全文获取类型
收费全文 | 3225篇 |
免费 | 82篇 |
国内免费 | 16篇 |
专业分类
化学 | 2181篇 |
晶体学 | 21篇 |
力学 | 127篇 |
数学 | 545篇 |
物理学 | 449篇 |
出版年
2022年 | 22篇 |
2021年 | 33篇 |
2020年 | 39篇 |
2019年 | 43篇 |
2018年 | 32篇 |
2017年 | 36篇 |
2016年 | 69篇 |
2015年 | 49篇 |
2014年 | 74篇 |
2013年 | 181篇 |
2012年 | 155篇 |
2011年 | 183篇 |
2010年 | 124篇 |
2009年 | 117篇 |
2008年 | 191篇 |
2007年 | 173篇 |
2006年 | 180篇 |
2005年 | 199篇 |
2004年 | 174篇 |
2003年 | 136篇 |
2002年 | 150篇 |
2001年 | 55篇 |
2000年 | 45篇 |
1999年 | 47篇 |
1998年 | 50篇 |
1997年 | 54篇 |
1996年 | 38篇 |
1995年 | 25篇 |
1994年 | 38篇 |
1993年 | 37篇 |
1992年 | 22篇 |
1991年 | 28篇 |
1990年 | 15篇 |
1989年 | 25篇 |
1988年 | 14篇 |
1987年 | 17篇 |
1986年 | 20篇 |
1985年 | 34篇 |
1984年 | 35篇 |
1983年 | 22篇 |
1982年 | 31篇 |
1981年 | 35篇 |
1980年 | 40篇 |
1979年 | 28篇 |
1978年 | 25篇 |
1977年 | 38篇 |
1976年 | 37篇 |
1975年 | 36篇 |
1974年 | 26篇 |
1973年 | 12篇 |
排序方式: 共有3323条查询结果,搜索用时 15 毫秒
911.
Starting from o-aminopyridine carboxylic acids, a general synthetic route leading to various pyridopyrimidin-4(3H)-ones is described. The first metallation and functionalization of the pyridine moiety has been studied and a regioselective metallation at the peri-position C5 of the pyridine ring has been highlighted. 相似文献
912.
Goeppert A Dinér P Ahlberg P Sommer J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(14):3277-3283
The H/D exchange observed when methane is contacted with D(2)SO(4) at 270-330 degrees C shows that the alkane behaves as a sigma base and undergoes rapid and reversible protonation at this temperature. DFT studies of the hydrogen exchange between a monomer and a dimer of sulfuric acid and methane show that the transition states involved in the exchange are bifunctional, that is one hydrogen atom is transferred from a hydroxy group in sulfuric acid to methane, while one hydrogen atom is abstracted from methane by a non-hydroxy oxygen atom in sulfuric acid. All the transition states include a CH(5) moiety, which shows similarities to the methanium ion CH(5) (+). The calculated potential activation energy of the hydrogen exchange for the monomer is 174 kJ mol(-1), which is close to the experimental value (176 kJ mol(-1)). Solvation of the monomer and the transition state of the monomer with an extra sulfuric acid molecule, decrease the potential activation energy by 6 kJ mol(-1). The acid-base process is in competition, however, with an oxidative process involving methane and sulfuric acid which leads to CO(2), SO(2), and water, and thus to a decrease of acidity and loss of reactivity of the medium. 相似文献
913.
We prepared and investigated oligonucleotide duplexes of the sequence d(GATGAC(X)nGCTAG).d(CTAGC(Y)nGTCATC), in which X and Y designate biphenyl- (bph) and pentafluorobiphenyl- ((5F)bph) C-nucleotides, respectively, and n varies from 0-4. These hydrophobic base substitutes are expected to adopt a zipperlike, interstrand stacking motif, in which not only bph/bph or (5F)bph/(5F)bph homo pairs, but also (5F)bph/bph mixed pairs can be formed. By performing UV-melting curve analysis we found that incorporation of a single (5F)bph/(5F)bph pair leads to a duplex that is essentially as stable as the unmodified duplex (n=0), and 2.4 K more stable than the duplex with the nonfluorinated bph/bph pair. The T(m) of the mixed bph/(5F)bph pair was in between the T(m) values of the respective homo pairs. Additional, unnatural aromatic pairs increased the T(m) by +3.0-4.4 K/couple, irrespective of the nature of the aromatic residue. A thermodynamic analysis using isothermal titration calorimetry (ITC) of a series of duplexes with n=3 revealed lower (less negative) duplex formation enthalpies (DeltaH) in the (5F)bph/(5F)bph case than in the bph/bph case, and confirmed the higher thermodynamic stability (DeltaG) of the fluorinated duplex, suggesting it to be of entropic origin. Our data are compatible with a model in which the stacking of (5F)bph versus bph is dominated by dehydration of the aromatic units upon duplex formation. They do not support a model in which van der Waals dispersive forces (induced dipoles) or electrostatic (quadrupole) interactions play a dominant role. 相似文献
914.
