全文获取类型
收费全文 | 24516篇 |
免费 | 580篇 |
国内免费 | 141篇 |
专业分类
化学 | 16743篇 |
晶体学 | 123篇 |
力学 | 577篇 |
数学 | 4339篇 |
物理学 | 3455篇 |
出版年
2022年 | 177篇 |
2021年 | 261篇 |
2020年 | 302篇 |
2019年 | 319篇 |
2018年 | 234篇 |
2017年 | 194篇 |
2016年 | 483篇 |
2015年 | 454篇 |
2014年 | 495篇 |
2013年 | 1305篇 |
2012年 | 1144篇 |
2011年 | 1433篇 |
2010年 | 725篇 |
2009年 | 709篇 |
2008年 | 1260篇 |
2007年 | 1282篇 |
2006年 | 1254篇 |
2005年 | 1213篇 |
2004年 | 1053篇 |
2003年 | 910篇 |
2002年 | 817篇 |
2001年 | 348篇 |
2000年 | 279篇 |
1999年 | 249篇 |
1998年 | 276篇 |
1997年 | 314篇 |
1996年 | 358篇 |
1995年 | 239篇 |
1994年 | 280篇 |
1993年 | 262篇 |
1992年 | 237篇 |
1991年 | 240篇 |
1990年 | 192篇 |
1989年 | 232篇 |
1988年 | 238篇 |
1987年 | 201篇 |
1986年 | 195篇 |
1985年 | 291篇 |
1984年 | 337篇 |
1983年 | 219篇 |
1982年 | 366篇 |
1981年 | 337篇 |
1980年 | 318篇 |
1979年 | 317篇 |
1978年 | 323篇 |
1977年 | 292篇 |
1976年 | 279篇 |
1975年 | 250篇 |
1974年 | 248篇 |
1973年 | 234篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
171.
Robert Piziak 《International Journal of Theoretical Physics》1992,31(5):871-879
We make a linkage with the ultimate generalization of Hilbert space, anR-module with an orthogonality relation, and certain constructs due to Foulis and Randall and other related structures suggested by Cattaneo, Franco, and Marino. 相似文献
172.
L.V. Natarajan Fredrick M. Stein Robert E. Blankenship Raymond Chang 《Chemical physics letters》1983,95(6):525-528
A study of the linear diehroism and fluorescence polarization of diphenylpolyenes (C6H5—(CH=CH)n—C6H5) with n = 1,2,3,4,6,8 in stretched polyethylene films shows that the polyenes orient in the anisotropic matrix with their transition dipole (emission and absorption, π → π*) aligned with the direction of stretch. The maximum dichroic ratio is observed for 1,6-diphenyl-1,3,5-hexatriene (DPH), with n = 3, whereas the values are substantially lower for n < 3 or n > 3. The high orientation of DPH and poor alignment of the higher polyenes may be accounted for in terms of the growing flexibility of the polyene chains with increasing chain length. The results justify the use of DPH as a convenient fluorescence probe for biological and synthetic membranes. 相似文献
173.
Minimal basis set (STO) molecular orbital and valence-bond calculations are reported for the3 B 1 and1 A 1 states of CH2. The open-shell molecular orbital calculations used the Roothaan formulation. The valence-bond calculations used the Prosser-Hagstrom biorthogonalisation technique to evaluate the cofactors required in using Löwdin's formulae. Optimisation of geometry and orbital exponents in the molecular orbital calculation on the3 B 1 state gave a geometry of RC-H=2.11 a.u. and H-C-H=123.2 °. The energy obtained was ?38.8355 a.u. The molecular orbital and valencebond calculations are compared. In the valence-bond calculations the variation with bond-length and bond-angle of the configuration energies was studied. Valence bond “build-up” studies are also reported. Valence-bond calculations using hybrid orbitals instead of natural atomic orbitals showed that the perfect-pairing approximation is not as good for CH2 as BeH2. The nature of the lone-pair and bonding orbitals is found to be significantly different between the3 B 1 and1 A 1 states. In the3 B 1 state the 2s and 2p orbitals are fairly equally mixed between both types of orbital. However in the1 A 1 state the bonding orbitals have mainly 2p character and the lone pair orbitals have mainly 2s character. As was found for H2O, the bonding hybrid orbitals do not follow the hydrogen nuclei as the bond angle varies but continue to point approximately in their equilibrium directions. 相似文献
174.
Robert G Brinson 《Tetrahedron letters》2004,45(32):6155-6158
The intramolecular keto-ene/cyclization tandem reaction of γ-N-allylamino ketones is an effective means of producing 1,3-oxazines. The reaction usually requires high temperatures and/or pressures. We discovered that N,N-diallyl amines undergo the reaction at lower temperatures than their monoallyl analogs. An extreme example, 1-N,N-diallylamino-9,10-anthraquinone, undergoes the keto-ene reaction near ambient temperature. In the case of 1-N,N′-dialkylaminoanthraquinones, electron deficient ene components can even be used, allowing the preparation of a broad spectrum of oxazines. Furthermore, the N-allyl-1,3-oxazine can be easily deallylated to produce a 1,3-oxazine that contains a secondary amine. 相似文献
175.
Werner Seebacher Reto Brun Robert Saf Robert Weis 《Monatshefte für Chemie / Chemical Monthly》2003,17(9):1411-1420
4-Aminobicyclo[2.2.2]octanones were converted to their N-oxides and to 4-aminobicyclo[2.2.2]octanes. Furthermore, the 6,7-bis-(4-methoxyphenyl) analogues were synthesized. All products were screened for their activities against Trypanosoma b. rhodesiense and Plasmodium falciparum. The pharmacological results were compared with those of formerly tested bicyclo[2.2.2]octanones and bicyclo[2.2.2]octanols. Structure-activity relationships are discussed. 相似文献
176.
