全文获取类型
收费全文 | 1558篇 |
免费 | 117篇 |
国内免费 | 115篇 |
专业分类
化学 | 1307篇 |
晶体学 | 13篇 |
力学 | 6篇 |
综合类 | 3篇 |
数学 | 351篇 |
物理学 | 110篇 |
出版年
2023年 | 20篇 |
2022年 | 27篇 |
2021年 | 24篇 |
2020年 | 43篇 |
2019年 | 39篇 |
2018年 | 37篇 |
2017年 | 39篇 |
2016年 | 45篇 |
2015年 | 42篇 |
2014年 | 58篇 |
2013年 | 176篇 |
2012年 | 120篇 |
2011年 | 75篇 |
2010年 | 85篇 |
2009年 | 87篇 |
2008年 | 99篇 |
2007年 | 100篇 |
2006年 | 92篇 |
2005年 | 73篇 |
2004年 | 72篇 |
2003年 | 71篇 |
2002年 | 50篇 |
2001年 | 38篇 |
2000年 | 43篇 |
1999年 | 29篇 |
1998年 | 28篇 |
1997年 | 26篇 |
1996年 | 22篇 |
1995年 | 20篇 |
1994年 | 13篇 |
1993年 | 13篇 |
1992年 | 15篇 |
1991年 | 6篇 |
1990年 | 9篇 |
1989年 | 8篇 |
1988年 | 6篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1970年 | 1篇 |
1967年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有1790条查询结果,搜索用时 0 毫秒
41.
Tetra-coordinated boron derivatives, (EtO)B(DTZ) and (DTZH)B(DTZ), (where DTZ−− and DTZ− represent the anions of the Schiff base DTZH2) have been synthesized by 1:1 and 1:2 molar reactions of triethoxyborane with bibasic tridentate Schiff bases, derived by
the equimolar condensation of S-methyl or S-benzyldithiocarbazate with acetyl acetone or benzoyl acetone. Further 1:1 derivatives
have been shown to undergo replacement reactions witht-butyl alcohol, showing thereby the labile nature of the ethoxy group. Based on infrared and proton magnetic resonance spectral
studies and monomeric nature, suitable structures have been assigned to these derivatives. 相似文献
42.
Houjou H Kanesato M Hiratani K Mandon D 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(18):4576-4583
Novel diiron complexes with an Fe2(mu-OMe)2 core were studied as models of the active site of nonheme iron-containing enzymes. X-ray crystal structures of the complexes showed the existence of two types of ligand folding-parallel and twisted-both of which have four virtually equivalent phenolato groups sticking out from the Fe2O2 rhombic plane. Cyclic voltammetry measurements revealed two or more distinct redox waves in a region of relatively high potential, in addition to known Fe(II)/Fe(III) redox waves in a region of lower potential. These new peaks were assigned to the high-valence state of iron atoms, that is, Fe(III)Fe(IV) and Fe(IV)Fe(IV), resonating with the phenoxyl radical(s). The split width of the redox waves ranged from 0.14 to 0.20 eV, which may be a measure of the electronic interaction of the phenolate groups through the Fe2(mu-OMe)2 core. 相似文献
43.
We have studied the basicity of 2-phenyl-5-R-1,3,4-oxadiazoles (R = H, Me, CH2Ph, t-Bu, CH2Cl, CCl3, CF3) in aqueous sulfuric acid solutions. These compounds are weak organic bases (pKBH
+ is −1.8 to −5.2). The values of pKBH
+ determined on the H0 and X acidity function scales agree well with each other. The substituent at the 5 position has a substantial effect on the
basicity of the 1,3,4-oxadiazole ring.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 748–756, May, 2006. 相似文献
44.
