全文获取类型
收费全文 | 973篇 |
免费 | 22篇 |
国内免费 | 11篇 |
专业分类
化学 | 628篇 |
晶体学 | 10篇 |
力学 | 8篇 |
数学 | 88篇 |
物理学 | 272篇 |
出版年
2021年 | 9篇 |
2020年 | 8篇 |
2019年 | 14篇 |
2018年 | 6篇 |
2017年 | 7篇 |
2016年 | 19篇 |
2015年 | 8篇 |
2014年 | 19篇 |
2013年 | 48篇 |
2012年 | 46篇 |
2011年 | 62篇 |
2010年 | 28篇 |
2009年 | 31篇 |
2008年 | 52篇 |
2007年 | 51篇 |
2006年 | 57篇 |
2005年 | 56篇 |
2004年 | 38篇 |
2003年 | 54篇 |
2002年 | 35篇 |
2001年 | 22篇 |
2000年 | 32篇 |
1999年 | 14篇 |
1998年 | 17篇 |
1997年 | 13篇 |
1996年 | 8篇 |
1995年 | 7篇 |
1994年 | 10篇 |
1993年 | 5篇 |
1992年 | 11篇 |
1991年 | 10篇 |
1990年 | 12篇 |
1989年 | 11篇 |
1988年 | 8篇 |
1987年 | 11篇 |
1986年 | 7篇 |
1985年 | 15篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1982年 | 7篇 |
1981年 | 14篇 |
1980年 | 12篇 |
1979年 | 13篇 |
1978年 | 9篇 |
1977年 | 10篇 |
1976年 | 5篇 |
1975年 | 9篇 |
1974年 | 9篇 |
1973年 | 9篇 |
1972年 | 11篇 |
排序方式: 共有1006条查询结果,搜索用时 375 毫秒
21.
The negative chemical ionization mass spectra of representative perfluorinated alkanes, cycloalkanes, ethers and tertiary amines have been examined, using Ar at about 0.5 torr as the reagent gas. The compounds chosen are typical of those under study as components of fluorochemical emulsion blood substitutes. Many such PFC's, particularly those with cyclic or branched structures, give intense molecular ions; most give simple spectra with a few major fragment ions at high mass, in marked contrast to the EI spectra which are dominated by m/e 69 (CF3+) and 131 (C3F5+) of no value for structure elucidation. NCI-GC/MS is more sensitive than conventional EI-GC/MS and promises to be more generally useful for structure determination. Specific examples from the various classes will be presented, and their NCI and EI mass spectra compared. 相似文献
22.
Ono K Yoshida A Saito N Fujishima T Honzawa S Suhara Y Kishimoto S Sugiura T Waku K Takayama H Kittaka A 《The Journal of organic chemistry》2003,68(19):7407-7415
Six novel 2-substituted analogues of 1alpha,25-dihydroxy-19-norvitamin D(3), 6a,b-8a,b, were efficiently synthesized utilizing (-)-quinic acid as the A-ring precursor. The C2-modified A-rings were prepared as 4-alkylated (3R,5R)-3,5-dihydroxycyclohexanones 12-15 from (-)-quinic acid based on radical allylation at the C4 position of methyl (-)-quinicate. The new type of the CD-ring coupling partner 23 was synthesized from 25-hydroxy Grundmann's ketone 19 to apply to the modified Julia olefination to construct a diene unit between the A-ring and the CD-ring. The coupling yields, including a deprotection step, were 47-62%. After the separation of the diastereomers based on C2 stereochemistry, the structure (2alpha or 2beta) was determined by (1)H NMR experiments and compared to DeLuca's 2-methyl- and 2-ethyl-1alpha,25-dihydroxy-19-norvitamin D(3). Thus, the synthesized 2alpha-(3-hydroxypropyl)-1alpha,25-dihydroxy-19-norvitamin D(3) (8a) showed almost the same potency in binding to the bovine thymus vitamin D receptor (VDR) as the natural hormone 1, while its beta-isomer 8b had only a 3% affinity. Both 2alpha-allyl- and 2alpha-propyl-1alpha,25-dihydroxy-19-norvitamin D(3) (6a and 7a) and their 2beta-analogues (6b and 7b) possessed a weak affinity for the VDR. The strong VDR ligand 8a was ca. 36-fold more potent in induction of HL-60 cell differentiation than 1, and interestingly, even the weaker ligand 8b showed a 6.7-fold higher potency in the cell differentiation activity than that of 1. 相似文献
23.
Moisture of several ppm in hydrogen chloride gas has been accurately determined by a new method using the Karl Fischer reagent, which hitherto had a limitation in accuracy because of a partial reaction with hydrogen chloride when direct titration was employed.The moisture in the sample gas was collected in a cold trap kept at about ?80 °C while the hydrogen chloride gas passed through and the frozen moisture was dissolved by pyridine-methanol (1:1) solvent. a KF Reagent of factoe: 0.35 mg H2O/ml.Titration by the Karl Fischer reagent having the equivalency factor of, e.g., 0.5 mg H2O/ml was carried out and the concentration of the moisture was calculated from the sample volume used. 相似文献
24.
