全文获取类型
收费全文 | 1006篇 |
免费 | 33篇 |
国内免费 | 3篇 |
专业分类
化学 | 598篇 |
晶体学 | 7篇 |
力学 | 29篇 |
数学 | 80篇 |
物理学 | 328篇 |
出版年
2023年 | 16篇 |
2022年 | 22篇 |
2021年 | 28篇 |
2020年 | 30篇 |
2019年 | 30篇 |
2018年 | 25篇 |
2017年 | 24篇 |
2016年 | 33篇 |
2015年 | 15篇 |
2014年 | 35篇 |
2013年 | 53篇 |
2012年 | 49篇 |
2011年 | 61篇 |
2010年 | 37篇 |
2009年 | 35篇 |
2008年 | 57篇 |
2007年 | 54篇 |
2006年 | 53篇 |
2005年 | 40篇 |
2004年 | 41篇 |
2003年 | 35篇 |
2002年 | 18篇 |
2001年 | 20篇 |
2000年 | 15篇 |
1999年 | 9篇 |
1998年 | 4篇 |
1997年 | 6篇 |
1995年 | 8篇 |
1994年 | 10篇 |
1993年 | 9篇 |
1992年 | 8篇 |
1991年 | 12篇 |
1990年 | 13篇 |
1989年 | 9篇 |
1988年 | 13篇 |
1987年 | 10篇 |
1986年 | 7篇 |
1985年 | 10篇 |
1984年 | 11篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 10篇 |
1980年 | 14篇 |
1979年 | 3篇 |
1978年 | 7篇 |
1977年 | 5篇 |
1976年 | 3篇 |
1974年 | 5篇 |
1973年 | 3篇 |
1933年 | 3篇 |
排序方式: 共有1042条查询结果,搜索用时 62 毫秒
951.
Lin HY Gonyea G Killeen S Chowdhury SK 《Rapid communications in mass spectrometry : RCM》2000,14(6):520-522
The most intense ion(s) in negative ion fast atom bombardment (FAB) mass spectra of 2- and 4-benzaldehyde sulfonic acid (BSA) in glycerol or 3-nitrobenzyl alcohol matrix corresponds to a covalent association of the analyte with one or two matrix molecules accompanied by the elimination of a molecule of water. The molecular ion [M - H](-), however, is of low abundance. The identity of the resulting ions [M + nA - H(2)O - H](-) (where M is the analyte and A is the matrix) was confirmed by exact mass measurement using the peak matching technique. These covalent matrix-analyte complexes were not observed when the sulfonic acid functionality in BSA was substituted with COOH, NO(2), and OH or when the sulfonic acid was in salt form. These observations indicate that the free sulfonic acid group in BSA is responsible for the covalent adduct formation. To our knowledge, analyte-matrix covalent association in negative ion FAB spectra of BSA has not been reported previously. 相似文献
952.
1,1'-bis(tert-butoxycarbonylamino)ferrocene (6), a protected derivative of 1,1'-diaminoferrocene, has been synthesized by a very convenient method and serves as a synthon for 1,1'-diaminoferrocene. Its structure in solid state and in solution has been studied by NMR and X-ray crystallography. 1,1'-bis(tert-butoxycarbonylamino)ferrocene serves as starting material for the synthesis of amino acid conjugates of L- and D-alanine. The structures of these bioconjugates have been studied by NMR and CD spectroscopy and X-ray crystallography and reveal that the chiral organization of the podant amino acid chains is controlled by the chirality of the attached amino acid. The substituents engage in strong intramolecular H-bonding generating 14-membered H-bonded rings, a motif previously unrealized in ferrocene-amino acid and peptide conjugates. 相似文献
953.
