全文获取类型
收费全文 | 8552篇 |
免费 | 746篇 |
国内免费 | 1039篇 |
专业分类
化学 | 5146篇 |
晶体学 | 186篇 |
力学 | 69篇 |
综合类 | 44篇 |
数学 | 189篇 |
物理学 | 4703篇 |
出版年
2023年 | 25篇 |
2022年 | 92篇 |
2021年 | 91篇 |
2020年 | 89篇 |
2019年 | 110篇 |
2018年 | 158篇 |
2017年 | 162篇 |
2016年 | 199篇 |
2015年 | 162篇 |
2014年 | 236篇 |
2013年 | 756篇 |
2012年 | 379篇 |
2011年 | 401篇 |
2010年 | 327篇 |
2009年 | 516篇 |
2008年 | 499篇 |
2007年 | 557篇 |
2006年 | 531篇 |
2005年 | 457篇 |
2004年 | 418篇 |
2003年 | 410篇 |
2002年 | 372篇 |
2001年 | 340篇 |
2000年 | 325篇 |
1999年 | 321篇 |
1998年 | 270篇 |
1997年 | 237篇 |
1996年 | 249篇 |
1995年 | 260篇 |
1994年 | 220篇 |
1993年 | 176篇 |
1992年 | 235篇 |
1991年 | 129篇 |
1990年 | 85篇 |
1989年 | 79篇 |
1988年 | 85篇 |
1987年 | 58篇 |
1986年 | 40篇 |
1985年 | 36篇 |
1984年 | 36篇 |
1983年 | 28篇 |
1982年 | 30篇 |
1981年 | 19篇 |
1980年 | 23篇 |
1979年 | 24篇 |
1978年 | 12篇 |
1976年 | 18篇 |
1975年 | 13篇 |
1974年 | 13篇 |
1973年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
171.
A. A. Hasanein G. M. El-Subruiti G. O. Younes M. H. Srour 《Journal of solution chemistry》2004,33(12):1481-1499
The electronic absorption spectra of 2-aminopyrimidine (compound I), 2-amino-4-methylpyrimidine (compound II), 2-amino-4,6-dimethylpyrimidine (compound III), 2-amino-4,6-dimethoxypyrimidine (compound IV), 4-amino-2,6-dimethylpyrimidine (compound V), and 4,5-diamopyrimidine (compound VI) have been measured in water and in a series of different organic solvents. The solvent effects on the spectra are discussed and the solvent induced spectral shifts are analyzed in terms of different solute–solvent interaction mechanisms, using the multiple linear regression technique. 相似文献
172.
Yu. M. Evtushenko B. E. Zaitsev V. M. Ivanov K. M. Gitis 《Chemistry of Heterocyclic Compounds》2000,36(9):1054-1057
The reaction of maleic anhydride with 2-methylimidazole in acetonitrile and DMF is accompanied by the appearance of strong absorption bands in the visible part of the spectrum due to the formation of molecular complexes. In acetonitrile the reaction proceeds by two routes via the formation of an amide and the molecular complex. Phthalic anhydride reacts with 2-methylimidazole to give amide but not to form molecular complexes. 相似文献
173.
UV absorption spectra of acetyl fluoride-h3 and -d3 (CH3COF and CD3COF) molecules in the region of S1S0 electronic transitions are investigated. The origins (00
0 or 140±
0) of these transitions are observed at 39912 and 39904 cm–1, respectively, and some of the fundamental frequencies of these molecules in the s0 and S1 states are determined. In particular, the systems of torsion and inversion (out-of-plane C=O vibrations) energy levels are studied. The geometric parameters of an acetyl fluoride-h3 molecule in the S1 state are estimated by the theoretical simulation of the rotational contours of the 00
0 (140±
0) band. These data are used to evaluate the potential barriers to internal rotation in the S0 and S1 states, which were found to be 360 and 560 cm–1 for acetyl fluoride-h3 and 380 and 770 cm–1 for acetyl fluoride-d3, respectively, as well as the potential barriers to inversion in the S1 states, which were found to be 2090 and 2370 cm–1 for acetyl fluoride-h3 and acetyl fluoride-d3, respectively.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1957–1964, November, 1994.In conclusion, the authors would like to acknowledge T. S. Kuznetsova for the synthesis and purification of the AF samples.The research presented here was supported in part by the International Science Foundation under grant No. MJ 1000. 相似文献
174.
