全文获取类型
收费全文 | 77738篇 |
免费 | 521篇 |
国内免费 | 112篇 |
专业分类
化学 | 41299篇 |
晶体学 | 1262篇 |
力学 | 3280篇 |
综合类 | 1篇 |
数学 | 7838篇 |
物理学 | 24691篇 |
出版年
2022年 | 629篇 |
2021年 | 872篇 |
2020年 | 966篇 |
2019年 | 1103篇 |
2018年 | 1597篇 |
2017年 | 1687篇 |
2016年 | 2151篇 |
2015年 | 1023篇 |
2014年 | 1720篇 |
2013年 | 3273篇 |
2012年 | 2902篇 |
2011年 | 3277篇 |
2010年 | 2639篇 |
2009年 | 2642篇 |
2008年 | 3001篇 |
2007年 | 2872篇 |
2006年 | 2770篇 |
2005年 | 2435篇 |
2004年 | 2239篇 |
2003年 | 2073篇 |
2002年 | 2136篇 |
2001年 | 2234篇 |
2000年 | 1623篇 |
1999年 | 1252篇 |
1998年 | 1126篇 |
1997年 | 1037篇 |
1996年 | 891篇 |
1995年 | 831篇 |
1994年 | 811篇 |
1993年 | 812篇 |
1992年 | 864篇 |
1991年 | 933篇 |
1990年 | 877篇 |
1989年 | 905篇 |
1988年 | 816篇 |
1987年 | 767篇 |
1986年 | 739篇 |
1985年 | 864篇 |
1984年 | 961篇 |
1983年 | 818篇 |
1982年 | 834篇 |
1981年 | 746篇 |
1980年 | 701篇 |
1979年 | 768篇 |
1978年 | 890篇 |
1977年 | 913篇 |
1976年 | 809篇 |
1975年 | 774篇 |
1974年 | 790篇 |
1973年 | 760篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
921.
O. R. Sergeeva V. V. Chenets V. G. Lipovich 《Reaction Kinetics and Catalysis Letters》1982,19(3-4):411-414
Kinetic isotope effect (KIE) measurements in the trans-alkylation of toluene and benzene with ethylbenzene in hexane in the presence of AlBr3 at 313 K indicate that deuterium in the toluene methyl group causes a small inverse KIE characteristic for a -complex transiton state. The small direct KIE found for the reaction with C2D5C6H5 does not agree with a mechanism involving alkyl-and -phenylalkyl cations.
() - AlBr3 313 . , , - C2D5C6H5 , - -.相似文献
922.
O. Yu. Nadzhafova M. V. Drozdova E. V. Nebesnaya V. B. Ishchenko 《Journal of Analytical Chemistry》2007,62(12):1136-1142
Conditions are optimized for preparing sol-gel modified composite films based on silicon oxide and polyvinylsulfonic acid in the presence of the Tween 20 nonionic surfactant as a template for the adsorption of Fe(II) and Zn(II) phenanthrolinates. The adsorption of Zn(II), Cu(II), Mn(II), Co(II), and Ni(II) phenanthrolinates followed by their development on the surface with Rose Bengal oxyxanthene dye was studied by spectrophotometry. The variation of the pH of the solution pH and the addition of 0.01 M of EDTA enhanced the selectivity of determining Zn(II) using a film obtained under optimum conditions. The applicability of the proposed thin-film coatings to the adsorption-spectrophotometric determination of Fe(II) and Zn(II) in the ranges of 0.2–5.6 and 0.3–5.0 mg/L, respectively, was shown. The procedure was used to determine zinc in vitamin preparations, a food supplement, and an insulin preparation. The accuracy of determination was checked by atomic absorption spectrometry. The detection limit for zinc was 0.06 mg/L at a liquid phase volume of 25 mL and absorption layer thickness of 0.015 ± 0.003 mm. 相似文献
923.
924.
A. N. Egorochkin S. Ya. Khorshev N. S. Vyazankin T. I. Chernysheva O. V. Kuz'min 《Russian Chemical Bulletin》1971,20(3):478-483
Conclusions The frequencies of the valence vibrations of the Si-H bond in trisubstituted silanes depends not only on the inductive effect of substituents, but also on the effects of d-p interaction, ,- and p,-conjugation. An analysis was made of the relative contributions of these effects, depending on the nature of the substituent.For the preceding communications, see [1, 2].Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 544–550, March, 1971. 相似文献
925.
