全文获取类型
收费全文 | 5542篇 |
免费 | 110篇 |
国内免费 | 28篇 |
专业分类
化学 | 3577篇 |
晶体学 | 26篇 |
力学 | 175篇 |
数学 | 831篇 |
物理学 | 1071篇 |
出版年
2020年 | 40篇 |
2019年 | 44篇 |
2018年 | 45篇 |
2017年 | 48篇 |
2016年 | 77篇 |
2015年 | 87篇 |
2014年 | 104篇 |
2013年 | 263篇 |
2012年 | 215篇 |
2011年 | 243篇 |
2010年 | 160篇 |
2009年 | 143篇 |
2008年 | 231篇 |
2007年 | 210篇 |
2006年 | 242篇 |
2005年 | 238篇 |
2004年 | 193篇 |
2003年 | 173篇 |
2002年 | 200篇 |
2001年 | 85篇 |
2000年 | 102篇 |
1999年 | 89篇 |
1998年 | 78篇 |
1997年 | 98篇 |
1996年 | 99篇 |
1995年 | 106篇 |
1994年 | 86篇 |
1993年 | 85篇 |
1992年 | 73篇 |
1991年 | 70篇 |
1990年 | 57篇 |
1989年 | 54篇 |
1988年 | 57篇 |
1987年 | 44篇 |
1986年 | 44篇 |
1985年 | 99篇 |
1984年 | 79篇 |
1983年 | 62篇 |
1982年 | 78篇 |
1981年 | 98篇 |
1980年 | 70篇 |
1979年 | 79篇 |
1978年 | 98篇 |
1977年 | 100篇 |
1976年 | 77篇 |
1975年 | 73篇 |
1974年 | 72篇 |
1973年 | 62篇 |
1971年 | 46篇 |
1970年 | 37篇 |
排序方式: 共有5680条查询结果,搜索用时 15 毫秒
71.
Eberhard Bothe Helmut Gorner Joachim Opitz Dietrich Schulte-Frohlinde Aslam Siddiqi Malgorzata Wala 《Photochemistry and photobiology》1990,52(5):949-959
Double-stranded (ds) calf thymus DNA (0.4 mM), excited by 20 ns laser pulses at 248 nm, was studied in deoxygenated aqueous solution at room temperature and pH 6.7 in the presence of a sodium salt (10 mM). The quantum yields for the formation of hydrated electrons (phi c-), single-strand breaks (phi ssb) and double-strand breaks (phi dsb) were determined for various laser pulse intensities (IL). phi c- and phi ssb increase linearly with increasing IL; however, phi ssb has a tendency to reach saturation at high IL (greater than 5 X 10(6) Wcm-2). The ratio phi ssb/phi c-, representing the number of ssb per radical cation, is about 0.08 at IL less than or equal to 5 X 10(6) Wcm-2. For comparison, the number of ssb per OH radical reacting with dsDNA is 0.22. On going from argon to N2O saturation, phi ssb and phi dsb become larger by factors of approximately 5 and 10-15, respectively. This enhancement is produced by attack on DNA bases by OH radicals generated by N2O-scavenging of the photoelectrons. While phi ssb is essentially independent of the dose (Etot), phi dsb depends linearly on Etot in both argon- and N2O-saturated solutions. The linear dependence of phi dsb implies a square dependence of the number of dsb on Etot. This portion of dsb formation is explained by the occurrence of two random ssb, generated within a critical distance (h) in opposite strands. For both argon- and N2O-saturated solutions h was found to be of the order of 40-70 phosphoric acid diester bonds. On addition of electron scavengers such as 2-chloroethanol (or N2O plus t-butanol), phi dsb is similar to that in neat, argon-saturated solutions. Thus, hydrated electrons are not involved in the chemical pathway leading to laser-pulse-induced dsb of DNA. 相似文献
72.
John Bethke Jürgen Kopf Paul Margaretha Bernard Pignon Léon Dupont Léon E. Christiaens 《Helvetica chimica acta》1997,80(6):1865-1868
Irradiation (λ = 350 nm) of 1H-[2]benzothiopyran-1-one ( 2 ) in the solid state affords selectively and in good yield 6aα, 6bα, 12bα, 12cα -tetrahydrocyclobuta[1, 2-c:4, 3-c′]bis([2]benzothiopyran)-5, 8-dione ( 3 ), the head-to-head (HH) cis-cisoid-cis-cyclodimer of 2 , X-Ray analysis of 2 confirms that this reaction proceeds according to the well-established topochemical principles. The same dimer 3 is obtained in low yields on irradiation of 10−1 M solutions of 2 in either MeOH or MeCN, while no conversion at all is observed in benzene. On irradiation of 2 in MeCN in the presence of tetrachloroethene, the [2 + 2] photocycloadduct 4 is formed in good yield, the conversion 2 → 4 being efficiently quenched by naphthalene. In contrast, no reaction is observed -on irradiation of 2 in the presence of 2, 3-dimethylbut-2-ene, neither in polar nor in apolar solvents. 相似文献
73.
