全文获取类型
收费全文 | 2549篇 |
免费 | 42篇 |
国内免费 | 13篇 |
专业分类
化学 | 1646篇 |
晶体学 | 13篇 |
力学 | 53篇 |
数学 | 416篇 |
物理学 | 476篇 |
出版年
2022年 | 21篇 |
2021年 | 42篇 |
2020年 | 28篇 |
2019年 | 28篇 |
2018年 | 25篇 |
2017年 | 24篇 |
2016年 | 43篇 |
2015年 | 33篇 |
2014年 | 41篇 |
2013年 | 127篇 |
2012年 | 113篇 |
2011年 | 134篇 |
2010年 | 75篇 |
2009年 | 72篇 |
2008年 | 118篇 |
2007年 | 119篇 |
2006年 | 129篇 |
2005年 | 138篇 |
2004年 | 98篇 |
2003年 | 94篇 |
2002年 | 90篇 |
2001年 | 37篇 |
2000年 | 37篇 |
1999年 | 23篇 |
1998年 | 24篇 |
1997年 | 25篇 |
1996年 | 30篇 |
1995年 | 34篇 |
1994年 | 29篇 |
1993年 | 29篇 |
1992年 | 39篇 |
1991年 | 28篇 |
1990年 | 18篇 |
1989年 | 21篇 |
1988年 | 28篇 |
1986年 | 20篇 |
1985年 | 48篇 |
1984年 | 45篇 |
1983年 | 34篇 |
1982年 | 47篇 |
1981年 | 42篇 |
1980年 | 50篇 |
1979年 | 29篇 |
1978年 | 39篇 |
1977年 | 35篇 |
1976年 | 27篇 |
1975年 | 30篇 |
1974年 | 24篇 |
1973年 | 23篇 |
1972年 | 16篇 |
排序方式: 共有2604条查询结果,搜索用时 0 毫秒
71.
The oxidation of [PtCl3(C2H4)]- by Cl2 in aqueous solution, to yield CH2ClCH2OH and [PtCl4]2-, has been shown to proceed through the following sequence of steps: [PtCl3(C2H4)] Cl2Cl [PtCl5(CH2CH2Cl)]2- [PtCl5(CH2CH2OH)]2- → [PtCl42- + CH2ClCH2OHEach of the steps and intermediates in this mechanistic sequence has been identified and characterized. 相似文献
72.
TNA, or threose nucleic acid, is capable of Watson-Crick base pairing with DNA, RNA, and TNA; coupled with its chemical simplicity, this suggests that TNA is a possible progenitor of RNA. As an initial step toward developing the molecular tools necessary to investigate the functional capabilities of TNA by in vitro selection, we have screened a variety of DNA polymerases for activity on a TNA template. We report that despite having a repeating unit that is one atom shorter than that of DNA, several polymerases showed surprisingly good ability to copy limited stretches of TNA. 相似文献
73.
MacLean EJ Harris KD Kariuki BM Kitchin SJ Tykwinski RR Swainson IP Dunitz JD 《Journal of the American Chemical Society》2003,125(47):14449-14451
The transformation of ammonium cyanate into urea, first studied over 170 years ago by W?hler and Liebig, has an important place in the history of chemistry. To understand the nature of this solid state reaction, knowledge of the crystal structure of ammonium cyanate is a prerequisite. Employing neutron powder diffraction, we demonstrate conclusively that, in the structure of ammonium cyanate, the NH(4)(+) cation forms N-H...N hydrogen bonds to four cyanate N atoms at alternate corners of a distorted cube, rather than our previously proposed alternative arrangement with N-H...O hydrogen bonds to cyanate O atoms at the other four corners. 相似文献
74.
A covalently linked porphyrin-quinone dyad crystallizes with two orientations of the quinone, extended away from (off) and cofacial with the porphyrin macrocycle (on), which induce different conformations of the macrocycle and model the recently proposed structural effect of a nearby residue on the heme prosthetic group of a nitric oxide synthase. 相似文献
75.
Jiong Jack Chen 《Tetrahedron letters》2003,44(17):3459-3462
Substituted anilines and vinyl sulfone undergo a facile double Michael addition to form substituted phenylthiomorpholine dioxide, catalyzed with AlCl3 or H3PO4. Scope and conditions were explored. 相似文献
76.
