全文获取类型
收费全文 | 1215篇 |
免费 | 60篇 |
国内免费 | 4篇 |
专业分类
化学 | 968篇 |
晶体学 | 9篇 |
力学 | 7篇 |
数学 | 71篇 |
物理学 | 224篇 |
出版年
2023年 | 7篇 |
2022年 | 22篇 |
2021年 | 20篇 |
2020年 | 24篇 |
2019年 | 24篇 |
2018年 | 23篇 |
2017年 | 8篇 |
2016年 | 29篇 |
2015年 | 33篇 |
2014年 | 39篇 |
2013年 | 76篇 |
2012年 | 72篇 |
2011年 | 86篇 |
2010年 | 33篇 |
2009年 | 47篇 |
2008年 | 52篇 |
2007年 | 58篇 |
2006年 | 46篇 |
2005年 | 55篇 |
2004年 | 39篇 |
2003年 | 57篇 |
2002年 | 38篇 |
2001年 | 14篇 |
2000年 | 14篇 |
1999年 | 16篇 |
1998年 | 9篇 |
1997年 | 16篇 |
1996年 | 12篇 |
1995年 | 10篇 |
1994年 | 15篇 |
1993年 | 10篇 |
1992年 | 18篇 |
1991年 | 7篇 |
1990年 | 15篇 |
1988年 | 17篇 |
1987年 | 12篇 |
1986年 | 10篇 |
1985年 | 21篇 |
1984年 | 30篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 14篇 |
1980年 | 11篇 |
1979年 | 16篇 |
1978年 | 17篇 |
1977年 | 13篇 |
1976年 | 9篇 |
1975年 | 7篇 |
1973年 | 6篇 |
1968年 | 6篇 |
排序方式: 共有1279条查询结果,搜索用时 562 毫秒
1.
Dr. Christopher R. Opie Dr. Hidetoshi Noda Prof. Dr. Masakatsu Shibasaki Dr. Naoya Kumagai 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(18):4648-4653
The B3NO2 six-membered heterocycle (1,3-dioxa-5-aza-2,4,6-triborinane=DATB), comprising three different non-carbon period 2 elements, has been recently demonstrated to be a powerful catalyst for dehydrative condensation of carboxylic acids and amines. The tedious synthesis of DATB, however, has significantly diminished its utility as a catalyst, and thus the inherent chemical properties of the ring system have remained virtually unexplored. Here, a general and facile synthetic strategy that harnesses a pyrimidine-containing scaffold for the reliable installation of boron atoms is disclosed, giving rise to a series of Pym-DATBs from inexpensive materials in a modular fashion. The identification of a soluble Pym-DATB derivative allowed for the investigation of the dynamic nature of the B3NO2 ring system, revealing differential ring-closing and -opening behaviors depending on the medium. Readily accessible Pym-DATBs proved their utility as efficient catalysts for dehydrative amidation with broad substrate scope and functional-group tolerance, offering a general and practical catalytic alternative to reagent-driven amidation. 相似文献
2.
We measured the entire region of mesopic and photopic vision to determine the color zone covering the visual field with unique red, yellow, green, and blue hue components. Eight kinds of test stimuli in the natural color system (NCS) color notation system were used. These stimuli were presented at horizontal and vertical meridians, and at meridians inclined at 45° angles. The illuminance level was set at six levels: 0.01 lx-1000 lx. The evaluation method measured chromatic, white, and black components as well as the hue component of the stimuli. Results show that the color zone of the retina extended further into the periphery with increasing illuminance; the response of opponent colors y-b was shown to be greater than that of the opponent colors r-g. 相似文献
3.
The electrode reaction of decamethylferrocene (DMFc) dissolved in a thin layer of a room-temperature molten salt (RTMS), 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (C8mimC1C1N) or 1-octyl-3-methylimidazolium bis(pentafluoroethylsulfonyl)imide (C8mimC2C2N), on a self-assembled monolayer-modified gold electrode is coupled with the ion transfer across the interface between the RTMS and the outer aqueous solution (W) to give a voltammogram whose shape resembles a voltammogram of a simple one-electron transfer process. The electroneutrality of the RTMS layer during the oxidation of DMFc to decamethylferricenium ion is maintained by the concomitant dissolution of C8mim+ ion from the RTMS phase to the W phase, and the reduction of decamethylferricenium ion to DMFc is accompanied by the transfer of either C1C1N- or C2C2N- from RTMS to W. The midpoint potential of the voltammogram varies with the concentration of the salt in the aqueous phase, C8mimCl or LiCnCnN (n = 1 or 2), in a Nernstian manner, showing that the phase-boundary potential between the RTMS and the W is controlled by the partition of these ions. Although the phase-boundary potential across the RTMS / W interface is Nernstian with respect to the ions common to both phases at the equilibrium, the polarization at the RTMS / W interface under current flow distorts the shape of the voltammograms, resulting in a wider peak separation in the voltammogram. 相似文献
4.
