全文获取类型
收费全文 | 992篇 |
免费 | 20篇 |
国内免费 | 8篇 |
专业分类
化学 | 667篇 |
晶体学 | 7篇 |
力学 | 18篇 |
数学 | 105篇 |
物理学 | 223篇 |
出版年
2022年 | 11篇 |
2020年 | 9篇 |
2019年 | 8篇 |
2018年 | 6篇 |
2016年 | 11篇 |
2015年 | 16篇 |
2014年 | 10篇 |
2013年 | 48篇 |
2012年 | 49篇 |
2011年 | 57篇 |
2010年 | 26篇 |
2009年 | 26篇 |
2008年 | 43篇 |
2007年 | 45篇 |
2006年 | 47篇 |
2005年 | 58篇 |
2004年 | 35篇 |
2003年 | 37篇 |
2002年 | 33篇 |
2001年 | 16篇 |
2000年 | 12篇 |
1999年 | 13篇 |
1998年 | 11篇 |
1997年 | 12篇 |
1996年 | 17篇 |
1995年 | 17篇 |
1994年 | 15篇 |
1993年 | 19篇 |
1992年 | 20篇 |
1991年 | 15篇 |
1990年 | 7篇 |
1989年 | 6篇 |
1988年 | 15篇 |
1987年 | 11篇 |
1986年 | 7篇 |
1985年 | 12篇 |
1984年 | 11篇 |
1983年 | 11篇 |
1981年 | 8篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1978年 | 9篇 |
1977年 | 12篇 |
1976年 | 7篇 |
1975年 | 13篇 |
1974年 | 12篇 |
1973年 | 13篇 |
1972年 | 6篇 |
1971年 | 10篇 |
1908年 | 8篇 |
排序方式: 共有1020条查询结果,搜索用时 0 毫秒
11.
12.
Burford N Cameron TS Clyburne JA Eichele K Robertson KN Sereda S Wasylishen RE Whitla WA 《Inorganic chemistry》1996,35(19):5460-5467
The phosphadiazonium cation [MesNP](+) reacts quantitatively with the fluorenylide anion, MesNH(2), and MesOH (Mes = 2,4,6-tri-tert-butylphenyl), resulting in formal insertion of the N-P moiety into the H-Y (Y = C, N, O) bonds. Specifically, reaction of MesNPCl with fluorenyllithium gives the aminofluorenylidenephosphine [crystal data: C(31)H(38)NP, monoclinic, P2(1)/c, a = 9.568(8) ?, b = 24.25(2) ?, c = 11.77(1) ?, beta = 101.38(8) degrees, Z = 4]. Similarly, reaction of [MesNP][GaCl(4)] with MesNH(2) gives the diaminophosphenium salt [MesN(H)PN(H)Mes][GaCl(4)] [crystal data: C(36)H(60)Cl(4)GaN(2)P, monoclinic, C2/c, a = 24.921(2) ?, b = 10.198(4) ?, c = 16.445(2) ?, beta = 93.32(1) degrees, Z = 4], and reaction with MesOH gives the first example of an aminooxyphosphenium salt [MesN(H)POMes][GaCl(4)]. It is proposed that the reactions involve nucleophilic attack at phosphorus followed by a 1,3-hydrogen migration from Y to N. Experimental evidence for the formation of sigma-complex intermediates is provided by the isolation of [MesNP-PPh(3)][SO(3)CF(3)] [crystal data: C(37)H(44)F(3)NO(3)P(2)S, triclinic, P&onemacr;, a = 10.663(1) ?, b = 19.439(1) ?, c = 10.502(1) ?, alpha = 103.100(7) degrees, beta = 113.311(7) degrees, gamma = 93.401(7) degrees, Z = 2]. As part of the unequivocal characterization of the aminooxyphosphenium salt, detailed solid-state (31)P NMR studies and GIAO calculations on the phosphenium cations have been performed. Contrary to popular belief, the phosphorus shielding in dicoordinate cations is not caused by the positive charge but results from efficient mixing between the phosphorus lone pair and pi orbitals. 相似文献
13.
D. D. MacNicol P. R. Mallinson A. Murphy C. D. Robertson 《Journal of inclusion phenomena and macrocyclic chemistry》1987,5(2):233-239
The title hexaphenol (1), a direct analogue of the -hydroquinone hexameric unit, has been prepared by six-fold demethylation of hexakis (p-methoxyphenyloxy)benzene (2) with BBr3. Host1 forms a trigonal adduct, space groupR3,a=22.088(3),c=12.232(3) Å, containing 6 molecules of pyridine per host molecule: a detailed X-ray study of this inclusion compound reveals a true clathrate structure, the closed cages of which accommodate a small, non-stoichiometric amount of water. 相似文献
14.
15.
