全文获取类型
收费全文 | 2930篇 |
免费 | 127篇 |
国内免费 | 23篇 |
专业分类
化学 | 2160篇 |
晶体学 | 15篇 |
力学 | 103篇 |
综合类 | 1篇 |
数学 | 383篇 |
物理学 | 418篇 |
出版年
2024年 | 16篇 |
2023年 | 39篇 |
2022年 | 277篇 |
2021年 | 194篇 |
2020年 | 123篇 |
2019年 | 107篇 |
2018年 | 103篇 |
2017年 | 88篇 |
2016年 | 165篇 |
2015年 | 90篇 |
2014年 | 108篇 |
2013年 | 265篇 |
2012年 | 162篇 |
2011年 | 176篇 |
2010年 | 104篇 |
2009年 | 100篇 |
2008年 | 132篇 |
2007年 | 116篇 |
2006年 | 96篇 |
2005年 | 101篇 |
2004年 | 78篇 |
2003年 | 74篇 |
2002年 | 50篇 |
2001年 | 32篇 |
2000年 | 18篇 |
1999年 | 21篇 |
1998年 | 9篇 |
1997年 | 9篇 |
1996年 | 22篇 |
1995年 | 8篇 |
1994年 | 9篇 |
1993年 | 15篇 |
1992年 | 18篇 |
1991年 | 14篇 |
1990年 | 9篇 |
1989年 | 6篇 |
1988年 | 5篇 |
1986年 | 12篇 |
1985年 | 12篇 |
1984年 | 7篇 |
1983年 | 10篇 |
1982年 | 13篇 |
1980年 | 11篇 |
1979年 | 9篇 |
1978年 | 11篇 |
1977年 | 4篇 |
1976年 | 4篇 |
1975年 | 5篇 |
1974年 | 3篇 |
1973年 | 4篇 |
排序方式: 共有3080条查询结果,搜索用时 26 毫秒
31.
Issa RM Fayed TA Awad MK El-Kony SM 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,62(4-5):980-986
The absorption spectra of mono- and bis-azo-derivatives obtained by coupling the diazonium salts of aromatic amines and 2,7-dihydroxynaphthalene have been studied in six organic solvents. The different absorption bands have been assigned and the effect of solvents on the charge transfer band is also discussed. The diagnostic IR spectral bands and 1H NMR signals are assigned and discussed in relation to molecular structure. Also, semi-empirical molecular orbital calculations using the atom superposition and electron delocalization molecular orbital (ASED-MO) theory have been performed to investigate the molecular and electronic structures of these compounds. According to these calculations, an intramolecular hydrogen bonding is essential for stabilization of such molecules. 相似文献
32.
Khan KM Shujaat S Rahat S Hayat S Atta-ur-Rahman Choudhary MI 《Chemical & pharmaceutical bulletin》2002,50(11):1443-1446
Eight new beta-N-substituted acyl hydrazides along with their corresponding acyl derivatives were synthesized and screened for in vitro beta-glucuronidase inhibition and found to be active against the enzyme. All of these compounds were found to be noncompetitive inhibitors except for N'-(2-cyanoethyl)-4-hydroxy benzohydrazide (10), which was found to be an uncompetitive inhibitor. Structure-activity relationship studies indicated that the benzyloxy group present in compounds 12 and 13 is responsible for the beta-glucuronidase inhibition activity. 相似文献
33.
B. G. Kovalev Fadel' Akhmed Nasser A. M. Sorochinskaya 《Chemistry of Natural Compounds》1995,30(4):515-517
A new synthesis of racemic 10-methyltridecan-2-one, the sex pheromone ofDiabrotica undecimpunctata Howardi has been effected.InterBAV Scientific Production Company RM, Kishinev. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 558–560, July–August, 1994. 相似文献
34.
Mostafa M. Amini Amirreza Azadmher Hamid Reza Bijanzadeh Nasser Hadipour 《Journal of inclusion phenomena and macrocyclic chemistry》2006,54(1-2):77-80
Behavior of {[(CH3)2SnCl2·H2O]2·18-crown-6}n in solution have been investigated by HMBC NMR. The results show that coordination of water to tin containing species and
its hydrogen bonding to crown ether remains intact in non-coordinating solvent in contrast to coordinating solvent. H-NMR
reveals that the chemical shift of water in complex varies by solvent of crystallization. 相似文献
35.
