全文获取类型
收费全文 | 1493篇 |
免费 | 30篇 |
国内免费 | 12篇 |
专业分类
化学 | 1125篇 |
晶体学 | 21篇 |
力学 | 20篇 |
数学 | 99篇 |
物理学 | 270篇 |
出版年
2023年 | 6篇 |
2021年 | 5篇 |
2020年 | 14篇 |
2019年 | 25篇 |
2018年 | 18篇 |
2017年 | 13篇 |
2016年 | 25篇 |
2015年 | 30篇 |
2014年 | 36篇 |
2013年 | 89篇 |
2012年 | 86篇 |
2011年 | 93篇 |
2010年 | 52篇 |
2009年 | 68篇 |
2008年 | 82篇 |
2007年 | 85篇 |
2006年 | 81篇 |
2005年 | 83篇 |
2004年 | 96篇 |
2003年 | 84篇 |
2002年 | 70篇 |
2001年 | 26篇 |
2000年 | 18篇 |
1999年 | 15篇 |
1998年 | 20篇 |
1997年 | 21篇 |
1996年 | 23篇 |
1995年 | 11篇 |
1993年 | 7篇 |
1992年 | 17篇 |
1991年 | 17篇 |
1990年 | 13篇 |
1989年 | 13篇 |
1988年 | 9篇 |
1987年 | 19篇 |
1986年 | 6篇 |
1985年 | 20篇 |
1984年 | 10篇 |
1983年 | 10篇 |
1982年 | 7篇 |
1981年 | 12篇 |
1980年 | 9篇 |
1979年 | 8篇 |
1978年 | 8篇 |
1977年 | 11篇 |
1976年 | 9篇 |
1975年 | 7篇 |
1974年 | 8篇 |
1973年 | 11篇 |
1969年 | 5篇 |
排序方式: 共有1535条查询结果,搜索用时 13 毫秒
71.
Seiko Nan'ya Etur Maekawa Hitoshi Hayakawa Yukio Kitaguchi Yoshio Ueno 《Journal of heterocyclic chemistry》1985,22(6):1483-1485
The 5H-pyrido[2,3-a]phenoxazin-5-one derivatives and 5H-pyrido[3,2-a]phenoxazin-5-one derivatives were prepared by the condensation of substituted 2-aminophenols with 6,7-dibromo-5,8-quinolinequinone followed by dehalogenation in the presence of sodium hydrosulfite dissolved in aqueous pyridine under a nitrogen atmosphere. 相似文献
72.
Miyasaka H Nezu T Sugimoto K Sugiura K Yamashita M Clérac R 《Inorganic chemistry》2004,43(18):5486-5488
Heterometallic linear tetramers [Mn(5-R-saltmen)Ni(pao)(bpy)(2)](2)(ClO(4))(4) (5-R-saltmen(2-) = N,N'-1,1,2,2-tetramethylethylene bis(5-R-salicylideneiminate); pao(-) = pyridine-2-aldoximate; bpy = 2,2'-bipyridine, R = H, 1; Cl, 2; Br, 3; MeO, 4) have been synthesized and structurally characterized. These compounds exhibit a [Ni(II)-NO-Mn(III)-(O)(2)-Mn(III)-ON-Ni(II)] skeleton where -ON- is an oximate bridge between Mn(III) and Ni(II) ions and -(O)(2)- is a bi-phenolate bridge between Mn(III) ions. These tetramers can be seen as oligomeric units of the heterometallic Mn(III)(2)-Ni(II) chain observed in a family of single-chain magnets (Clérac, R.; Miyasaka, H.; Yamashita, M.; Coulon, C. J. Am. Chem. Soc. 2002, 124, 12837. Miyasaka, H.; Clérac, R.; Mizushima, K.; Sugiura, K.; Yamashita, M.; Wernsdorfer, W.; Coulon, C. Inorg. Chem. 2003, 42, 8203.). Magnetic measurements on these tetramers confirm the nature of the magnetic interactions reported for the Mn(III)(2)-Ni(II) chains: a strong antiferromagnetic Mn(III)/Ni(II) coupling via the oximate bridge (J(Ni-Mn) ranges from -23.7 to -26.1 K) and a weak ferromagnetic Mn(III)/Mn(III) coupling through the bi-phenolate bridge (J(Mn-Mn) ranges from +0.4 to +0.9 K). These magnetic interactions lead to tetramers with an S = 2 ground state. 相似文献
73.
