全文获取类型
收费全文 | 46058篇 |
免费 | 3313篇 |
国内免费 | 2660篇 |
专业分类
化学 | 22843篇 |
晶体学 | 459篇 |
力学 | 2650篇 |
综合类 | 171篇 |
数学 | 10732篇 |
物理学 | 15176篇 |
出版年
2024年 | 63篇 |
2023年 | 380篇 |
2022年 | 645篇 |
2021年 | 655篇 |
2020年 | 765篇 |
2019年 | 667篇 |
2018年 | 1756篇 |
2017年 | 1952篇 |
2016年 | 1503篇 |
2015年 | 1351篇 |
2014年 | 1459篇 |
2013年 | 1838篇 |
2012年 | 4695篇 |
2011年 | 3917篇 |
2010年 | 2872篇 |
2009年 | 2543篇 |
2008年 | 1702篇 |
2007年 | 1599篇 |
2006年 | 1621篇 |
2005年 | 5262篇 |
2004年 | 4551篇 |
2003年 | 2838篇 |
2002年 | 1029篇 |
2001年 | 747篇 |
2000年 | 476篇 |
1999年 | 618篇 |
1998年 | 422篇 |
1997年 | 360篇 |
1996年 | 379篇 |
1995年 | 325篇 |
1994年 | 314篇 |
1993年 | 221篇 |
1992年 | 358篇 |
1991年 | 307篇 |
1990年 | 267篇 |
1989年 | 219篇 |
1988年 | 204篇 |
1987年 | 122篇 |
1986年 | 101篇 |
1985年 | 79篇 |
1984年 | 59篇 |
1983年 | 52篇 |
1981年 | 36篇 |
1976年 | 79篇 |
1975年 | 37篇 |
1974年 | 40篇 |
1973年 | 48篇 |
1972年 | 39篇 |
1969年 | 37篇 |
1966年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
831.
This work describes a hybrid procedure for eliminating major interference sources in aqueous near-infrared (NIR) spectra, that include aqueous influence, noise, and systemic variations irrelevant to concentration. The scheme consists of two parts: extension of wavelet prism (WPe) and orthogonal signal correction (OSC). First, WPe is employed to remove variations due to aqueous absorbance and noise; then OSC is applied to remove systemic spectral variations irrelevant to concentration. Although water possesses strong absorption bands that overshadow and overlap the absorption bands of analytes, along with noise and systematic interference, successful calibration models can be generated by employing the method proposed here. We show that the elimination of major interference sources from the aqueous NIR spectra results in a substantial improvement in the precision of prediction, and reduces the required number of PLS components in the model. In addition, the strategy proposed here can be applied to various analytical data for quantitative purposes as well. 相似文献
832.
A new fluorescent probe based on calix[6]arene functionalized with three naphthoic acid groups was synthesized and showed selective fluorescence enhancement in the presence of La3+ and Y3+. In addition, the fluorescence enhancement behaviors depended on the pH values of the solution. 相似文献
833.
The title reduction of nitroaromatics ArNO(2) by vinyl halide radical cations CH(2)[double bond]CH-X(+*)(X = Cl or Br) to form arylnitrenium ions ArNH(+) involves a change in oxidation number of nitrogen from +3 to -1. This novel reaction provides a new route for the generation of arylnitrenium ions, a highly selective method for the detection of explosives in mixtures, and offers clues to the carcinogenic activity of nitroaromatics in vivo. 相似文献
834.
Jurchen JC Cooper RE Williams ER 《Journal of the American Society for Mass Spectrometry》2003,14(12):1477-1487
Gas-phase H/D exchange is widely used for characterizing the structure of ions. However, many structural parameters that affect the rate of H/D exchange are poorly understood, which complicates the interpretation of experimental data. Here, the effects of sodium ion adduction on the rate of H/D exchange with D2O for a series of peptides and peptide dimers with varying numbers of acidic residues are described. The maximum number of sodium ion adducts that can be accommodated by the peptides and peptide dimers in this study is N + 1, where N is the number of free carboxylic acid groups. The formation of methyl-esters at all carboxylic acid groups, or the replacement of all the acidic hydrogens with sodium ions, effectively shuts down H/D exchange with D2O. In contrast, both the rate and the extent of H/D exchange with D2O are increased for most of the peptides and peptide dimers by the adduction of an intermediate number of sodium ions. These results are consistent with the H/D exchange occurring via a salt-bridge mechanism and show that the presence of two carboxylic acid groups is much better than one. The results with peptide dimers also indicate that surface accessibility may not be a dominant factor in the extent of H/D exchange for these ions. 相似文献
835.
Unprotected 5-(4-nitrophenyl)-10,15,20-triphenylporphyrin and 5,10-bis(4-nitrophenyl)-15,20-diphenylporphyrin react in the presence of a base at low temperature with carbanions (which bear a leaving group X at the carbanionic center) affording vicarious nucleophilic substitution of hydrogen (VNS) products in good yields (50-89%). The reactivity is explained in terms of the predominance of the porphyrin N-anion resonance forms at this temperature. 相似文献
836.