Mojumdar S. C. Chowdhury B. Varshney K. G. Mazanec K. 《Journal of Thermal Analysis and Calorimetry》2004,78(1):135-144
The results, presented here discusses the Macro-Defect-Free (MDF) cements prepared from the blends of sulfoaluminate ferrite
belite (SAFB) clinkers, ordinary Portland cement (OPC), Al2O3 and poly(butyl acrylate) (PBA), styrene/acrylonitrile co-polymer (SACP) or sodium polyphosphate (poly-P). Though MDF cements
have several attractive properties, their utilization has been limited due to the insufficient moisture resistance. It is
a very challenging task for scientists and technologist to improve the moisture resistance of MDF cements. Therefore, the
new MDF cements were subjected to various moist atmospheres to investigate their moisture resistance. The most significance
of this work is the improvement of moisture resistance of the studied MDF cements. The aim of this work was to understand
the effects of polymers, Al2O3, OPC and SAFB clinkers in the raw mix and delayed drying on MDF cements and also on their subsequent moisture resistance
and thermal stability as well as to discover the new properties of these materials. Their chemical, thermal and scanning electron
microscopic (SEM) analysis was also carried out before and after exposure to moisture. PBA was found to be the most suitable
polymer for MDF cement synthesis, since the samples containing PBA showed the highest resistance to moisture. There are three
main temperature regions on TG curves of both series of MDF cement samples. The significant differences in SEM of MDF cements
before and after moisture attack and with different polymers were observed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
954.
Halder M Chowdhury PK Das R Mukherjee P Atkins WM Petrich JW 《The journal of physical chemistry. B》2005,109(41):19484-19489
The photophysics of hypericin have been studied in its complex with two different isoforms, A1-1 and P1-1, of the protein glutathione S-transferase (GST). One molecule of hypericin binds to each of the two GST subunits. Comparisons are made with our previous results for the hypericin/human serum albumin complex (Photochem. Photobiol. 1999, 69, 633-645). Hypericin binds with high affinity to the GSTs: 0.65 microM for the A1-1 isoform and 0.51 microM for the P1-1 isoform (Biochemistry 2004, 43, 12761-12769). The photophysics and activity of hypericin are strongly modulated by the binding protein. Intramolecular hydrogen-atom transfer is suppressed in both cases. Most importantly, while there is significant singlet oxygen generation from hypericin bound to GST A1-1, binding to GST P1-1 suppresses singlet oxygen generation to almost negligible levels. The data are rationalized in terms of a simple model in which the hypericin photophysics depends entirely upon the decay of the triplet state by two competing processes, quenching by oxygen to yield singlet oxygen and ionization, the latter of these two are proposed to be modulated by A1-1 and P1-1. 相似文献
955.
The 4A2 → 2E transition of potassium chromicyanide has been studied under magnetic field. It has been found that the g-value is nearly isotropic, quite unlike the case of ruby. An attempt has been made to explain the zero-field splittings of 2E, 2T1 and 2T2 states and the g-value of the 2E state on the assumption that the distant ions are mainly responsible for lowering of symmetry of the crystal field from Oh. A reasonable choice of parameters can make the calculated values agree with the observed ones. 相似文献
956.
Reactivity of ionic liquids 总被引:2,自引:0,他引:2
Shahana Chowdhury Author Vitae Author Vitae Janet L. Scott Author Vitae 《Tetrahedron》2007,63(11):2363-2389
Ionic liquids are becoming widely used in synthetic organic chemistry and yet relatively little attention has been paid to the intrinsic reactivity of these low temperature molten salts. Clues to the non-innocent nature of many ionic liquids are contained in the reports of altered reactivity of dissolved substrates, unexpected catalytic activity and unforeseen by-product formation. In this review, we focus on the reactivity of ionic liquids, as opposed to reactivity in ionic liquids (although discussion of the latter is often included where it aids understanding of the former). 相似文献
957.
M.A.Chowdhury M.M.Alam M.F.Mina F.Akhtar S.E.Kabir 《高分子科学》2004,(3):253-258
Hydrogels have been synthesized from 10%, 20%, 30%, 40%, 50% and 60% aqueous solutions of acrylamide monomer by gamma radiation employing doses in the range of 0.2-30 kGy from a Co-60 source. The effect of solution concentration, y-ray dose, pH and time was studied in order to observe the optimizing conditions in the characterization of hydrogels. Gel fraction increases with dose for all concentrations indicating hundred-percentage conversion of gel at doses ≥5 kGy for homogenous solutions in the range of 20%-50% concentration. On the other hand, 10% solution provides conversion less than 86% even at 30 kGy, whereas 60% monomer makes an inhomogeneous solution which stile gives about 100% gel fraction. Swelling of hydrogels under water with respect to time varies with both the doses and concentrations due to the change of crosslinking density in the gels. The maximum volume change of hydrogels during swelling and water desorption mainly occurs within 24 h. Swelling is also enhanced with the rise of pH due to change of ionic content of the solvent. Considering the amount of gel fraction and the properties of hydrogel, the samples prepared from 20% solution at 5 kGy show better results. Moreover, the effect of bacteria on hydrogel was found to be nil, suggesting a prohibition of growth of microorganism in it. 相似文献
958.
