全文获取类型
收费全文 | 3512篇 |
免费 | 101篇 |
国内免费 | 15篇 |
专业分类
化学 | 2169篇 |
晶体学 | 15篇 |
力学 | 196篇 |
数学 | 744篇 |
物理学 | 504篇 |
出版年
2023年 | 16篇 |
2022年 | 38篇 |
2021年 | 98篇 |
2020年 | 73篇 |
2019年 | 51篇 |
2018年 | 65篇 |
2017年 | 57篇 |
2016年 | 111篇 |
2015年 | 104篇 |
2014年 | 80篇 |
2013年 | 186篇 |
2012年 | 206篇 |
2011年 | 247篇 |
2010年 | 147篇 |
2009年 | 121篇 |
2008年 | 202篇 |
2007年 | 221篇 |
2006年 | 162篇 |
2005年 | 155篇 |
2004年 | 146篇 |
2003年 | 117篇 |
2002年 | 98篇 |
2001年 | 44篇 |
2000年 | 25篇 |
1999年 | 37篇 |
1998年 | 34篇 |
1997年 | 34篇 |
1996年 | 36篇 |
1995年 | 26篇 |
1994年 | 30篇 |
1993年 | 47篇 |
1992年 | 20篇 |
1991年 | 27篇 |
1990年 | 29篇 |
1989年 | 17篇 |
1988年 | 26篇 |
1987年 | 22篇 |
1986年 | 23篇 |
1985年 | 47篇 |
1984年 | 41篇 |
1983年 | 24篇 |
1982年 | 25篇 |
1981年 | 29篇 |
1980年 | 27篇 |
1979年 | 22篇 |
1978年 | 26篇 |
1977年 | 23篇 |
1976年 | 22篇 |
1975年 | 13篇 |
1974年 | 21篇 |
排序方式: 共有3628条查询结果,搜索用时 515 毫秒
81.
Wolfgang Oppolzer Giovanni Poli Arend J. Kingma Christian Starkemann Grald Bernardinelli 《Helvetica chimica acta》1987,70(8):2201-2214
The 1,4-addition of alkylmagnesium chlorides to conjugated N-enoylsultams and subsequent ‘enolate trapping’ with aq. NH4Cl or MeI/hexamethylphosphoric triamide generated centers of asymmetry at C(β) and/or at C(α) with good to excellent π-face defferentiation as demonstrated by the conversions 1 → 2 , 1 → 4 , and 8 → 9 . This holds also for the regioselective 1,4-addition of EtMgC1 to a dienoylsultam ( 15 → 16 ). Reactive conformations 1 ≠, 8 ≠, 13 , and 14 are postulated in agreement with X-ray evidence which also served for the structure determination of the product 9j . 相似文献
82.
Paolo Domiano Amos Musatti Mario Nardelli Corrado Pelizzi Giovanni Predieri 《Transition Metal Chemistry》1979,4(6):351-354
Summary The x-ray crystal structure of the title complex is described Crystals are monoclinic, space groupP21/n, with unit-cell dimensions:a=18.070(2),b=13.471(2),c=6.788(2) Å,=94.70(1),Z=4. The structure was solved from diffractometer data by Patterson and Fourier methods and refined by least-squares techniques toR=5.0% for 2451 independent reflections. It consists of complex molecules, in which the copper atom square planar coordination comprises the chlorine atom, Cu-Cl=2.240(3) Å, and the organic ligand which acts as terdentate through the oxygen atom [Cu-O=1.948(3) Å] and a nitrogen atom, [Cu-N=1.933(5) Å] from the hydrazidic chain and the oxygen atom, [Cu-O = 1.894(4) Å] from the pyridoxal group. 相似文献
83.
In presence of tin(II) bromide, noble metals give coloured products which are suitable for spcctrophotometric determinations. The colours are red (platinum), yellow-orange (rhodium), yellow-brown (palladium), yellow (iridium) and violet (gold) They are extracted, except for gold, with isoamyl alcohol Platinum, rhodium and palladium can be separated from irdium, and rhodium and platinum from palladium. Rhodium and platinum can be determined simultaneously. 相似文献
84.
Giuseppe Fachinetti Giovanni Fochi Carlo Floriani 《Journal of organometallic chemistry》1973,57(2):C51-C54
Reactions of azobenzene with dicarbonyldicyclopentadienyltitanium(II), Ti(π-C5H5)2(CO)2, and dicyclopentadienylvanadium(II), V(π-C5H5)2, have yielded the corresponding dicyclopentadienylmetal-azobenzene complexes. 相似文献
85.
86.
