全文获取类型
收费全文 | 37030篇 |
免费 | 3443篇 |
国内免费 | 6434篇 |
专业分类
化学 | 30754篇 |
晶体学 | 191篇 |
力学 | 666篇 |
综合类 | 208篇 |
数学 | 1479篇 |
物理学 | 4473篇 |
综合类 | 9136篇 |
出版年
2024年 | 135篇 |
2023年 | 516篇 |
2022年 | 1075篇 |
2021年 | 1245篇 |
2020年 | 1732篇 |
2019年 | 1371篇 |
2018年 | 1378篇 |
2017年 | 1322篇 |
2016年 | 1484篇 |
2015年 | 1517篇 |
2014年 | 2075篇 |
2013年 | 3085篇 |
2012年 | 2000篇 |
2011年 | 2335篇 |
2010年 | 1743篇 |
2009年 | 2013篇 |
2008年 | 2081篇 |
2007年 | 2349篇 |
2006年 | 2164篇 |
2005年 | 1974篇 |
2004年 | 1939篇 |
2003年 | 1616篇 |
2002年 | 1172篇 |
2001年 | 939篇 |
2000年 | 951篇 |
1999年 | 854篇 |
1998年 | 756篇 |
1997年 | 694篇 |
1996年 | 603篇 |
1995年 | 581篇 |
1994年 | 511篇 |
1993年 | 423篇 |
1992年 | 411篇 |
1991年 | 330篇 |
1990年 | 294篇 |
1989年 | 256篇 |
1988年 | 202篇 |
1987年 | 129篇 |
1986年 | 86篇 |
1985年 | 83篇 |
1984年 | 70篇 |
1983年 | 38篇 |
1982年 | 46篇 |
1981年 | 49篇 |
1980年 | 32篇 |
1979年 | 30篇 |
1978年 | 30篇 |
1977年 | 35篇 |
1976年 | 36篇 |
1974年 | 45篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
DFT calculations have been carried out for 2-, 3- and 4-methoxybenzyl alcohol radical cations (1+, 3+ and 4+, respectively) and the α-methyl derivatives 2+ and 5+ using the UB3LYP/6-31G(d) method. The theoretical results have been compared with the experimental rate constants for deprotonation of 1+-5+ under acidic and basic conditions. In acidic solution, the decay of 1+-5+ proceeds by cleavage of the C-H bond, while in the presence of −OH all the radical cations undergo deprotonation from the α-OH group. This pH-dependent change in mechanism has been interpreted qualitatively in terms of simple frontier molecular orbital theory. The −OH induced α-O-H deprotonation is consistent with a charge controlled reaction, whereas the C-H deprotonation, observed when the base is H2O, appears to be affected by frontier orbital interactions. 相似文献
92.
氯化血红素催化新型荧光反应测定葡萄糖的研究 总被引:3,自引:0,他引:3
本文用流动注射分析了研究了过氧化物模拟酶催化过氧新型荧光底物(N,N-二氰甲基邻苯二胺,DCMOPA)的最优条件。结果表明,该系统可用于定量测定氧化氢及催化氯化血红素的浓度。 相似文献
93.
2,2-Dimethyl-3-(2-methyl-3-indolyl)cyclopropylacetic acid, its amide and esters, and the corresponding alcohol, viz., the product of ester reduction by LiAlH4, were synthesized. The chemoselectivity of N- and O-alkylation of these compounds was studied. Selective monoalkylation at the nitrogen atom of the heterocycle, O-alkylation to the side chain, or dialkylation at both nucleophilic sites can be carried out under conditions of phase-transfer catalysis. The N-acylation at the indole fragment of nitrile of this acid occurs only under the Vilsmeier—Haak formylation conditions. 相似文献
94.
We describe the first studies on the chemistry of 2-(2-oxo-3-phenylpropyl)benzaldehydes, which were converted into 3-benzylisochromen-1-ones via the corresponding 2-(2-oxo-3-phenylpropyl)benzoic acid. The 2-(2-oxo-3-phenylpropyl)benzaldehydes proved to be convenient starting materials for the synthesis of 3-phenyl-2-naphthols. Oxidation of the latter compounds resulted in a novel, efficient synthesis of 3-phenyl-1,2-naphthoquinones, which were efficiently transformed into 2-hydroxy-3-phenyl-1,4-naphthoquinones. 相似文献
95.
