全文获取类型
收费全文 | 14399篇 |
免费 | 418篇 |
国内免费 | 52篇 |
专业分类
化学 | 10164篇 |
晶体学 | 36篇 |
力学 | 228篇 |
综合类 | 1篇 |
数学 | 2719篇 |
物理学 | 1721篇 |
出版年
2020年 | 133篇 |
2019年 | 161篇 |
2018年 | 101篇 |
2016年 | 284篇 |
2015年 | 282篇 |
2014年 | 362篇 |
2013年 | 561篇 |
2012年 | 478篇 |
2011年 | 512篇 |
2010年 | 371篇 |
2009年 | 332篇 |
2008年 | 485篇 |
2007年 | 492篇 |
2006年 | 520篇 |
2005年 | 466篇 |
2004年 | 409篇 |
2003年 | 399篇 |
2002年 | 409篇 |
2001年 | 257篇 |
2000年 | 227篇 |
1999年 | 180篇 |
1998年 | 162篇 |
1997年 | 178篇 |
1996年 | 178篇 |
1995年 | 164篇 |
1994年 | 173篇 |
1993年 | 179篇 |
1992年 | 199篇 |
1991年 | 153篇 |
1990年 | 167篇 |
1989年 | 165篇 |
1988年 | 179篇 |
1987年 | 186篇 |
1986年 | 159篇 |
1985年 | 279篇 |
1984年 | 237篇 |
1983年 | 152篇 |
1982年 | 201篇 |
1981年 | 196篇 |
1980年 | 237篇 |
1979年 | 227篇 |
1978年 | 233篇 |
1977年 | 257篇 |
1976年 | 219篇 |
1975年 | 204篇 |
1974年 | 176篇 |
1973年 | 211篇 |
1972年 | 114篇 |
1971年 | 107篇 |
1970年 | 86篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
101.
Hans Kummer 《Foundations of Physics》1987,17(1):1-62
An axiomatic theory is formulated which describes a class of yes-no experiments, involving a fixed basic source, a fixed basic detector, and various filters. It is assumed that all filters considered can be constructed from a setP of primitive filters by composition and stochastic selection. Two physically plausible axioms are formulated which allow us to define the concept of asystem in the present context (cf. Definition2.4). To each system we can attach anorder unit module (
) (cf. Definition5.1) whereby (
) is acomplete, separable order unit space. Two additional axioms are proposed which have the effect that the space (
) becomes isomorphic to the order unit space underlying a JB-algebra, at least in the case where
isfinite dimensional (cf. Corollary7.9). 相似文献
102.
Òscar Rubio-Pons Boris Minaev Oleksandr Loboda Hans Ågren 《Theoretical chemistry accounts》2005,113(1):15-27
The phosphorescence spectrum of p-dichlorobenzene has been calculated using multiconfiguration self-consistent-field wave functions and the quadratic response technique. Attention has been paid to the intensity distribution of the singlet–triplet (3B1u1Ag) transition through a number of vibronic subbands. The second order spin–orbit coupling (SOC) contribution to the spin splitting of the 3B1u (3*) state is found to be almost negligible, and the calculations therefore provide a good estimate for the zero-field splitting (ZFS) parameters based only on the electron spin–spin coupling expectation values. Nuclear quadrupole resonance constants for the different Cl isotopes are also calculated to accomplish the ZFS assignment. The electric dipole activity of the spin sublevels in the triplet–singlet transitions to the ground-state vibrational levels is estimated by calculations of derivatives using distorted geometries which are shifted from the equilibrium position along different vibrational modes. A vibrational analysis of the phosphorescence spectrum, based on the SOC-induced mixing of the singlet and triplet states calculated along different vibrational modes, provides reasonable agreement with experimental data.Acknowledgment O. R.-P. would like to thank the European MOLPROP network for support. The authors thank Alexander Baev for fruitful discussions. This work was supported by the Swedish Royal Academy of Science (KVA). 相似文献
103.
