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   3239篇
  免费   109篇
  国内免费   17篇
化学   2042篇
晶体学   20篇
力学   128篇
数学   456篇
物理学   719篇
  2023年   29篇
  2021年   58篇
  2020年   70篇
  2019年   67篇
  2018年   48篇
  2017年   36篇
  2016年   81篇
  2015年   68篇
  2014年   69篇
  2013年   145篇
  2012年   185篇
  2011年   200篇
  2010年   105篇
  2009年   89篇
  2008年   171篇
  2007年   173篇
  2006年   168篇
  2005年   144篇
  2004年   88篇
  2003年   87篇
  2002年   67篇
  2001年   52篇
  2000年   43篇
  1999年   35篇
  1998年   41篇
  1997年   36篇
  1996年   37篇
  1995年   32篇
  1994年   38篇
  1993年   44篇
  1992年   44篇
  1991年   32篇
  1990年   33篇
  1989年   30篇
  1988年   26篇
  1987年   22篇
  1986年   32篇
  1985年   24篇
  1984年   39篇
  1983年   29篇
  1982年   38篇
  1981年   43篇
  1980年   48篇
  1979年   29篇
  1978年   33篇
  1977年   42篇
  1976年   35篇
  1975年   32篇
  1974年   25篇
  1973年   28篇
排序方式: 共有3365条查询结果,搜索用时 33 毫秒
101.
The mono-ortho chlorobiphenyls (CBs) 60, 74, 114, 123, 157, 167 and 189 were determined in Aroclor mixtures and aquatic organisms by multidimensional gas chromatography with electron capture detection (MDGC/ECD), using a combination of an Ultra 2 and an FFAP column. MDGC/ECD is recommended as the most suitable technique for direct determination of these CBs, without a liquid chromatographic (LC) pre-separation of mono-ortho CBs from the other CBs. Dependent, to some extent, on the stationary phase used, single-column determinations of these CBs easily yield too high results due to the presence of interferences. The contribution of the mono-ortho CBs studied to the total 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) equivalents (TEQs) from CBs in fish is highly dependent on the toxic equivalency factors (TEFs) used and varies between 0.02 and 22%. This contribution is mainly due to CBs 74, 114, 157 and 167 which may easily be determined by taking three heart-cuts and combining them into one MDGC/ECD run. The analytical error is negligible compared with the huge uncertainty of the TEFs. A need for more precise TEFs is emphasized.  相似文献   
102.
The cross-polarization magic angle spinning 13C NMR spectra of Hg(SbF6)2 - 2 Arene (Arene = C6HMe5, 1,2,4,5-C6H2Me4, 1,2,3,4-C6H2Me4, or C6H6) have been measured. The spectra of the complexes of C6HMe5 and 1,2,4,5-C6H2Me4 are consistent with static η1-bonding of the mercury to the arene at an unsubstituted carbon atom, while the spectra of the 1,2,3,4-C6H2Me4 and C6H6 complexes show the arene to have time-averaged Cs or C2, and C6 symmetry respectively, at the temperature of measurement (300 K).The reduced temperature 13C NMR spectra of Hg(Arene)n2+ (n = 1 or 2; Arene = 1,3,5-C6H3R3 (R = Me, i-Pr, or t-Bu)) in SO2 solution are also reported and affirm that in these intramolecularly mobile species the mercury bonds in an η1-manner, with unsubstituted aryl carbon atoms being the strongly preferred point of mercury attachment. This site preference is further demonstrated by the solution 13C NMR spectra of Hg(Arene)n2+ (Arene = 1,2,3,4-C6H2-Me4, n = 1 or 2; Arene = 1,4-C6H4R2, R = Me or t-Bu, n = 1). The spectra of the 1,4-C6H4R2 complexes and Hg(p-C6H4-t-BuMe)2+ provide clear evidence for steric influence of the binding site.Like Hg(C6Me6)22+, but unlike most of the complexes of substituted benzenes which have been studied, Hg(1,3,5-C6H3-i-Pr3)22+ exchanges only slowly with excess free ligand.  相似文献   
103.
