全文获取类型
收费全文 | 495篇 |
免费 | 7篇 |
国内免费 | 5篇 |
专业分类
化学 | 341篇 |
晶体学 | 1篇 |
力学 | 3篇 |
数学 | 122篇 |
物理学 | 40篇 |
出版年
2023年 | 3篇 |
2022年 | 5篇 |
2021年 | 2篇 |
2020年 | 4篇 |
2019年 | 2篇 |
2018年 | 11篇 |
2017年 | 5篇 |
2016年 | 16篇 |
2015年 | 15篇 |
2014年 | 8篇 |
2013年 | 35篇 |
2012年 | 14篇 |
2011年 | 25篇 |
2010年 | 14篇 |
2009年 | 23篇 |
2008年 | 21篇 |
2007年 | 32篇 |
2006年 | 24篇 |
2005年 | 23篇 |
2004年 | 36篇 |
2003年 | 18篇 |
2002年 | 26篇 |
2001年 | 15篇 |
2000年 | 13篇 |
1999年 | 6篇 |
1998年 | 9篇 |
1997年 | 4篇 |
1996年 | 7篇 |
1995年 | 3篇 |
1994年 | 7篇 |
1993年 | 5篇 |
1992年 | 6篇 |
1991年 | 10篇 |
1990年 | 3篇 |
1989年 | 6篇 |
1988年 | 2篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 8篇 |
1983年 | 2篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1977年 | 4篇 |
1954年 | 1篇 |
1948年 | 1篇 |
1940年 | 1篇 |
1937年 | 1篇 |
1929年 | 3篇 |
排序方式: 共有507条查询结果,搜索用时 15 毫秒
101.
This note concerns the f-parity subgraph problem, i.e., we are given an undirected graph G and a positive integer value function \({f : V(G) \rightarrow \mathbb{N}}\), and our goal is to find a spanning subgraph F of G with deg F ≤ f and minimizing the number of vertices x with \({\deg_F(x) \not\equiv f(x) \, {\rm mod} \, {2}}\) . First we prove a Gallai–Edmonds type structure theorem and some other known results on the f-parity subgraph problem, using an easy reduction to the matching problem. Then we use this reduction to investigate barriers and elementary graphs with respect to f-parity factors, where an elementary graph is a graph such that the union of f-parity factors form a connected spanning subgraph. 相似文献
102.
Kövér KE Batta G Fehér K 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2006,181(1):89-97
Here, we present a modified CPMG-HSQMBC experiment which is capable to reduce the detrimental phase twists in the "long-range" connectivity multiplets caused by proton-proton couplings. We demonstrate that concerted CPMG pulse trains applied on both nuclei in the starting CPMG-INEPT transfer step can considerably be improved by composite pi pulses that compensate for pulse imperfections and off-resonance effects. Experimental optimization of the interpulse delay within the CPMG cycle was found to be crucial in order to achieve the best possible "decoupling" of homonuclear coupling modulation. 相似文献
103.
104.
Z Mucsi M Porcs-Makkay G Simig IG Csizmadia B Volk 《The Journal of organic chemistry》2012,77(17):7282-7290
The routine prediction of the reactivity of a complex, multifunctional molecule is a challenging and time-consuming procedure. In the last step of the synthesis of the well-known drug substance tenidap, a nonexpected difference was observed between the reactivities of two closely related carbamate moieties, the N-ethoxycarbonyl and the N-phenoxycarbonyl group. A detailed kinetic study, necessitating a significant computational effort, is described in the present paper for this reaction step. On the other hand, the systems chemistry concept, by analyzing the details of the electronic structure and the connections between functional groups in a fast and simple way, is also able to answer this question using various "-icity" parameters (aromaticity, carbonylicity, olefinicity). The complete systems chemistry approach involves all these conjugativicity parameters, while its further simplified version is based on only one key parameter, which is carbonylicity in the present case. The above methods were compared in terms of their predictive power. The results show that the systems chemistry concept, even its one-parameter version, is applicable for the characterization of this challenging reactivity issue. 相似文献
105.
Tibor Csupsz Dniel Szücs Ferenc Krisztin Klmn Oldamur Hollczki Anik Fekete Dezs Szikra va Tth Imre Tth Gyula Tircs 《Molecules (Basel, Switzerland)》2022,27(2)
A new pyclen-3,9-diacetate derivative ligand (H23,9-OPC2A) was synthesized possessing an etheric O-atom opposite to the pyridine ring, to improve the dissociation kinetics of its Mn(II) complex (pyclen = 3,6,9,15-tetraazabicyclo(9.3.1)pentadeca-1(15),11,13-triene). The new ligand is less basic than the N-containing analogue (H23,9-PC2A) due to the non-protonable O-atom. In spite of its lower basicity, the conditional stability of the [Mn(3,9-OPC2A)] (pMn = −log(Mn(II)), cL = cMn(II) = 0.01 mM. pH = 7.4) remains unaffected (pMn = 8.69), compared to the [Mn(3,9-PC2A)] (pMn = 8.64). The [Mn(3,9-OPC2A)] possesses one water molecule, having a lower exchange rate with bulk solvents (kex298 = 5.3 ± 0.4 × 107 s−1) than [Mn(3,9-PC2A)] (kex298 = 1.26 × 108 s−1). These mild differences are rationalized by density-functional theory (DFT) calculations. The acid assisted dissociation of [Mn(3,9-OPC2A)] is considerably slower (k1 = 2.81 ± 0.07 M−1 s−1) than that of the complexes of diacetates or bisamides of various 12-membered macrocycles and the parent H23,9-PC2A. The [Mn(3,9-OPC2A)] is inert in rat/human serum as confirmed by 52Mn labeling (nM range), as well as by relaxometry (mM range). However, a 600-fold excess of EDTA (pH = 7.4) or a mixture of essential metal ions, propagated some transchelation/transmetalation in 7 days. The H23,9-OPC2A is labeled efficiently with 52Mn at elevated temperatures, yet at 37 °C the parent H23,9-PC2A performs slightly better. Ultimately, the H23,9-OPC2A shows advantageous features for further ligand designs for bifunctional chelators. 相似文献
106.
