全文获取类型
收费全文 | 79380篇 |
免费 | 388篇 |
国内免费 | 383篇 |
专业分类
化学 | 24999篇 |
晶体学 | 799篇 |
力学 | 6753篇 |
数学 | 32133篇 |
物理学 | 15467篇 |
出版年
2019年 | 30篇 |
2018年 | 10444篇 |
2017年 | 10265篇 |
2016年 | 6084篇 |
2015年 | 862篇 |
2014年 | 320篇 |
2013年 | 348篇 |
2012年 | 3851篇 |
2011年 | 10587篇 |
2010年 | 5673篇 |
2009年 | 6088篇 |
2008年 | 6653篇 |
2007年 | 8836篇 |
2006年 | 305篇 |
2005年 | 1391篇 |
2004年 | 1575篇 |
2003年 | 2029篇 |
2002年 | 1067篇 |
2001年 | 282篇 |
2000年 | 323篇 |
1999年 | 165篇 |
1998年 | 199篇 |
1997年 | 162篇 |
1996年 | 226篇 |
1995年 | 140篇 |
1994年 | 85篇 |
1993年 | 113篇 |
1992年 | 69篇 |
1991年 | 76篇 |
1990年 | 74篇 |
1989年 | 69篇 |
1988年 | 70篇 |
1987年 | 71篇 |
1986年 | 77篇 |
1985年 | 63篇 |
1984年 | 57篇 |
1983年 | 54篇 |
1982年 | 52篇 |
1981年 | 52篇 |
1980年 | 58篇 |
1979年 | 57篇 |
1978年 | 57篇 |
1973年 | 40篇 |
1914年 | 48篇 |
1913年 | 40篇 |
1912年 | 40篇 |
1909年 | 41篇 |
1908年 | 41篇 |
1907年 | 32篇 |
1904年 | 32篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
961.
Young-a Lee Sang Myung jung Shin Won kang Ok-sang Jung 《Transition Metal Chemistry》2004,29(7):710-713
A facile procedure for synthesizing the mono(hydroxo)tris(carboxylato)platinum(IV) species has been achieved. The reaction of [PtII(OH)2(dmpda)] (dmpda=2,2-dimethyl-1,3-propanediamine) with a 30% aqueous solution of H2O2 in the presence of a carboxylic acid produces a stable [PtIV(OOCR)3(OH)(dmpda)] (R=Me, Et) complex in high yield. The crystal structures of [PtIV(OOCMe)3(OH)(dmpda)] . H2O (triclinic P1 bar, a=8.761(2) Å, b=9.245(3) Å, c=10.659(2) Å, =106.25(2)°, =93.90(2)°, =98.92(2)°, V=813.1(3) Å3, Z=2, R= 0.0474) and [PtIV(OOCEt)3(OH)(dmpda)] (monoclinic P21/c, a=12.777(4) Å, b=10.514(2) Å, c=14.971(3) Å, =107.40(2)°, V=1919.2(8) Å3, Z=4, R=0.0611) show that the hydroxyl group has been selectively positioned at an axial site. The intramolecular hydrogen bond between the OH and C=O moiety exists (O(H)...=C, 2.83 Å for [PtIV(OOCMe)3(OH)(dmpda)] · H2O; 2.72 Å for [PtIV(OOCEt)3(OH)(dmpda)]. Formation of the axial-mono(hydroxo)tris(carboxylato)platinum(IV) species may be ascribed to a combination of `reactive-equatorial effects' with `cis-addition' in the carboxylic acid. 相似文献
962.
A rapid, robust, sensitive and selective time-based flow injection (FI) on-line solvent extraction system interfaced with electrothermal atomic absorption spectrometry (ETAAS) is described for analyzing ultra-trace amounts of Cr(VI). The sample is initially mixed on-line with isobutyl methyl ketone (IBMK). The Cr(VI) is complexed by reaction with ammonium pyrrolidine dithiocarbamate (APDC), and the non-charged Cr(VI)–PDC chelate formed is extracted into IBMK in a knotted reactor made from PTFE tubing. The organic extractant is separated from the aqueous phase by a gravity phase separator with a small conical cavity and delivered into a collector tube, from which 55 μl organic concentrate is subsequently introduced via an air flow into the graphite tube of the ETAAS instrument. The operations of the FI-system and the ETAAS detector are synchronously coupled. A significant advantage of the approach is that matrix constituents, such as high salt contents, effectively are eliminated. The extraction procedure was optimized by a simplex approach. A central composite design was subsequently employed to verify the estimated operational optimum. An 18-fold enhancement in sensitivity of Cr(VI) was achieved after preconcentration for 99 s at a sample flow rate of 5.5 ml min−1, as compared to direct introduction of 55 μl of sample, yielding a detection limit (3σ) of 3.3 ng l−1. The sampling frequency was 24.2 samples h−1. The proposed method was successfully evaluated by analyzing a NIST Cr(VI)-reference material, synthetic seawater and waste waters, and waste water samples from an incineration plant and a desulphurization plant, respectively. 相似文献
963.