175.
M. V. Alfimov S. P. Gromov O. B. Stanislavskii E. N. Ushakov O. A. Fedorova 《Russian Chemical Bulletin》1993,42(8):1385-1389
Styryl dyes containing a crown ether group and a heteroaromatic moiety with a sulfoalkylN-substituent (1a,b) undergo photocyclodimerization in acetonitrile in the presence of Mg(ClO4)2 to give only the typeA isomer of cyclobutane derivative (2a,b). The photochemical regio- and stereoselectivity of the cycloaddition is explained by self-organization of thetrans-isomers of the styryl dyes upon complexation with the Mg2+ cations into dimers with a fixed mutual arrangement of multiple bonds.For part 7, see ref. 1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1449–1452, August, 1993. 相似文献
176.
V. P. Feshin M. Yu. Kon'shin A. V. Radushev E. V. Feshina V. Yu. Gusev A. E. Lesnov V. I. Karmanov 《Russian Chemical Bulletin》1996,45(11):2505-2508
The basicity of hydrazides of the highest aliphatic carboxylic acids RC(O)NHNH2 (R = CnH2n+1,n = 5-12) has been studied by potentiometric titration, and IR and1H NMR spectroscopy.Ab initio Hartree-Fork calculations using the 6–31G* basis set with full optimization of geometry were carried out on the simplest acy1hydrazines and their possible protonated forms. Based on these calculations, and the 1R and1H NMR spectra, the tautomerism of alkylhydrazides and the structures of their protonated forms are discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2645–2649, November, 1996. 相似文献
177.
Enrique J. Baran 《Monatshefte für Chemie / Chemical Monthly》1979,110(2):297-300
Mean amplitudes of vibration for various octahedral complex compounds of the typeLnCl6
3– andLnBr6
3– (withLn=Nd, Eu, Gd, Dy, Er, and Yb) have been calculated, at different temperatures, using known spectroscopic data. The results are briefly discussed and it is found that theLn–Cl andLn–Br amplitude values are highly characteristic. 相似文献
178.
Far-IR absorption and reflection spectra, as well as laser Mandelshtam–Brillouin and Raman scattering spectra of -glycine, β-alanine,
-histidine,
-tryptophane single crystals in the 0.2–400 cm−1 range were investigated. It was revealed that the far-IR and Raman spectra of the amino acids under study contain more bands than predicted by factor-group analysis, thus indicating a possible contribution of low-energy intramolecular vibrations and overtones, as well as an emergence of forbidden vibrations. Some of the low-frequency bands have never, to our knowledge, been detected previously. 相似文献
179.
Yu. V. Tomilov I. V. Kostyuchenko E. V. Shulishov O. M. Nefedov 《Russian Chemical Bulletin》1997,46(3):511-518
The reaction of diazocyclopropane generatedin situ with acrylonitrile or methyl acrylate to give 1∶1, 1∶2, and 2∶1 cycloadducts was carried out. The products resulting from
1,3-dipolar cycloaddition and subsequent isomeriation,viz., 3-cyano- and 3-methoxycarbonylspiro(2-pyrazoline-5,1′-cyclopropanes), isolated in the first step in ∼70% yield, react in
an alkaline solution with the above acrylates or diazocyclopropane as C(3)-nucleophiles to give the corresponding 3-(2′-cyanoethyl)-,
3-(2′-methoxycarbonylethyl)-, or 3-(cyclopropylazo)-1-pyrazolines. The thermal deazotization of these pyrazolines to spiropentane
derivatives was investigated.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 532–539, March, 1997. 相似文献
180.
In this paper, the nanosecond transient absorption spectra and the fluorescence spectraof Hypocrellin A(HA) are examined in solvents of varying polarity. There are three absorp-tion bands in dilute HA solutions: Ⅰa, Ⅱa are defined as the triplet-triplet absorption ofHA, the band Ⅲa is supposed to be the absorption of the solvent--sepatated ion pair (SSIP).In more concentrated solutions, only the absorption of SSIP was observed. The effects ofsolvent polarity on transient absorption spectra and fluorescence spectra of HA and the effectof oxygen on the intensities of the fluorescence of HA are discussed. Then a reasonablemechanism for the photolysis of HA is proposed. 相似文献