全文获取类型
收费全文 | 2207篇 |
免费 | 171篇 |
国内免费 | 10篇 |
专业分类
化学 | 1621篇 |
晶体学 | 29篇 |
力学 | 67篇 |
数学 | 258篇 |
物理学 | 413篇 |
出版年
2023年 | 14篇 |
2022年 | 24篇 |
2021年 | 79篇 |
2020年 | 66篇 |
2019年 | 96篇 |
2018年 | 77篇 |
2017年 | 56篇 |
2016年 | 111篇 |
2015年 | 80篇 |
2014年 | 98篇 |
2013年 | 184篇 |
2012年 | 125篇 |
2011年 | 127篇 |
2010年 | 101篇 |
2009年 | 99篇 |
2008年 | 112篇 |
2007年 | 122篇 |
2006年 | 100篇 |
2005年 | 72篇 |
2004年 | 72篇 |
2003年 | 64篇 |
2002年 | 62篇 |
2001年 | 52篇 |
2000年 | 39篇 |
1999年 | 29篇 |
1998年 | 11篇 |
1997年 | 18篇 |
1996年 | 19篇 |
1995年 | 14篇 |
1994年 | 10篇 |
1993年 | 6篇 |
1992年 | 7篇 |
1991年 | 9篇 |
1990年 | 10篇 |
1988年 | 8篇 |
1987年 | 10篇 |
1986年 | 5篇 |
1985年 | 10篇 |
1984年 | 23篇 |
1983年 | 10篇 |
1981年 | 13篇 |
1980年 | 17篇 |
1979年 | 9篇 |
1978年 | 11篇 |
1977年 | 6篇 |
1976年 | 9篇 |
1975年 | 7篇 |
1973年 | 6篇 |
1971年 | 6篇 |
1862年 | 8篇 |
排序方式: 共有2388条查询结果,搜索用时 484 毫秒
991.
E. D. Totchasov M. Yu. Nikiforov G. A. Al’per 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2007,81(7):1088-1091
A method for calculating the limiting activity coefficients of liquid nonelectrolytes in binary solvents based on the associated solution + lattice model is suggested. The method can be used to calculate limiting activity coefficients in mixed solvents with the use of only molecular association parameters and mutual exchange energies ωij determined from the data on phase equilibria in mixed solvents. 相似文献
992.
Jose M. Alía Howell G. M. Edwards Yolanda Díaz de Mera Emma E. Lawson 《Journal of solution chemistry》1997,26(5):497-526
Solutions of silver trifluoromethanesulfonate (“triflate”) and silver perchlorate in acrylonitrile, over a range of concentrations
between 0.5 and 4 mol.kg-1 have been analyzed by dispersive infrared and Fourier-transform Raman spectroscopy. The spectral regions studied include
the solvent v(C≡N) fundamental and several anion internal modes. The silver ion solvation number in the infinite dilution
limit is 3 in both systems. This number falls when the salt concentration is increased because of the ionic pairing, which
is slightly more intense for silver perchlorate solutions, with a calculated spectroscopic association constant 0.23 ±0.08
kg-mol-1. For silver triflate solutions, the spectroscopic association constant is 0.17±0.05 kg-mol-1. In both cases, the ionic pair structure is bidentate and silver ion preserves two molecules of acrylonitrile in its first
solvation shell. 相似文献
993.
First order solvolysis rates of the trans-dichlorobis (N-methylethylenediamine) cobalt(III) ion have been measured over a wide range of solvent compositions and temperatures in water–propan-2-ol and water–acetonitrile mixtures. The rate of solvolysis is faster in the former mixtures rather than the latter. Plots of log(rate constant) versus the reciprocal of the dielectric constant of the co-solvent, and also versus the Grunwald–Winstein Y-values are non-linear for both co-solvents; this non-linearity is derived from a large differential effect of solvent structure between the initial and transition states. However, extrema in the variation of enthalpy H and entropy S of activation correlate well with the extrema in physical properties of the mixtures which are related to changes in solvent structure. Linear plots of H versus S were obtained and the isokinetic temperature indicates that the reaction is entropy controlled. The application of a free-energy cycle shows that changes in solvent structure affect the pentacoordinated cobalt(III) ion in the transition state more than the hexacoordinated cobalt(III) ion in the initial state. In addition, the stabilizing influence of changes in solvent structure is greater in propan-2-ol–water mixtures than in acetonitrile–water mixtures, and the difference becomes greater as the mole fraction, x2 of the organic co-solvent increases. 相似文献
994.
