全文获取类型
收费全文 | 1847篇 |
免费 | 169篇 |
国内免费 | 92篇 |
专业分类
化学 | 1957篇 |
晶体学 | 2篇 |
力学 | 11篇 |
综合类 | 7篇 |
数学 | 6篇 |
物理学 | 125篇 |
出版年
2024年 | 2篇 |
2023年 | 14篇 |
2022年 | 35篇 |
2021年 | 46篇 |
2020年 | 86篇 |
2019年 | 70篇 |
2018年 | 56篇 |
2017年 | 58篇 |
2016年 | 81篇 |
2015年 | 76篇 |
2014年 | 74篇 |
2013年 | 160篇 |
2012年 | 122篇 |
2011年 | 79篇 |
2010年 | 77篇 |
2009年 | 82篇 |
2008年 | 72篇 |
2007年 | 76篇 |
2006年 | 81篇 |
2005年 | 74篇 |
2004年 | 62篇 |
2003年 | 70篇 |
2002年 | 52篇 |
2001年 | 33篇 |
2000年 | 33篇 |
1999年 | 37篇 |
1998年 | 37篇 |
1997年 | 23篇 |
1996年 | 46篇 |
1995年 | 29篇 |
1994年 | 25篇 |
1993年 | 16篇 |
1992年 | 24篇 |
1991年 | 23篇 |
1990年 | 13篇 |
1989年 | 11篇 |
1988年 | 24篇 |
1987年 | 10篇 |
1986年 | 21篇 |
1985年 | 23篇 |
1984年 | 18篇 |
1983年 | 6篇 |
1982年 | 16篇 |
1981年 | 13篇 |
1980年 | 11篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1976年 | 1篇 |
1974年 | 2篇 |
1973年 | 1篇 |
排序方式: 共有2108条查询结果,搜索用时 625 毫秒
61.
Tomislav P. Živković 《Theoretical chemistry accounts》1988,73(2-3):207-219
A new method for the calculation of bond orders in alternant hydrocarbons is presented. The method requires a summation over the contributions of various superposition diagrams. Quantitatively, the method is almost as reliable as PPP, and due to its simplicity it can be used for fast and relatively accurate calculation of bond orders. In addition some simple rules are derived, which in many cases can be used to predict the signs of bond orders between nonbonded atoms.Dedicated to Professor J. Koutecký on the occasion of his 65th birthdayResearch supported by the Robert A. Welch Foundation of Houston, Texas 相似文献
62.
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
63.
Tricyclo[2.2.0.0]hexane: a new hypothetical molecule which should have only one inverted carbon atom
HF and MP2 calculations with the 6-31G** and 6-311G** basis sets and those at MP2/cc-pVTZ level were carried out for the hypothetical tricyclo[2.2.0.01,3]hexane. The results indicate that the molecule under study should have one carbon atom with highly unusual inverted configuration. The analysis of the vibrational frequencies of this molecule as well as the analysis of its plausible decomposition routes performed at the DFT level indicate that this unique molecule could be a plausible synthetic target. 相似文献
64.
Two different injectors, a split/splitless injector and a programmed temperature vaporizer (PTV) injector were investigated as the interface in on-line supercritical fluid extraction (SFE)-capillary gas chromatography (cGC). The parameters affecting the chromatographic peak shapes as well as the quantitative performance of the interfaces in on-line SFE-cGC were identified and studied. Particular attention was paid to the case where modified extraction fluids were used. Experiments were performed on two different samples. The first sample consisted of PAHs spiked on sand at different concentration levels. The other sample was a polymeric material. 相似文献
65.
L. Bardi C. Martini F. Opsi E. Bertolone S. Belviso G. Masoero M. Marzona F. Ajmone Marsan 《Journal of inclusion phenomena and macrocyclic chemistry》2007,57(1-4):439-444
In situ bioremediation of polycyclic aromatic hydrocarbons (PAH) polluted soils can be improved by the augmentation of degrading
microbial populations and by the increase of hydrocarbon bioavailability. β-cyclodextrin (β-CD) significantly accelerate the
induction of hydrocarbon biodegradation, but it is not still clear its effectiveness during final, slower stages of degradation.
Moreover, it is yet not known if the PAH uptake from plants is influenced by the presence of CD. A field study was carried
out by creating two plots (A and B). Diesel fuel was spread on the surface, and on plot B a commercial microbial consortium
and β-CD were spread. Soybean was seeded in both plots. Soil samples were withdrawn every 10 cm layers from 0 to 60 cm depth,
before fuel spreading, immediately after seeding and after soya harvesting. Chemical and microbial analyses were carried out
throughout the process to characterize the soil and to determine residual PAHs. Soybean seeds were analyzed for PAH content.
