption (TPD) spectra have shown that the desorption energies of R-3-methylcyclohexanone (R-3-MCHO) on the chiral Cu(643)(R) and Cu(643)(S) surfaces are enantiospecific (J. Am. Chem. Soc. 2002, 124, 2384). Here, a comparison of the TPD spectra from Cu(111), Cu(221), Cu(533), Cu(653)(R&S), and Cu(643)(R&S) surfaces reveals that the enantiospecific desorption occurs from the chiral kink sites on the Cu(643) surfaces. Titration of the chiral kink sites with I atoms confirms this assignment of desorption features in the TPD spectra. Finally, the enantiospecific difference in the desorption energies of R- and S-3-MCHO has been used as the basis for demonstration of an enantioselective, kinetic separation of racemic 3-MCHO into its purified components during adsorption and desorption on the Cu(643)(R&S) surfaces. 相似文献
188.
Journal of Theoretical Probability - Let $$ \{X, X_{n};~n \ge 1 \}$$ be a sequence of independent and identically distributed Banach space valued random variables. This paper is devoted to... 相似文献
189.
Ying-Sing Fung Andrew Shin-Kwan Tang 《Fresenius' Journal of Analytical Chemistry》1994,350(12):721-723
A fast, simple and reliable method for the analysis of DEHP in PVC products was developed using a one-step Soxhlet extraction prior to HPLC determination. The linear range from 1.25 /g to 250 mg/g of DEHP in PVC was found sufficient to cover the normal concentrations encountered. The reliability of the method was established by comparison with the ASTM method and no potential interferent was identified except a large concentration of DIMP. 相似文献
190.
Elliptic genera of level N have been defined by F. Hirzebruch,generalising the earlier notion of elliptic genus due to S.Ochanine. We show that there are corresponding elliptic cohomologytheories which are naturally associated to such genera and thatthese are obtained from the level 1 case by algebraic extensionof the coefficient rings from level 1 to level N modular forms. 相似文献