全文获取类型
收费全文 | 52394篇 |
免费 | 2035篇 |
国内免费 | 72篇 |
专业分类
化学 | 33507篇 |
晶体学 | 250篇 |
力学 | 1691篇 |
数学 | 8217篇 |
物理学 | 10836篇 |
出版年
2023年 | 384篇 |
2022年 | 323篇 |
2021年 | 538篇 |
2020年 | 705篇 |
2019年 | 678篇 |
2018年 | 1066篇 |
2017年 | 871篇 |
2016年 | 1601篇 |
2015年 | 1473篇 |
2014年 | 1475篇 |
2013年 | 3072篇 |
2012年 | 3569篇 |
2011年 | 3697篇 |
2010年 | 2229篇 |
2009年 | 1849篇 |
2008年 | 3118篇 |
2007年 | 3138篇 |
2006年 | 2761篇 |
2005年 | 2688篇 |
2004年 | 2298篇 |
2003年 | 1864篇 |
2002年 | 1618篇 |
2001年 | 971篇 |
2000年 | 816篇 |
1999年 | 575篇 |
1998年 | 435篇 |
1997年 | 403篇 |
1996年 | 499篇 |
1995年 | 321篇 |
1994年 | 381篇 |
1993年 | 371篇 |
1992年 | 405篇 |
1991年 | 349篇 |
1990年 | 332篇 |
1989年 | 325篇 |
1988年 | 267篇 |
1987年 | 271篇 |
1986年 | 234篇 |
1985年 | 407篇 |
1984年 | 366篇 |
1983年 | 265篇 |
1982年 | 346篇 |
1981年 | 338篇 |
1980年 | 294篇 |
1979年 | 283篇 |
1978年 | 266篇 |
1976年 | 286篇 |
1975年 | 258篇 |
1974年 | 268篇 |
1973年 | 262篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
A study of the absorption spectra of the food colourants Tartrazine (E-102), Amaranth (E-123) and Curcumin (E-100) in various organic solvents and pH media, have been carried out. In addition, the spectral behaviour in aqueous micellar systems of anionic, cationic and non-ionic surfactants, as well as beta-cyclodextrin, was investigated. Analytical methods are proposed for the determination of the three dyes in mixtures without any prior separation step, by the application of the derivative technique to the normal absorption spectra, based on the use of cetyltrimethylammonium bromide (CTAB) in 0.1 M NaOH, over the concentration range 2-50 mug ml(-1). The methods are applied for determining the three dyes in commercial food products. 相似文献
942.
Nicole Desrosiers Gérald Perron John G. Mathieson Brian E. Conway Jacques E. Desnoyers 《Journal of solution chemistry》1974,3(10):789-806
The apparent molal volume φv, expansibility φE, compressibility φK, and heat capacity φc of NaCl were measured in urea-water mixtures, as a function of salt (<1.5m) and urea (<13m) concentrations at 25°C. At a fixed urea concentration, the transfer functions from H2O to 3m urea are linear functions of the NaCl aquamolality. At a fixed salt aquamolality, (0.1m), the sign of the transfer functions is in the direction of a decrease in the structure-breaking effect, and the absolute values of the transfer functions tend to level off at high urea concentrations (13m). The functions φv, φE, φK, φc, and (?φv/?T)p were measured for the sodium halides and alkali, bromides (chlorides in the case of φK) at a fixed salt aquamolality of 0.1m and fixed urea molality of 3m. The corresponding transfer functions from H2O to 3m urea are opposite those from H2O to D2O and similarly are relatively independent of ionic size. This suggests that urea, shows no specific interaction affinity for ions and that the overall number of water molecules influenced by the ions is relatively constant for all alkali halides. The lithium halides are an exception in that Li+ seems to have hardly any structure-breaking effect. 相似文献
943.
Sánchez-Moreno MJ Gómez-Coca RB Fernández-Botello A Ochocki J Kotynski A Griesser R Sigel H 《Organic & biomolecular chemistry》2003,1(10):1819-1826
The synthesis of (1H-benzimidazol-2-yl-methyl)phosphonic acid, H2(Bimp)+/-, is described: 2-chloromethylbenzimidazole was reacted with ethylchloroformate to give 1-carboethoxy-2-chloromethylbenzimidazole which was treated with trimethyl phosphite and after hydrolysis with aqueous HBr H2(Bimp)+/- was obtained. In H2(Bimp)+/- one proton is at the N-3 site and the other at the phosphonate group; both acidity constants were determined in aqueous solution by potentiometric pH titrations (25 degrees C; I = 0.1 M, NaNO3) and this furnished the pKa values of 5.37 +/- 0.02 and 7.41 +/- 0.02, respectively. The acidity constant for the release of the primary proton from the P(O)(OH)2 group of H3(Bimp)+ was estimated: pKa = 1.5 +/- 0.2. Moreover, Bimp2- can be further deprotonated at its neutral (N-1/N-3)H site to give the benzimidazolate residue, but this reaction occurs only in strongly alkaline solution (KOH); application of the H_ scale developed by G. Yagil (J. Phys. Chem., 1967, 71, 1034) together with UV spectrophotometric measurements gave pKa = 14.65 +/- 0.12. Comparisons with acidity constants taken from the literature show that this latter pKa value is far too large and this allows the conclusion that an intramolecular hydrogen bond is formed between the (N-1/N-3)H site and the phosphonate group of Bimp2-; the formation degree of this hydrogen-bonded isomer is estimated to be 98 +/- 2%. The general relevance of this and the other results are shortly discussed and the species distribution for the Bimp system in dependence on pH is provided. 相似文献
944.
