全文获取类型
收费全文 | 3271篇 |
免费 | 316篇 |
国内免费 | 144篇 |
专业分类
化学 | 1581篇 |
晶体学 | 19篇 |
力学 | 262篇 |
综合类 | 59篇 |
数学 | 979篇 |
物理学 | 831篇 |
出版年
2024年 | 6篇 |
2023年 | 53篇 |
2022年 | 277篇 |
2021年 | 237篇 |
2020年 | 126篇 |
2019年 | 81篇 |
2018年 | 63篇 |
2017年 | 107篇 |
2016年 | 133篇 |
2015年 | 76篇 |
2014年 | 125篇 |
2013年 | 187篇 |
2012年 | 169篇 |
2011年 | 195篇 |
2010年 | 135篇 |
2009年 | 197篇 |
2008年 | 153篇 |
2007年 | 202篇 |
2006年 | 150篇 |
2005年 | 132篇 |
2004年 | 130篇 |
2003年 | 117篇 |
2002年 | 95篇 |
2001年 | 66篇 |
2000年 | 69篇 |
1999年 | 77篇 |
1998年 | 45篇 |
1997年 | 38篇 |
1996年 | 34篇 |
1995年 | 35篇 |
1994年 | 29篇 |
1993年 | 32篇 |
1992年 | 19篇 |
1991年 | 23篇 |
1990年 | 14篇 |
1989年 | 13篇 |
1988年 | 12篇 |
1987年 | 15篇 |
1986年 | 15篇 |
1985年 | 9篇 |
1984年 | 6篇 |
1983年 | 4篇 |
1980年 | 3篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1977年 | 2篇 |
1976年 | 4篇 |
1975年 | 2篇 |
1959年 | 2篇 |
1957年 | 2篇 |
排序方式: 共有3731条查询结果,搜索用时 15 毫秒
51.
Molecular parameters and retention characteristics of unsubstituted polyaromatic hydrocarbons in HPLC 总被引:1,自引:0,他引:1
Summary The present research studies the possibility of using the correlation dependence between molecular parameters of unsubstituted
polyaromatic hydrocarbons (PAH) and their retention in reversed-phase liquid chromatography to optimize the conditions for
the separation and identification of unknown peaks on the chromatograms of multicomponent mixtures. A linear correlation equation,
that takes the number and environment of the carbon atom in the PAH molecule into account as well as the differences in the
specific interactions of isomeric molecules with polar eluent, has been proposed. The adequacy of the proposed PAH retention
model was verified by comparing the calculated retention values with the experimental data. The possibility of identifying
unsubstituted PAH according to the number of carbon atoms of various types and according to the values of the molecules lengths
(calculated on the basis of the retention of these substances under different eluent compositions) was exemplified by various
chromatographic systems (reversed phase-eluent-PAH molecules). 相似文献
52.
Synthesis of 6α-methyl-16α,17α-cyclohexano-19-norprogesterone from a 19-methyl-6-desmethyl precursor
After prolonged refluxing of 19-tosyloxy-16α,17α-cyclohexanopregn-5-en-3β-ol-20-one (3) with NaI in 2-propanol, the initially formed 19-iodo derivative (4) undergoes supraface migration of the CH2I group from the C(10) atom to the C(6) atom, probably through involvement of a homoallyl cation. The resulting 6β-iodomethyl-16α,17α-cyclohexano-19-norpregn-5(10)-en-3β-ol
(5) was transformed in three steps into 6α-methyl-16α,17α-cyclohexano-19-norprogesterone (6α-methyl-19-nor-D′
6-pentarane,8). The transformation of compound5 into the target product8 also gave a side product, a pentarane with aromatic ringA (10), which was isolated and characterized by spectroscopic methods.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1688–1691, September, 1997. 相似文献
53.