Sbastien Richard Gildas Pri Alain Guignard Jrme Thibonnet J.‐Luc Parrain Alain Duchêne Mohamed Abarbri 《Helvetica chimica acta》2003,86(3):726-732
Optically active (perfluoroalkyl)‐oxazepin‐7‐ones were synthesized in two steps starting from ethyl perfluorobut‐2‐ynoate by direct addition of optically active amino alcohols via intermolecular Michael addition and lactone formation. 相似文献
915.
Laurent Dreesen Christophe Silien Cédric Volcke Yannick Sartenaer Paul A Thiry André Peremans Jerome Grugier Jacqueline Marchand-Brynaert Alain Brans Stana Grubisic Bernard Joris 《Chemphyschem》2007,8(7):1071-1076
Despite the large number of articles and patents dealing with penicillin and other beta-lactam antibiotics, there have been no reports about the self-assembly of such substances as monolayers on gold surfaces. The main reason stems from the high reactivity of the beta-lactam ring, which hinders the development of molecules possessing this entity together with a metal-anchoring function. Herein, we present the synthesis of a novel molecule, 6-[(R,S)-5-(1,2-dithiolan-3-yl)pentanoyl-amino]-penicillanic acid, which combines the beta-lactam ring and a metal-anchoring group. Using spectroscopic tools, we demonstrate the chemisorption of this compound on gold as self-assembled monolayers without any alteration of the penicillin pharmacophore and document its reactivity towards a penicillin-binding protein, BlaR-CTD. Our work is a preliminary step towards the development of new biosensors and well-ordered protein arrays, both based on the high affinity of penicillin for penicillin-binding proteins. 相似文献
916.
Emile Bisagni Alain Civier Jean-Pierre Marquet 《Journal of heterocyclic chemistry》1975,12(3):461-465
Treatment of 4-hydroxy-6-methylpyran-2-one with chloracetalhyde in basic aqueous medium gave 6-methylfuro[3,2-c]pyran-4-one. This compound reacted with ammonium hydroxide and some primary amines to form the corresponding N-substituted furo[3,2-c]pyrid-4-ones which may also be obtained from 4-hydroxy-α-pyridones. Furo[3,2-c]pyran-4-one was acylated at the 2 position and 4-chloro-6-methylfuro[3,2-c]pyridine easily gave 4-substituted derivatives by displacement of the halogen atom. 相似文献
917.
Abstract— Steady-state and multifrequency phase fluorometry were used to characterize the conformational state and conformational dynamics of recombinant tick anticoagulant peptide ( Ornithodorus moubata ) (TAP). The TAP contains two tryptophan residues at positions 11 and 37. The fluorescence emission varies sigmoidally as a function of pH with a pKa of 6.01 ± 0.07. This pH dependency suggests that tryptophan fluorescence is quenched by His43 at low pH. This is confirmed by modification of the his-tidine with diethylpyrocarbonate. At pH 9 the fluorescence decay is well described by a sum of three exponentials (0.52,1.9 and 5.4 ns), which decrease all three at pH 4 (0.25, 1.61 and 4.4 ns). From the reactivity of the fluorescence lifetimes toward N -bromosuccinimide and from the calculation of the accessibility we can attribute the long lifetime to Trpll, the short one to Trp37 and the middle one to both. The anisotropy decay was resolved into two components of 3.85 ns and 0.27 ns at pH 4 and 4.5 ns and 0.6 ns at pH 9. The long anisotropy decay time corresponds to the rotational correlation time of the protein, the short one to local mobility of the tryptophan residues. 相似文献
918.
Ibon Alkorta Jos Elguero Nadine Jagerovic Alain Fruchier Glenn P. A. Yap 《Journal of heterocyclic chemistry》2004,41(2):285-289
1‐Hydroxymethylindazole and 1‐hydroxymethylbenzotriazole have been studied in solution by 1H, 13C and 15N NMR spectroscopy and the X‐ray structure of the second compound determined. DFT and GIAO calculations have been used to discuss geometries, energies (comparatively with 2‐substituted isomers) and NMR chemical shifts. 相似文献
919.
Jean DAngelo Jacqueline Ficini Serge Martinon Claude Riche Alain Sevin 《Journal of organometallic chemistry》1979,177(1):265-272
A cyclobutadiene—palladium complex, obtained by dimerization of an ynamine is described. The crystal structure of this compound shows, as salient feature, that the cyclobutadiene ring is not planar, but highly puckered. Anti-bonding interactions have been suggested as an explanation for this puckering. 相似文献
920.
Libert F Coudoré F Richard D Durif F Eschalier A 《Journal of mass spectrometry : JMS》2005,40(12):1521-1525
A method was developed and validated for the analysis of R(-)-apomorphine, (R-)-apocodeine and R(-)-norapomorphine in human plasma and urine with N-propylnorapomorphine as internal standard using gas chromatography/mass spectrometry (GC/MS) and single-ion monitoring after a single liquid-liquid extraction and silylation of compounds. The quantification limits were 1 ng/ml for apomorphine and apocodeine and 25 ng/ml for norapomorphine. Calibration curves were linear, within the range 1-100 ng/ml. Variation in intraday and interday precision was below 10%. This method was applied to study apomorphine bioavailability in nine patients with Parkinson's disease before and after coadministration of a catechol-O-methyl transferase inhibitor. 相似文献