Ionic liquids are salts that are liquid at or near room temperature. Their wide liquid range, good thermal stability, and very low vapor pressure make them attractive for numerous applications. The general approach to creating ionic liquids is to employ a large, unreactive, low symmetry cation with and an anion that largely controls the physical and chemical properties. The most common cations used in ionic liquids are N-alkylpyridinium and N,N′-dialkylimidazolium. Another very effective cation for the creation of ionic liquids is tetraalkylphosphonium, [PR1R2R3R4]+. The alkyl groups, Rn, generally are large and not all the same. The halide salts of several phosphonium cations are available as starting materials for metathesis reactions used to prepare ionic liquids. The large phosphonium cations can combine with relatively large anions to make viscous but free flowing liquids with formula mass greater than 1000 g mol−1. Some other more massive salts are waxes and glasses. The synthesis and the physical, chemical, and optical properties of phosphonium-ionic liquids having anions with a wide range of masses were measured and are reported here. 相似文献
995.
The synthesis of a certain series of s-triazoles and fused s-triazoles namely: 3-(3,4-dimethoxyphenyl)-4-arylideneamino-5-mercapto-s-triazoles 3.8 , 3-(3,4-dimethoxyphenyl)-6-substituted-s-triazolo[3,4-b]-1,3,4-thiadiazoles 11-17 , 3-(3,4-dimethoxyphenyl)-6-substituted thio-s-triazolo[3,4-b]-1,3,4-thiadiazoles 19, 20 and 3-(3,4-dimethoxyphenyl)-6-substituted-7H-s-triazolo[3,4-b]-1,3,4-thiadiazines 21, 22 , is described. 相似文献
996.
3‐Amino‐4‐aryl‐5‐ethoxycarbonyl‐6‐methylthieno[2,3‐b]pyridine‐2‐carboxamides 3a‐c were prepared from ethyl 4‐aryl‐3‐cyano‐6‐methyl‐2‐thioxo‐1,2‐dihydropyridine‐5‐carbonylates 1a‐c and reacted with some carbonyl compounds to give tetrahydropyridothienopyrimidine derivatives 6a‐c, 7a‐c and 8a‐c , respectively. Treatment of compound 3c with chloroacetyl chloride led to the formation of a next key compound, ethyl 2‐chloromethyl‐4‐oxo‐3,4‐dihydropyrido[3′,2′:4,5]thieno[3,2‐d]pyrimidine‐8‐carboxylate 9 . Also, 3‐amino‐2‐benzimidazolylthieno[2,3‐b]pyridine‐5‐carboxylate 5 and 2‐(3′‐aminothieno [2,3‐b]pyridin‐2′‐yl)‐4‐oxo‐3,4‐dihydropyrido[3′,2′:4,5]thieno[3,2‐d]pyrimidine‐8‐carboxylate 17 were prepared from 1c. The compounds 5, 9 and 17 were used as good synthons for other pyridothienopyrimidines and pyridothienopyrimidobenzimidazoles as well as for related fused polyheterocyclic systems. 相似文献
997.
Gehan Moustafa El‐Subruiti 《国际化学动力学杂志》2002,34(8):495-499
The kinetics of solvolysis of trans‐dichlorobis(N‐methylethylenediamine)cobalt(III) complex have been investigated in aqua‐organic solvent media (0–60% (v/v) cosolvent) at 25 ≤ t°C ≤ 60, using n‐propanol and tert‐butyl alcohol as cosolvents. The first‐order rate constant increased nonlinearly with the reciprocal of the dielectric constant Ds?1, and xorg, reflecting the individuality of the cosolvents and thereby suggesting that the relative stabilities of the transition state and initial state were governed by the preferential solvation effect. The thermodynamic parameters (ΔH≠ and ΔS≠) were sensitive to the structural changes in the bulk solvent phase. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 495–499, 2002 相似文献
998.
