全文获取类型
收费全文 | 3092篇 |
免费 | 46篇 |
国内免费 | 33篇 |
专业分类
化学 | 1766篇 |
晶体学 | 62篇 |
力学 | 70篇 |
数学 | 668篇 |
物理学 | 605篇 |
出版年
2022年 | 50篇 |
2021年 | 64篇 |
2020年 | 30篇 |
2019年 | 35篇 |
2018年 | 38篇 |
2017年 | 56篇 |
2016年 | 79篇 |
2015年 | 77篇 |
2014年 | 89篇 |
2013年 | 211篇 |
2012年 | 134篇 |
2011年 | 180篇 |
2010年 | 114篇 |
2009年 | 125篇 |
2008年 | 172篇 |
2007年 | 161篇 |
2006年 | 150篇 |
2005年 | 125篇 |
2004年 | 142篇 |
2003年 | 100篇 |
2002年 | 105篇 |
2001年 | 50篇 |
2000年 | 43篇 |
1999年 | 44篇 |
1998年 | 40篇 |
1997年 | 51篇 |
1996年 | 55篇 |
1995年 | 31篇 |
1994年 | 34篇 |
1993年 | 41篇 |
1992年 | 28篇 |
1991年 | 30篇 |
1990年 | 34篇 |
1989年 | 32篇 |
1988年 | 20篇 |
1987年 | 28篇 |
1986年 | 19篇 |
1985年 | 30篇 |
1984年 | 41篇 |
1983年 | 24篇 |
1982年 | 24篇 |
1981年 | 30篇 |
1980年 | 23篇 |
1979年 | 21篇 |
1978年 | 20篇 |
1977年 | 29篇 |
1976年 | 13篇 |
1975年 | 12篇 |
1974年 | 11篇 |
1972年 | 11篇 |
排序方式: 共有3171条查询结果,搜索用时 31 毫秒
91.
1-Ethyl-6-fluoro-1,4-dihydro-7-methyl-4-oxo-1,8-naphthyridine-3-carboxylic acid ( 8 ) has been prepared in large quantities by a highly efficient process. It has in turn been degraded to give 7-chloro-1-ethyl-6-fluoro-1,4-dihydro-4-oxo-1,8-naphthyridine-3-carboxylic acid ( 11 ). This intermediate has been reacted with piperazine to give the known antibacterial agent, enoxacin ( 12 ). 相似文献
92.
The preparation of 1,4-dihydro-4-oxo-3′-alkylthio-3,4′-diquinolinyl sulfides 3 or 1,4-dihydro-4-oxo-3-(alkylthio)quinolines 4 by acid catalysed hydrolysis of 4-methoxy-3′-alkylthio-3,4′-diquinolinyl sulfides 1 or 4-methoxy-3-(alkylthio)-quinolines 2 is described. The reactions of 4-methoxy-3′-alkylthio-3,4′-diquinolinyl sulfides 1 or 1,4-dihydro-4-oxo-3′-alkylthio-3,4′-diquinolinyl sulfides 3 with phosphoryl chloride in DMF afforded 4-chloro-3′-alkylthio-3,4′-diquinolinyl sulfides 5 . Treatment of the title compounds 1 or 3 with boiling phosphoryl chloride systems:leads to 4-chloro-3-(alkylthio)quinolines 6 and thioquinanthrene but those of alkoxy- or oxo-quinolines 2 or 4 lead to 4-chloro-3-(alkylthio)quinolines 6 . The reactions of N-methyl-4(1H)-quinolinones 3n and 4n with phosphoryl chloride directed to 4-chloro-3-(alkylthio)quinolines 6 were studied as well. 相似文献
93.
