全文获取类型
收费全文 | 76033篇 |
免费 | 4546篇 |
国内免费 | 2005篇 |
专业分类
化学 | 42047篇 |
晶体学 | 624篇 |
力学 | 2968篇 |
综合类 | 79篇 |
数学 | 15819篇 |
物理学 | 21047篇 |
出版年
2023年 | 598篇 |
2022年 | 1036篇 |
2021年 | 1150篇 |
2020年 | 1225篇 |
2019年 | 1236篇 |
2018年 | 2455篇 |
2017年 | 2514篇 |
2016年 | 2722篇 |
2015年 | 2267篇 |
2014年 | 2450篇 |
2013年 | 3903篇 |
2012年 | 6506篇 |
2011年 | 5858篇 |
2010年 | 3983篇 |
2009年 | 3529篇 |
2008年 | 3360篇 |
2007年 | 3258篇 |
2006年 | 3040篇 |
2005年 | 6468篇 |
2004年 | 5572篇 |
2003年 | 3633篇 |
2002年 | 1729篇 |
2001年 | 1234篇 |
2000年 | 933篇 |
1999年 | 979篇 |
1998年 | 686篇 |
1997年 | 641篇 |
1996年 | 686篇 |
1995年 | 542篇 |
1994年 | 556篇 |
1993年 | 480篇 |
1992年 | 549篇 |
1991年 | 554篇 |
1990年 | 498篇 |
1989年 | 418篇 |
1988年 | 374篇 |
1987年 | 313篇 |
1986年 | 279篇 |
1985年 | 349篇 |
1984年 | 354篇 |
1983年 | 230篇 |
1982年 | 238篇 |
1981年 | 236篇 |
1980年 | 232篇 |
1979年 | 182篇 |
1978年 | 176篇 |
1977年 | 189篇 |
1976年 | 220篇 |
1975年 | 182篇 |
1973年 | 177篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The room-temperature phosphorescence (r.t.p.) and fluorescence spectra of benzoquinoline isomers are investigated. The isomers can be resolved into the linear or angular subgroups on the basis of their fluorescence and r.t.p. spectra by using conventional fixed excitation. Second-derivative and synchronous scanning techniques can be combined to improve the selectivity of the r.t.p. and fluorescence methods. These simple luminescence techniques were used to estimate three benzoquinoline isomers in a coal tar fraction. Direct analysis of this complex sample allowed acridine to be estimated and upper limits to be provided for benzo(h)quinoline and phenanthridine; the presence of three other isomers was not detected. Comparative studies with data obtained by high-performance liquid chromatography are reported. 相似文献
992.
An algorithm for identification of the IR-active substructures has been developed. In the algorithm, which recognizes about 100 substructures, the analysis is based on three parameters for a set of characteristic bands: position, intensity and arbitrarily chosen diagnostics. The algorithm provides two ways of identification: a. standard method, which compares an IR spectrum with the correlation tables contained in the algorithm, b. dynamic method advanced here which consists in that the recognized substructures cause blocking of their characteristic bands making them almost inaccessible for further analysis. 相似文献
993.
Borba A Gómez-Zavaglia A Simões PN Fausto R 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(7):1461-1470
The preferred conformations of dimethyl sulfate and their vibrational spectra were studied by matrix-isolation FT-IR spectroscopy and theoretical methods (DFT and MP2, with basis sets of different sizes, including the quadruple-zeta, aug-cc-pVQZ basis). Conformer GG (of C2 symmetry and exhibiting OSOC dihedral angles of 74.3 degrees ) was found to be the most stable conformer in both the gaseous phase and isolated in argon. Upon annealing of the matrix, the less stable observed conformer (GT; with C1 symmetry) quickly converts to the GG conformer, with the resulting species being embedded in a matrix-cage which corresponds to the most stable matrix-site for GG form. The highest energy TT conformer, which was assumed to be the most stable conformer in previous studies, is predicted by the calculations to have a relative energy of ca. 10 kJ mol-1 and was not observed in the spectra of the matrix-isolated compound. 相似文献
994.