Jean M. J. Tronchet Bernard Gentile Alain P. Bonenfant Olivier R. Martin 《Helvetica chimica acta》1979,62(3):696-699
Further examples of O-isopropylidenaldehydosugars in the furanose series Formyl-bearing furanose derivatives 1–3 (α-D -xylo or α-D -erythro configurations) and 5–8 (configurations α-D -ribo, α-D -lyxo, β-L -threo and D -arabino) have been prepared by classical synthetic steps and their properties mainly spectroscopic, reported. The coupling constant between the formyl and the vicinal proton is always small. Like other members of the series, these new aldehydosugars constitute very useful and flexible synthetic intermediates. 相似文献
74.
Zusammenfassung Mit Hilfe der Dünnschicht-Chromatographie lassen sich Gemische von Tetraorganozinnverbindungen, welche keine polaren Gruppen enthalten, auftrennen und identifizieren, sofern sie sich entweder in der Zahl der Phenylgruppen pro Molekel oder in der Natur des aliphatischen Restes (Alkyl oder Allyl) unterscheiden.Für die analytische Dünnschicht-Chromatographie eignet sich als Fließmittel eine Mischung von 80 Vol-% Hexan und 20 Vol-% Benzol, für die präparative Schicht-Chromatographie (mit Mehrfachentwicklung) je nach Trennproblem reines Hexan oder Hexan-Benzol-Mischungen mit 10 bis 20 Vol-% Benzol.In diesen Fließmitteln hängen die Rf-Werte innerhalb der Verbindungsreihen der Phenyl-isobutyl-stannane Ph
x
SniBu4–x
und der Phenyl-allylstannane Ph
x
SnAll4–x
linear mit der Zahl der Phenylreste x zusammen und fallen mit steigendem x. Die sie verbindenden Geraden schneiden sich bei x=4 (Tetraphenylzinn).Zur Anfärbung von Organozinnverbindungen eignen sich Dithizon und Silbernitrat. Durch Besprühen der Platten mit Dithizon lassen sich auf Grund der entstehenden Farben Tetra-, Tri- und Diorganozinnverbindungen sofort unterscheiden. Mit Silbernitrat lassen sich Allyl-Zinnverbindungen und Polystannane von Alkylzinnverbindungen unterscheiden; Tetraphenylzinn wird dabei nicht angefärbt.
On the thin-layer chromatography of organotin compounds
With the aid of thinlayer chromatography mixtures of tetraorganotin compounds containing no polar groups can be separated and identified if they differ in the number of phenyl groups per molecule or in the nature of their aliphatic residues (alkyl or allyl).For analytical purposes mixtures of 80% hexane and 20% benzene by volume are suitable as mobile phase. For preparative purposes (requiring multiple development) pure hexane or hexane containing 10 to 20% benzene by volume are best suited as mobile phase, where the amount of hexane depends on the problem of separation at hand.The Rf values of the mobile phases vary linearly with the number of the phenyl groups x and decrease with increasing x. The straight lines connecting the Rf values cross at x=4 (tetraphenyltin).Dithizone and silver nitrate are suitable spray reagents for organotin compounds. By spraying the plates with dithizone tetra-, tri-, and di-organotin compounds can be immediately distinguished on the basis of the ensuing colourations. With silver nitrate allyltin compounds and polystannanes can be distinguished from alkyltin compounds; tetra-phenyltin is not coloured by this reagent.相似文献
75.
Bernard Tinland 《Theoretical chemistry accounts》1968,11(5):452-454
The conformation of biphenyl is studied by the CNDO method. A fixed length of the twisted bond allows no minimum for the total energy between the values 0° and 90° of the angle of twist. 相似文献
76.
The electrostatic field associated with one complete turn of B-DNA is presented. Two base sequences poly (dG) · poly (dC) and poly (dA) · poly (dT) are studied and the effects of sodium counterions bound to the nucleic acid are investigated. The contrasts between the electrostatic potential and the electrostatic field of the macromolecules are discussed and the possible applications of the field are considered. 相似文献
77.