“软平板印刷”微结构制备技术为微米和亚微米器件的制备提供了一条新的途径 [1] ,已被电子学家和材料学家所应用 ,近年来进入了生物学领域[2 ] .本实验室将这一方法与生物分子电子学相结合 ,提出了用于 DNA芯片在片合成的分子印章法 [3,4 ] .分子印章法的实质是接触压印与组合化学相结合的固相界面反应 .聚二甲氧基硅氧烷 ( PDMS)是一种软印刷的优良材料 [5] ,但是由于其疏水性和较差的机械性能 ,必须对其进行改性才能用来制备 DNA分子印章 [6 ] . 聚氨酯作为一种功能材料 ,由于分子中交替的软、硬链段及其不同的热动力学性能而形成… 相似文献
837.
The regio- and stereospecific glycosylation of 8,3′,5′-trideuterodaidzein 1 with α-acetobromoglucose to provide 8,3′,5′-trideuterodaidzein-7-O-β-glucopyranoside 2 is presented. 相似文献
838.
Yoshiaki?YuguchiEmail author Takahiro?Hirotsu Jun?Hosokawa 《Cellulose (London, England)》2005,12(5):469-477
Xyloglucan is a type of hemicellulose with a cellulose backbone containing (1→6)-α-xylose or (1→2)-β-galactoxylose as a side chain. It is soluble in water. Its aqueous solution forms a gel or gel-like precipitate by addition
of Congo red. Xyloglucan gel structures with various concentrations of Congo red were observed by small angle X-ray scattering
(SAXS) at the nano-level. SAXS results indicated that the xyloglucan chains interacted with Congo red, and that an increase
of concentration of Congo red induced a characteristic cross-linking domain, which consisted of a flat structure containing
stacked xyloglucan chain assemblies. The Congo red molecules are inserted between the xyloglucan chains. 相似文献
839.
Hans-Jürgen?BuschmannEmail author Lucia?Mutihac Eckhard?Schollmeyer 《Journal of inclusion phenomena and macrocyclic chemistry》2005,51(1-2):53-57
The formation of complexes between α-cyclodextrin and n-alkylamines and their hydrochlorides has been studied in aqueous solution using calorimetric titrations. All alkylamines form stronger complexes than the corresponding hydrochlorides. The values of the reaction enthalpies are smaller for the alkylamine hydrochlorides compared with the alkylamines. By increasing the number of methylene groups, these differences become smaller. In addition, the reaction enthalpies for protonation of the alkylamines and their complexes with α-cyclodextrin have been measured. The heat of protonation of these complexes is always smaller compared with the alkylamines. Due to the protonation and the formation of a strong solvation shell around the ammonium group the interactions with α-cyclodextrins are weakened. From a thermodynamic cycle using all measured reactions, it can be concluded that the aggregation of the alkylamines with long alkyl chains (heptyl-, octyl-, and nonylamine) has an influence on the values of the reaction enthalpies and entropies for the protonated form only.This revised version was published online in July 2005 with a corrected issue number. 相似文献
840.
Alessandro?De?Robertis Concetta?De?Stefano Demetrio?Milea Silvio?SammartanoEmail author 《Journal of solution chemistry》2005,34(10):1211-1226
Complex formation constants were determined potentiometrically (by a ISE-H+, glass electrode) in the systems, M2+ – Lz – H+ [M2+ = (C2H5)2Sn2+, Lz = malonate, glycinate and ethylenediamine] at t = 25 ∘C and 0.1 mol-L−1≤ I/ ≤ 1 mol-L−1 in NaClaq (0.1 mol-L−1 ≤ I ≤ 0.75 mol-L−1 for the ethylenediamine system). Thermodynamic values of formation constants, at infinite dilution, are [± 95% confidence
interval, Tβpqr refer to the equilibrium, pM2+ + qLz + rH+ = MpLqHr(2+z+r)]: for malonate, log10 Tβ110 = (5.47 ± 0.10); for glycinate, log10 Tβ110 = (9.54 ± 0.08), log10 Tβ111 = (12.97 ± 0.10); and for ethylenediamine, log10 Tβ110 = (10.47 ± 0.10), log10 Tβ120 = (16.17 ± 0.12) and log10 Tβ111 = (15.46 ± 0.10). The dependence on ionic strength of the formation constants was modeled by a simple Debye–Hückel type equation
and by the SIT approach. By analyzing the stability of the species in the three different systems we found a simple additivity
rule that can be expressed by the relationship: log10 K = 6.46 nN + 3.96 nO − 0.60 (nN2+ nO2), with a mean deviation, ε(log10 K) = 0.15 (K = equilibrium constant for the interaction of the organometal cation with the unprotonated or protonated ligand, nN = number of amino groups and nO = number of carboxylic groups of the ligand(s) involved in the formation reaction of complex species). 相似文献