Haldar D Banerjee A Drew MG Das AK Banerjee A 《Chemical communications (Cambridge, England)》2003,(12):1406-1407
A terminally protected acyclic tetrapeptide Boc-Aib-Val-Aib-beta-Ala-OMe 1 (Aib: alpha-aminoisobutyric acid, beta-Ala: beta-Alanine) self-assembles into a continuous hydrogen-bonded supramolecular helix with an average diameter of 10 A (1 nm) starting from a double bend molecular conformation in crystals and further self-assembly of this supramolecular architecture leads to the formation of polydisperse nanorods of diameters 10-40 nm. 相似文献
959.
A new copper(II) complex [Cu(HL)(ClO4)](ClO4) (1), where HL is a multidentate Schiff base N,N′-(2-hydroxypropane-1,3-diyl)bis(pyridine-2-aldimine), is prepared, structurally characterized by X-ray crystallography and its spectral and electrochemical properties studied. The complex forms a one-dimensional chain in the solid state structure in which the monomeric Cu(HL) units are linked by the perchlorate ligand. The complex has an axially elongated six coordinate geometry (4+2) with a CuN4O2 core in which the Schiff base ligand displays a tetradentate mode of bonding in the basal plane. The axial ligand is perchlorate with a significantly long Cu–O bond of ca. 2.6 Å. The one-electron paramagnetic complex displays a cyclic voltammetric response for the Cu(II)/Cu(I) couple at 0.01 V versus SCE in MeCN–0.1 M TBAP. The azomethine bond of the Schiff base in 1 on treatment with H2O2 undergoes oxidative conversion to form a bis(picolinato)copper(II) · dihydrate species through the formation of an amido intermediate as evidenced from the solution infrared spectral studies. 相似文献
960.
Summary Application of hexamethylphosphoramide (HMPA) in the iron(III)-thiocyanate system gives a simple, sensitive and highly selective method for the spectrophotometric determination of Fe(III) in the organic phase (CHCl3). The color is stable for more than 24 hours in CHCl3. The system obeys Beer's law for 7g Fe/ml and the optimum concentration range is found to be 0.5–5g/ml. The molar absorptivity of 1.33×104 l·mole–1·cm–1, and Sandell's sensitivity is 0.0044/cm2 at 460 nm The composition of Fe(III) to thiocyanate is found to be 13 in the presence of HMPA in CHCl3. The method is free from interferences of many anions and cations. The method has been used to determine iron in bauxite ore.
Flüssig-flüssig-Extraktion von Eisenrhodanid nach Zusatz von Hexatnethyl-pbosphoramid: Direkte spektrophotometrische Bestimmung in der organischen Phase
Zusammenfassung Der Zusatz von Hexamethylphosphoramid (HMPA) zu Eisen(III)-Thiocyanat führt zu einer einfachen, empfindlichen und sehr selektiven Methode der spektrophotometrischen Bestimmung von Fe(III) in organischer Phase (CHCl3). Die Farbe bleibt in Chloroform mehr als 24 Stunden beständig. Das System entspricht dem Beerschen Gesetz unterhalb 7g Fe/ml. Das Konzentrationsoptimum liegt zwischen 0,5 und 5g/ml. Die molare Extinktion beträgt 1,33·104 l·Mol–1·cm–1, die Empfindlichkeit nach Sandell 0,0044g/cm2 bei 460 nm. Das Verhältnis Fe(III): Rhodanid in CHCl3 ist 13 in Gegenwart von HMPA. Die Methode wird von vielen Anionen und Kationen nicht gestört. Sie wurde zur Bestimmung von Eisen in Bauxit verwendet.相似文献