全文获取类型
收费全文 | 1251篇 |
免费 | 43篇 |
专业分类
化学 | 975篇 |
晶体学 | 1篇 |
力学 | 37篇 |
数学 | 97篇 |
物理学 | 184篇 |
出版年
2023年 | 7篇 |
2022年 | 22篇 |
2021年 | 48篇 |
2020年 | 21篇 |
2019年 | 25篇 |
2018年 | 22篇 |
2017年 | 13篇 |
2016年 | 37篇 |
2015年 | 34篇 |
2014年 | 37篇 |
2013年 | 94篇 |
2012年 | 106篇 |
2011年 | 77篇 |
2010年 | 43篇 |
2009年 | 52篇 |
2008年 | 66篇 |
2007年 | 60篇 |
2006年 | 72篇 |
2005年 | 54篇 |
2004年 | 55篇 |
2003年 | 57篇 |
2002年 | 45篇 |
2001年 | 16篇 |
2000年 | 10篇 |
1999年 | 14篇 |
1998年 | 6篇 |
1997年 | 5篇 |
1996年 | 27篇 |
1995年 | 11篇 |
1994年 | 16篇 |
1993年 | 6篇 |
1992年 | 14篇 |
1991年 | 4篇 |
1990年 | 9篇 |
1989年 | 7篇 |
1988年 | 4篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 12篇 |
1984年 | 4篇 |
1983年 | 5篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1978年 | 8篇 |
1977年 | 6篇 |
1974年 | 4篇 |
1973年 | 3篇 |
1927年 | 3篇 |
排序方式: 共有1294条查询结果,搜索用时 46 毫秒
41.
Hélio A. Stefani Claudio M.P. Pereira Roberta B. Almeida Karla P. Guzen 《Tetrahedron letters》2005,46(40):6833-6837
The 4-halo-3,5-dimethyl pyrazoles have been synthetisized in good yields in short reaction times in the absence of a catalyst by reaction of 3,5-dimethyl pyrazoles with N-halosuccinimides (NBS, NCS and NIS) under ultrasound irradiation. Finally, the halogenation of pyrazoles with Br2, ICl and I2 was showed in similar conditions. 相似文献
42.
43.
Roberta Costi Roberto Di Santo Marino Artico Silvio Massa 《Journal of heterocyclic chemistry》2002,39(1):81-90
The synthesis of derivatives of 2,3‐dihydroimidazo[1,5,4‐ef][1,2,5]benzothiadiazepin‐6(4H,7H)‐thione 1,1‐dioxide is reported starting from N‐substituted ethyl 2‐(5‐chloro‐2‐nitrobenzenesulfonamido)‐2‐alkyl‐acetates. Fundamental steps of the synthetic pathway were: i) intramolecular cyclization of N‐substituted 2‐(2‐amino‐5‐chlorobenzenesulfonamido)‐2‐alkylacetic acids in the presence of N‐(3‐dimethyl‐aminopropyl)‐N′‐ethyl carbodiimide hydrochloride‐N,N‐dimethylaminopyridine complex; ii) building of imidazole ring from 2‐alkyl‐8‐chloro‐2,3‐dihydro‐3‐methyl‐1,2,5‐benzothiadiazepin‐4(5H)‐one 1,1‐dioxide to achieve 2‐alkyl‐9‐chloro‐2,3‐dihydro‐3‐methylimidazo[1,5,4‐ef][1,2,5]benzothiadiazepin‐6(4H,7H)‐one 1,1‐dioxide; iii) preparation of thiocarbonyl derivative by treatment with Lawesson's reagent. Introduction of a 3‐methyl‐2‐butenyl chain at position 2 of above imidazobenzothiadiazepinone required protection at the 7 position with thermally removable tert‐butoxycarbonyl moiety, due to the fact that alkylation of unprotected structure proved to be regioselective for the 7 position. 相似文献
44.
By measuring the relative CO quantum yields from ketene photolysis as a function of photolysis wavelength we have determined the threshold energy at 25° for CH2CO(1A1) → CH2(3B1) + CO(1Σ+) to be 75.7 ± 1.0 kcal/mole. This corresponds to a value of 90.7 ± 1.0 kcal/mole for ΔHf2980[CH2(3B1)]. By measuring the relative ratio of CH2(1A1)/CH2(3B1) from ketene photolysis as a function of photolysis wavelength we have determined the threshold energy at 25°C for CH2CO(1A1) → CH2(1A1) + CO(1Σ+) to be 84.0 ± 0.6 kcal/mole. This corresponds to a value of 99.0 ± 0.6 kcal/mole for ΔHf2980[CH2(1A1)]. Thus a value for the CH2(3B1) ? CH2(1A1) energy splitting of 8.3 ± 1 kcal/mole is determined, which agrees with three other recent independent experimental estimates and the most recent quantum theoretical calculations. 相似文献
45.