Self-association system of(R)-1,3-butanediol in dilute carbon tetrachloride(CCl4)solution is studied as a model of molecular association mixture.Analysis methods including FSMWEFA(fixed-size moving window evolving factor analysis)combined with PCA(principal component analysis),SIMPLISMA (simple-to-use interactive self-modeling mixture analysis),and ITTFA(iterative target transformation factor analysis)are adopted to resolve infrared spectra of(R)-1,3-butanediol solution.Association number and equilibrium constant are computed.(R)-1,3-butanediol in dilute inert solution is determined as a monomer-trimer equilibrium system.Theoretical investigation of trimer structures is carried out with DFT(density functional theory),and structural factors are analyzed. 相似文献
964.
Keith E. Johnson Richard M. Pagni John Bartmess 《Monatshefte für Chemie / Chemical Monthly》2007,138(11):1077-1101
Summary. A background for studying acids in various solvents is developed, emphasizing the importance of knowing to what extent a solvent
conducts electricity and is therefore ionized, the dissociation equilibria of common molecular solvents and the acidic and
basic species generated by solvent leveling. Acidity measurements in the atypical solvent water are discussed and the common
method of expressing acidity in other systems – by Hammett values – is introduced. Representative examples of reactions involving Br?nsted acids in ionic liquids are presented and attention paid to the questions of speciation and acidity values. It is found that
the gas phase proton affinity of a base is often a better guide to the acidity of its conjugate acid in an ionic liquid than
is the dissociation constant of the said acid in water. 相似文献
965.
Miranda LP Lubell WD Halkes KM Groth T Grøtli M Rademann J Gotfredsen CH Meldal M 《Journal of combinatorial chemistry》2002,4(5):523-529
A novel polymer matrix for solid-phase synthesis, SPOCC(194) resin (1), was designed featuring a backbone of homogeneous tetraethylene glycol (TEG(194)) macromonomer linked by quaternary carbon junctions and terminating in primary alcohol functionality. Beaded SPOCC(194) resin was effectively prepared by suspension polymerization of oxetanylated TEG macromonomer 5 in stirred silicon oil. Mechanically stable and inert to a diverse range of reaction conditions, SPOCC(194) possessed a high hydroxyl group loading (0.9-1.2 mmol/g) for substrate attachment and swelled effectively ( approximately 2-4 mL/g) in a variety of organic and aqueous solvents. Developed for solid-phase synthesis at high reactant concentrations for driving organic and aqueous reactions to completion, SPOCC(194) exhibited high functional group density (mmol/mL) similar to that of low-loaded aminomethylated polystyrene-divinylbenzene copolymer (PS-1%DVB) yet significantly higher than that of PEGA(1900), SPOCC(1500), and TentaGel S. High-resolution MAS NMR spectra of Fmoc-derivatized SPOCC(194) indicate that monitoring of functional group transformation is possible. Moreover, by employment of a nonaromatic resin-linker combination, electrophilic chemistry, such as Lewis acid catalyzed glycosylation and Friedel-Crafts acylation, was selectively performed on substrate bound to SPOCC(194) resin. Such properties make SPOCC(194) resin a promising new polymer matrix for the support-bound construction of small organic molecules by parallel and combinatorial synthesis and the scavenging of solution-phase reactants or byproducts. 相似文献
966.
The synthesis and anion binding properties of new ruthenium(II) and cobalt(II) phenanthroline complexes, containing two amide
subunits are described. Evidence for anion binding in dimethyl sulfoxide (DMSO) solution was obtained from u.v.–vis titration
experiments. Results indicated that these receptors showed strong affinity for F− and AcO−, and showed weak affinity for OH− and H2PO
4
−
, and showed no affinity for Cl−, Br−, I−. These receptors interacted with various anions examined through hydrogen-bond formation. 相似文献
967.