Yvan Giroud Pierre-Alain Carrupt Alessandra Pagliara Bernard Testa Ronald G. Dickinson 《Helvetica chimica acta》1998,81(2):330-341
In this study, we compared the lipophilicity of O-glucuronides and their aglycones. Distribution coefficients (log D) and P values of neutral species (log P) were determined by centrifugal partition chromatography (CPC) in octanol/buffer systems. Two-phase potentiometry was also used to measure the log P value of some lipophilic solutes. The experimentally determined global influence of glucuronidation on lipophilicity, obtained as the difference (decrement) log P(glucuronide) ? log P(aglycone), was found to be ?1.30 ± 0.16 (n = 4) for glucuronides of alcohols (methyl, menthyl, neomenthyl, and chloramphenicol O-glucuronide). The mean decrement was ?2.06 ± 0.31 (n = 9) for glucuronides of phenols (phenyl, p-nitrophenyl, 1-naphthyl, 6-bromo-2-naphthyl, 4-methylumbelliferyl, 3-coumarinyl, phenolphthalein, 4′-benzophenonyl O-glucuronide, and diflunisal phenolic glucuronide). For the acylglucuronide of diflunisal and its rearrangement isomers, the mean decrement was ?1.80 ± 0.08 (n = 4; range ?1.7 to ?1.9). Differences in through-bond proximity effects as parametrized in the CLOGP algorithm seem to account for much of this difference. Conformational factors may also play a role, although it appears modest and unassessable for the glucuronides investigated here. The results imply that in vivo glucuronidation should have a stronger influence on the excretion of phenols than on that of alcohols. 相似文献
78.
In distinction to Extended Hückel Theory which predicts as the most stable conformation of free zwitterionic GABA a totally extended form, PCILO and SCF ab initio studies show that the intrinsically preferred conformation of the isolated molecule is a highly folded one, resulting from strong interactions between the two charged ends. Computations are also carried out for hydrated GABA in the supermolecule approach allowing moreover for the flexibility of binding of some of the water molecules of the first hydration shell. They predict the coexistence in solution of a large number of conformations showing different degrees of folding (or extension), a result confirmed by recent NMR studies. This and a number of similar results show that we have to adapt our thinking on the role of conformations in pharmacological activity to this situation, which was frequently obscured by the more abundant results of X-ray crystallography yielding a single conformation. 相似文献
79.
Cheguillaume G Buchmann W Desmazières B Tortajada J 《Journal of mass spectrometry : JMS》2004,39(4):368-377
Two model peptides, des-Arg1-bradykinin (DAB) and bradykinin (B), were cationized by Ag+ after their separation by reversed-phase liquid chromatography (RPLC) prior to mass spectrometry (MS). Silver nitrate solution was used as a post-column reagent. The RPLC and MS experimental conditions were optimized using flow injection in order to obtain sufficiently abundant silver adducts to permit MS/MS experiments. The use of water-methanol with 0.1% formic acid as mobile phase allowed a good chromatographic separation of the two peptides with a polymeric stationary phase and sufficiently abundant silver-containing adducts, [M + Ag + H]2+ and [M + 2Ag]2+. The gas-phase dissociation of [DAB + Ag + H]2+ and [DAB + 2Ag]2+ led to interpretable mass spectra during the on-line cationization experiment. Most of the ions obtained by dissociating [DAB + Ag + H]2+ and [DAB + 2Ag]2+ species are silver-containing ions but the ions produced depend on the parent. The ions coming from the dissociation of the doubly charged silver adducts [DAB + Ag + H]2+ or [DAB + 2Ag]2+ are of interest compared with those coming from the singly charged silver species or doubly charged protonated species. The fragmentation of the doubly charged silver adducts provides ions over the entire mass range. Although the presence of several prolines in des-Arg1-bradykinin prevents the formation of some expected ions, the observation of triplets [an-H + Ag]+, [bn-H + Ag]+ and [bn + OH + Ag]+ produced by the dissociation of on-line Ag(+)-cationized peptides could contribute to greater success of automatic sequencing of peptides. 相似文献
80.
The detection and dosimetry of radiosterilization of pharmaceuticals is a growing concern to numerous government regulatory agencies worldwide. In the absence of suitable detection methods, attention was focused on electron spin resonance (ESR) spectrometry. This paper reports experimental data on ESR dosimetry of irradiated nitrofurans (nitrofurantoin, nifuroxazide, nifurzide and nifurtoinol). Whereas the ESR spectrum of a non-irradiated sample shows no signal, a signal, that is dependent on the irradiation dose is observed with irradiated samples. The number of free radicals was calculated by comparing the second integral from radiosterilized samples and a Bruker strong pitch reference; the values were in the range 1.1 x 10(18)-1.5 x 10(19) radicals mol-1 (G = 0.003-0.03). In addition to qualitative detection, ESR spectrometry can be used for dose determination. When a bi-exponential function is applied to the variation of the peak-to-peak amplitude versus dose, the function correlates well with the data. However, an exponential function, easier to use than a bi-exponential function, will probably be sufficient for dose determination by retrospective dosimetry. Decay of radicals upon storage was modelled using a bi-exponential function. From this, the time limit from the irradiation (25 kGy) for identification of irradiated nitrofurans by ESR can be evaluated. With regard to the commercial aspects of drugs, radicals should be detected up to 2 years after irradiation. The kinetic decrease makes discrimination between irradiated and non-irradiated nitrofurans possible even after storage for up to 2 years, except for nifurtoinol. 相似文献