Three novel Schiff base cadmium(II) complexes, derived from the end‐on (μ‐1,1‐N3) azide or end‐to‐end (μ‐1,3‐NCS) thio cyanate bridges and similar tridentate Schiff base ligands, have been synthesized under similar synthetic procedures and their crystal structures determined by X‐ray diffraction methods. They are the dinuclear double end‐on azide‐bridged [Cd2(L1)2(N3)2(μ‐1,1‐N3)2] ( 1 ), the dinuclear double end‐on azide‐bridged [Cd2(L2)2(N3)2(μ‐1,1‐N3)2] ( 2 ), and the dinuclear double end‐to‐end thiocyanate‐bridged [Cd2(L3)2(NCS)2(μ1,3‐NCS)2] ( 3 ), where L1, L2 and L3 are three similar tridentate Schiff bases obtained by condensation of 2‐pyridylaldehyde with N,N‐diethylethane‐1,2‐diamine, of 2‐pyridylaldehyde with N‐isopropylethane‐1,2‐diamine, and of 2‐pyridylaldehyde with N,N‐dimethylpropane‐1,3‐diamine, respectively. Each cadmium(II) centre in the complexes is in a distorted octahedral coordination. There is a crystallographic inversion centre in each of the complexes. The similar small ligands used as the secondary ligands in the preparation of the cadmium(II) complexes with similar Schiff bases can result in similar structures. 相似文献
45.
The electrochemical oxidation of anodic cobalt, nickel, and zinc in acetonitrile containing both 2-pyrrole-[N-(o-hydroxyphenyl)methylimines] (H2L) and a bidentate ligand (1, 10-phenanthroline(phen) or 2,2′-bipyridine(bipy)) yielded compounds of general formula M(HL)2 · phen and M(HL)2 · bipy (M = Co, Ni, Zn). The crystal structure of 2,2′-bipyridine bis{2-[(2-pyrrole)methylimino]5-methylphenolato}nickel(II) was determined by X-ray diffraction. This compound crystallizes in the orthorhombic space group Pccn with a = 19.430(2), b = 28.488(2), c = 17.567(1) Å. The nickel atom has a distorted octahedral geometry, and the pyrrole nitrogen is not coordinated. The IR, 1H-NMR and UV-visible spectra of the complexes are discussed and related to the structure. 相似文献
46.
Biswajit Saha 《Tetrahedron letters》2007,48(8):1379-1383
A mild and efficient protocol for the Pictet-Spengler reaction in water using an acid catalyst has been described. The condensation of tryptophan, tryptamine, and Nb-benzyl tryptophan with different aldehydes having both electron-withdrawing and -donating substituents in the presence of a catalytic amount of TFA in water furnished tetrahydro-β-carbolines in good isolated yields. A salient feature of the water mediated Pictet-Spengler reaction was the general trend observed during the condensation of Trp-OMe and aryl/aliphatic aldehydes furnishing diastereomeric mixtures with a preference for the cis-isomer. 相似文献
47.
Three linear trinuclear Schiff base complexes, {Zn[Zn(CH3COO)(C17H16N2O2)]2} ( 1 ), {Zn[Zn(CH3COO)(C25H20N2O2)]2} ( 2 ), and {Cd[Cd(CH3COO)(C18H18N2O2)]2} ( 3 ), were synthesized for the first time under solvolthermal conditions. Their structures have been characterized by elemental analyses, X-ray single crystal determinations, and infrared spectroscopy. There are three bridges across the M-M atom pairs (M is Zn for 1 and 2 , or Cd for 3 ) in each complex, involving two O atoms of a Schiff base ligand (N,N′-bis(salicylidene)-1, 3-propanediaminate (SALPD2-) for 1 , N, N′-bis(2-hydroxy-naphthalmethenylimino)-1, 3-propanediaminate (NAPTPD2-) for 2 , and N,N′-bis-(salicylidene)-1,4-butanediaminate (SALBD2-) for 3 ), and an O-C-O moiety of a μ-acetato group. In each of the complexes, the central M2+ ion is located on an inversion center and has a distorted octahedral coordination involving four bridging O atoms from two Schiff base ligands in the equatorial plane and one O atom from each bridging acetate group in the axial positions. The coordination around the terminal M2+ ions is irregular square pyramidal, with two O atoms and two N atoms of the Schiff base ligand in the basal plane and one O atom from an acetate group in the apical position. The acetate bridges linking the central and terminal M2+ ions are mutually trans. The M…M distances are 3.050(3) Å in 1 , 3.139(2) Å in 2 , and 3.287(6) Å in 3 . 相似文献
48.