高锰酸盐指数的灰色GM(1,1)预测 总被引:1,自引:0,他引:1
以中山市岐江河近7年的高锰酸盐指数监测数据为基础,采用灰色系统理论中的预测方法,建立了岐江河高锰酸盐指数的灰色预测模型。用该模型预测岐江河2006年、2007年、2008年高锰酸盐指数的值分别为5.3,5.4和5.4。 相似文献
54.
The prediction of migration time of electroosmotic flow (EOF) marker was achieved by applying artificial neural networks (ANN) model based on principal component analysis (PCA) and standard normal distribution simulation to the input variables. The voltage of performance, the temperature in the capillary, the pH and the ionic strength of background electrolytes (BGE) were applied as the input variables to ANN. The range of the performance voltage studied was from 15 to 27 kV, and that of the temperature in the capillary was from 20 to 30 °C. For the pH values studied, the range was from 5.15 to 8.04. The range of the ionic strength investigated in this paper was from 0.040 to 0.097. The prediction abilities of ANN with different pre-processing procedure to the input variables were compared. Under the same performance conditions, the average prediction error of the migration time of the EOF marker was 5.46% with RSD = 1.76% according to 10 parallel runs of the optimized ANN structure by the proposed approach, and that of the 10 parallel predictions of the optimal ANN structure for the different performance conditions was 12.95% with RSD = 2.29% according to the proposed approach. The study showed that the proposed method could give better predicted results than other approaches discussed. 相似文献
55.
Preparation, 19F NMR Spectroscopic Evidence and Study of the Formation of Metal-Mixed Cluster Anions [(Mo6?nWnCl )F ]2?, n = 0?6 The complete system of metal-mixed octahedral cluster ions [(Mo6?nWnCl)F]2?, n = 0?6, is prepared by tempering Mo powder with WCl6 at 600°C. A mixture containing inclusively the geometric isomers (n = 2, 3, 4) all ten possible species is transferred into the tetra-n-butylammonium salts (TBA)2[(Mo6?nWnCl)F]. In the 19F nmr spectrum the 24 expected signals are observed, assigned on the basis of their chemical shifts, multiplicities and intensities, and confirmed by a 2D-19F-19F COSY spectrum. From the integrated intensities the distribution of the different components is derived revealing a non-statistical formation, in that isomers with Mo…?Mo or W…?W atoms in trans-positions in comparision to those with mixed Mo…?W axes are favoured, and that especially the homoleptic compounds Mo6 and W6 are present to an over-average extent. Evaluation of 19F chemical shifts reveals that F bound to W which is in antipodal position to Mo resonates at higher field compared to F bound to W in a W…?W arrangement, caused by an increased shielding, which is synonymous to a positive antipodal-effect by Mo. Vice versa F bound to Mo with an antipodal W resonates at lower field compared with F bound to Mo in an Mo…?Mo arrangement caused by an increased deshielding and synonymous a negative antipodal-effect by W. The chemical shifts, resulting from antipodal-effects, are different for the compounds within the [(Mo6?nWnCl)F]2? - system. The difference of the antipodal effect of successive substitution products results in characteristic values designated as antipodal shift constants, depending on the kind of substituents, which is valid for other cluster systems, too. 相似文献
56.
T. V. Magdesieva I. I. Kukhareva D. P. Krut'ko G. A. Artamkina K. P. Butin I. P. Beletskaya 《Russian Chemical Bulletin》1994,43(5):881-883
Interaction of the 5-cyclopentadienyldicarbonyliron(II) anion with pentafluorobenzoyl chloride affords C6F5COFe(CO)2Cp, CpFe(CO)2C6F4COFe(CO)2Cp, and CpFe(CO)2C6F4C6F4Fe(CO)2Cp. The reaction products are characterized by1H,13C and19F NMR spectra.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 935–937, May, 1994. 相似文献
57.
