全文获取类型
收费全文 | 796篇 |
免费 | 31篇 |
国内免费 | 2篇 |
专业分类
化学 | 566篇 |
晶体学 | 2篇 |
力学 | 13篇 |
数学 | 73篇 |
物理学 | 175篇 |
出版年
2021年 | 5篇 |
2020年 | 16篇 |
2019年 | 11篇 |
2018年 | 9篇 |
2017年 | 12篇 |
2016年 | 19篇 |
2015年 | 19篇 |
2014年 | 19篇 |
2013年 | 35篇 |
2012年 | 38篇 |
2011年 | 49篇 |
2010年 | 21篇 |
2009年 | 10篇 |
2008年 | 44篇 |
2007年 | 60篇 |
2006年 | 44篇 |
2005年 | 37篇 |
2004年 | 21篇 |
2003年 | 21篇 |
2002年 | 24篇 |
2001年 | 15篇 |
2000年 | 22篇 |
1999年 | 9篇 |
1998年 | 6篇 |
1997年 | 6篇 |
1996年 | 12篇 |
1995年 | 13篇 |
1994年 | 14篇 |
1993年 | 6篇 |
1992年 | 11篇 |
1991年 | 14篇 |
1990年 | 5篇 |
1989年 | 7篇 |
1988年 | 6篇 |
1986年 | 5篇 |
1985年 | 11篇 |
1984年 | 9篇 |
1983年 | 6篇 |
1982年 | 11篇 |
1981年 | 9篇 |
1980年 | 10篇 |
1979年 | 7篇 |
1978年 | 6篇 |
1977年 | 14篇 |
1976年 | 7篇 |
1975年 | 15篇 |
1974年 | 7篇 |
1973年 | 9篇 |
1972年 | 6篇 |
1966年 | 3篇 |
排序方式: 共有829条查询结果,搜索用时 156 毫秒
21.
Ethylene tetramerization: a new route to produce 1-octene in exceptionally high selectivities 总被引:11,自引:0,他引:11
Bollmann A Blann K Dixon JT Hess FM Killian E Maumela H McGuinness DS Morgan DH Neveling A Otto S Overett M Slawin AM Wasserscheid P Kuhlmann S 《Journal of the American Chemical Society》2004,126(45):14712-14713
Linear alpha-olefins, such as 1-hexene and 1-octene, are important comonomers in the production of linear low-density polyethylene (LLDPE). The conventional method of producing 1-hexene and 1-octene is by oligomerization of ethylene, which yields a wide spectrum of linear alpha-olefins (LAOs). While there exists several processes for producing 1-hexene via ethylene trimerization, a similar route for the selective production of 1-octene has so far been elusive. We now, for the first time, report an unprecedented ethylene tetramerization reaction that produces 1-octene in selectivities exceeding 70%, using an aluminoxane-activated chromium/((R2)2P)2NR1 catalyst system. 相似文献
22.
E. Weigold S. Dey A.J. Dixon I.E. McCarthy P.J.O. Teubner 《Chemical physics letters》1976,41(1):21-24
Measurements are reported on the spectroscopy of methane using the symmetric (e,2e) technique at energies of 600 eV and 1200 eV. The angular correlations of the states with separation energies of 14.2 and 23.1 eV have been measured and compared with the orbital wavefunctions of Snyder and Basch and with some earlier data at 400eV. The angular correlation of the configuration interaction state at 31 eV shows that this state definetely results from the removal of an electron in the 2a1 orbital. Other structure at high separation energy is also identified with this orbital. Relative strengths of the It2 and 2a1 states are compared and found to be in agreement with the theory at 1200eV. 相似文献
23.
24.
Wilk A Grajkowski A Bull TE Dixon AM Freedberg DI Beaucage SL 《Journal of the American Chemical Society》2002,124(7):1180-1181
The determination of the absolute configuration of deoxyribonucleoside cyclic N-acylphosphoramidites at phosphorus toward the synthesis of P-stereodifined phosphorothioated oligodeoxyribonucleotides is easily accomplished with computer-assisted molecular modeling and M-GOESY NMR spectroscopy. Specifically, computer-modeling diasteromeric phosphoramidite 3 has identifed a proximal (2.55 A) through-space interaction between benzylic H-5 and sugar H-2' ', which can predictably be detected by M-GOESY NMR in SP-3 but not in RP-3 because of being too distant (5.85 A). Consistent with computer-assisted modeling predictions, M-GOESY NMR spectra of SP-3 and RP-3 revealed NOE signals generated from nuclei near the selectively excited H-2' ' that are common to both SP-3 and RP-3, namely those of H-2', H-4', H-3', and H-1'. In addition, a diagnostic NOE signal at 5.5 ppm (benzylic H-5) is, as predicted, only detected in SP-3 and thus provides an unequivocal assessment of the configuration of the diastereomer at phosphorus. M-GOESY NMR data also confirm that the condensation of deoxyribonucleoside cyclic N-acylphosphoramidites with base-activated nucleosidic or nucleotidic 5'-hydroxyls proceeds via a single nucleophilic event. 相似文献
25.
