全文获取类型
收费全文 | 3445篇 |
免费 | 205篇 |
国内免费 | 18篇 |
专业分类
化学 | 2424篇 |
晶体学 | 13篇 |
力学 | 76篇 |
数学 | 601篇 |
物理学 | 554篇 |
出版年
2023年 | 31篇 |
2022年 | 40篇 |
2021年 | 61篇 |
2020年 | 79篇 |
2019年 | 76篇 |
2018年 | 70篇 |
2017年 | 76篇 |
2016年 | 177篇 |
2015年 | 138篇 |
2014年 | 122篇 |
2013年 | 223篇 |
2012年 | 266篇 |
2011年 | 280篇 |
2010年 | 163篇 |
2009年 | 117篇 |
2008年 | 229篇 |
2007年 | 193篇 |
2006年 | 163篇 |
2005年 | 169篇 |
2004年 | 150篇 |
2003年 | 137篇 |
2002年 | 123篇 |
2001年 | 39篇 |
2000年 | 30篇 |
1999年 | 37篇 |
1998年 | 37篇 |
1997年 | 25篇 |
1996年 | 38篇 |
1995年 | 12篇 |
1994年 | 20篇 |
1993年 | 28篇 |
1992年 | 19篇 |
1991年 | 15篇 |
1990年 | 15篇 |
1989年 | 11篇 |
1988年 | 15篇 |
1987年 | 13篇 |
1986年 | 14篇 |
1985年 | 31篇 |
1984年 | 23篇 |
1983年 | 11篇 |
1982年 | 13篇 |
1981年 | 9篇 |
1980年 | 10篇 |
1979年 | 18篇 |
1978年 | 12篇 |
1977年 | 12篇 |
1975年 | 9篇 |
1974年 | 13篇 |
1973年 | 12篇 |
排序方式: 共有3668条查询结果,搜索用时 421 毫秒
41.
M. Bešter-Rogač M. Tomšič J. Barthel R. Neueder A. Apelblat 《Journal of solution chemistry》2002,31(1):1-18
Conductivity measurements of oxalic acid and neutral oxalates (disodium oxalate, dipotassium oxalate, dicesium, and diammonium oxalate) were performed on dilute aqueous solutions, c < 3 × 10–3 mol-dm–3, from 5 to 35°C. These data and those available from the literature were analyzed in terms of dissociation steps of oxalic acid, the Onsager conductivity equation for neutral oxalates, the Quint–Viallard conductivity equation for the acid, and the Debye–Hückel equation for activity coefficients, to give the limiting equivalent conductances of bioxalate anion ;(HC2O4
–) and oxalate anion (1/2C2O4
2–) and the corresponding dissociation constants K
1 and K
2. 相似文献
42.
Substituent effects on monoboronic acid sensors for saccharides based on N-phenyl-1,8-naphthalenedicarboximides 总被引:4,自引:0,他引:4
In the course of our investigations on new monoboronic acid saccharide sensors with C(0) spacers, a series of probes 1-6 based on 1,8-naphthalenedicarboximide were synthesized. Sensor 1 displays features typical of PET monoboronic acid sensors and shows high selectivity to fructose. Sensor 2 exhibits a novel dual emission and remarkable sensitivity for glucose relative to fructose and galactose through subtle changes in pH. Sensor 3 displays significantly enhanced fluorescence in the presence of galactose at low pH. Although probes 4-6 exhibit unique properties such as high quantum yield (Phi(F) = 0.407) and excellent solubility in water, they did not show significant change in fluorescence intensity in the presence of monosaccharides. The effects of substituent on all six probes lend support to the proposed photoelectrochemical model. 相似文献
43.