According to the classical Erd?s-Szekeres theorem, every sufficiently large set of points in general position in the plane contains a large subset in convex position. Parallel to the Erd?s-Hajnal problem in graph-Ramsey theory, we investigate how large such subsets must a configuration contain if it does not have any sub-configuration belonging to a fixed order type. 相似文献
107.
Du
an Uhrín Gyula Batta Victor J. Hruby Paul N. Barlow Katalin E. Kvr 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1998,130(2)
An enhanced version of the X(ω1) half-filtered TOCSY experiment for measurement of long-range heteronuclear coupling constants is proposed which yields high-quality spectra with substantially increased sensitivity and resolution. The modified method features gradient-enhanced X filtering sequences, broadband homonuclear decoupling duringt1, optional1JXHscaling in theF1domain, and gradient coherence selection in combination with the sensitivity-enhanced protocol for the TOCSY transfer. These modifications extend the applicability of the method—coupling constants can be measured accurately for natural abundance samples at low concentrations and for compounds yielding complex spectra. Computer-aided analysis of E.COSY-type multiplets is applied for the determination of heteronuclear long-range coupling constants. 相似文献
108.
Norbert Szoboszlai Andrea Réti Barna Budai Zsuzsa Szabó Judit Kralovánszky Gyula Záray 《Spectrochimica Acta Part B: Atomic Spectroscopy》2008
The elemental content of Cu, Fe and Zn in two human adenocarcinoma cell lines was investigated by total reflection X-ray fluorescence (TXRF) spectrometry. Cancer cells were sedimented directly to the quartz plates using a modified cytospin slide holder setup. Special glass stands and caps were also constructed to hold the quartz plates with the cells during the vapour-phase microwave assisted digestion. The method was validated by analysis of certified reference materials. The signal-to-noise ratio was optimized by washing the cells with different solutions. The technique was applied to the determination of Cu, Fe and Zn content of HT-29 and HCA-7 colorectal adenocarcinoma cell lines. Dry mass of the centrifuged cells were determined and the elemental analysis data reported for the two cell lines were referred either to cell numbers, to the total protein content or to the dry mass. 相似文献
109.
Gábor Bernáth Ferenc Fülöp Alajos Kálmán Gyula Argay Pál Sohár István Pelczer 《Tetrahedron》1984,40(18):3587-3593
The rapid, spontaneous epimerization occurring at the C(2) chirality centre of a new diastereomeric (r-4,c-2,c-5)-2-(p-nitrophenyl)-3-methyl-4,5-tetramethylenetetrahydro-1,3-oxazine led to the conclusion that the configuration at C(2) of the bicyclic 1,3-oxazines formed by the cyclization of alicyclic 1,3-aminoalcohols with aldehydes is determined by the dominant conformation of the product. The first X-ray diffraction evidence is given for the N-outside conformation of compounds of this type. 相似文献
110.
Zsuzsanna Cs. Gyarmati Pter Csoms Gbor Bernth Pauliina Valtamo Henri Kivel Karel D. Klika Kalevi Pihlaja Gyula Argay Alajos Klmn 《Journal of heterocyclic chemistry》2004,41(2):187-199
The reactions of ethyl 5‐oxotricyclo[4.3.1.13,8]undecane‐4‐carboxylate (2) with methyl‐substituted 2‐aminopyridines in polyphosphoric acid (PPA) gave two products, linearly‐condensed pyridopyrimidinones 4a‐c and 2‐pyridylcarboxamides 5a‐c , whereas the reactions with amino, hydroxy and nitro derivatives of 2‐aminopyridine furnished only linearly‐condensed pyridopyrimidinones (4g‐j). Use of a mixture of PPA and phosphorus oxychloride as solvent afforded both linearly‐ (4a‐c,e,f) and angularly‐condensed (6a–c,e,f) pyridopyrimidinones. In toluene, with p‐toluenesulfonic acid as catalyst, 2‐pyridylcarboxamides 5a‐f were obtained. In a mixture of PPA and phosphorus oxychloride at 80–120 °C, 5a‐f yielded angularly‐condensed pyridopyrimidinones 6a‐f. All the products exhibited characteristic features, as determined by NMR and electron ionization mass spectrometry and X‐ray crystallography. 相似文献