Timosheva NV Chandrasekaran A Prakasha TK Day RO Holmes RR 《Inorganic chemistry》1996,35(22):6552-6560
Analogous to the formation of CH(2)[(t-Bu)(2)C(6)H(2)O](2)P(Ph)(O(2)C(6)Cl(4)) (1), the new bicyclic tetraoxyphosphoranes CH(2)[(t-Bu)(2)C(6)H(2)O](2)P(Et)(O(2)C(6)Cl(4)) (3) and CH(2)[ClC(6)H(3)O](2)P(Ph)(O(2)C(6)Cl(4)) (4) were synthesized by the oxidative addition of the appropriate cyclic phosphines with o-tetrachlorobenzoquinone. For the formation of CH(2)[(t-Bu)(2)C(6)H(2)O](2)P(Ph)(O(2)C(2)Ph(2)) (2), a similar reaction was followed with the use of benzil (PhCOCOPh) in place of o-tetrachlorobenzoquinone. X-ray analysis of 1-3 revealed trigonal bipyramidal geometries and provided evidence for the first series of complexes in the absence of ring strain in which the least electronegative group, ethyl or phenyl, is located in an axial position, in violation of the electronegativity rule. Thus, the two oxygen-containing ring systems occupied two different sets of positions in the trigonal bipyramid (TBP) with the eight-membered rings at diequatorial sites. X-ray analysis of 4 revealed a trigonal bipyramidal geometry with electron-withdrawing chlorine substituents on each ring assumed the more conventional geometry with the rings occupying axial-equatorial positions and the phenyl group located in the remaining equatorial site. The fact that molecular mechanics calculations favorably reproduced the observed geometries suggests that a steric contribution associated with the ring tert-butyl groups for 1-3 is partly responsible in favoring diequatorial ring occupancy for the eight-membered ring. NMR data supported rigid pentacoordinated structures in solution at 23 degrees C. Phosphorane 1 crystallizes in the orthorhombic space group Fdd2 with a = 44.787(5) ?, b = 34.648(8) ?, c = 10.3709(9) ?, and Z = 16. Phosphorane 2 crystallizes in the orthorhombic space group Pna2(1) with a = 20.658(8) ?, b = 10.342(2) ?, c = 19.879(6) ?, and Z = 4. Phosphorane 3 crystallizes in the orthorhombic space group Pcmn with a = 9.807(2) ?, b = 16.632(4) ?, c = 23.355(3) ?, and Z = 4. Phosphorane 4 crystallizes in the monoclinic space group C2/c with a = 35.699(5) ?, b = 12.187(2) ?, c = 14.284(3) ?, beta = 107.08(1) degrees, and Z = 8. The final conventional unweighted residuals are 0.0395 (1), 0.0518 (2), 0.0540 (3), and 0.0868 (4). 相似文献
46.
2-and 4-Ethoxymethoxybenzyl methyl ethers were employied as useful starting materials for the synthesis of 1,2- or 1,4-dicarbo-substituted benzenes. The proposed reaction sequence involves connection between the reductive lithiation of benzyl alkyl ethers and the metal-catalyzed cross-coupling reaction of aromatic triflates. 相似文献
47.
The synthesis of some novel bakkenolides and their epi-spiro analogues was achieved by a new approach. Photolysis of allyl 1-(phenylseleno)-2-oxocyclopentanecarboxylates 7-9 afforded the corresponding spiro lactones 10-12 by radical cyclization via group transfer of the phenylseleno group. Selenoxide elimination of 11 and 12 produced the corresponding beta-methylene lactones 14 and 15. Diels-Alder cycloaddition of lactone11 with piperylene failed at ambient pressure, but proceeded in generally good yield in the presence of various Lewis acids at pressures of ca. 16 kbar, to give mixtures of beta-exo, alpha-endo, and beta-endo cycloadducts 19, 21, and 23, respectively. The preponderance of endo products 21 and 23, formed via highly hindered, but more compact, transition states was attributed to the high pressure and resulted in trans-dimethyl configurations of the products. The facial selectivity was dependent upon the Lewis acids, and the greatest alpha:beta ratio was observed with catalysts of the type TiCl(2)(OR)(2). Epimerization of the C-4 methyl group in 21 and 23 to furnish the corresponding cis-dimethyl analogues was achieved via exo-epoxidation, regioselective reduction, oxidation to the corresponding 3-keto derivatives, and base-catalyzed equilibration, thereby affording (+/-)-3,6-dioxobakkenolide-A (39) and its epi-spiro derivative 28, respectively. When the radical cyclization step was performed subsequent to the Diels-Alder cycloaddition by photolysis of perhydrindane 43, only the epi-spiro product 44 was obtained. 相似文献
48.
49.
50.