The nitro group in tertiary or secondary aliphatic nitro compounds is replaced by hydrogen or deuterium on treatment with tributyltin hydride or tributyltin deuteride, respectively. 相似文献
25.
Yasushi Mizobe Reiko Ono Yasuzo Uchida Masanobu Hidai Meguru Tezuka Suehiro Moue Atsuhiko Tsuchiya 《Journal of organometallic chemistry》1980,204(3):377-385
Diazoalkane complexes of type [MF(NNCRR′)(dpe)2][BF4] (M = Mo or W; dpe = Ph2PCH2CH2PPh2), which are easily derived from bis(dinitrogen) complexes [M(N2)2(dpe)2], undergo consecutive one- and two-electron oxidations and reductions under voltammetric conditions at a platinum electrode. The ESR spectra of the species generated by the controlled potential electrolysis show that primary oxidation occurred on the metal atom (M = Mo) and reduction on the two nitrogen atoms in the diazoalkane ligands (M = Mo or W). 相似文献
26.
Dipping versus spraying: exploring the deposition conditions for speeding up layer-by-layer assembly
Izquierdo A Ono SS Voegel JC Schaaf P Decher G 《Langmuir : the ACS journal of surfaces and colloids》2005,21(16):7558-7567
Polyelectrolyte film fabrication by successive spraying of polycation and polyanion solutions is described and compared to classic dipping. The poly(styrenesulfonate)/poly(allylamine) system is examined in detail. The influence of various parameters such as spraying time, polyelectrolyte concentration, and effect of film drying during multilayer construction is investigated. It is found that film deposition by spraying is easily controlled and very reliable. The thickness of the multilayers grows linearly with the number of deposition cycles similarly to what is observed when dipping substrates or when polyelectrolyte solutions flow over a surface. The assembly of films is very fast and leads to films with small surface roughness as estimated by atomic force microscopy and X-ray reflectometry. Spray deposition allows achieving regular multilayer growth even under conditions for which dipping fails to produce homogeneous films (e.g., extremely short contact times). Moreover, because drainage constantly removes a certain quantity of the excess material arriving at the surface, one can even skip the rinsing step and, thus, speed up even further the whole buildup process. 相似文献
27.
Yoshizawa M Ono K Kumazawa K Kato T Fujita M 《Journal of the American Chemical Society》2005,127(31):10800-10801
Two molecules of planar MII(acac)2 complexes (M = Pt, Pd, and, Cu; acac = acetylacetonato) are efficiently stacked within an organic-pillared coordination cage, exhibiting characteristic spectroscopies (for M = Pt and Pd) and electron spin-spin coupling (for M = Cu) attributable to metal-metal interaction. 相似文献
28.
The competitive inclusion complexations in the ternary phenacetin/competitors/beta-cyclodextrin (beta-CyD) systems were investigated by the solubility method, where m-bromobenzoic acid (m-BBA) and o-toluic acid (o-TA) were used as competitors. The solubility changes of the drug and competitors as a function of beta-CyD concentration in the ternary systems were formulated using their stability constants and intrinsic solubilities. The decrease in solubility of phenacetin by the addition of competitors could be quantitatively simulated by the formulation, when both drug and competitor give A(L) type solubility diagrams. On the other hand, when one of the guests gives a B(S) type solubility diagram, its solubility change was clearly reflected in that of the another guest, i.e., phenacetin gave an A(L) type solubility diagram in the binary phenacetin/beta-CyD system and o-TA gave a B(S) type diagram in the binary o-TA/beta-CyD system, but in the ternary phenacetin/o-TA/beta-CyD system, a new plateau region appeared in the original A(L) type diagram of phenacetin. This was explained by the solubilization theory of Higuchi and Connors. The solubility analysis of the ternary drug/competitor/CyD systems may be particularly useful for determination of the stability constant of a drug whose physicochemical and spectroscopic analyses are difficult, because they can be calculated by monitoring the solubility change of a competitor, without monitoring that of a drug. Furthermore, the present results suggest that attention should be paid to the type of the phase solubility diagram, as well as the magnitude of the stability constant and the solubility of the complex, for a rational formulation design of CyD complexes. 相似文献
29.
A simple, specific and sensitive micro-scale method for the assay of the antiarrhythmic agent mexiletine in human serum is described. The method uses high-performance liquid chromatography, with pre-column fluorimetric derivatization by fluorescamine. Following extraction with diethyl ether, mexiletine and 4-methylmexiletine (an internal standard) were derivatized with fluorescamine under weakly alkaline condition (pH 9.0) and chromatographed on a reversed-phase column with aqueous methanol-2-propanol as the mobile phase. The two fluorescent derivatives of mexiletine and the internal standard were separated as clear single peaks, and no interfering peaks were observed on the chromatograms. The detection limit for mexiletine was 0.005 micrograms/ml from only 100 microliters of serum, and the calibration curves in the range 0.01-5 micrograms/ml were linear, with an overall coefficient of variation of less than 5%. The analytical recovery of a known amount of mexiletine added to serum was almost 100%. This method proved to be effective in the rapid monitoring of the serum concentrations in patients who received this potent antiarrhythmic drug. 相似文献
30.