《Electroanalysis》2005,17(2):186-190
We report for the first time on the electroassisted biomimetic activation of molecular oxygen by a newly prepared electropolymerized polypyrrole‐manganese phthalocyanine film. The prepared films and their intervention in the electroassisted catalytic reduction of molecular oxygen were analyzed by cyclic voltammetry and UV‐visible spectrophotometry on optically transparent electrodes. The obtained results demonstrate the probable existence of the key‐steps responsible for the suggested formation of the highly reactive manganese oxo intermediate. 相似文献
96.
Bernd Schetter 《Journal of organometallic chemistry》2004,689(9):1472-1480
N,N′-dicyclohexyl-N-ferrocenoylurea 2, N,N′-diisopropyl-N-ferrocenoylurea 3, N,N′-di-p-tolyl-N-ferrocenoylurea 4 and N,N′-di-tert-butyl-N-ferrocenoylurea 5 were obtained by reaction of ferrocenecarboxylic acid 1 with N,N′-dicyclohexylcarbodiimide (DCC), N,N′-diisopropylcarbodiimide (DIC), N,N′-di-p-tolylcarbodiimide 10 and N,N′-di-tert-butylcarbodiimide 11, respectively. Both N-tert-butyl-N′-ethyl-N-ferrocenoylurea 6 and N′-tert-butyl-N-ethyl-N-ferrocenoylurea 7 were obtained by reaction of 1 with N-tert-butyl-N′-ethylcarbodiimide 12. In all cases a small amount of ferrocenecarboxylic anhydride 8 was formed as a by-product. All compounds were characterized by 1H NMR, 13C NMR, IR and MS. Single crystal X-ray structural analyses were made of 2, 3 and 4. From the consistent results, the reaction products of 1 with carbodiimides appear different from those proposed by some earlier workers. With N-(3-dimethylaminopropyl)-N′-ethylcarbodiimidehydrochloride 9 ferrocenoylurea was not isolated, but the main product was rather 8. The suitability of 8 as acylation reagent was applied by using 9 to obtain N-(3-triethoxysilyl)-propylferrocenecarboxamide in a one-pot reaction from 1 and 3-(triethoxysilyl)-propylamine. 相似文献
97.
α-Bromo benzolymethylene triphenylphosphorane 3 has been synthesized by the reaction of benzoylmethylene triphenylphosphorane 1 with N-bromosuccinimide in the yield of 87% and can react with aromatic aldehydes 4 to give α-bromochalcones 5 in good yields. 相似文献
98.
The detection of oxygen in magneto-optical layers is of fundamental importance for the characterization of the stability of RE-TM material. The magnetic properties are directly influenced by oxide formation. Oxygen depth profiles are carried out by using the SIMS technique. A comparison with magnetic measurements showed a clear conformity. We were able to study the oxidation behaviour of various layers at room temperature and at higher values up to 250° C for several hours. It could be shown that aluminum is a successful material for the protection of RE-films against oxidation. The difficulties of translating SIMS counting rates into concentration values were overcome by using EPMA. Specially prepared reference samples were measured by this technique and could then be used as standard samples for SIMS. 相似文献
99.
Isabelle FavierElisabet Duñach 《Tetrahedron》2003,59(10):1823-1830
Mandelic acid and some aryl substituted derivatives were oxidised under molecular oxygen and a catalytic amount of Bi(0). The corresponding aldehydes and/or the carboxylic acids were obtained selectively depending on the nature of the substituent. Aldehydes and α-keto acids were oxidised under the same Bi(0)/O2 system and α-keto acids were proposed as intermediates in the formation of benzoic acid derivatives. 相似文献
100.
Ulrich Jordis 《Monatshefte für Chemie / Chemical Monthly》1986,117(11):1339-1342
5-Amino-benzo-1,3-dithiol, available by reduction of 5-amino-benzo-1,3-dithiol-2-thion, was elaborated into dithio-oxolinic Acid (6 a). 相似文献