Mohammed Akhter Hossain Keiko Takahashi Hisakazu Mihara Akihiko Ueno 《Journal of inclusion phenomena and macrocyclic chemistry》2002,43(3-4):271-277
-Helix peptides bearing one unit of -cyclodextrin (-CD), one unit of pyrene and one unit of nitrobenzene (NB) in their side chains have been designed and synthesized as novel molecule-responsive devices.In both the CD-peptides,
-PR17 and
-PL17, the NB unit is separated from the CD unit by two turns of the helix. Two reference peptides (PL17, and -P17,) have also been synthesized. The circular dichroism studies in the peptide absorption region (200–250 nm)of
-PR17 and
-PL17 suggestthat the CD-peptides form stable-helixstructures (83–77%), which was destabilized by accommodating guest molecules (e.g., n-pentanol) into the CD cavity. It suggests that formation of intramolecular host–guest(CD–NB) complex stabilized thehelical structure and exogenous guest molecule excluded the appending NB moiety from inside to outside of the CD cavity, thereby causing destabilization of the helical structure and increasing the random coil content. The ICD spectra of the peptides in the pyrene and nitrobenzene absorption region (250–40 nm) suggest that NB forms inclusion complex with CD. The fluorescence studies revealed that the fluorescence of the pyrene unit is quenched by the NB unit in
-PR17 and
-PL17. The fluorescence intensity increases with increasing guest concentration for the CD-peptides.This guest-responsiveenhancement in the fluorescence intensity can be explained in terms of increased distance between the pyrene and NB moieties, which is caused by exclusion of the NB moiety from the CD cavity by guest accommodation. Using the guest-responsive fluorescence quenching properties of the CD-peptides, we have obtained binding constants for various short chain alkanols.
-PL17 has higher binding affinity to the guest molecules than its isomer,
-PR17, indicating that the location of functional groups on the peptide scaffold is important in molecule detection. 相似文献
36.
Gamil A. Al-Hazmi Mohammed S. El-Shahawi Ahmed A. El-Asmy 《Transition Metal Chemistry》2005,30(4):464-470
The redox properties of the title mono- and binuclear copper(II) chelates have been investigated by cyclic voltammetry in DMF at a working platinum electrode. The cathodic reduction and anodic oxidation of the investigated chelates produced the corresponding electrochemical CuI and CuIIIspecies stable only in the voltammetric time scale, The effects of substituents on E1/2, redox properties and stability towards oxidation of the complexes were related to the electron-withdrawing or releasing ability of the substituents on the C=N1[H, CH3 or C6H5] and/or N4H [H, C2H5, C6H5 or pClC6H4] groups, The electron attracting substituents stabilize the Cu(II) complexes while electron-donating groups favor oxidation to Cu(III). Changes in the E1/2 for the complexes due to remote substituent effects could be related to changes in basicity of N4H.Thus, variation in N41-J has more influence on E1/2 than changes in C=N1. The correlation between E1/2 of the complexes and pKa of the ligands has been attributed to the spherical potential generated by the electron density of the donor atoms at the antibonding d orbitals. 相似文献
37.
Aluminium dodecyl sulfate trihydrate [Al(DS)3].3H2O is easily prepared and can be used as a Lewis acid surfactant catalyst in water to conduct the highly efficient Michael addition of indoles and pyrrole to alpha,beta-unsaturated electron-deficient compounds at room temperature. 相似文献
38.
Protection of various thiols with diphenylmethanol was achieved in high yields at room temperature using catalytic amounts of AlPW12O40 in CH2Cl2. In the presence of this catalyst, protection of SH versus OH was achieved with high chemoselectivity and yields. The catalyst can be easily recovered and reused. Deprotection of DPM thioethers was also achieved using molecular iodine at reflux in CH2Cl2 in high yields. 相似文献
39.
The redox behaviour of adsorbed species of 2,4,6-tri(2-pyridyl)-1,3-5-triazine (TPTZ) at the surface of a mercury electrode was examined using cycling voltammetry. Based on the adsorption and accumulation of TPTZ at the charged mercury surface cathodic adsorptive stripping voltammetry (CSV) is applied for the trace determination of TPTZ using a differential pulse (DP) technique. Experimental and operational parameters for the quantitative analysis of TPTZ are optimized and the detection limit was found to be 3×10–9 mol/l. The effect of some interferences, e.g. organic compounds, cations and anions, are investigated. 相似文献
40.
Summary 3,4-Dicyano-5-aminopyrazole, H3,4(CN)25NH2pz (L) reacts either with anhydrous MCl2 or with [M(PPh3)2Cl2] to yield ML4Cl2 complexes (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd or Hg), whose monomeric and covalent natures have been confirmed by their solubility in most non-polar solvents and their low electrical conductivities. The bonding mode of substituted pyrazole is inferred from the position of the (C-N) band in the i.r. spectra. The electronic spectra and the magnetic moments of these compounds were recorded. 相似文献