全文获取类型
收费全文 | 5008篇 |
免费 | 94篇 |
国内免费 | 29篇 |
专业分类
化学 | 2636篇 |
晶体学 | 49篇 |
力学 | 169篇 |
数学 | 931篇 |
物理学 | 1346篇 |
出版年
2020年 | 42篇 |
2019年 | 39篇 |
2018年 | 29篇 |
2017年 | 30篇 |
2016年 | 64篇 |
2015年 | 57篇 |
2014年 | 74篇 |
2013年 | 224篇 |
2012年 | 190篇 |
2011年 | 256篇 |
2010年 | 115篇 |
2009年 | 102篇 |
2008年 | 203篇 |
2007年 | 193篇 |
2006年 | 206篇 |
2005年 | 204篇 |
2004年 | 153篇 |
2003年 | 146篇 |
2002年 | 141篇 |
2001年 | 134篇 |
2000年 | 112篇 |
1999年 | 77篇 |
1998年 | 51篇 |
1997年 | 64篇 |
1996年 | 83篇 |
1995年 | 80篇 |
1994年 | 101篇 |
1993年 | 106篇 |
1992年 | 95篇 |
1991年 | 89篇 |
1990年 | 75篇 |
1989年 | 85篇 |
1988年 | 58篇 |
1987年 | 74篇 |
1986年 | 58篇 |
1985年 | 102篇 |
1984年 | 82篇 |
1983年 | 48篇 |
1982年 | 75篇 |
1981年 | 70篇 |
1980年 | 61篇 |
1979年 | 69篇 |
1978年 | 72篇 |
1977年 | 56篇 |
1976年 | 67篇 |
1975年 | 65篇 |
1974年 | 69篇 |
1973年 | 101篇 |
1972年 | 38篇 |
1971年 | 31篇 |
排序方式: 共有5131条查询结果,搜索用时 15 毫秒
151.
152.
153.
154.
P. Duane Walker Paul G. Mezey Gerald M. Maggiora Mark A. Johnson James D. Petke 《Journal of computational chemistry》1995,16(12):1474-1482
The shape group method (SGM) and the associated (a,b)-parameter maps provide a detailed shape characterization of molecular charge distributions. This method is applied to the study of the variations of shape and conjugation of conformers of 2-phenyl pyrimidine in their electronic ground state. Within the SGM framework, the method of (a,b)-parameter maps provides a concise, nonvisual, algorithmic technique for shape characterization of molecules with fixed nuclear geometries. Moreover, shape codes derived from the (a,b)-parameter maps afford a practical means for efficiently storing the shape properties of molecules in an electronic database. The shape codes of two or more charge distributions can be compared directly, and numerical measures of molecular shape similarity can be computed using a technique that is simple, fast, and inexpensive, especially in relation to direct, pairwise comparisons of electronic charge densities. The quantitative and automated nature of the method suggests applications in the field of computer-aided molecular design. In this study, the method is used for the first time to determine detailed numerical shape codes and shape similarity measures for a nontrivial conformational problem involving changes in energy and in conjugation. Numerical shape similarity measures of eight conformers of 2-phenyl pyrimidine are determined and correlated with variations in conformational energy and conjugation. The competing effects of steric repulsion and conjugation lead to important correlations between conformational energy and shape. © 1995 John Wiley & Sons, Inc. 相似文献
155.
Cheeseman M Feuillet FJ Johnson AL Bull SD 《Chemical communications (Cambridge, England)》2005,(18):2372-2374
A new way of combining chiral auxiliaries and substrate-directable reactions for asymmetric synthesis is described that employs a three-step sequence of aldol-cyclopropanation-retro-aldol reactions for the stereoselective synthesis of enantiopure cyclopropane carboxaldehydes. 相似文献
156.
M. Robbins P. Gibart D.W. Johnson R.C. Sherwood V.G. Lambrecht 《Journal of solid state chemistry》1974,9(2):170-172
Phase relationships between spinel and defect NiAs structures in the systems M1?xNixCr2S4 (where M = Mn, Fe, Co) were investigated. It was found that the spinel structure is stable between x = 0 and x = 0.3 when M = Mn or Fe. When M = Co the spinel is formed in the region x = 0 to x ~ 0.4. The apparent stabilization of the defect NiAs phase by Ni2+ may be related to the strong sixfold site preference of Ni2+. Curie temperatures of all three ferrimagnetic systems increases with increasing Ni2+ substitution. This is probably due to higher NiS covalency. 相似文献
157.
Contrary to an earlier report, lithiation of tris(trimethylsilyl)methane followed by carbonation does not yield the acid (Me3Si)3CCO2H, (I), if an alkali extraction step is included in the work up; the product is actually the acid (Me3Si)2CHCO2H, (II). The acid (I) can be obtained if the alkali extraction is omitted. Separate experiments have confirmed that (I) is rapidly converted into (II) by aqueous or methanolic base. 相似文献
158.
159.
U Dahmen C J D Hetherington V Radmilovic E Johnson S Q Xiao C P Luo 《Microscopy and microanalysis》2002,8(4):247-256
Twinning plays an important role in phase transformations and can have significant effects on microstructural evolution. Different roles of twinning in the development of microstructures during precipitation and phase transformations are reviewed and illustrated with examples from investigations by high-resolution electron microscopy, including the effect of multiple twinning on the development of Ge precipitates in Al-Ge and Ag-Ge alloys, the twin dissociation of grain boundaries in Au, the formation of hexagonal Si at twin intersections and the effect of twin boundaries on the equilibrium shape of Pb inclusions in Al. 相似文献
160.
Fatty acid amides are a class of compounds with newly discovered biological activity. The ion trap mass spectrometric characteristics of silylated fatty acid amides were examined. Silylation of primary fatty acid amides is required prior to gas chromatography owing to thermal instability of the underivatized compound. The trimethylsilylated amides do not yield a molecular ion under normal electron ionization conditions (70 eV). With methane as a chemical ionization gas, the [M+H]+ ion appears. The [M+H]+ ion also appears when the helium buffer gas pressure is increased in the ion trap. There are three fragments other than the [M+H]+ peak that are predominant in the ion trap mass spectra of these compounds. Two of the fragments have been reported previously, namely the m/z 59 and the [M-71]+ fragments. The fragment of m/z 72 was identified and is the result of a rearrangement. Isotopic labeling was used to confirm fragment identity and the composition of the rearrangement products. Fragmentation patterns were affected by the amide chain length and concentration. 相似文献