Wilk A Grajkowski A Bull TE Dixon AM Freedberg DI Beaucage SL 《Journal of the American Chemical Society》2002,124(7):1180-1181
The determination of the absolute configuration of deoxyribonucleoside cyclic N-acylphosphoramidites at phosphorus toward the synthesis of P-stereodifined phosphorothioated oligodeoxyribonucleotides is easily accomplished with computer-assisted molecular modeling and M-GOESY NMR spectroscopy. Specifically, computer-modeling diasteromeric phosphoramidite 3 has identifed a proximal (2.55 A) through-space interaction between benzylic H-5 and sugar H-2' ', which can predictably be detected by M-GOESY NMR in SP-3 but not in RP-3 because of being too distant (5.85 A). Consistent with computer-assisted modeling predictions, M-GOESY NMR spectra of SP-3 and RP-3 revealed NOE signals generated from nuclei near the selectively excited H-2' ' that are common to both SP-3 and RP-3, namely those of H-2', H-4', H-3', and H-1'. In addition, a diagnostic NOE signal at 5.5 ppm (benzylic H-5) is, as predicted, only detected in SP-3 and thus provides an unequivocal assessment of the configuration of the diastereomer at phosphorus. M-GOESY NMR data also confirm that the condensation of deoxyribonucleoside cyclic N-acylphosphoramidites with base-activated nucleosidic or nucleotidic 5'-hydroxyls proceeds via a single nucleophilic event. 相似文献
94.
(1S,2R,3S)-, (1R,2R,3S)- and (1S,2R,3R)-4-amino-1,2,3-trihydroxybutylphosphonic acids were synthesised. The synthetic strategy involved preparation of the respective 4-azido-2,3-O-isopropylidene-l-threose or -d-erythrose, addition of dialkyl phosphites, separation of C-1 epimeric O,O-dibenzyl phosphonates, the reduction of azides and the removal of the protecting groups. The (2R,3S) and (2R,3R) configurations in the final products were secured by employing diethyl l-tartrate and d-isoascorbic acid as starting materials. The stereochemical course of the addition to the carbonyl groups in 4-azido-2,3-O-isopropylidene-l-threose or -d-erythrose followed that established earlier for 2,3-O-isopropylidene-d-glyceraldehyde and similar (3:1-4:1) diastereoselectivities were achieved. 相似文献
95.
Nancy J. McMillan Russell S. Harmon Frank C. De Lucia Andrzej M. Miziolek 《Spectrochimica Acta Part B: Atomic Spectroscopy》2007,62(12):1528-1536
Laser-induced breakdown spectroscopy (LIBS) provides an alternative chemical analytical technique that obviates the issues of sample preparation and sample destruction common to most laboratory-based analytical methods. This contribution explores the capability of LIBS analysis to identify carbonate and silicate minerals rapidly and accurately. Fifty-two mineral samples (18 carbonates, 9 pyroxenes and pyroxenoids, 6 amphiboles, 8 phyllosilicates, and 11 feldspars) were analyzed by LIBS. Two composite broadband spectra (averages of 10 shots each) were calculated for each sample to produce two databases each containing the composite LIBS spectra for the same 52 mineral samples. By using correlation coefficients resulting from the regression of the intensities of pairs of LIBS spectra, all 52 minerals were correctly identified in the database. If the LIBS spectra of each sample were compared to a database containing the other 51 minerals, 65% were identified as a mineral of similar composition from the same mineral family. The remaining minerals were misidentified for two reasons: 1) the mineral had high concentrations of an element not present in the database; and 2) the mineral was identified as a mineral with similar elemental composition from a different family. For instance, the Ca–Mg carbonate dolomite was misidentified as the Ca–Mg silicate diopside. This pilot study suggests that LIBS has promise in mineral identification and in situ analysis of minerals that record geological processes. 相似文献
96.
Mieczys?aw Makosza Tadeusz Lemek Andrzej Kwast Fran?ois Terrier 《The Journal of organic chemistry》2002,67(2):394-400
Relations of rates of the vicarious nucleophilic substitution of hydrogen (VNS) and S(N)Ar substitution of fluorine in 2-fluoronitrobenzenes with chloroalkyl aryl sulfone carbanions were determined from competitive experiments carried out at various concentrations of base. The observed dependence of the VNS/S(N)Ar rate ratio on the base concentration confirmed the two-step mechanism of the VNS, which consists of reversible formation of sigma(H) adducts of the alpha-chlorocarbanion to nitroarene, followed by base-induced beta-elimination of HCl. It was also evidenced that both of these processes can be the rate-limiting steps: the beta-elimination at low base concentration and the nucleophilic addition at high base concentration. Consistent with that conclusion is the finding that the kinetic isotope effect in the VNS reaction decreases from 4.2 (a value typical of a primary KIE) to 0.8 (a value typical of a secondary KIE) with increasing base concentration. Also reported is our discovery that the S(N)Ar substitution of the 2-fluoronitrobenzenes studied in this work was subject to base catalysis under some of the experimental conditions employed in our competitive experiments. 相似文献
97.
