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   653篇
  免费   6篇
  国内免费   2篇
化学   581篇
晶体学   2篇
力学   4篇
数学   19篇
物理学   55篇
  2022年   4篇
  2021年   4篇
  2019年   4篇
  2018年   5篇
  2017年   5篇
  2016年   10篇
  2015年   6篇
  2014年   13篇
  2013年   31篇
  2012年   33篇
  2011年   43篇
  2010年   11篇
  2009年   21篇
  2008年   39篇
  2007年   33篇
  2006年   31篇
  2005年   34篇
  2004年   38篇
  2003年   42篇
  2002年   27篇
  2001年   10篇
  2000年   4篇
  1999年   10篇
  1998年   12篇
  1997年   4篇
  1996年   7篇
  1992年   3篇
  1991年   7篇
  1989年   5篇
  1988年   5篇
  1987年   8篇
  1986年   5篇
  1985年   12篇
  1984年   8篇
  1983年   3篇
  1982年   5篇
  1981年   12篇
  1980年   9篇
  1979年   9篇
  1978年   11篇
  1977年   11篇
  1976年   9篇
  1975年   12篇
  1974年   3篇
  1973年   6篇
  1972年   4篇
  1971年   3篇
  1970年   3篇
  1969年   5篇
  1967年   5篇
排序方式: 共有661条查询结果,搜索用时 171 毫秒
101.
We propose a class of displacement- and laser-noise-free gravitational-wave-interferometer configurations, which does not sense nongeodesic mirror motion and laser noise, but provides a nonvanishing gravitational-wave signal. Our interferometers consist of four mirrors and two beam splitters, which form four Mach-Zehnder interferometers. By contrast to previous works, no composite mirrors with multiple reflective surfaces are required. Each mirror in our configuration is sensed redundantly, by at least two pairs of incident and reflected beams. Displacement- and laser-noise-free detection is achieved when output signals from these four interferometers are combined appropriately. Our 3-dimensional interferometer configuration has a low-frequency response proportional to f2, which is better than the f3 achievable by previous 2-dimensional configurations.  相似文献   
102.
We investigated the effects of NaF, NaCl, NaBr, and NaI on the molecular organization of H2O by a calorimetric methodology developed by us earlier. We use the third derivative quantities of G pertaining to 1-propanol (1P) in ternary 1P-a salt-H2O as a probe to elucidate the effects of a salt on H2O. We found that NaF and NaCl worked as hydration centers. The hydration numbers were 19 +/- 2 for NaF and 7.5 +/- 0.6 for NaCl. Furthermore, the bulk H2O away from the hydration shell was found unaffected by the presence of Na+, F-, and Cl-. For NaBr and NaI, in addition to the hydration to Na+, Br- and I- acted like a hydrophilic moiety such as urea. Namely, they formed a hydrogen bond to the existing H2O network and retarded the fluctuation nature of H2O. These findings were discussed with respect to the Hofmeister ranking. We suggested that more chaotropic anions Br- and I- are characterized as hydrophiles, whereas kosmotropes, F- and Cl-, are hydration centers.  相似文献   
103.

Enzymes are gradually increasingly preferred over chemical processes, but commercial enzyme applications remain limited due to their low stability and low product recovery, so the application of an immobilization technique is required for repeated use. The aims of this work were to produce stable enzyme complexes of cross-linked xylanase on magnetic chitosan, to describe some characteristics of these complexes, and to evaluate the thermal stability of the immobilized enzyme and its reusability. A xylanase was cross-linked to magnetite particles prepared by in situ co-precipitation of iron salts in a chitosan template. The effect of temperature, pH, kinetic parameters, and reusability on free and immobilized xylanase was evaluated. Magnetization, morphology, size, structural change, and thermal behavior of immobilized enzyme were described. 1.0?±?0.1 μg of xylanase was immobilized per milligram of superparamagnetic chitosan nanoparticles via covalent bonds formed with genipin. Immobilized xylanase showed thermal, pH, and catalytic velocity improvement compared to the free enzyme and can be reused three times. Heterogeneous aggregates of 254 nm were obtained after enzyme immobilization. The immobilization protocol used in this work was successful in retaining enzyme thermal stability and could be important in using natural compounds such as Fe3O4@Chitosan@Xylanase in the harsh temperature condition of relevant industries.

  相似文献   
104.
A novel thermosetting imide compound having a respective phenylethynyl carbonyl group at both terminal ends was newly synthesized from an acid anhydride having a phenylethynyl carbonyl group and various diamine compounds. The thermosetting behavior of the obtained novel thermosetting imide compound having phenylethynyl carbonyl groups was analyzed through differential scanning calorimetry measurements and infrared spectroscopic analysis. As a result, it became clear that a curing reaction of phenylethynyl carbonyl groups proceeds at approximately 200°C and that the curing reaction thereof proceeds at a temperature that is lower by 150°C or more compared with that of phenylethynyl groups. Examination of the polymerization reaction of the imide compounds having phenylethynyl carbonyl groups using model compounds revealed that a reaction that imparted an alkene C=C and polycyclic aromatic structure progressed. Moreover, a network polymer obtained from a thermosetting imide compound having respective phenylethynyl carbonyl groups at both terminal ends exhibited extremely superior heat resistance and thermal decomposition resistance. These superior thermal properties are thought to be due to the strong molecular interaction (molecular packing) that results from the polycyclic aromatic structures and alkenes produced through polymerization of the phenylethynyl carbonyl groups and to the suppression of the movement of the molecular chains.  相似文献   
105.