Miyasaka H Nezu T Sugimoto K Sugiura K Yamashita M Clérac R 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(5):1592-1602
Mn(III)-Ni(II)-Mn(III) linear-type trinuclear complexes bridged by oximate groups were selectively synthesized by the assembly reaction of [Mn2(5-Rsaltmen)2(H2O)2](ClO4)2 (5-Rsaltmen2-=N,N'-(1,1,2,2-tetramethylethylene) bis(5-R-salicylideneiminate); R=Cl, Br) with [Ni(pao)2(phen)] (pao-=pyridine-2-aldoximate; phen=1,10-phenanthroline) in methanol/water: [Mn2(5-Rsaltmen)2Ni(pao)2(phen)](ClO4)2 (R=Cl, 1; R=Br, 2). Structural analysis revealed that the [Mn(III)-ON-Ni(II)-NO-Mn(III)] skeleton of these trimers is in every respect similar to the repeating unit found in the previously reported series of 1D materials [Mn2(saltmen)2Ni(pao)2(L1)2](A)(2) (L(1)=pyridine, 4-picoline, 4-tert-butylpyridine, N-methylimidazole; A=ClO4-, BF4-, PF6-, ReO4-). Recently, these 1D compounds have attracted a great deal of attention for their magnetic properties, since they exhibit slow relaxation of the magnetization (also called single-chain magnet (SCM) behavior). This unique magnetic behavior was explained in the framework of Glauber's theory, generalized for chains of ferromagnetically coupled anisotropic spins. Thus, in these 1D compounds, the [Mn(III)-ON-Ni(II)-NO-Mn(III)] unit was considered as an S(T)=3 anisotropic spin. Direct-current magnetic measurements on 1 and 2 confirm their S(T)=3 ground state and strong uniaxial anisotropy (D/k(B) approximately -2.4 K), in excellent agreement with the magnetic characteristic deduced in the study on the SCM series. The ac magnetic susceptibility of these trimers is strongly frequency-dependent and characteristic of single-molecule magnet (SMM) behavior. The relaxation time tau shows a thermally activated (Arrhenius) behavior with tau0 approximately 1x10(-7) s and Delta(eff)/k(B) approximately 18 K. The effective energy barrier for reversal of the magnetization Delta(eff) is consistent with the theoretical value (21 K) estimated from |D| S2T. The present results reinforce consistently the interpretation of the SCM behavior observed in the [Mn2(saltmen)2Ni(pao)2(L1)2](A)2 series and opens new perspectives to design single-chain magnets. 相似文献
74.
75.
Belik AA Azuma M Matsuo A Whangbo MH Koo HJ Kikuchi J Kaji T Okubo S Ohta H Kindo K Takano M 《Inorganic chemistry》2005,44(19):6632-6640
SrCu2(PO4)2 was prepared by the solid-state method at 1153 K. Its structure was solved by direct methods in the space group Pccn (No. 56) with Z = 8 from synchrotron X-ray powder diffraction data measured at room temperature. Structure parameters were then refined by the Rietveld method to obtain the lattice parameters, a = 7.94217(8) A, b = 15.36918(14) A, and c = 10.37036(10) A. SrCu2(PO4)2 presents a new structure type and is built up from Sr2O16 and Cu1Cu2O8 units with Cu1...Cu2 = 3.256 A. The magnetic properties of SrCu2(PO4)2 were investigated by magnetic susceptibility, magnetization up to 65 T, Cu nuclear quadrupole resonance (NQR), electron-spin resonance, and specific heat measurements. With spin-dimer analysis, it was shown that the two strongest spin-exchange interactions between Cu sites result from the Cu1-O...O-Cu2 and Cu2-O...O-Cu2 super-superexchange paths with Cu1...Cu2 = 5.861 A and Cu2...Cu2 = 5.251 A, and the superexchange associated with the structural dimer Cu1Cu2O8 is negligible. The magnetic susceptibility data were analyzed in terms of a linear four-spin cluster model, Cu1-Cu2-Cu2-Cu1 with -2J(1)/kB = 82.4 K for Cu1-Cu2 and -2J(2)/k(B) = 59 K for Cu2-Cu2. A spin gap deduced from this model (Delta/kB = 63 K) is in agreement with that obtained from the Cu NQR data (Delta/kB = 65 K). A one-half magnetization plateau was observed between approximately 50 and 63 T at 1.3 K. Specific heat data show that SrCu2(PO4)2 does not undergo a long-range magnetic ordering down to 0.45 K. SrCu2(PO4)2 melts incongruently at 1189 K. We also report its vibrational properties studied with Raman spectroscopy. 相似文献
76.
