首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   1244篇
  免费   26篇
  国内免费   6篇
化学   1046篇
晶体学   13篇
力学   8篇
数学   48篇
物理学   161篇
  2023年   5篇
  2022年   8篇
  2021年   8篇
  2020年   13篇
  2019年   15篇
  2018年   7篇
  2017年   8篇
  2016年   22篇
  2015年   22篇
  2014年   39篇
  2013年   35篇
  2012年   64篇
  2011年   89篇
  2010年   43篇
  2009年   44篇
  2008年   82篇
  2007年   88篇
  2006年   89篇
  2005年   76篇
  2004年   83篇
  2003年   59篇
  2002年   74篇
  2001年   22篇
  2000年   17篇
  1999年   11篇
  1998年   9篇
  1997年   15篇
  1996年   18篇
  1995年   10篇
  1994年   13篇
  1993年   8篇
  1992年   13篇
  1991年   9篇
  1990年   12篇
  1989年   6篇
  1988年   9篇
  1987年   7篇
  1986年   7篇
  1985年   13篇
  1984年   15篇
  1983年   13篇
  1982年   15篇
  1981年   15篇
  1980年   9篇
  1979年   4篇
  1978年   6篇
  1977年   5篇
  1976年   6篇
  1975年   5篇
  1968年   3篇
排序方式: 共有1276条查询结果,搜索用时 16 毫秒
951.
In the course of our investigation on biologically active substances from inedible mushrooms in Japan, Germany, and Vietnam, we studied the chemical constituents of 22 species belonging to five families: Scutigeraceae, Polyporaceae, Xylariaceae, Thelephoraceae, and Paxillaceae. Various types of chemical substances were purified and characterized based on the modern spectroscopic methods and also on chemical reactions. These metabolites have shown a broad activity in many biological systems, such as antimicrobial, nematicidal, inhibition of NO production, anti-human immunodeficiency virus, tumor necrosis factor-alpha, and antioxidant activities. These isolated metabolites did not only show interesting activities, but also are employed as chemical markers supported for chemosystematics of these families. This review paper deals with the chemical constituents of 22 species, their biological activities, and also a discussion on chemosystematics.  相似文献   
952.
Novel supramolecular complexes have been prepared from an alpha-cyclodextrin-beta-cyclodextrin hetero-dimer (alpha-CD-beta-CD hetero-dimer) and hetero-cinnamamide guest dimers, G-t-Boc and G-NH2, having adamantyl groups in aqueous solutions. On addition of the competitive guest, the supramolecular structure formed by a mixture of the alpha-CD-beta-CD hetero-dimer and G-t-Boc was found to be different from that of a mixture of the alpha-CD-beta-CD hetero-dimer and G-NH2 by the 1H NMR spectroscopy, the ROESY NMR spectroscopy, and the circular dichroism spectroscopy. The size of the supramolecular complex from the mixture of the alpha-CD-beta-CD hetero-dimer and G-NH2 is larger than that from the mixture of the alpha-CD-beta-CD hetero-dimer and G-t-Boc, which was proved by the pulse field gradient spin-echo NMR and the atomic force microscopy. These results suggest that the mixture of the alpha-CD-beta-CD hetero-dimer and G-t-Boc formed a pinching-type complex, and the mixture of the alpha-CD-beta-CD hetero-dimer and G-NH2 formed a supramolecular polymer-type complex.  相似文献   
953.
The electronic and infrared spectra of 2-fluoropyridine-methanol clusters were observed in a supersonic free jet. The structure of hydrogen-bonded clusters of 2-fluoropyridine with methanol was studied on the basis of the molecular orbital calculations. The IR spectra of 2-fluoropyridine-(CH3OH)n(n = 1-3) clusters were observed with a fluorescence-detected infrared depletion (FDIR) technique in the OH and CH stretching vibrational regions. The structures of the clusters are similar to those observed for 2-fluoropyridine-(H2O)n (n = 1-3) clusters. The existence of weak hydrogen bond interaction through aromatic hydrogen was observed in the IR spectra. The theoretical calculation also supports the result. The vibrational frequencies of CH bonds in CH3 group are affected by hydrogen bond formation although these bonds do not directly relate to the hydrogen bond interaction. The B3LYP/6-311 ++G(d,p) calculations reproduce well the vibrational frequency of the hydrogen-bonded OH stretching vibrations. However, the calculated frequency of CH stretching vibration could not reproduce the IR spectra because of anharmonic interaction with closely lying overtone or combination bands for nu3 and nu9 vibrations. The vibrational shift of nu2 vibration is reproduced well with molecular orbital calculations. The calculation also shows that the frequency shift of nu2 vibration is closely related to the CH bond length at the trans position against the OH bond in hydrogen-bonded methanol.  相似文献   
954.
Antihyperglycemic effect of insulin from self-dissolving micropiles in dogs   总被引:1,自引:0,他引:1  
As a percutaneous delivery device, self-dissolving micropiles (SDMPs) composed of chondroitin sulfate and insulin were prepared under room temperature from highly concentrated solution, glue. The mean weight of SDMP was 1.03+/-0.04 mg. One insulin SDMP was percutaneously administered to the shaved abdominal skin of four beagle dogs at insulin dose level of 1.0 and 2.0 IU/dog. After administration, blood samples were collected for 6 h and plasma glucose levels were measured. The time when minimum plasma glucose level appeared, T(min), was 1.38+/-0.2 h for 1.0 IU study and 1.38+/-0.1 h for 2.0 IU study and clear dose-dependent hypoglycemic effect of insulin was observed in the dose range. By comparing the area above the plasma glucose level vs. time curve (AAC) between insulin SDMP and subcutaneous (s.c.) injection solution, the relative pharmacological availabilities were 99% (1.0 IU) and 90% (2.0 IU), respectively. To ascertain the usefulness of insulin SDMP, oral glucose tolerance test (OGTT) was performed. When dogs were treated with insulin SDMPs, 2.0 IU, followed by an OGTT 30 min, glycemia did not appear for 5 h. On the other hand, when OGTT was performed at 1 h after insulin SDMP administration, hypoglycemia appeared as in the case of s.c. injection of insulin solution, 2.0 IU. Insulin SDMP improved the oral glucose challenge for 3 h, with a maximum effect at 30 min before the administration of glucose. Those results suggest the usefulness of a SDMP for the percutaneous delivery of peptide/protein drugs like insulin.  相似文献   
955.