Theoretical determination of chromophores in the chromogenic effects of aromatic neurotoxicants 总被引:3,自引:0,他引:3
Zhan CG Dixon DA Sabri MI Kim MS Spencer PS 《Journal of the American Chemical Society》2002,124(11):2744-2752
We report the first computational study of the chromophores responsible for the chromogenic effects of aromatic neurotoxicants containing a 1,2-diacetyl moiety in their oxidation metabolites. A series of ab initio electronic structure calculations was performed on two representative aromatic compounds, 1,2-diacetylbenzene (1,2-DAB) and 1,2-diacetyl tetramethyl tetralin (1,2-DATT), the putative active metabolites of the neurotoxic aromatic hydrocarbon compounds 1,2-diethylbenzene (1,2-DEB) and acetyl ethyl tetramethyl tetralin (AETT), and on the products of their possible reactions with proteins that result in chromogenic effects. The electronic excitation energies determined by three different computational approaches were found to be consistent with each other. The calculated results are consistent with the conclusion/prediction that the chromogenic effects of 1,2-DAB (or 1,2-DEB) and 1,2-DATT (or AETT) could result from ninhydrin-like reactions, rather than the formation of pyrrole-like compounds. Our pK(a) calculations further indicate that the chromophore, i.e., the product of the ninhydrin-like reaction showing the blue color, is deprotonated in neutral aqueous solution. The corresponding protonated structure has a different color as it absorbs in the blue region of the visible spectrum, and its chromogenic contribution would be significant in solution at low pH. 相似文献
26.
In this paper, we examine three algorithms in the ABS family and consider their storage requirements on sparse band systems. It is shown that, when using the implicit Cholesky algorithm on a band matrix with band width 2q+1, onlyq additional vectors are required. Indeed, for any matrix with upper band widthq, onlyq additional vectors are needed. More generally, ifa
kj
0,j>k, then thejth row ofH
i
is effectively nonzero ifj>i>k. The arithmetic operations involved in solving a band matrix by this method are dominated by (1/2)n
2
q. Special results are obtained forq-band tridiagonal matrices and cyclic band matrices.The implicit Cholesky algorithm may require pivoting if the matrixA does not possess positive-definite principal minors, so two further algorithms were considered that do not require this property. When using the implicit QR algorithm, a matrix with band widthq needs at most 2q additional vectors. Similar results forq-band tridiagonal matrices and cyclic band matrices are obtained.For the symmetric Huang algorithm, a matrix with band widthq requiresq–1 additional vectors. The storage required forq-band tridiagonal matrices and cyclic band matrices are again analyzed.This work was undertaken during the visit of Dr. J. Abaffy to Hatfield Polytechnic, sponsored by SERC Grant No. GR/E-07760. 相似文献
27.
28.
29.
Tandem mass spectrometry is used to monitor the contamination of banknotes by cocaine. By introducing a series of banknotes into an instrument a distribution of contamination can be obtained. The distribution of samples arising from defendants where the banknotes have been in close proximity to cocaine should differ from the distribution from the general background population. Peak picking and integration is used to produce a series of intensity readings for a batch of banknotes. By visually inspecting these distribution, and applying a variety of chemometric methods (principal components analysis, cluster analysis and class modelling via Mahalanobis distance) it is possible to discriminate effectively between the two classes of distribution (7157 background notes and 4826 case notes alleged to be from drug dealers). By calculating the Mahalonobis distance over 100 bootstrap iterations, background samples were correctly classified 96.48% of the time, while case samples were correctly classified 89.37% of the time. 相似文献
30.
Maxwell A. Cairns Keith R. Dixon Martin A.R. Smith 《Journal of organometallic chemistry》1977,135(1):C33-C34
Syntheses of cis-[PtCl(CH2COCH3)(PEt3)2], cis-[PtCl(CH2NO2) (PEt3)2], and trans-[Pt(CCPh)2 (PEt3)2] are described. The procedure involves reaction of cis-[PtCl2(PEt3)2] with Ag2O and acidic CH bonds to precipitate AgC1 and generate a PtC bond. The method may represent a new general route to platinum—carbon bonds. 相似文献