Leonard S Van Schepdael A Iványi T Lázár I Rosier J Vanstockem M Vermeersch H Hoogmartens J 《Electrophoresis》2005,26(3):627-632
A capillary electrophoretic (CE) method was developed for the separation of diastereoisomers of a new human immunodeficiency virus (HIV) protease inhibitor TMC114. In total 16 isomers of this drug have been synthesized (eight pairs of enantiomers). We succeeded in the separation of the eight diastereoisomers, but no enantiomers could be separated. Because of the high similarity and water-insolubility of these isomers, the separation is a real challenge. Different CE modes were tried out: capillary zone electrophoresis (CZE), nonaqueous capillary electrophoresis (NACE), micellar electrokinetic capillary chromatography (MEKC), and microemulsion electrokinetic capillary chromatography (MEEKC). Only MEEKC offered resolution of these compounds. 相似文献
104.
Gilles Gosselin Jean Louis Imbach Leroy B. Townsend Raymond P. Panzica 《Journal of heterocyclic chemistry》1979,16(6):1185-1191
Ribosylation of the trimethylsilyl derivative ( 1b ) of imidazole-2-thione ( 1a ) using either stannic chloride or silver perchlorate as catalyst resulted in the formation of the acylated derivatives of 1-(β-D-ribofuranosyl)imidazole-2-thione ( 3c ) and 1,3-di-(β-D-ribofuranosyl)imidazole-2-thione ( 4c ) with the latter predominating ( 4c:3c , ca. 2:1 ). The diribosylated nucleoside 4c was shown to be the N,N-disubstituted product rather than the N,S-disubstituted product by 1H nmr and 13C nmr spectroscopy. Employment of the iodine-catalyzed fusion procedure reversed the aforementioned product ratios and provided the monoriboside 3c in excellent yield. When the trimethylsilyl derivative ( 5b ) of 2-methylthioimidazole ( 5a ) was reacted with 2,3,5-tri-O-benzoyl-D-ribofuranosyl bromide ( 2d ) in acetonitrile, the major product was 1,3-di-(2,3,5-tri-O-benzoyl-β-D-ribofuranosyl)-imidazole-2-thione ( 4b ). The formation of 4b in this reaction is thought to arise via the Hilbert-Johnson mechanism. 相似文献
105.
Thomas R. Gengenbach Zoran R. Vasic Sheng Li Ronald C. Chatelier Hans J. Griesser 《Plasmas and Polymers》1997,2(2):91-114
The properties and composition of plasma polymer surfaces stored in air can change considerably over time, especially as a
result of oxidative reactions. When plasma polymers contain an element other than O, it is possible to probe for mechanisms
in addition to oxidation that contribute to the aging of the surface. Plasma polymers containing N were fabricated from either
1,3-diaminopropane (DAP),n-heptylamine (nHA), or allylamine (AA), and studied by X-ray photo-electron spectroscopy (XPS) and air/water contact angles
(CA). For each of the plasma polymers, a multiexponential increase in the O/C ratio was observed over time using XPS. The
N/C ratios remained constant (AA) or decreased somewhat (nHA and DAP). In contrast, the trends in CA values differed, declining
for the nHA surfaces, rising for the AA, and changing little for the DAP. Surface roughness, assessed by scanning tunnelling
or atomic force microscopy, did not change over time. The diverse adjustments in the polarity of each surface and the similar
compositional changes between them are reconcilable if the aging of the plasma polymer surface is a manifestation of the superposition
of concurrent oxidative reactions and partial surface reorientation; the former introduce polar groups and the latter transports
then from the surface to deeper regions beyond the CA probe depth but within the XPS analysis depth. These processes vary
between different plasma polymers. Data for the alkylamine plasma polymers is also compared with that for two plasma polymers
fabricated from methanol. The change in composition, but not polarity, of the DAP surface after 4 days of storage demonstrates
the importance of using multiple techniques to characterize the aging of plasma polymer surfaces. 相似文献
106.