Henriksen T Juhler RK Svensmark B Cech NB 《Journal of the American Society for Mass Spectrometry》2005,16(4):446-455
The purpose of the work presented here was to evaluate the influence of solution composition and analyte characteristics on responsiveness to analysis with negative ion electrospray ionization mass spectrometry. The responses of a series of structurally diverse acidic molecules were compared in various solvents. Response was generally observed to be higher in methanol than acetonitrile and response for all analytes was poorer when water was mixed with the organic solvent. A positive correlation between negative ion ESI-MS response and log P was observed when either acetonitrile or methanol was used as the electrospray solvent. This result was expected because analytes with significant nonpolar character should be particularly responsive to ESI-MS analysis due to their higher affinity for electrospray droplet surfaces. It was also predicted that highly acidic analytes would be most responsive to analysis with negative ion ESI-MS due to their tendency to form negative ions. However, for the analytes studied here, acidity was found not to have a consistent influence on ESI-MS response. Many of the highly acidic molecules were quite polar and, consequently, were poorly responsive. Furthermore, the deprotonated molecular ion was detected for a number of molecules with very high pKa values, which would not be expected to form negative ions in the bulk solution. Ultimately, these results indicate that acidity is not a conclusive parameter for prediction of the relative magnitudes of negative ion ESI-MS response among a diverse series of analytes. Analyte polarity does; however, appear to be useful for this purpose. 相似文献
945.
Based on the similarity in properties of arsenate and phosphate, the colorimetric method using the molybdene blue complex was tested in order to determine low As(V) concentration in waters. The influence of complex formation time, daylight, temperature and competitive anions (silicate and sulphate) upon complex formation was determined. Optimal complex formation was reached in 1 h at 20±1 °C and was slightly favoured when developed in daylight. The formation rate declined with decreasing reaction temperature and no influence of any of the competitive anions tested (at concentrations usually found in natural waters of granitic areas) was noted. The detection limit of this method was 20 μg As(V) l−1. This simple, fast and sensitive arsenic determination method is suitable for field analysis, especially for waters containing low levels of phosphate and organic matter. Through arsenate determination, this colorimetric method allowed the arsenite oxidation efficiency of five common industrial oxidants to be compared. H2O2 and MnO2(s) were not considered as effective oxidants as a high excess was necessary to ensure As(III) oxidation. NaOCl and KMnO4 were promising oxidants as they allowed complete arsenite oxidation with a small excess for NaOCl or even less than the electron stoichiometric ratio in the case of KMnO4. FeCl3 was the most effective oxidant among the reagents tested here. 相似文献
946.
Petrov P Stassin F Pagnoulle C Jérôme R 《Chemical communications (Cambridge, England)》2003,(23):2904-2905
Multi-walled carbon nanotubes (MWNTs) have been solubilized in water and in various organic solvents by noncovalent side-wall functionalization by pyrene containing polymers. 相似文献
947.
García-Granados A López PE Melguizo E Moliz JN Parra A Simeó Y Dobado JA 《The Journal of organic chemistry》2003,68(12):4833-4844
Several triterpenic derivatives, with the A-ring functionalized, were semisynthesized from oleanolic and maslinic acids. The reactivities of sulfites, sulfate, and epoxides in these triterpene compounds were investigated under different reaction conditions. Moreover, contracted A-ring triterpenes (five-membered rings) were obtained, by different treatments of the sulfate 7. From the epoxide 8, deoxygenated and halohydrin derivatives were semisynthesized with several nucleophiles. Ozonolysis and Beckmann reactions were used to yield 4-aza compounds, from five-membered ring olanediene triterpenes. The X-ray structure of sulfate 7 is given and compared with density functional theory geometries. Theoretical (13)C and (1)H chemical shifts (gauge-invariant atomic orbital method at the B3LYP/6-31G*//B3LYP/6-31G* level) and (3)J(H,H) coupling constants were calculated for compounds 5-9 and 34-36, identifying the (R)- or (S)-sulfur and alpha- or beta-epoxide configurations together with 4-aza or 3-aza structures. 相似文献
948.