Yoshitaka Ogiwara Yuji Kimura Zenjiro Osawa Hitoshi Kubota 《Journal of polymer science. Part A, Polymer chemistry》1977,15(7):1667-1675
Effects of ferric chloride (FeCl3) on photodegradation of high-density polyethylene (PE) were investigated by using ESR and infrared spectrometry. In the system with irradiation at light of λ > 220 nm, PE irradiated at 77°K yielded an 8-line spectrum, the intensity of which was markedly weakened by using FeCl3 with the sample, indicating a distinct depression of radical formation. On the other hand, PE with the use of FeCl3 yielded radicals under irradiation only with light of λ > 300 nm, showing a singlet spectrum with a line width of 15 gauss. For photooxidized PE, almost the same effect of FeCl3 was observed. On irradiation at room temperature, PE samples with and without FeCl3 showed a singlet spectrum with line widths of 15 and 25 gauss, respectively. On the other hand, the unsaturated double bond contained in a small amount in PE sample was observed by infrared study to be decreased with photoirradiation; however, the decrease was sharply depressed with the addition of FeCl3 to the sample. The degradation of carbonyl group in a photooxidized sample was markedly affected by photoirradiation, and the decay was obviously reduced for the sample on addition of FeCl3. It is concluded that FeCl3 works upon photodegradation of PE to enhance the Norrish type II reaction and to accelerate the formation of unsaturated double bond in the chain. 相似文献
77.
Ayumi Yoshida Hitoshi Yamauchi Hideki Sakai Norimichi Kawashima Masahiko Abe 《Colloids and surfaces. B, Biointerfaces》1997,8(6):191-342
Molecular interactions between lipid bilayers (liposomes) and chondroitin sulfate C (CS), a water soluble polymer, have been investigated in terms of zeta-potential, particle size, microscopic-viscosity, microscopic-polarity of liposomes and permeability of calcein. Microscopic morphology is dramatically changed by the addition of CS to the positively charged liposomes (Pos.L), while it is not changed by the addition to uncharged liposomes (Unc.L) or negatively charged liposomes (Neg.L). The absolute value of the particle size of Pos.L increases with the addition of CS, while the zeta- potential of Pos.L decreases. Permeability of Pos.L decreases with an increase in the concentration of CS. Phase transition temperature of Pos.L is changed after the addition of CS. These values, however, are not changed for the other liposomes by the addition of CS. The results of gel filtration chromatography show that CS is absorbed on the Pos.L surface. Microscopic viscosity is also increased by the addition of CS to Pos.L due to the adsorption of CS. 相似文献
78.
Czesław Rudowicz Ireneusz Stefaniuk Roman Dziembaj Hitoshi Ohta Marcin Molenda Susumu Okubo Makoto Yoshida 《Research on Chemical Intermediates》2007,33(8):853-862
This paper deals with the analysis of the temperature dependence of high-frequency EMR (HF-EMR) spectra due to Mn3+ and Mn4+ ions in the lithium manganese spinel LiMn2O4. A range of powder samples obtained by the sol-gel method with calcinations in several temperature ranges were prepared for
this study. Based on the initial characterization carried out by a number of techniques, the physicochemical and structural
properties of the samples were earlier determined. Independently, temperature magnetization and HF-EMR measurements were carried
out. The EMR spectra vary strongly between samples, indicating possible structural or chemical changes. Quantitative analysis
of the temperature dependence of the HF-EMR spectra due to Mn3+ and Mn4+ ions in LiMn2O4 is presented in this paper. The spectral analysis concerns the line shape, linewidth, intensity and g-factors. Fittings using the Lorentzian spectral shape and, to a certain extent, the Gaussian spectral shape have been carried
out in order to parameterize the temperature dependence of the HF-EMR spectra. This parameterization of the HF-EMR experimental
data enables a deeper characterization of the samples. Subsequently, a better insight into the role of the Mn3+ and Mn4+ ions in accounting for the characteristics most suitable for application of LiMn2O4 as a cathode material may be gained. 相似文献
79.