Robert R Hudgins Fang Huang Gabriela Gramlich Werner M Nau 《Journal of the American Chemical Society》2002,124(4):556-564
A fluorescent amino acid derivative (Fmoc-DBO) has been synthesized, which contains 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) as a small, hydrophilic fluorophore with an extremely long fluorescence lifetime (325 ns in H2O and 505 ns in D2O under air). Polypeptides containing both the DBO residue and an efficient fluorescence quencher allow the measurement of rate constants for intramolecular end-to-end contact formation. Bimolecular quenching experiments indicated that Trp, Cys, Met, and Tyr are efficient quenchers of DBO (k(q) = 20, 5.1, 4.5, and 3.6 x 10(8) M(-1) x s(-1) in D2O), while the other amino acids are inefficient. The quenching by Trp, which was selected as an intrinsic quencher, is presumed to involve exciplex-induced deactivation. Flexible, structureless polypeptides, Trp-(Gly-Ser)n-DBO-NH2, were prepared by standard solid-phase synthesis, and the rates of contact formation were measured through the intramolecular fluorescence quenching of DBO by Trp with time-correlated single-photon counting, laser flash photolysis, and steady-state fluorometry. Rate constants of 4.1, 6.8, 4.9, 3.1, 2.0, and 1.1 x 10(7) s(-1) for n = 0, 1, 2, 4, 6, and 10 were obtained. Noteworthy was the relatively slow quenching for the shortest peptide (n = 0). The kinetic data are in agreement with recent transient absorption studies of triplet probes for related peptides, but the rate constants are significantly larger. In contrast to the flexible structureless Gly-Ser polypeptides, the polyproline Trp-Pro4-DBO-NH2 showed insignificant fluorescence quenching, suggesting that a high polypeptide flexibility and the possibility of probe-quencher contact is essential to induce quenching. Advantages of the new fluorescence-based method for measuring contact formation rates in biopolymers include high accuracy, fast time range (100 ps-1 micros), and the possibility to perform measurements in water under air. 相似文献
177.
Nucleophilic Ring Opening of Aryl α-Nitrocyclopropanecarboxylates with Sterically Protected but Electronically Effective Carbonyl and Nitro Group. A New Principle of α-Amino Acid Synthesis (2-Aminobutanoic Acid a4-Synthon) The readily available 2,4,6-tri(tert-butyl)-and 2,6-di(tert-butyl)-4-methoxypahenol esters 2 of α-nitrocyclo-propanecarboxaylic acid ring opening with C-, N-, O-, and S-nucleophiles (cyanide, malonate, azide, anilines, alkoxides, phenoxides, thiolates) in DMF or alcohol solvents (80–95% yield). The products 6 – 14 are 2-nitrobutanoates with the newly introduced substituent in the 4-position. Reduction of the NO2 group with Zn/AcOH/Ac2O gives N-acetyl-α-amino acid esters 16 – 22 (40–90% yield). Subsequent oxidative cleavage (H2O2/HCOOH) of The p-methoxy-phenyl esters 18 and 20 produces free amino acids (65% 23 and 67% 24 , respectively). Thus, the nitro ester 2 corresponds to a 2-aminobutanoic-acid a4-synthon, it is a ‘homo-Michael acceptor’ producing γ-substituted α-amino acids. 相似文献
178.
The ab initio energies, nuclear and electron repulsions and charge distributions have been calculated using moderately large basis sets as a function of the RC N angle (R NH2, NF2 or PF2). The optimum RC N angles were calculated to be 178.9°, 176.6°, and 175° for NH2CN, NF2CN, and PF2CN, respectively. A rationalization of the differing bends is presented in terms of nuclear-nuclear and electron-electron repulsions. 相似文献
179.
Acylation of diene Fe(CO)3 complexes using the Perrier complexes RCOCl/AlCl3 in methylene chloride at 0°C gives dienone complexes in high yield. Substitution occurs only at unsubstituted terminal carbons of the diene unit. Quenching the reaction mixtures in cold aqueous ammonia gives cis dienone complexes only. Trans dienone complexes are prepared by subsequent isomerization in methanolic sodium methoxide. Formylation of diene Fe(CO)3 complexes proceeds in modest yield using dichloromethylmethyleter/AlCl3 in methylene chloride to give trans-dienal complexes. Reduction of the dienone and dienal complexes as well as those of dienols and dienoic esters with 4 : 1 AlCl3/LiAlH4 results in complete removal of the oxygen function to give trans-diene complexes in good yield. 相似文献
180.
Electroosmotic flow in channels with step changes in zeta potential and cross section 总被引:2,自引:0,他引:2
We consider the effects that step changes in zeta potential and cross section have on electroosmosis in long-and-narrow channels with arbitrary cross-sectional shapes. The Stokes equation of flow is solved analytically utilizing the thin Debye layer approximation to provide effective slip velocities on the channel walls. The effects of channel dimensions, surface potentials, applied pressure drop, and applied voltage are discussed. One anecdotal case, a two-region rectangular channel, is presented to illustrate the solution. The flow in each region is a combination of a uniform electroosmotic flow and a nonuniform pressure-driven flow. The electroosmotic pumping causes the pressure gradient in each region to adjust so that the flow rate is the same in each region and the overall applied pressure drop is met, resulting in convex velocity profiles in some regions and concave velocity profiles in other regions. By appropriate choice of the applied pressure drop, flat velocity profiles may be achieved in one or more regions. 相似文献