Zusammenfassung Es wurde die thermische Zersetzung dercis-undtrans-Isomeren des [PtAox
2Cl2] sowie des [PtAox
3Cl]Cl mit Hilfe derTG-undTD-Kurven im Falle des reinen Komplexes sowie im Falle einer Mischung mit Al2O3 (1:3) geklärt. Dietrans-Form ist stabiler als diecis-Form, die Zersetzung wird durch Al2O3 nicht beeinflu\t. [PtAox
3Cl]Cl wird infolge der Erhitzung in die stabileretrans-Form [PtAox
2Cl2] umgewandelt und verhält sich auch weiterhin wie diese.
Mit 4 Abbildung 相似文献
A contribution to the study of the compounds of Pt(II) with acetoxime
The thermal decomposition ofcis- andtrans-[PtAox 2Cl2] and of [PtAox 3Cl]Cl was studied by means ofTGA andTDA on pure substances as well as on mixtures with Al2O3 (1:3). Thetrans form is more stable then thecis form, the presence of Al2O3 has no influence on its decomposition. The [PtAox 3Cl]Cl is transformed by heating into the more stabletrans-isomer and its further behaviour is identical with that of thetrans isomer.
Mit 4 Abbildung 相似文献
949.
Íñigo X. García-Zubiri Gustavo González-Gaitano Miguel Sánchez Josér; Ramón Isasi 《Journal of inclusion phenomena and macrocyclic chemistry》2004,49(3-4):291-302
The inclusion complexes of β-cyclodextrin (β-CD) with naphthalene and some of its derivatives in the solid state have been studied by infrared spectroscopy. Digital subtraction, deconvolution and curve fitting have been used to investigate the interactions between the naphthalene derivatives andβ-CD. Several preparation methods for the solid dispersions have been tested, using FTIR as an effective tool to evaluate the interactions at the molecular level. The effects of temperature and humidity on the spectra have been also analyzed. A carbonyl moiety in the guest molecule can increase the stability of the complex by establishing specific interactions with the hydroxyl groups of the CD cavity rims. The stability of the complexes is higher for 2-naphthylacetate than that for 1-naphthyl acetate, andso is the degree of association of its carbonyl groups. On the other hand, 2-acetylnaphthaleneforms very stable inclusioncomplexes although its carbonyl groups appear to be significantly less associated. 相似文献
950.
Platas-Iglesias C Esteban-Gómez D Enríquez-Pérez T Avecilla F de Blas A Rodríguez-Blas T 《Inorganic chemistry》2005,44(7):2224-2233
Compounds of formula [Pb(L2)(NCS)2] (1) and [Pb(L4)(SCN)2] (2) (where L2 is the lariat crown ether N,N'-bis(3-aminobenzyl)-4,13-diaza-18-crown-6 and L4 is the Schiff-base lariat crown ether N,N'-bis(3-(salicylaldimino)benzyl)-4,13-diaza-18-crown-6) were isolated and structurally characterized by X-ray diffraction analyses. The X-ray crystal structures of both compounds show the metal ion coordinated to the six donor atoms of the crown moiety, leaving the corresponding pendant arms uncoordinated. The coordination sphere of lead(II) is completed by two thiocyanate groups that coordinate either through their nitrogen (1) or sulfur (2) atoms. The organic receptor adopts a syn conformation in 1, while in 2 it shows an anti conformation. To rationalize these unexpected different conformations of the L2 and L4 receptors in compounds 1 and 2, as well as the different binding modes found for the thiocyanate ligands, we have carried out theoretical calculations at the DFT (B3LYP) level. These calculations predict the syn conformation being the most stable in both 1 and 2 complexes. So, the anti conformation found for 2 in the solid state is tentatively attributed to the presence of intermolecular pi-pi interactions between phenol rings, for which the dihedral angle between the least-squares planes of both rings amounts to 2.6 degrees and the distance between the center of both rings is 3.766 A. On the other hand, the analysis of the electronic structure has revealed that the Pb-ligand bonds present highly ionic character in this family of compounds. They also suggest a greater transfer of electron density from the NCS- ligands when they coordinate through the less electronegative S atom. The Pb-SCN covalent bond formation mainly occurs due to an effective overlap of the occupied 3p z orbitals of the S atoms and the unoccupied 6p z AO of the Pb atom, while the Pb-NCS bonding interaction is primarily due to the overlap of the 6s and 7s AO of Pb with sp(1.10) hybrids of the N donor atoms. Our electronic structure calculations can rationalize the different coordination of the thiocyanate groups in compounds 1 and 2: the simultaneous formation of two Pb-SCN bonds is more favorable for S-Pb-S angles close to 180 degrees , for which the overlap between the occupied 3p z orbitals of the S atoms and the unoccupied 6 pz AO of the Pb atom is maximized. 相似文献