X-ray diffraction analysis reveals the thiogermanic acid H(4)Ge(4)S(10) possesses discrete adamantane-like Ge(4)S(10)(4)(-) complex anions. Each thioanion is composed of four corner shared GeS(2.5)(-) tetrahedral units. Crystals were grown from anhydrous liquid hydrogen sulfide reactions with glassy germanium sulfide at room temperature. The crystal structure was solved and refined from single crystal diffractometer data (Mo Kalpha radiation) obtained at 173 K. H(4)Ge(4)S(10) is triclinic, centrosymmetric space group Ponemacr;, with a = 8.621(4) A, b = 9.899(4) A, c = 10.009(4) A, alpha = 85.963(7) degrees, beta = 64.714(7) degrees, gamma = 89.501(8) degrees, and Z = 2. Average bridging and terminal d(Ge-S) distances are 2.229 and 2.206 A, respectively. Vibrational mode assignments are reported from Raman scattering and IR absorption spectra of polycrystalline samples. The nu(s)(Ge-S-Ge) and nu(s)(Ge-S(-)) stretching modes are observed at 354 and 405 cm(-)(1), respectively.  相似文献   
104.
2,4-Diamino-5,7-dihydro-6H-thiopyrano[4′,3′:4,5]thieno[2,3-d]pyrirnidine, 2,4-diamino-9H-mdeno[1′,2′:4,5]thieno[2,3-d]pyrimidine, 2,4-diamino-5H-indeno[2′,1′:4,5]thieno[2,3-d]pyrimidine, 9,11-diamino-5,6-dihydronaphtho[1′,2′:4,5]thieno[2,3-d]pyrimidine, 7,9-diamino-5,6-dihydronaphtho[2′,1′:4,5]thieno[2,3-d]pyrimidine, 2,4-diamino-7-benzy]-5,6,7,8-tetrahydropyrido[4′,3′:4,5]thieno[2,3-d]pyrimidine, and various 2,4-diamino-5,6,7,8-tetrahydro-[1]benzothieno[2,3-d]pyrimidines were synthesized by cyclization of the requisite fused 2-aminothio-phenene-3-carbonitriles utilizing chloroformamidine hydrochloride in diglyme. Several compounds exhibited strong inhibitory effects against Streptococcus faecalis (MGH-2), Staphylococcus aureus (UC-76), Streptococcus faecium (ATCC 8043), Lactobacillus casei (ATCC 7469), and Pediococcus cerevisiae (ATCC 8081) in vitro, and three compounds displayed antimalarial activity against Plasmodium berghei in mice and P. falciparum (Uganda I) in vitro.  相似文献   
105.
Temperature-programmed desorption mass spectrometry (TPD-MS) measurements on [(18)O]water-enriched copper sulfate pentahydrate (CuSO(4).5H(2)(18)O) reveal an unambiguous occurrence of efficient oxygen isotope exchange between the water of crystallization and the sulfate in its CuSO(4) solid phase. To the best of our knowledge, the occurrence of such an exchange was never observed in a solid phase. The exchange process was observed during the stepwise dehydration (50-300 degrees C) of the compound. Specifically, the exchange promptly occurs somewhere between 160 and 250 degrees C; however, the exact temperature could not be resolved conclusively. It is shown that only the fifth, sulfate-associated, anionic H(2)O molecule participates in the exchange process and that the exchange seems to occur in a preferable fashion with, at the most, one oxygen atom in SO(4). Such an exchange, occurring below 250 degrees C, questions the common conviction of unfeasible oxygen exchange under geothermic conditions. This new oxygen exchange phenomenon is not exclusive to copper sulfate but is unambiguously observed also in other sulfate- and nitrate-containing minerals.  相似文献   
106.
The electrochemical oxidation of indium metal in cells of the type leads to the formation of RInX2 compounds; if 2,2′-bipyridine is also present, the products are the adducts RInX2·bipy (R = CH3, C2H5, C6H5, C6H5CH2, C6F5; X = Cl, Br, I (not all combinations)). When R′4NX is present instead of bipy, the products are the salts R′4N[RInX3]. The electrochemical oxidation apparently proceeds via the general mechanism discussed previously. Anomalous results with CH3I or C2H5I are discussed in the light of the known solution chemistry of organoindium(III) compounds.  相似文献   
107.
108.
109.
Abstract

An analytical procedure for the sampling and quantitative determination of biogenic hydrocarbons in the atmosphere is presented. The method is based on the adsorptive preconcentration of the hydrocarbons using Tenax TA/Carbopack B filled sampling tubes followed by thermodesorption of the trapped compounds and gas chromatographic analysis. To avoid losses of the biogenic alkenes as a consequence of their reaction with ozone on the adsorbents during the sampling step, an ozone scrubber is used in front of the adsorption tubes. Diurnal variations of the monoterpene- and ozone-concentrations are determined at different heights in and above a forest stand (coniferous and deciduous trees) in the Vosges (Col du Donon, 760 m a.s.l.) in order to elucidate the importance of terpene-oxidation reactions on the formation of peroxidic products in forest air.  相似文献   
110.
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