The perfect pairing (PP) approximation from generalized valence bond theory is formulated in an unrestricted fashion for both closed- and open-shell systems using a coupled cluster ansatz. In the model chemistry proposed here, active electron pairs are correlated, but the unpaired or radical electrons remain uncorrelated, leading to a linear number of decoupled cluster amplitudes which can be solved for analytically. The alpha and beta spatial orbitals are variationally optimized independently. This minimal treatment of electron-electron correlation noticeably improves upon symmetry-breaking problems and other pathologies in Hartree-Fock (HF) theory and may be computed using the resolution of the identity approximation at only a factor of several times more effort than HF itself. PP also generally predicts improved molecular structures over HF. This compact, correlated wave function potentially provides a useful starting point for dynamical correlation corrections. 相似文献
926.
P. Laggner O. Kratky W. H. Palm A. Holasek 《Monatshefte für Chemie / Chemical Monthly》1971,102(6):1729-1744
Zusammenfassung Schweineserum-Albumin wurde nach sorgfältiger Reinigung bei den pH-Werten 7,0, 5,1 und 3,7 mit Hilfe der Röntgenkleinwinkelstreuung untersucht. Es wurde bei allen Messungen praktisch dasselbe Molekulargewicht gefunden: der Mittelwert liegt bei 70 300±2 500. Die Ermittlung der Streumassenradien ergab bei pH 7,0 und pH 5,1 ähnliche Werte (33,0 Å bzw. 31,1 Å), bei pH 3,7 dagegen einen Wert von 37,8 Å. Die Formbestimmung ergab bei pH 7,0 und pH 5,1 identische Achsenverhältnisse (0,75 : 1 : 2), während bei pH 3,7 ein solches von 0,2 : 1 : 1 gefunden wurde.
Herrn Prof. Dr.H. Nowotny gewidmet. 相似文献
X-ray small-angle scattering investigations on solutions of porcine serum albumin at differentpH-values
Higly purified samples of porcine serum albumin were measured by the X-ray small-angle scattering method at pH 7.0, 5.1, and 3.7. The molecular weight is found to be the same in all three cases: the mean value is 70 300±2 500. The radius of gyration is 31.1 Å and 33.0 Å at pH 5.1 and 7.0 resp., whereas at pH 3.7 a value of 37.8 Å is found. The determination of the molecular conformation yields a similar axial ratio at pH 5.1 and pH 7.0 of 0.75 : 1 : 2; at pH 3,7 we find an axial ratio of 0.2 : 1 : 1 to be consistent with the scattering curves.
Herrn Prof. Dr.H. Nowotny gewidmet. 相似文献
927.
Ab initio calculations of the structure, force field, frequencies and intensities of normal modes of the LiBeF3 molecule have been carried out, using basis sets of Huzinaga and Dunning in a double-zeta contraction. The calculated results are compared with the infrared spectrum of LiBeF3 in an inert matrix obtained by Snelson et al. 相似文献
928.
V. G. Granik A. G. Sukhoruchkin N. S. Kuryatov V. P. Pakhomov O. S. Anisimova R. G. Glushkov 《Chemistry of Heterocyclic Compounds》1973,9(7):880-883
The diethyl acetal of N-methylpiperidin-2-one has been synthesized and its reactions with compounds having active CH2 and CH3 groups have been studied. In the reaction of diethyl acetal of N-methylpiperidin-2-one with malonic ester, in addition to the condensation product, ethylmalonic ester is formed. The reaction of the acetal with benzoic acid leads to O-alkylation and the formation of ethyl benzoate.For communication IV, see [2].Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 954–957, July, 1973. 相似文献
929.
H. Durchschlag O. Kratky O. F. Olaj J. W. Breitenbach 《Journal of polymer science. Part A, Polymer chemistry》1973,11(6):1327-1338
Small-angle x-ray investigations on poly-o-bromostyrene in benzene solution at extremely high resolution (corresponding to a Bragg's value of 3200 Å) proved to be able to link together the ranges covered by small-angle x-ray scattering and light scattering. On the other hand, there are problems encountered in molecular weight determination of chain molecules, since both methods at extremely small angles and at finite concentrations reach the limits of particle scattering. The differing results obtained by means of the two methods for the molecular weight of a rather high molecular weight sample of poly-o-bromostyrene are ascribed to entangelements stabilized by dipole–dipole interactions. These, due to the differing Bragg's angles and concentration ranges in the two methods, affect the experimental results to a different degree. 相似文献
930.
The reflectivity and transmitivity of a spatially dispersive crystal for S-polarized incident light are calculated for a microscopical model in which the interaction among the crystal planes is assumed to fall exponentially with distance. The angle of incidence is arbitrary and the deduced formulae are valid from X-ray to the visible light region. The generalization to the more complicated models is briefly discussed. For solving the problem an algebraic formulation of the Ewald dynamical theory of diffraction is used. 相似文献