Alexopoulos T Allen C Anderson EW Balamurali V Banerjee S Beery PD Bhat P Bishop JM Biswas NN Bujak A Carmony DD Carter T Choi Y Cole P DeBonte R DeCarlo V Erwin AR Findeisen C Goshaw AT Gutay LJ Hirsch AS Hojvat C Jennings JR Kenney VP Lindsey CS Loomis C LoSecco JM McMahon T McManus AP Morgan N Nelson K Oh SH Porile NT Reeves D Rimai A Robertson WJ Scharenberg RP Stampke SR Stringfellow BC Thompson M Turkot F Walker WD Wang CH Warchol J Wesson DK Zhan Y 《Physical review D: Particles and fields》1993,48(3):984-997
16.
Pierre de la Harpe A. Guyan Robertson Alain Valette 《Israel Journal of Mathematics》1993,81(1-2):65-96
Let Γ be a finitely generated group. In the group algebra ℂ[Γ], form the averageh of a finite setS of generators of Γ. Given a unitary representation π of Γ, we relate spectral properties of the operator π(h) to properties of Γ and π.
For the universal representationπ
un
of Γ, we prove in particular the following results. First, the spectrum Sp(π
un
(h)) contains the complex numberz of modulus one iff Sp(π
un
(h)) is invariant under multiplication byz, iff there exists a character
such that η(S)={z}. Second, forS
−1=S, the group Γ has Kazhdan’s property (T) if and only if 1 is isolated in Sp(π
un
(h)); in this case, the distance between 1 and other points of the spectrum gives a lower bound on the Kazhdan constants. Numerous
examples illustrate the results. 相似文献
17.
The S(1)<--S(0) transition of 1-phenylimidazole (1PI) has been studied in a supersonic jet expansion by resonant two-photon ionization. The origin band at 36 075 cm(-1) is accompanied by a low frequency progression associated with torsion about the bond connecting phenyl and imidazole groups. Torsional potentials have been determined for both states. In S(0), phi(min)=37.2+/-0.5 degrees and the planar barrier is 339+/-20 cm(-1), while in S(1), phi(min)=17.6+/-0.5 degrees and the planar barrier is 57+/-2 cm(-1). The transition moment alignment is observed to be consistent with an excited state of L(b) character, in spite of the "off-axis" conjugation provided by the imidazole ring. These results are compared with ab initio calculations on both states, performed using Hartree-Fock, M?ller-Plesset second-order perturbation, density functional theory with the Becke3-Lee-Yang-Parr functional, time-dependent density functional theory, configuration interaction singles, and complete active space self-consistent field methods. Solution-phase UV spectra of neutral and protonated 1PI are also reported. 相似文献
18.
The α,β:γ,δ-unsaturated tosylhydrazone lithium salts (4) undergo photolysis at ?60°C to give vinylcyclopropenes (10) and [l,2]diazeto[l,4-a] pyrroles (11) and/or (12). The formation of (11, X=H) establishes the intermediacy of (8, X=H) in the formation of the 3-1,2-diazepine (9). The diazoalkene (5, X=Me) showed opposite electrocyclisation periselectivity to its thermal cyclisation and gave (11, X=Me) (8, X=Me), rather than the pyrazoles (2) and (3). 相似文献
19.
Zhao S Prasad C Robertson HJ Liu YM 《Fresenius' Journal of Analytical Chemistry》2001,369(3-4):220-224
A capillary electrophoresis (CE) method with laser induced fluorescence (LIF) detection is described for quantification of enterostatin (Val-Pro-Asp-Pro-Arg), a pentapeptide involved in appetite regulation and insulin secretion. Enterostatin and two other pentapeptides belonging to the enterostatin family (i.e. Ala-Pro-Gly-Pro-Arg and Val-Pro-Gly-Pro-Arg) were well separated from each other. The peptides were fluorescently tagged with naphthalene-2,3- dicarboxaldehyde (NDA) and separated by micellar electrokinetic chromatography (MEKC) in the presence of methanol as an organic modifier. Coupled with LIF detection, the method had a detection limit of 4.8 x 10(-6) M for enterostatin. The relative standard deviation was to be 4.0% from five determinations of enterostatin at 37.2 microM in a human cerebrospinal fluid (CSF) sample. Twenty-three human CSF samples were analyzed. The level of enterostatin ranged from 24 microM to 51 microM with a mean (+/- SEM) value of 41.7 +/- 2.0 microM. 相似文献
20.
J. K. Haywood I. Robertson K. Boddy 《Journal of Radioanalytical and Nuclear Chemistry》1979,48(1-2):117-124
A 14 MeV neutron generator system with a new sealed tube (Philips 18604) was evaluated for total body in vivo neutron activation
analysis (TBIVNAA). The neutron output, angular distribution, rise time, reproducibility of short irradiations and leakage
of tritium were investigated. The 18604 tube meets all the requirements of TBIVNAA within its normal operating specification. 相似文献