A liquid chromatographic tandem mass spectrometric (LC-MS/MS) assay was developed and validated to determine valproic acid in human plasma. The method involved a solid-phase extraction of valproic acid and betamethasone valerate, an internal standard, from plasma and detection using an LC-MS/MS system with electrospray ionization source in negative ion mode. Separation was achieved within 3 min on a non-porous silica column with mobile phase containing ammonium acetate and methanol. Multiple reaction monitoring was utilized for detection monitoring at 142.89-142.89 for valproic acid and 457.21-457.21 for the internal standard. The calibration curve for valproic acid was linear over the range of 0.5-150 microg/mL. The limit of detection was 0.17 microg/mL and the lower limit of quantification was 0.5 microg/mL, when 0.2 mL plasma was used for extraction. The percentage coefficient of validation for accuracy and precision (inter- and intra-day) for this method was less than 9.5% with recovery ranging from 82.3 to 86.9% for valproic acid.  相似文献   
956.
Polybutadienes (PBs) are found to form inclusion complexes with cyclodextrins (CDs) stereoselectively to give crystalline compounds in bulk. These complexes have been isolated and characterized by means of 1H NMR and 13C CP/MAS NMR spectroscopy, and X‐ray diffraction. Although α‐CD did not form inclusion complexes with any kinds of PBs in aqueous solutions, α‐CD did form inclusion complexes with PBs having 1,4‐cis‐ and 1,4‐trans‐butadiene units in bulk by heating at 100 °C. On the other hand, PB having 79% of a 1,2‐structure did not form inclusion complexes with α‐CD. The yield of the inclusion complexes increases with an increase in the content of the 1,4‐cis‐structure of PB and decreases with the molecular weights of the PBs.

  相似文献   

957.
A new pyrochlore-type Na0.32Bi1.68Ti2O6.46(OH)0.44 with the cubic cell of a=10.339(5) Å was prepared by hydrothermal reaction using TiO2 (anatase) and Bi2O3 in NaOH solution. This compound was obtained when the molar ratio of NaOH/TiO2 was above 2 and the reaction temperature was above 240 °C. The TG-curve of as-prepared sample showed a mass loss of 0.8 mass% which was caused by release of OH group. This compound decomposed to a pyrochlore-type compound and a layered-type Na0.5Bi4.5Ti4O15 above 800 °C. The optical band gap of Na0.32Bi1.68Ti2O6.46(OH)0.44 was estimated to be 2.5 eV.  相似文献   
958.
Ferroelectricity as well as characteristic proton-transfer dynamics are achieved by combining a 2,3,5,6-tetra(2'-pyridyl)pyrazine (TPPZ) molecule with anilic acids (H2xa). Dielectric measurements revealed phase transitions at T(c) = 334 and 172 K for bromanilate (Hba(-)) and chloranilate (Hca(-)) salts, respectively. The room-temperature ferroelectricity of the (H2-TPPZ)(Hba)2 crystal is evidenced by the slow polarization reversal with modest pyroelectricity. In accord with the observed large deuteration effect, synchrotron X-ray diffraction studies disclosed proton dynamics in an intramolecular N...H(+)...N bond of the H2-TPPZ(2+) dication and in an O-H...O(-) hydrogen-bonded cyclic dimer of the ortho-quinoid Hxa(-) anions. The disordered (Hxa(-))2 dimer in two-fold orientation manifests its double-proton transfer process above T(c), whereas these protons are ordered in the ferroelectric phase. The H2-TPPZ(2+) dication acts as a proton sponge by forming two intramolecular N...H(+)...N hydrogen bridges between the pyridyl units with a very short N...N distance. The dication in the paraelectric state adopts a nonpolar geometry due to the delocalization of the protons over two sites in the respective N...H(+)...N bonds. Below T(c), only one of the two protons gets localized, and the resultant acentric H2-TPPZ(2+) ion generates the dipole moment responsible for the ferroelectricity.  相似文献   
959.
Low-density tin dioxide (SnO2) is required for radiating monochromatic extreme ultraviolet (EUV) light with low debris and high conversion efficiency from a laser. In this paper, tin dioxide nanoparticle hollow microcapsules were successfully fabricated by a layer-by-layer template technique. The obtained capsules have a rougher surface (30 nm in rms) compared to the freshly prepared polyelectrolyte capsules. Based on the X-ray diffraction (XRD) results, the tin dioxide nanoparticles well maintained their size after they were assembled on the capsules' surfaces. In order to remove the polymer template, a heat treatment was introduced, and after the heat treatment the capsule sizes shrank about 71% (the average size was from 4.9 to 3.5 mum), and the obtained capsules maintained their round shape after water evaporation. The narrowest bandwidth at the 13.5 nm emission in the EUV region was observed when the capsules were irradiated by a CO2 laser with an intensity of 2.9 x 10(10) W/cm (2).  相似文献   
960.
Regioselective halogenations and Suzuki reactions ensure proper linkage of the aromatic rings in two total syntheses of terprenin ( 1 ). Both routes make it possible to prepare 1 efficiently and in large quantity.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号