Simon Doherty Julian G. Knight Tom H. Scanlan Mark R. J. Elsegood William Clegg 《Journal of organometallic chemistry》2002,650(1-2):231-248
The aprotic and protic bi- and multidentate iminophosphines 2-Ph2PC6H4N=CR1R2 (R1=H, R2=Ph=2a; R1=Me R2=Ph=2b; R1=H, R2=2-thienyl=2c; R1=H, R2=C6H4-2-PPh2=2d; R1=H, R2=C6H4-2-OH=2e, R1=H, R2=C6H4-2-OH-3-But=2f; R1=H, R2=CH2C(O)Me=2g) have been prepared by the acid catalyzed condensation of 2-(diphenylphosphino)aniline with the corresponding aldehyde–ketone. Iminophosphine 2d can be reduced with sodium cyanoborohydride to give the corresponding amino-diphosphine 2-Ph2PC6H4N(H)CH2C6H4-2-PPh2 (2h). In the presence of a stoichiometric quantity of acid, 2-(diphenylphosphino)aniline reacts in an unexpected manner with benzaldehyde, salicylaldehyde, or acetophenone to give the corresponding 2,3-dihydro-1H-benzo[1,3]azaphosphol-3-ium salts and with pyridine-2-carboxaldehyde to give N-(pyridin-2-ylmethyl)-2-diphenylphosphinoylaniline, the latter of which has been characterized by single-crystal X-ray crystallography, as its palladium dichloride derivative. The attempted condensation of 2-(diphenylphosphino)aniline with pyridine-2-carboxaldehyde to give the corresponding pyridine-functionalized iminophosphine resulted in an unusual transformation involving the diastereoselective addition of two equivalents of aldehyde to give 1,2-dipyridin-2-yl-2-(o-diphenylphosphinoyl)phenylamino-ethanol, which has been characterized by a single-crystal X-ray structure determination. The bidentate iminophosphine 2-Ph2PC6H4N=C(H)Ph reacts with [(cycloocta-1,5-diene)PdClX] X=Cl, Me) to give [Pd{2-Ph2PC6H4N=C(H)Ph}ClX] and the imino-diphosphine 2-Ph2PC6H4N=C(H)C6H4-PPh2 reacts with [(cycloocta-1,5-diene)PdClMe] to give [Pd{2-Ph2PC6H4N=C(H)C6H4---PPh2}ClMe] and each has been characterized by single-crystal X-ray crystallography. The monobasic iminophosphine 2-Ph2PC6H4N=C(Me)CH2C(O)Me reacts with [Ni(PPh3)2Cl2] in the presence of NaH to give the phosphino–ketoiminate complex [Ni{2-Ph2PC6H4N=C(Me)CHC(O)Me}Cl], which has been structurally characterized. Mixtures of iminophosphines 2a–h and a palladium source catalyze the Suzuki cross coupling of 4-bromoacetophenone with phenyl boronic acid. The efficiency of these catalysts show a marked dependence on the palladium source, catalysts formed from [Pd2(OAc)6] giving consistently higher conversions than those formed from [Pd2(dba)3] and [PdCl2(MeCN)2]. Catalysts formed from neutral bi- and terdentate iminophosphines 2a–d gave significantly higher conversions than those formed from their monobasic counterparts 2e–f. Notably, under our conditions the conversions obtained with 2a–c compare favorably with those of the standards; catalysts formed from tris(2-tolyl)phosphine and tris(2,4-di-tert-butylphenyl)phosphite and a source of palladium. In addition, mixtures of [Ir(COD)Cl]2 and 2a–h are active for the hydrosilylation of acetophenone; in this case catalysts formed from monobasic iminophosphines 2e–f giving the highest conversions. 相似文献
44.
Tomás Pérez-Ruiz Carmen Martínez-Lozano Virginia Tomás Antonio Sanz Jesús Martín 《Fresenius' Journal of Analytical Chemistry》1998,362(4):399-403
A flow-injection configuration is proposed for the fluorimetric determination of ethylenethiourea. The procedure is based
on the inhibitory effect of ethylenethiourea on the oxidation of thiamine to thiochrome by mercury(II). A linear calibration
graph was obtained between 0.1 and 2.0 μg mL–1, with a sampling rate of 40 samples per hour and a relative standard deviation of about 1.11%. The usefulness of the method
was tested for the determination of ethylenethiourea residues in water, milk, potatoes, pear, grape and apple.
Received: 26 January 1998 / Revised: 6 April 1998 / Accepted: 9 April 1998 相似文献
45.
Time-dependent density functional theory (TD-DFT) is applied to the CD spectra of Lambda(deltadeltadelta)-(+)-[Co(S-pn)n(en)(3-n)]3+ (n = 1, 2, 3) and Lambda(deltadeltadelta)-(+)-[Co(en)3]3+ as well as the stereoisomers Delta-((delta)n(lambda)(3-n))-(-)-[Co(S-pn)n(en)(3-n)]3+ (n = 1, 2, 3) and Delta(deltadeltadelta)-(-)-[Co(en)3]3+. Theory is able to reproduce the major differences in the CD spectra of the species with a Lambda-configuration and their isomers with a Delta-configuration in both the d-d and ligand-to-metal CT region. It is further possible to rationalize the trend in terms of a larger azimuthal distortion away from the octahedral geometry in the Lambda-conformation compared to the Delta-configuration. Considerations were also given to the CD spectra of the lel3-isomer, Delta(lambdalambdalambda)-(-)-[Rh(R-pn)3]3+ and the ob-isomer, Lambda(lambdalambdalambda)-(+)-[Rh(S-pn)3]3+. 相似文献
46.