I. Iovel L. Golomba J. Popelis S. Grinberga S. Belyakov E. Lukevics 《Chemistry of Heterocyclic Compounds》2002,38(10):1210-1229
We have studied condensation of 2-, 3-, 4-pyridinealdehydes and 6-methylpyridine-2-aldehyde with 2-aminopyridine and its 3-, 4-, and 6-methyl derivatives in benzene in the presence of molecular sieves. The reactions occur even at room temperature to form the corresponding pyridyl-pyridyl azomethines, and also aminals. We have determined the optimal conditions for carrying out the processes with the aim of obtaining both types of products. We consider the characteristics of the mass spectra for the synthesized aldimines. We present the X-ray diffraction results for the two aminals. 相似文献
49.
Takeda N Kajiwara T Suzuki H Okazaki R Tokitoh N 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(15):3530-3543
The first stable silylene-isocyanide complexes, [Tbt(Mes)SiCNAr] (5 c: Ar=Tip, 5 d: Ar=Tbt, 5 e: Ar=Mes*; Tbt=2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Mes=mesityl, Tip=2,4,6-triisopropylphenyl, Mes*=2,4,6-tri-tert-butylphenyl) were successfully synthesized by the reaction of a kinetically stabilized disilene, [Tbt(Mes)Si=Si(Mes)Tbt] (1), with bulky isocyanides, ArNC (3c-e). The spectroscopic data of 5 c-e and theoretical calculations for a model molecule indicated that 5 c-e are not classical cumulative compounds but the first stable silylene-Lewis base complexes. The reactions of 5 c-e with triethylsilane and 1,3-dienes gave the corresponding silylene adducts, and they underwent isocyanide-exchange reactions in the presence of another isocyanide at room temperature. These results indicate dissociation of complexes 5 c-e to the corresponding silylene 2 and isocyanides 3 c-e under very mild conditions. The reaction of 5 c with methanol gave the MeOH adduct 16, [Tbt(Mes)SiHC(OMe)NTip], which has a hydrogen atom on the silicon atom. This regioselectivity can be explained in terms of the contribution of zwitterionic resonance structures D and E, which have an anion on the silicon atom. This result indicates that 5 c is not a classical cumulene having Si=C double bonds that should react with methanol to give adducts bearing a methoxyl group on the silicon atom. Although the reactions of 5 c-e with electrophilic reagents such as methanol, hydrogen chloride, and methyl iodide gave the formal silylene adducts, the studies on the reaction mechanism by trapping experiments and the observation of the intermediate suggested that the reaction mainly or partially proceeds by initial nucleophilic attack of the silicon atom, as is the case in the formation of 16 in the reaction of 5 c with methanol. It was revealed that 5 c-e show the nucleophilicity of the silicon atom, most likely resulting from the contribution of the zwitterionic resonance structures D and E. 相似文献
50.
Evdoxia Coutouli-Argyropoulou Christos Sideris 《Journal of organometallic chemistry》2006,691(18):3909-3918
Unsymmetrical 1,1′-disubstituted ferrocenes bearing an amino acid moiety and a conjugated electron density controlling substituent were synthesized conveniently starting from 1,1′-ferrocenedicarbaldehyde. The novel ferrocene amino acid derivatives were completely characterized from their MS, 1H NMR and 13C NMR spectra. Their electrochemical behavior was studied by cyclic voltammetry. Their formal redox potentials Ef were slightly influenced by the nature of the amino acid and mainly by the kind of the ethenyl substituent. Furthermore all the (Z)-isomers exhibited a slight anodic shift compared with the corresponding (E)-isomers. 相似文献