Na-montmorillonite hydrates in presence of ethane molecules are studied by means of hybrid Monte Carlo simulations in the NP(zz)T and muP(zz)T ensembles. The NP(zz)T ensemble allows us to study the interlaminar distance as a function of water and ethane content. These data show clear plateaus for lower ethane contents and mainly for water contents consistent with the formation of a single water layer. In addition, from this ensemble the structure for some of these interlaminar compositions were analyzed. For systems containing few ethane molecules and water enough to complete a single layer, it was observed that ethane mainly situates close to the interlayer midplane and adopts a nearly parallel arrangement to the clay surface. On the other hand, the muP(zz)T ensemble allows us to determine the interlaminar distance and water-ethane content for any specific reservoir. Here, some important findings are the following: the partial exchange of water by ethane molecules that enhances for decreasing the water vapor pressure; the obtention of a practically constant interlaminar space distance as a function of the water vapor pressure; the conservation of ion solvation shells; the enhancement of the water-ethane exchange for burial conditions; and finally, the incapability for a dehydrated clay mineral to swell in a dry and rich ethane atmosphere. 相似文献
995.
Molecular dynamics simulation of a linear soft polymer has been performed and the free volume properties of the system have been analyzed in detail in terms of the Voronoi polyhedra of the monomers. It is found that there are only small density fluctuations present in the system. The local environment of the monomers is found to be rather spherical, even in comparison with liquids of atoms or small molecules. The monomers are found to be, on average, eight coordinated by their nearest geometric neighbors, including intra-chain and inter-chain ones. The packing of the monomers is found to be rather compact, in a configuration of 1900 monomers there are, on average, only three voids large enough to incorporate a spherical particle as large as a monomer, indicating that the density of the large vacancies in the system is considerably, i.e., by a few orders of magnitude lower than in molecular liquids corresponding to roughly the same reduced densities. 相似文献
996.
Zorica?Crnjak Orel Egon?Matijevi? Dan?V.?GoiaEmail author 《Colloid and polymer science》2003,281(8):754-759
Uniform, spherical CuCl particles were obtained by mixing aqueous solutions of CuCl 2 and ascorbic acid in the presence of polyvinylpyrrolidone (PVP) as dispersing agent. The size and the uniformity of the resulting particles depended on the volume ratio of the reactant solutions, their concentrations, the distribution of the stabilizers, and the mixing method. The single jet precipitation yielded large spheres of broad size distributions, while the particles obtained by the double jet technique were rather uniform in size. The final colloidal CuCl particles were formed by the aggregation of nanocrystals, initially generated in the system. Depending on the pH of the reaction mixture, these particles slowly change to large CuCl crystals on aging in the mother liquor. 相似文献
997.
998.
al-Maharik NI Kaltia SA Mutikainen I Wähälä K 《The Journal of organic chemistry》2000,65(8):2305-2308
Unique C-C-bridged bis-isoflavones 5, 8, and 9 were obtained by reaction of 2-bromomethyl-7,4'-dimethoxyisoflavone 4 with ethyl cyanoacetate anion or tetraethylammonium cyanide or by Pd-catalyzed ethoxycarbonylation, respectively. The phenolic carboxylic acid 7 is available from 5 in two steps. 相似文献
999.
Van T.H. Nguyen Joachim T. Anders Qingjun Ma Regine Herbst-Irmer Peter Langer 《Tetrahedron》2007,63(52):12975-12985
Acid-mediated ring-transformations of 5-alkylidene-2,5-dihydropyrrol-2-ones, available by cyclization of 1,3-diketone dianions with bis(imidoyl) dichlorides of oxalic acid, resulted in formation of functionalized pyran-4-ones, such as 5,6-dihydrobenzo[h]chromones and 5,6,7,8-tetrahydrochromones. 相似文献
1000.
A new method of group identification was established for selected aliphatic compounds giving homologous series, based upon constant ΔRM and directional coefficient “a” values (the “a” values being tangents of the angle between the course of the RM = f(nc) function and the “x” axis).The results presented concern identification of higher fatty acids and their ethyl esters, amides of higher fatty acids, higher aliphatic amines, ethylalkyl ketones, and dicarboxylic acids.Simultaneously, another method of identifying groups of aliphatic compounds was established, taking advantage of differentiated visualizing effects. 相似文献