A new additive scheme is proposed for the precalculation of gas chromatographic retention indices of complex organic compounds. The principal feature of this approach is the absence of previously calculated I increments for any structural fragments or functional groups in the molecule. Instead, arithmetical operations involving I values of simpler structural analogues of target compounds are used directly. I precalculation for polychlorinated hydroxybiphenyls (839 congeners) on the HP-5 stationary phase was chosen as one of the most important applications of the method under discussion. Such a large number of congeners cannot be obtained as reference samples and their gas chromatographic (GC)-mass spectrometric (MS) identification should therefore be based currently on precalculated I values. 相似文献
58.
D. N. Kravtsov V. M. Pachevskaya A. S. Peregudov E. I. Fedin 《Russian Chemical Bulletin》1995,44(7):1311-1315
A number of compounds of the type oftrans-4-FC6H4Pt(PAr3)2SC6H4F-4, where Ar is a substituted phenyl group, have been prepared starting from the corresponding chlorides. By exchange reactions oftrans-4-FC6H4Pt[P(C6H4F-4)3]2SC6H4F-4 with the above-mentioned compounds or Ar3P,trans-4-FC6H4Pt[P(C6H4F-4)3][PAr3]SC6H4F-4 have been generated in solution. For the latter compounds, the effect of Ar3P oncis- andtrans-ligands has been studied by the19F NMR technique. It has been shown that thecis- andtrans-effects of Ar3P run parallel and are well described by pK
a
values and ionization potentials of the unshared electron pair in Ar3P, as well as by 0 constants of the aryl groups.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1359–1363, July. 1995. 相似文献
59.
Correlation of antioxidant depletion and mechanical performance during thermal degradation of an HTPB elastomer 总被引:1,自引:0,他引:1
M. Celina J.M. Skutnik Elliott S.T. Winters R.A. Assink L.M. Minier 《Polymer Degradation and Stability》2006,91(8):1870-1879
Thermal degradation studies of a stabilized HTPB based elastomer were conducted at temperatures from 50 °C to 110 °C. The concentration of extractable antioxidant (AO2246) in the polymer was quantified via AO extraction and a gas chromatography-based method using internal standards. The decrease in extractable AO levels as a function of time and temperature was evaluated and correlated with mechanical property changes. Most importantly, AO depletion features were found to be temperature dependent. At elevated temperatures (>80 °C) extractable AO levels decreased rapidly and faster than the concurrent loss in mechanical properties. While extractable AO concentrations decrease quickly, the material is able to maintain some useful mechanical properties, perhaps via non-extractable or grafted AO species formed during degradation providing additional protection. At lower aging temperatures extractable or free AO levels decreased more slowly than the mechanical properties. Therefore, for condition monitoring purposes a universal correlation between AO levels and aging state or material condition could not be established. Most importantly, however, loss of mechanical properties and oxidative degradation is observed at lower temperatures despite significant levels of free antioxidant in the material. The antioxidant appears to be limited in its effectiveness to completely prevent degradation reactions, or only fractions of the total AO available are actually involved in the inhibition process. 相似文献
60.
In isotachophoresis (ITP), the sample constituents migrate, depending on their concentrations in the loaded sample, either in fully developed zones or in the boundary layers between the zones of constituents of the corresponding effective mobilities. The latter (spike) migration mode is analytically beneficial in selective detections of trace analytes, especially, when appropriately chosen discrete spacers minimize detection interferences due to matrix constituents. To facilitate a search for suitable mixtures of discrete spacers, a two-step calculation procedure was developed in this work. Using a pool of discrete spacers consisting of 42 anionic and zwitterionic constituents, this procedure was shown effective in the anionic ITP separations performed at pH = 6.5-10.0. Besides the predictions of the migration orders, it was helpful in identifying the spacing constituents that could cause resolution problems due to an uncertainty with which pH of the leading electrolyte solution is known. The ionic mobility and pKa data, taken for the spacing constituents from the literature and the ones obtained from the ITP experiments carried out in this work, were used in the calculations performed in a context with the choice of spacers. Although the data obtained from the ITP experiments provided better results, small uncertainties with which they were acquired (attributable to fluctuations in the experimental conditions) set practical limits in the calculation based choice of multi-component mixtures of the spacing constituents. 相似文献