Hans Bürger 《Angewandte Chemie (International ed. in English)》1973,12(6):474-486
In contrast to carbon, silicon fails to form multiple bonds that are stable at room temperature. Consequently molecules in which silicon exhibits coordination numbers (CN) of 1, 2, and 3 may only be obtained at very high or low temperatures. Under these conditions their structural features, including multiple bonds, resemble those of carbon. On the other hand, silicon is capable of forming various hexacoordinated compounds making use of its d orbitals. Nitrogen and oxygen bonded to silicon develop an unusual stereochemistry: planar nitrogen, nearly or completely linear oxygen, and considerable shortening of SiN, SiO, and SiF bonds are specific examples. N(SiR3)2 and CH2SiR3 ligands permit stabilization of unusually low CNs of many metals and give rise to amino and alkyl derivatives of unexpectedly high stability due to the particular electronic, the R3Si group. 相似文献
107.
Hans. R. Kricheldorf William E. Hull 《Journal of polymer science. Part A, Polymer chemistry》1978,16(3):583-595
The tripeptides Phe-Gly-Gly, β-Ala-Gly-Gly, and ?-Aca-Gly-Gly as well as the peptide derivatives δ-isothiocyanatovaleroyl-Gly-Gly and ?-isothiocyanatocaproyl-Gly-Gly were synthesized by using known methods so that the peptide nitrogen between the two glycyl residues was isotopically enriched in 15N to a level of 0.8–0.9%. These monomer units were then used to produce the sequence polymers (Phe-Gly-Gly)n, (β-Ala-Gly-Gly)n, (δ-Ava-Gly-Gly)n, and (?-Aca-Gly-Gly)n. The 18.24 MHz 15N-NMR spectra of the oligo- and polypeptides were obtained by using trifluoroacetic acid as solvent, since the solutions have relatively low viscosity and exhibit a strong negative nuclear Overhauser enhancement of the 15N signals. For comparison, 15N-NMR spectra of the homopolymers (Gly)n, (β-Ala)n, (γ-Abu)n, (δ-Ava)n, and (?-Aca)n were also recorded. The 15N signals from the ω-aminoacyl residues in the sequence polymers appear up to 11 ppm upfield of the signals observed for the homopolyamides. The 15N signals from the two glycyl residues are separated by 3–7 ppm. Comparison with the 13C-NMR spectra of the same polymers indicates that 15N-NMR is better suited for the characterization and sequence analysis of these types of polymers. 相似文献
108.
Harald Schfer 《无机化学与普通化学杂志》1974,410(3):269-273
The Chemical Transport of Platinum with Chlorine Experiments show that the chemical transport of platinum by means of chlorine within a temperature gradient at temperatures below ≈ 1000°K goes into the hot temperature region, but at higher temperatures in the reverse direction. From the thermodynamic discussion it can be seen, that the platinum content of the gas phase at low temperatures is governed by the exothermic formation of Pt6Cl12,g, and at higher temperatures by the endothermic formation of PtCl3,g and PtCl2,g. The platinum content of the gas phase passes a minimum at ≈ 1000°K, if P(Cl2) = 3.5 atm. This result is in agreement with the observed inversion of the transport direction. 相似文献
109.
Hydroxylamine-O-sulfonic acid: a convenient reagent for the conversion of aldehydes into nitriles . Hydroxylamine-O-sulfonic acid (HAS) proved to be an excellent reagent for the conversion of aliphatic, aromatic and heteroatomic aldehydes into nitriles. There, aldehydes in the form of suspension in HAS water solution, react rapidely to give the corresponding nitriles in high yield. Only with formylpyridines could the intermediate oxime-O-sulfonic acids be isolated. 相似文献
110.
Gas Molecules Pd2Al2Cl10 and PdAlCl5 as Accompanists of PdAl2Cl8 Mass spectrometric observations using a double cell showed that the reaction of gaseous Al2Cl6 with solid PdCl2 besides the known gaseous complex PdAl2Cl8 gives PdAlCl5 and the unexpected complex Pd2Al2Cl10. For the equilibrium (with ΔCp? ?1 cal/K) ΔH°(298) = 7.5 kcal/Mol and ΔS°(298) = 5.3 ± 2 cl have been obtained. 相似文献