The excess partial molar enthalpy of 1-propanol (1P), H(E) (1P), was experimentally measured in ternary 1P-[NaPF(6), NaCF(3)SO(3) (OTF) or NaN(SO(2)CF(3))(2) (TFSI)]-H(2)O system. From the H(E) (1P), the enthalpic 1P-1P interaction function, H(E) (1P-1P), which is the compositional derivative of H(E) (1P), was evaluated graphically. On addition of the Na salt, the x(1P)-dependence pattern of H(E) (1P-1P) showed a characteristic change. This induced change is used as a probe to elucidate the effect of the sample Na-salt on H(2)O. Because we know the effect of Na(+) from our previous work, we show that each anion works as an amphiphile with hydrophobic and hydrophilic effects. Furthermore, the present method can quantify its relative hydrophobicity and hydrophilicity separately. The results indicate that the relative hydrophobicity ranking was in the order of TFSI(-) > PF(6-) approximately OTF(-), and the hydrophilicity TFSI(-) > PF6(- )> OTF-. Namely, TFSI- is the strongest amphiphile with the strongest hydrophobicity and the strongest hydrophilicity among the ionic liquid (IL) anions studied here. Using our earlier similar studies for normal ions, we map their relative hydrophobicity/hydrophilicity scales on a two-dimensional map together with those of the IL ions. The resulting map shows that the typical constituent ions for "ionic liquids" are strong amphiphiles; with more strongly hydrophobic and more strongly hydrophilic propensities than normal ions. Although the number of data points is limited, the melting points of ionic liquids consisting of TFSI(-) with the strongest hydrophobicity and the strongest hydrophilicity within the anions studied here are the lowest.  相似文献   
106.
β-Selective galactosylation was studied using a series of 2-O-benzylated phenyl 1-thio-galactosides and glycosyl acceptors in propionitrile with BSP-TTBP-Tf2O. The glycosylation enabled us to synthesize useful precursors of N-acetyllactosamine and core 1 O-glycoserine derivatives in a highly convergent manner.  相似文献   
107.
108.
An efficient free energy (FE) calculation of a water molecule to go across lipid membranes is presented. Both overlapping distribution and cavity insertion Widom methods are complementarily used. The former is useful for a dense region where water molecules are found, i.e., from the interfacial to bulk water region, while the latter works well in the low density region, i.e., the hydrocarbon region. Since both methods evaluate the excess chemical potential of water, the obtained FE profile is free from the fitting problem usually arisen when a FE difference method is used. A diphytanyl phosphatidylcholine bilayer is used for our test calculations. An excellent and fast convergence of the chemical potential is obtained when each method is applied for the appropriate region. The estimated FE barrier using the Ewald method for the electrostatic interaction is approximately 7.2 kcal/mol, which is higher than that using the interaction cutoff of 20 A by about 0.9 kcal/mol.  相似文献   
109.
We synthesized a series of bis(bromomethyl)‐1,10‐phenanthrolines as novel anticancer lead compounds and examined their DNA‐binding properties. 5,6‐Bis(bromomethyl)‐1,10‐phenanthroline showed DNA intercalating activity and DNA crosslinking activity, furthermore it is stable in aqueous solution.  相似文献   
110.
The synthesis of three- and four-layered [3.3]paracyclophanes ([3.3]PCPs) 3-5 has been accomplished by utilizing the (p-ethylbenzenesulfonyl)methyl isocyanide (EbsMIC) method. The structures of the three- to four-layered [3.3]PCPs 3- 5 and their diones 8, 10, and 11 have been elucidated based on the (1)H NMR spectra and finally by X-ray structural analysis. In the three-layered [3.3]PCP-dione 8, the trimethylene bridges of the [3.3]PCP unit assume a chair conformation similar to that of 2, while the [3.3]PCP-2,11-dione unit assumes a boat conformation different from that of [3.3]PCP-dione 1 with a chair conformation. On the other hand, the two [3.3]PCP units in three-layered [3.3]PCP 3 both assume a boat conformation. In the four-layered [3.3]PCP-dione 10, the two outer [3.3]PCP units assume a boat conformation while the inner dione unit has a chair conformation. The trimethylene bridges in the four-layered [3.3]PCP 4 are highly disordered even at -150 degrees C. All the outer benzene rings are distorted into a boat form while the inner ones are distorted into a twist form. In the electronic spectra, bathochromic shift and hyperchromic effect are observed, but the magnitude decreases with an increase in the number of layers and the spectra become structureless. In the charge-transfer (CT) bands of the three- to four-layered [3.3]PCPs 3- 5 with tetracyanoethylene (TCNE), two absorption maxima (lambda(max)) are observed. The effect of an increase in the layers becomes significant, and the changes in the longest wavelength lambda(max) values from two to three and three to four are ca. 60 and 50 nm, respectively. By comparison of the stereoisomeric four-layered [3.3]PCPs 4 (meso) and 5 (racemic), the helical arrangement of the trimethylene bridges of 5 shows a more efficient transannular pi-electronic interaction. In the three- to four-layered [3.3]PCP-diones, a magnitude of the CT interaction almost comparable to that of [3.3]PCP 2 was observed, and this indicates that the -CH(2)COCH(2)- bridges inhibit the CT interaction and that this tendency is supported by the calculated HOMO energy levels and observed oxidation potentials. Three- and four-layered [3.3]PCPs 3- 5 show reversible redox processes, and 4 and 5 show an electron-donating ability almost comparable to that of [3 6]CP. Very good correlation between the lambda(max) of the CT bands with TCNE and the oxidation potentials is observed.  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