In the present work, three polymeric surfactants were prepared and used as demulsifiers; polyalkyl phenol formaldehyde monoethanol amine ethoxylate, eo, 136(D1), polyalkyl phenol formaldehyde diethanol amine ethoxylate, eo, 37(D2) and polyalkyl phenol formaldehyde triethanol amine ethoxylate, eo, 21.5(D3). Their demulsification potency in breaking water‐in‐crude oil emulsions was investigated. In this respect, two naturally occurring Egyptian water‐in‐oil (w/o) emulsions, one of them was waxy and the other was asphaltenic, were used in order to study the demulsification power of these compounds. The data revealed that, the resolution of water from waxy crude emulsion was easier than asphaltenic crude emulsion. The demulsification efficiency increases with increasing demulsifier concentration, contact time and temperature. The interfacial tension (IFT) at the crude oil–water interface was measured, it was found that the concentration of demulsifiers required to cause a minimum IFT are always less than these indicating a maximum demulsification efficiency. All the results were discussed in relation to emulsifier chemical structure and crude oil composition. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
999.
Shengfu Yang Hong Wu Qiquan Luo Aula M. Al Hindawi Berlian Sitorus Andrew M. Ellis Jinlong Yang 《Chemical science》2020,11(32):8502
We report that Au atoms within van der Waals complexes serve as catalysts for the first time. This was observed in ionization-induced chemistry of 1,6-hexanediol–Au and 1,8-octanediol–Au complexes formed in superfluid helium nanodroplets, where the addition of Au atom(s) made C2H4+ the sole prominent product in dissociative reactions. Density functional theory (DFT) calculations showed that the Au atom significantly strengthens all of the C–C bonds and weakens the C–O bonds in the meantime, making the C–C bonds stronger than the two C–O bonds in the ionized complexes. This leads to a preferential cleavage of the C–O bonds and thus a strong catalytic effect of the Au atoms in the reactions.Single Au atoms within van der Waals complexes are found to serve as catalysts in ionisation-induced chemistry for the first time. 相似文献
1000.
Sk. Asrof Ali Hasan A. Al‐Muallem Mohamed I. M. Wazeer 《Journal of polymer science. Part A, Polymer chemistry》2002,40(14):2464-2477
The amine salt N,N‐diallyl‐N‐5‐carbomethoxypentylammonium chloride was copolymerized with sulfur dioxide in dimethyl sulfoxide with ammonium persulfate or 2,2′‐azobisisobutyronitrile and afforded a cationic polyelectrolyte (CPE) with a five‐membered cyclic structure on the polymeric backbone. The CPE, upon acidic hydrolysis of the pendent ester groups, gave a corresponding cationic acid salt (CAS) having the equivalent of chloride salt of 6‐N,N‐diallylammoniohexanoic acid as the monomeric unit. The CAS was converted into an anionic polyelectrolyte (APE) and a polybetaine (PB), having the monomeric unit equivalent of sodium 6‐N,N‐diallylaminohexanoate and 6‐N,N‐diallylammoniohexanoate, by treatment with 2 and 1 equiv of base, respectively. The solution properties of APE were investigated by potentiometric and viscometric techniques. The basicity constant of the amine functionality in APE was apparent and as such followed the modified Henderson–Hasselbalch equation; the protonation of the APE became more and more difficult as the degree of protonation of the whole macromolecule increased. The compositions and phase diagrams of the aqueous two‐phase systems of APE and poly(ethylene glycol) were studied. The PB was found to be insoluble in water, and this paves the way for the potential use of APE in aqueous two‐phase polymer systems for protein purification and its removal and recycling by conversion into PB. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2464–2477, 2002 相似文献