全文获取类型
收费全文 | 7105篇 |
免费 | 190篇 |
国内免费 | 520篇 |
专业分类
化学 | 6722篇 |
晶体学 | 62篇 |
力学 | 79篇 |
综合类 | 20篇 |
数学 | 566篇 |
物理学 | 366篇 |
出版年
2024年 | 11篇 |
2023年 | 52篇 |
2022年 | 172篇 |
2021年 | 160篇 |
2020年 | 151篇 |
2019年 | 161篇 |
2018年 | 141篇 |
2017年 | 180篇 |
2016年 | 162篇 |
2015年 | 136篇 |
2014年 | 190篇 |
2013年 | 593篇 |
2012年 | 246篇 |
2011年 | 230篇 |
2010年 | 187篇 |
2009年 | 257篇 |
2008年 | 287篇 |
2007年 | 311篇 |
2006年 | 252篇 |
2005年 | 260篇 |
2004年 | 266篇 |
2003年 | 255篇 |
2002年 | 1745篇 |
2001年 | 213篇 |
2000年 | 136篇 |
1999年 | 112篇 |
1998年 | 90篇 |
1997年 | 101篇 |
1996年 | 94篇 |
1995年 | 109篇 |
1994年 | 76篇 |
1993年 | 76篇 |
1992年 | 70篇 |
1991年 | 42篇 |
1990年 | 28篇 |
1989年 | 22篇 |
1988年 | 33篇 |
1987年 | 27篇 |
1986年 | 18篇 |
1985年 | 22篇 |
1984年 | 26篇 |
1983年 | 6篇 |
1982年 | 15篇 |
1981年 | 9篇 |
1980年 | 13篇 |
1979年 | 6篇 |
1978年 | 8篇 |
1973年 | 3篇 |
1971年 | 32篇 |
1970年 | 16篇 |
排序方式: 共有7815条查询结果,搜索用时 15 毫秒
131.
A novel organoantimony complex of 6-cyclohexyl-6,7-dihydrodibenzo[c,f] [1,5]azastibocin-12(5H)-yl nitrate ( 2 ) was synthesized and systematically characterized by techniques such as NMR spectra, TG-DSC, and X-ray diffraction. It was found that the complex 2 exhibits relatively strong Lewis acidity (3.3 < Ho ≤ 4.8) and could be employed as a water tolerant Lewis acid catalyst for the synthesis of synthetically valuable benzimidazole derivatives starting from aldehydes and arylenediamines. This catalytic system shows excellent tolerance toward a wide variety of functional groups, such as methyl, methoxyl, fluoro, chloro, bromo, nitro, cyan, trifluoromethyl, 1-naphthaldehyde, furfural and n-butyl, together with facile reusability in 5 times scale enlarged synthesis. 相似文献
132.
Dr. Anton A. Anisimov Dr. Fedor V. Drozdov Yulia S. Vysochinskaya Ekaterina O. Minyaylo Prof. Alexander S. Peregudov Prof. Fedor M. Dolgushin Dr. Olga I. Shchegolikhina Prof. Aziz M. Muzafarov 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(50):11404-11407
This study presents the synthesis of organoboron derivatives of stereoregular 4-, 6-, and 12-unit phenylcyclosilsesquioxanes. All compounds obtained were isolated in good yields (70–80 %) and were fully characterized by 1H, 13C, 29Si, 11B NMR, IR spectroscopy, HRMS ESI, and elemental microanalysis. The structure of the key modifier, obtained for the first time, 4-(tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl) dimethylvinylsilane, was also confirmed by single-crystal XRD. 相似文献
133.
《中国化学快报》2020,31(7):1855-1858
By virtue of electrochemistry, a series of α,α-dihaloacetophenones were easily obtained with good to excellent yields. This electrochemical procedure was taken in a divided cell with constant current in aqueous media. The reaction can be carried out smoothly at room temperature under metal and oxidant free condition, which provides an eco-friendly synthesis for the α,α-dihaloacetophenone derivatives. 相似文献
134.
The UV absorption spectra of six structurally related derivatives of anisole and veratrole, i.e., anisaldehyde, (E)-anethole, estragole, veratraldehyde, methyleugenol and (E)-methylisoeugenol, were recorded at various concentrations of the anionic surfactants, either sodium lauryl sulfate (SLS) or sodium lauryl ether sulfate (SLES) at T = 298 K. In addition, conductivity and density measurements were made for the SLS and SLES solutions to determine the volumetric properties of the studied surfactants. Next, using the W. Al-Soufi, L. Pińeiro and M. Novo model (APN model) including the pseudo-phase model for micellar solubilization, the values of micelle-water partition coefficients for each perfume-surfactant system were determined. In addition, the relations between the molecular structures of the solute and the head group of the surfactant and the value of the micelle-water partition coefficient as well as the octanol-water one were discussed. 相似文献
135.