González-Núñez ME Mello R Royo J Asensio G Monzó I Tomás F López JG Ortiz FL 《The Journal of organic chemistry》2005,70(9):3450-3457
[reaction: see text] Data on the apparent dipole moment of thianthrene-5-oxide (1) and (1)H NMR spectra in different solvents support the conformational mobility of 1, which flaps between two limit boat conformations with the sulfinyl group in pseudoequatorial and pseudoaxial positions, respectively. The conformational equilibrium of 1 occurs too fast for the (1)H NMR (500 MHz) time-scale even at -130 degrees C, and the equilibrium constant has not been determined. The apparent dipole moments of 1 in n-hexane and 1,4-dioxane and the (1)H NMR spectra of 1 and the model compounds cis- and trans-thianthrene-5,10-dioxides (2) and thianthrene (5) in different solvents and at various temperatures confirm that the relative position of the conformational equilibrium of 1 is solvent-dependent, and more polar solvents favor the conformation with the sulfoxide group in the pseudoaxial position (1(')(ax)). Variable-temperature (1)H NMR spectra have established the interconversion barrier of trans-2 and confirmed that the conformational equilibrium of cis-2 is strongly displaced toward the conformation with both sulfinyl groups in the pseudoequatorial position. The (1)H NMR data support the transannular interaction of the functional groups in 1 and trans-2. 相似文献
47.
This paper reports the development of a method based on capillary electrophoresis with laser-induced fluorescence detection for the simultaneous determination of thiouracil (TU) and phenylthiouracil (PhTU) with high sensitivity (nanomolar range, i.e., attomoles detected). After derivatization with 5-iodoacetamidofluorescein, the analytes were separated by capillary zone electrophoresis using 20 mM phosphate buffer (pH 10.0) and quantified by fluorescence detection. The linearity range, precision, recovery, and detection limits were determined, and the method was shown to be applicable for the determination of TU and PhTU in spiked feed samples and urine. 相似文献
48.
The rotational potential around the interannular bond in 2,2-bipyrrole has been calculated making use of standard minimal STO-3G and split valence 4-31G basis sets. Geometrical optimization concerning the most significant interannular internal parameters has been performed with both basis sets. The trans conformer is predicted to be more stable than the cis. The minimal basis set predicts the existence of a cisoid-gauche minimum which after limited optimization becomes very shallow and it seems to be an artifact of the rigid rotor approximation. At 4-31G level, both the trans and cis conformers represent maxima in the potential curve and two gauche minima appear at =46.0° and =147.6°, the latter being the absolute minimum. The absolute maximum of the potential curve corresponds to the cis conformer. 相似文献
49.
50.
Campomanes P Menéndez MI Cárdenas-Jirón GI Sordo TL 《The journal of physical chemistry. A》2005,109(34):7822-7831
A theoretical analysis of the silver-induced ring expansion of N-chloro-N-methyl-1-hydroxycyclopropylamine to form N-methyl-2-azetidinone, and of the Cl(-) elimination from this substrate without Ag(+) assistance, was performed using the B3LYP method and the 6-31+G(d) basis set for C, N, O, H, and Cl atoms and the relativistic effective core pseudopotential LANL2DZ complemented with one set of f polarization functions (zeta(f) = 0.473) for the Ag atom. The partial Ag(+)-assisted extrusion of Cl(-) at the rate-determining transition state provokes an important change in the nodal properties of the frontier molecular orbitals of the H(3)CClNCOHAg(+) fragment, thus making very stabilizing HOMO-LUMO interactions between this fragment and the C(2)H(4) moiety possible. This interaction leads to the ring opening and release of most of the strain energy, giving rise to a low energy barrier for the process. Also, by assisting the Cl(-) extrusion, Ag(+) avoids the elimination of the hydroxyl hydrogen atom, which would provoke the fragmentation of the system instead of the formation of the beta-lactam. 相似文献