Giovanni Brigati Paola Franchi Marco Lucarini Gian Franco Pedulli Luca Valgimigli 《Research on Chemical Intermediates》2002,28(2-3):131-141
An EPR investigation of the kinetics of the exit, k -, and entrance, k +, processes in micelles of sodium dodecyl sulfate, hexadecyltrimethylammonium bromide and polyoxyethylene(6)decanol of a family of para-substituted benzyl tert -butyl nitroxides and para-substituted benzyl hydroxyalkyl nitroxides is reported. The inclusion of nitroxide probes in the hydrophobic environment of the micelle gives rise to a reduction of the value of both nitrogen and β-proton splittings, with the result that the resonance fields for the MI(2H β ) = ±1lines of the free and included species are significantly different. The rate constants were obtained by analyzing the EPR line shape variations as function of surfactant concentration and temperature. The experimental value of k + obtained from the study of benzyl tert-butyl nitroxide indicates that the association reaction is very close to being controlled by diffusion. The value of the exit rate, k -, instead, depends on the probe hydrocarbon chain length. A comparison of our results with those obtained by luminescence quenching techniques is also reported. 相似文献
87.
Giancarlo Cravotto Marina Beggiato Giovanni Palmisano Stefano Tollari Werner Bonrath 《Tetrahedron letters》2005,46(13):2267-2271
Pd-catalyzed homo- and cross-couplings of boronic acids and aryl halides were successfully carried out both in aqueous media under high-intensity ultrasound (US) and in DME under microwave (MW). Heterogeneous catalysis with Pd/C was employed, avoiding phosphine ligands and phase-transfer catalysts. In a trial series involving 15 different iodo- and bromoaryls and 7 boronic acids, both energy sources drastically reduced reaction times affording biaryls in acceptable to good yields. With palladium(II) acetate as catalyst, electron-deficient aryl chlorides also reacted, affording a few biaryls in acceptable yields. Ullmann-type zinc-mediated homocoupling of iodo- and bromoaryls in the presence of Pd/C under CO2 atmosphere was achieved in aqueous media under US, though not under MW. Suzuki homo- and cross-couplings were also carried out in a new reactor developed in our laboratory, featuring combined US and MW irradiation, further improving a green synthetic method. 相似文献
88.
The preparation and crystal structure determination of the iron(III) compound of formula [(NH(4))(2)[Fe(2)O(ox)(2)Cl(2)].2H(2)O](n) (1) (ox = oxalate dianion) are reported here. Complex 1 crystallizes in the orthorhombic system, space group Fdd2, with a = 14.956(7) A, b = 23.671(9) A, c = 9.026(4) A, and Z = 8. The structure of complex 1 consists of the chiral anionic three-dimensional network [Fe(2)O(ox)(2)Cl(2)](2-) where the iron(III) ions are connected by single oxo and bisbidentate oxalato groups. The metal-metal separations through these bridging ligands are 3.384(2) and 5.496(2) A, respectively. Ammonium cations and crystallization water molecules are located in the helical pseudohexagonal tunnels defined by iron atoms. The longest iron-iron distance in the pseudohexagonal tunnel is 15.778(2) A whereas the shortest one is 8.734(2) A. The iron atoms are hexacoordinated: a terminal chloro ligand and five oxygen atoms, that of the oxo group and four from two cis coordinated oxalate ligands, build a distorted octahedral environment around the metal atom. The Fe-O(oxo) bond distance [1.825(2) A] is significantly shorter than the Fe(III)-O(ox) [average value 2.103(4) A] and Fe(III)-Cl bond distances [2.314(2) A]. Magnetic susceptibility measurements of 1 in the temperature range 2.0-300 K reveal the occurrence of a susceptibility maximum at 195 K and a transition toward a magnetically ordered state in the lower temperature region with T(c) = 40 K. The strong antiferromagnetic coupling through the oxo bridge (J = -46.4 cm(-1), the Hamiltonian being H = -JS(A).S(B)) accounts for the susceptibility maximum whereas a weak spin canting of approximately 0.3 degrees due to the antisymmetric magnetic exchange within the chiral three-dimensional network is responsible for the magnetic ordering. The values of coercive field (H(c)) and remnant magnetization (M(r)) obtained from the hysteresis loop of 1 at 5 K are 4000 G and 0.016 micro(B). 相似文献
89.
Conformational properties of polymers, such as average dihedral angles or molecular alpha-helicity, display a rather weak dependence on the detailed arrangement of the elementary constituents (atoms). We propose a computer simulation method to explore the polymer phase space using a variant of the standard multicanonical method, in which the density of states associated to suitably chosen configurational variables is considered in place of the standard energy density of states. This configurational density of states is used in the Metropolis acceptance/rejection test when configurations are generated with the help of a hybrid Monte Carlo algorithm. The resulting configurational probability distribution is then modulated by exponential factors derived from the general principle of the maximal constrained entropy by requiring that certain average configurational quantities take preassigned (possibly temperature dependent) values. Thermal averages of other configurational quantities can be computed by using the probability distributions obtained in this way. Moments of the energy distribution require an extra canonical sampling of the system phase space at the desired temperature, in order to locally thermalize the configurational degrees of freedom. As an application of these ideas we present the study of the structural properties of two simple models: a bead-and-spring model of polyethylene with independent hindered torsions and an all-atom model of alanine and glycine oligomers with 12 amino acids in vacuum. 相似文献