K. Ramachandra 《Proceedings Mathematical Sciences》1995,105(3):273-279
Some very precise results (see Theorems 4 and 5) are proved about thea-values of thelth derivative of a class of generalized Dirichlet series, forl≥l o =l o(a) (l o being a large constant). In particular for the precise results on the zeros ofζ (1) (s) —a (a any complex constant andl≥l o) see Theorems 1 and 2 of the introduction. 相似文献
136.
Yubin Yan 《BIT Numerical Mathematics》2003,43(3):647-669
We study smoothing properties and approximation of time derivatives for time discretization schemes with variable time steps for a homogeneous parabolic problem formulated as an abstract initial value problem in a Banach space. The time stepping methods are based on using rational approximations to the exponential function which are A()-stable for suitable (0,/2] with unit bounded maximum norm. First- and second-order approximations of time derivatives based on using difference quotients are considered. Smoothing properties are derived and error estimates are established under the so-called increasing quasi-quasiuniform assumption on the time steps. 相似文献
137.
Abstract With comparable molar quantities, chromotropic acid reacts with compounds containing aldehyde groups -CHO e.g. with formadehyde or glyoxylic acid to form cyclic compounds with rings containing four chromotropic acid fragments — cyclotetrachromotropylene derivatives. The cyclic compounds, due to the presence of a cavity in the molecule, can play the part of host and complex smaller molecules, which has been confirmed on the basis of triethylamine by the 1H-NMR measurements. Using spectrophotometric method the stability constant of 1:1 complex formed between Ti(IV) and cyclic compound chromotropic acid with formaldehyde has been determined - K = (9,0 ± 0,1) 104. 相似文献
138.
139.
Maria Gil Elena Bermejo Alfonso Castieiras Heloisa Beraldo Douglas X. West 《无机化学与普通化学杂志》2000,626(11):2353-2360
Pyridyl bis(N(4)‐substituted thiosemicarbazones), in which the substituents replacing the NH2 group on the thiosemicarbazone moieties are piperidyl, H2Plpip; hexamethyleneiminyl, H2Plhexim; diethylamino, H2Pl4DE; and dipropylamino, H2Pl4DP, have been synthesized. These bis(thiosemicarbazones) and their nickel(II) complexes have been characterized with IR, electronic, mass, and 1H and 13C NMR spectra. Crystal structures have been solved for H2Plpip and all four nickel(II) complexes. H2Plpip does not possess hydrogen bonding between the thiosemicarbazone moieties, but is in the Z isomeric form with intramolecular hydrogen bonding from both thiosemicarbazone moieties to pyridine nitrogen atoms. The nickel(II) complexes possess square‐planar N2S2 (i. e., imine nitrogen and thiolato sulfur atoms) centers and the two pyridine ring nitrogen atoms are not coordinated. 相似文献
140.
Effects of substituent and solvent on the structure and spectral properties of maleimide derivatives
Chin-Kuen Tai Yih-Jiun Lin Pao-Ling Yeh Yi-Ren Tzeng Yu-Ma Chou Bo-Cheng Wang 《Journal of Molecular Structure》2008,860(1-3):58-63
The maximum absorption wavelength , emission wavelength (λem) and the related oscillator strength (f) of the maleimides in the ground and first excited states were calculated by using the DFT, CIS and the time-dependent density functional theory (TD-DFT) methods, where the molecular structures were optimized by DFT/B3LYP/6-31G* calculation. Solvent effects on the maleimides were examined using the PCM simulation at DFT/B3LYP level with the 6-31G* basis set. For N-substituted maleimide, the substituent gives only a slight influence on the maleimide chromophore, while planar conformation of PhMLH leads to the improvement in π-delocalization from substituent to maleimide unit. For 3,4-substituted maleimide, the steric repulsion between substituent and maleimide chromophore influences the extent of π-delocalization and the molecular conformation. The calculated and λem of maleimides are in good agreement with the experimental data. In the gas phase, both absorption and emission peaks are red-shift as compared to the non-substituted maleimide. Under solvent environment, the more planar conformation of PhMLH shows a blue-shift in the calculated and λem as compared with other N-substituted maleimides. For 3,4-substituted maleimides, the effect of substitution produces the most significant spectral red-shift as compared to other maleimides. 相似文献