5.
This paper proposes a two step algorithm for solving a large scale semi-definite logit model, which is appreciated as a powerful
model in failure discriminant analysis. This problem has been successfully solved by a cutting plane (outer approximation)
algorithm. However, it requires much more computation time than the corresponding linear logit model. A two step algorithm
to be proposed in this paper is intended to reduce the amount of computation time by eliminating a certain portion of the
data based on the information obtained by solving an associated linear logit model. It will be shown that this algorithm can
generate a solution with almost the same quality as the solution obtained by solving the original large scale semi-definite
model within a fraction of computation time. 相似文献
6.
7.
Naoya Inazumi Shunzo Yamamoto Yoshimi Sueishi 《Journal of inclusion phenomena and macrocyclic chemistry》2007,59(1-2):33-39
The inclusion complexation of p-sulfonatocalix[6]arene (Calix-S6) with three kinds of phenothiazine dyes was studied spectrophotometrically in a mixture
of a room-temperature ionic liquid [bmim]BF4 (1-butyl-3-methylimidazolium tetrafluoroborate) and ethanol. We have determined the association constants of Calix-S6 with
phenothiazine dyes under external static pressure up to 767 bar in the [bmim]BF4-ethanol and alcohol-water mixtures. With increasing external pressure, the inclusion equilibrium in the alcohol-water mixtures
was shifted to the dissociation side. Conversely, the inclusion equilibrium of methylene blue (MB) and azure A (AA) in the
ionic liquid mixture was shifted to the association side. From the analysis of the pressure effects, the reaction volumes
ΔV for inclusion complexation were estimated as −7 to 9 cm3 mol−1 in the [bmim]BF4-ethanol mixture and 20–32 cm3 mol−1 in the alcohol-water mixtures. Based on the results, we have suggested that there is a competing complexation between the
included dye and [bmim]BF4 molecules in the ionic liquid. 相似文献
8.
9.
Fumio Hamada Kyoko Ishikawa Riko Ito Hisanori Shibuya Sanyo Hamai Iwao Suzuki Tetsuo Osa Akihiko Ueno 《Journal of inclusion phenomena and macrocyclic chemistry》1994,20(1):43-51
-Cyclodextrin with two sodium anthranilate moieties (1) has been prepared as a sensor for detecting organic compounds including terpenoids and steroids. Compound1 shows a pure monomer fluorescence whose intensity is increased or decreased upon addition of the guest species examined. In this system, the sodium anthranilate moieties act either as a spacer, which enables the cyclodextrin to form a 1:1 host-guest complex by narrowing the -cyclodextrin cavity, or as a hydrophobic cap.1 recognizes steroids with much higher sensitivity than terpenoids, in which the appended moieties act as a hydrophobic cap for terpenoids and a spacer for steroids, respectively. 相似文献
10.
M Horie K Saito Y Hoshino N Nose N Hamada H Nakazawa 《Journal of chromatography. A》1990,502(2):371-378
A simple and selective method for the determination of sulphamethazine (SMT) and its metabolite, N4-acetylsulphamethazine (N4-AcSMT), in meat by high-performance liquid chromatography (HPLC) with photodiode-array detection was developed. The drugs were extracted from meat with 0.2% metaphosphoric acid-methanol (6:4), followed by a Bond-Elut C18 clean-up procedure. The HPLC separation was carried out on a Supersphere RP-18e column (125 X 4.0 mm I.D.) using 0.05 M sodium dihydrogenphosphate (pH 4.5)-acetonitrile (8:2) as the mobile phase at a flow-rate of 0.5 ml/min, and monitored with a photodiode-array detector. The recoveries of SMT and N4-AcSMT from meat fortified at 0.5 micrograms/g were 90.1-93.3 and 93.0-94.4%, respectively, with coefficients of variation of 1.9-3.2 and 1.5-2.7%. The limits of detection were 0.02 micrograms/g for each drug. SMT was found in ten samples of imported meat (12.5%) at levels ranging from 0.05 to 1.05 micrograms/g. 相似文献