The dinuclear complex [Co2(μ-OAc)2(OAc)2(μ-H2O)(phen)2] has been prepared and its structure was determined. The compound crystallizes in the monoclinic space group P2(1)/c. The Co–Co distance is 3.574 Å and is similar to the Fe–Fe distance in the reduced methane monooxygenase hydroxylase. The electronic and IR spectra of the complex confirm octahedral coordination of the cobalt atoms and formation of strong O–HO hydrogen bonds in the solid state. The dependence of the magnetic susceptibility of the complex on temperature indicates an antiferromagnetic interaction, the value of the isotropic exchange parameter J was estimated to be −2.1 cm−1. The 1H NMR spectra show that in organic solvents the structure of compound is the same as in the solid state, however, in water solution the complex dissociates giving compounds with different Co:phen ratios. 相似文献
98.
Frank C. De Lucia Jr. Jennifer L. Gottfried Chase A. Munson Andrzej W. Miziolek 《Spectrochimica Acta Part B: Atomic Spectroscopy》2007,62(12):1399-1404
Detecting trace explosive residues at standoff distances in real-time is a difficult problem. One method ideally suited for real-time standoff detection is laser-induced breakdown spectroscopy (LIBS). However, atmospheric oxygen and nitrogen contributes to the LIBS signal from the oxygen- and nitrogen-containing explosive compounds, complicating the discrimination of explosives from other organic materials. While bathing the sample in an inert gas will remove atmospheric oxygen and nitrogen interference, it cannot practically be applied for standoff LIBS. Alternatively, we have investigated the potential of double pulse LIBS to improve the discrimination of explosives by diminishing the contribution of atmospheric oxygen and nitrogen to the LIBS signal. These initial studies compare the close-contact (< 1 m) LIBS spectra of explosives using single pulse LIBS in argon with double pulse LIBS in atmosphere. We have demonstrated improved discrimination of an explosive and an organic interferent using double pulse LIBS to reduce the air entrained in the analytical plasma. 相似文献
99.
Maksymilian Ignaczak Andrzej Grzejdziak Barbara Olejniczak 《Monatshefte für Chemie / Chemical Monthly》1989,120(6-7):515-527
Summary The conditional protonation constants (=0.1) for 2,2:6,2-terpyridine, logK
1=4.93, logK
2=3.69, were determined by thepH-metric method. The compositions of complexes of Ag2+ and Ag+ ions with 2,2:6,2-terpyridine (tp) were studied and equilibria of the complex formation process were described. The values of conditional complex formation constants are as follows: for Ag(tp)
2
+
:log01=5.79, log02=9.68, for Ag(tp)
2
2+
:log02=25.31, while the conditional constant of the Ag(tp)NO3 precipitate formation is:K
SO=2.45·104. Using coulometric and chronovoltamperometric measurements, the redox systems being formed in the complex solutions of Ag(II) and Ag(I) were determined and described including their formal potentials.
Komplexibildungsgleichgewichte und Redoxpotentiale des Systems Ag(II)/Ag(I) in Gegenwart von 2,2:6,2-Terpyridin in Wasser
Zusammenfassung Mit Hilfe derpH-metrischen Methode wurden die konditionalen Protonationskonstanten (=0.1) von 2,2:6,2-Terpyridin bestimmt: logK 1=4.93, logK 2=3.69. Es wurde auch die Zusammensetzung der Komplexe von Ag(II) und Ag(I) mit 2,2:6,2-Terpyridin(tp) bestimmt sowie die Gleichgewichte der komplexbildung beschrieben. Die Werte der Konditionalkomplexbildungskonstanten sind: für Ag(tp) 2 + :log01=5.79, log02=9.68, für Ag(tp) 2 2+ :log02=25.31 und für das Löslichkeitsprodukt Ag(tp)NO3:K SO –1 =4.08·10–5. Die in Komplexlösungen von Ag(II) und Ag(I) vorliegenden Redoxsysteme wurden mittels cyclischer Voltametrie und Coulometrie untersucht und die Formalpotentialwerte dieser Systeme in Wasser bestimmt.相似文献
100.