This study shows the ESR spectra of oxoiron(IV) porphyrin pi-cation radicals of 1-8 in dichloromethane-methanol (5:1) mixture. We reported in a previous paper that oxoiron(IV) porphyrin pi-cation radicals of 1-4 are in an a(1u) radical state while those of 5-8 are in an a(2u) radical. The ESR spectra (g( perpendicular)(eff) approximately 3.1 and g( parallel)(eff) approximately 2.0) for the a(1u) radical complexes, 1-4, appear quite different from those reported previously for the oxoiron(IV) porphyrin pi-cation radical of 5 (g(y) = 4.5, g(x) = 3.6, and g(z) = 1.99). The unique ESR spectra of the a(1u) radical complexes rather resemble those of compound I from Micrococcus lysodeikticus catalase (CAT) and ascorbate peroxidase (ASP). This is the first examples to mimic the ESR spectra of compound I in the enzymes. From spectral analysis based on a spin Hamiltonian containing an exchange interaction, the ESR spectra of 1-4 can be explained as a moderate ferromagnetic state (J/D approximately 0.3) between ferryl S = 1 and the porphyrin pi-cation radical S' = (1)/(2). The magnitudes of zero-field splitting (D) for ferryl iron and isotropic J value, estimated from the temperature-dependence of the half-saturation power of the ESR signals, are approximately 28 and approximately +8 cm(-1), respectively. A change in the electronegativity of the beta-pyrrole substituent hardly changes the ESR spectral feature while that of the meso-substituent slightly does owing to the change in the E/D value. On the basis of the present ESR results, we propose the a(1u) radical state for compound I of CAT and ASP. 相似文献
80.
The title compound, (NEt(4))[[Mn(salen)](2)Fe(CN)(6)] (1), was synthesized via a 1:1 reaction of [Mn(salen)(H(2)O)]ClO(4) with (NEt(4))(3)[Fe(CN)(6)] in a methanol/ethanol medium (NEt(4)(+) = tetraethylammonium cation, salen(2)(-) = N,N'-ethylenebis(salicylidene)iminate). The two-dimensional layered structure of 1 was revealed by X-ray crystallographic analysis: 1 crystallizes in monoclinic space group P2(1)/c with cell dimensions of a = 12.3660(8) A, b = 15.311(1) A, c = 12.918(1) A, beta = 110.971(4) degrees, Z = 2 and is isostructural to the previously synthesized compound, (NEt(4))[[Mn(5-Clsalen)](2)Fe(CN)(6)] (5-Clsalen(2-) = N,N'-ethylenebis(5-chlorosalicylidene)iminate; Miyasaka, H.; Matsumoto, N.; Re, N.; Gallo, E.; Floriani, C. Inorg. Chem. 1997, 36, 670). The Mn ion is surrounded by an equatorial salen quadridentate ligand and two axial nitrogen atoms from the [Fe(CN)(6)](3-) unit, the four Fe[bond]CN groups of which coordinate to the Mn ions of [Mn(salen)](+) units, forming a two-dimensional network having [[bond]Mn[bond]NC[bond]Fe[bond]CN[bond]](4) cyclic repeating units. The network is spread over the bc-plane of the unit cell, and the layers are stacked along the a-axis. The countercation NEt(4)(+) is located between the layers. Compound 1 is a ferrimagnet with T(c) = 7.7 K and exhibits hysteresis with a remnant magnetization of 13.44 cm(3).mol(-1) (M/N mu(B) = 2.4) at zero field and a coercivity of 1000 Oe when the powder sample was measured at 1.9 K. Magnetic measurements of a direction-arranged single crystal were also carried out. The orientation of the crystallographic axes of a selected single crystal was determined by X-ray analysis, and magnetization was measured when an external field was applied in the a*, b, and c directions. The magnetization in the a* direction increased more easily than those in the b and c directions below the critical temperature. No hysteresis was observed only for the measurement in the a* direction, indicating the presence of strong structural anisotropy with potential anisotropy on Mn(III) ions. 相似文献