Wang YN Topol IA Collins JR Burt SK 《Journal of the American Chemical Society》2003,125(43):13265-13273
Phosphate hydrolysis by GTPases plays an important role as a molecular switch in signal transduction and as an initiator of many other biological processes. Despite the centrality of this ubiquitous reaction, the mechanism is still poorly understood. As a first step to understand the mechanisms of this process, the nonenzymatic hydrolysis of mono-phosphate and tri-phosphate esters were systematically studied in gas phase and aqueous solution using hybrid density functional methods. The dielectric effect of the environment on the energetics of these processes was also explored. Theoretical results show that for mono-phosphate ester, the dissociative pathway is much more favorable than the associative pathway. However, the reaction barriers for the dissociative and associative pathways of tri-phosphate hydrolysis are very close in aqueous solution, though the dissociative pathway is more favorable in the gas phase. High dielectric solvents, such as water, significantly lower the activation barrier of the associative pathway due to the greater solvation energy of the associative transition states than that of the reactant complex. By contrast, the barrier of the dissociative pathway, with respect to the gas phase, is less sensitive to the surrounding dielectric. In the associative hydrolysis pathway of the tri-phosphate ester, negative charge is transferred from the gamma-phosphate to beta-phosphate through the bridging ester oxygen and results in Pgamma-O bond dissociation. No analogous charge transfer was observed in the dissociative pathway, where Pgamma-O bond dissociation resulted from proton transfer from the gamma-phosphate to the bridge oxygen. Finally, the active participation of local water molecules can significantly lower the activation energy of the dissociative pathway for both mono-phosphate and tri-phosphate. 相似文献
77.
Angelo J. Amoroso Brian F. G. Johnson Jack Lewis Chi-Keung Li Catherine A. Morewood Paul R. Raithby Maria D. Vargas Wing-Tak Wong 《Journal of Cluster Science》1995,6(1):163-173
Reduction of the heptaosmium cluster [Os7(CO)21] With [Et4N][NH4) gives the cluster dianion [Os7(CO)20]2–,1, in high yield. The reaction of the dianion with [AuPR
3Cl] (R=Et or Ph) in the presence of TlPF6 forms [Os7((CO)20(AuPR
3)2] [R=Et (2a);R = Ph(2b)] in 80% yield, while the corresponding reaction with (Os(C6H6)(CH3CN)3]2+ gives [Os8(CO)20 (
6-C6H6)] (3) in reasonable yield (ca. 30%). The dianion,1, and the clusters2 and3 have been fully characterized by bout spectroscopic and crystallographic methods. The crystal structure of the [Ph4P]+ salt of1 shows that the metals in the anion adopt a capped octahedral geometry, with all twenty carbonyl ligands in terminal sites. The metal core geometry in2a is best described as a tricapped octahedron, and is based on the structure of the dianion1 with two adjacent octahedral faces capped by the Au atoms of the two AuPEt3 groups. In a similar fashion, the geometry of3 is related to that of1 with the addition of an Os(C6H6) unit capped to a triangular face, to give a bicapped octahedral framework. 相似文献
78.
Ifk
1 andk
2 are positive integers, the partitionP = (1,2,...,
n
) ofk
1+k
2 is said to be a Ramsey partition for the pairk
1,k
2 if for any sublistL ofP, either there is a sublist ofL which sums tok
1 or a sublist ofP –L which sums tok
2. Properties of Ramsey partitions are discussed. In particular it is shown that there is a unique Ramsey partition fork
1,k
2 having the smallest numbern of terms, and in this casen is one more than the sum of the quotients in the Euclidean algorithm fork
1 andk
2.An application of Ramsey partitions to the following fair division problem is also discussed: Suppose two persons are to divide a cake fairly in the ratiok
1k
2. This can be done trivially usingk
1+k
2-1 cuts. However, every Ramsey partition ofk
1+k
2 also yields a fair division algorithm. This method yields fewer cuts except whenk
1=1 andk
2=1, 2 or 4. 相似文献
79.
Jack Huet 《Tetrahedron》1978,34(16):2473-2479
The measure of relative stabilities of β-alcoxystyrene isomers I: C6H5-CHCH-OR shows that the trans compound is the most stable when RCH3 and C2H5 and the cis compound is the most stable when Ri-C3H7 and t-C4H9. The orientation of the OR group can be determined by RMN 13C. The stabilities of these molecules are discussed in terms of non bounded attractive interactions. This interpretation is confirmed by the measure of relative stabilities of α-methyl β-alcoxy (and acetoxy)-styrene isomers II: C6H5-C(CH3)CH-OR. (RCH3, C2H5 et COCH3). 相似文献
80.
[3-3H]-valine was efficiently synthesised from sodium α-ketoisovalerate. With a β-lactam negative mutant of C. acremonium, l-[1-14C-3-3H]-valine and dl-[1-14C-3-3H]-valine were independently incorporated into the Arnstein tripeptide dimer, i.e. Bis-δ-(l-α-aminodipyl)-l-cystinyl-bis-d-valine, with full retention of trieium at C-3 of the d-valine residue. This result strongly suggested retention of configuration at C-3 of valine when the tripeptide was biosynthesised, and further limited the number of possible mechanisms for the biosynthesis of penicillins. 相似文献