Jingjing Xu Yidong Lu Baohong Liu Chunhe Xu Jilie Kong 《Journal of Solid State Electrochemistry》2007,11(12):1689-1695
An ultrathin, ordered, and packed protein film, consisting of the 2-mercaptoacetic acid (MAA), polydimethyldiallylammonium
chloride (PDDA), and wild-type (WT) photosynthetic reaction center (RC; termed as WT-RC) or its pheophytin (Phe)-replaced
counterpart (termed as Phe-RC), was fabricated by self-assembling technique onto gold electrode for facilitating the electron
transfer (ET) between RC and the electrode surface. Near-infrared (NIR)-visible (Vis) absorption and fluorescence (FL) emission
spectra revealed the influence of pigment substitution on the cofactors arrangement and excitation relaxation of the proteins,
respectively. Square wave voltammetry (SWV) and photoelectric tests were employed to systematically address the differences
between the WT-RC films and mutant ones on the direct and photo-induced ET. The electrochemical results demonstrated that
ET initiated by the oxidation of the primary donor (P) was obviously slowed down, and the formed P+ had more population as well as more positive redox potential in the Phe-RC films compared with those in the WT ones. The
photoelectrochemical results displayed the dramatically enhanced photoelectric performances of the mutant ones, further suggesting
the slow-down formation of final charge-separated state in Phe-RC. The functionalized protein films introduced in this paper
provided an efficient approach to sensitively probe the redox cofactors and ET differences resulting from only minor changes
in pigment arrangement in the pigment–protein complex. The favored ET process observed for the membrane proteins RC was potentially
valuable for a deep understanding of the multi-step biological ET process and development of versatile bioelectronic devices. 相似文献
968.
M. Blaauw I. H. Degenaar J. J. M. de Goeij 《Journal of Radioanalytical and Nuclear Chemistry》2007,271(3):765-770
In order to correct for neutron self-shielding in large-sample prompt gamma NAA, a method has been developed to determine
the macroscopic scattering and absorption cross sections, i.e., Σ
a and Σ
s, using four Cu flux monitors placed around the sample. With Monte Carlo computations, the neutron densities throughout the
sample and the resulting and the corresponding self-shielding factor as calculated from the Σ
a and Σ
s as obtained through the Cu monitors were compared to the true values. The derived Σ
a and Σ
s were found to be sufficiently accurate as long as Σ
t = Σ
a + Σ
s was less than 0.6 cm−1 and Σ
s/Σ
t was greater than 0.1. 相似文献
969.
J. Kaloustian Paulette Lechene De La Porte T. El-Moselhy H. Lafont H. Portugal 《Journal of Thermal Analysis and Calorimetry》2005,82(2):331-338
Summary Cholesterol constitutes the major component of most gallstones. It was identified and determined in gallstones by thermal
analysis technique (DSC and TG-DTA), mainly by the use of the melting temperature (Tonset=145°C and Tmax=149°C) and by DTG peak decomposition (Tmax=364°C). Cholesterol anhydrous (ChA), which showed endothermic polymorphic peak, Tmax=40°C, without mass loss, was differentiated from cholesterol monohydrate (ChH), which showed a broad endothermic peak, Tmax=59°C, attributed to loss of water of crystallization (theoretical 4.45%). Morphological studies of gallstones were performed
by optical microscopy and scanning electron microscopy (SEM). The stones consisted of a pigmented core with a variably-sized
irregular central cavity, surrounded by a radially arranged deposits of plate-like ChH. The outer part of the stones showed
ChA crystal arborescences. X-ray microanalysis gave a typical spectrum rich in C and O, and in some instances the presence
of P, which was attributed to the presence of phospholipids. CaCO3 was easily characterized by TG with the use of DTG decomposition peak at 674°C. 相似文献
970.
In this paper, zinc tetraaminophthalocyanine (Zn-APc) was immobilized on cellulosic fiber by covalent bond to obtain a novel cellulosic fiber supported metallophthalocyanine, named Zn-TDTAPc-F. At pH 11, upon visible light irradiation for 6 h in the presence of O2, Zn-TDTAPc-F was found to be highly effective for the degradation of phenol in aqueous solution, and the degradation rate of phenol was more than 95%. HPLC was used to confirm formic acid, fumaric acid and maleic acid as its main degradation products. 相似文献