It was observed that β-CD induced a significant increase of PAH degradation rate. The microbial inoculum did not improve the
degradation; biodegradation activity was strong in superficial layers, and some PAH leaching was observed, that was reduced
by CD. The analysis of PAH in soyabeans revealed that an uptake of hydrocarbons occurred, and that it was more significant
in plot B. This suggests that the β-CD-enhanced bioremediation process can further be improved by phytoremediation, that could
also allow to simultaneously reach an additional profit from a non-food yield for biofuel production. 相似文献
66.
Summary Procedures for the quantitative analysis of industrial effluents which involve concentration by solvent extraction or the
purge-and-trap method are time-consuming, labor-intensive, and prone to error. Direct aqueous injection gas chromatography
using an electron-capture detector for the analysis of volatile halocarbons at the ppb level is in routine use in many laboratories.
We now discuss the development of a similar protocol for the analysis of volatile polar organics such as acetone, methyl ethyl
ketone, methyl isobutyl ketone, and tretrahydrofuran using a flame-ionization detector. 相似文献
67.
Fernando J. S. Oliveira Francisca P. De França 《Applied biochemistry and biotechnology》2005,122(1-3):593-603
A 23 full factorial experimental design was adopted to estimate the effects of three variables on the biodegradation of oil during
soil bioremediation: bioaugmentation seeding a mixed culture, addition of fertilizer or mineral media, and correction of initial
pH of the soil to 7.0. The tests were carried out in polyvinyl chloride reactors with 5.0 kg of crude oil-contaminated soil
at 14 g/kg. After screening the variables, soil bioremediation tests were conduced with varied C:N ratios, yielding an increase
in biodegradation of the oil heavy fraction from 24 to 65%, consumption of total n-paraffins, and a remarkable decrease in the concentration of residual polycyclic aromatic hydrocarbons of the soil. 相似文献
68.
J. N. Spencer S. K. Berger C. R. Powell B. D. Henning G. S. Furman W. M. Loffredo E. M. Rydberg R. A. Neubert C. E. Shoop D. N. Blauch 《Journal of solution chemistry》1981,10(7):501-509
Enthalpies of solution and of transfer of amides for the solvents chloroform (CHCl3), methylenechloride (CH2Cl2), carbontetrachloride (CCl4), cyclohexane (C6H12), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), and ethylacetate (EtOAc) have been used to isolate and quantify the solvation interactions of amides in chlorinated solvents. Specific interactions at the aminde carbonyl and N–H groups have been identified. An analysis of the transfer enthalpies of pyrrole and methylpyrrole from cyclohexane to the chlorinated solvents shows that specific interactions between the pyrroles and these solvents are similar in nature. A means of calculating differences in the transfers of different solutes between the same solvent pair is given.Work done at Lebanon Valley College. 相似文献
69.
C. W. Wright 《Journal of separation science》1984,7(2):83-88
Complex organic mixtures, such as coal liquefaction and oil shale products and by-products, are comprised of hundreds or thousands of individual components. State-of-the-art high resolution gas chromatography does not always provide sufficient resolution to allow accurate quantitation or identification of many compounds of interest. The concept of dual capillary column chromatography combines the different resolving characteristics of two capillary columns coated with different stationary phases into a single chromatographic run. In this approach, both columns are connected to the same injection port. Analysis of complex mixtures in this fashion can confirm the identification and quantitation of components on two columns of different polarity with little increased analysis time, can provide a means of obtaining quantitative data for individual components which are known to coelute on any one column, and can alert one to unknown coelution problems that would be undetected by gas chromatographic analysis on a single capillary column. Simultaneous dual column analysis was applied to three samples, the neutral polycyclic aromatic hydrocarbon (PAH) fraction of a Solvent Refined Coal-II (SRC-II) heavy distillate, the nitrogen-containing polycyclic aromatic compound (N-PAC) fraction of an SRC-II heavy distillate, and the basic fraction from a shale oil process water. Fused silica capillary columns coated with SE-54 and Durawax 3 were used for the analyses of the heavy distillate, while SE-54 an Carbowax 20M capillary columns were used for the analysis of the process water. 相似文献
70.
Summary An improved procedure for the determination of the less volatile air pollutants of recycled closed atmospheres by activated carbon-Soxhlet extraction is proposed. After total treatment of desorbate, PAH determination is then accomplished by RP-HPLC using both adsorptiometric and fluorimetric UV detection; phenolic compounds are determined by GC-MS.This technique is suitable for very low levels of PAH and phenolic compounds. 相似文献