全文获取类型
收费全文 | 2032篇 |
免费 | 356篇 |
国内免费 | 226篇 |
专业分类
化学 | 1440篇 |
晶体学 | 15篇 |
力学 | 96篇 |
综合类 | 8篇 |
数学 | 286篇 |
物理学 | 769篇 |
出版年
2024年 | 9篇 |
2023年 | 55篇 |
2022年 | 56篇 |
2021年 | 50篇 |
2020年 | 71篇 |
2019年 | 69篇 |
2018年 | 71篇 |
2017年 | 62篇 |
2016年 | 90篇 |
2015年 | 74篇 |
2014年 | 95篇 |
2013年 | 127篇 |
2012年 | 157篇 |
2011年 | 169篇 |
2010年 | 98篇 |
2009年 | 129篇 |
2008年 | 122篇 |
2007年 | 118篇 |
2006年 | 117篇 |
2005年 | 102篇 |
2004年 | 73篇 |
2003年 | 59篇 |
2002年 | 56篇 |
2001年 | 64篇 |
2000年 | 52篇 |
1999年 | 63篇 |
1998年 | 35篇 |
1997年 | 45篇 |
1996年 | 47篇 |
1995年 | 33篇 |
1994年 | 19篇 |
1993年 | 26篇 |
1992年 | 30篇 |
1991年 | 28篇 |
1990年 | 22篇 |
1989年 | 10篇 |
1988年 | 9篇 |
1987年 | 10篇 |
1986年 | 9篇 |
1985年 | 8篇 |
1984年 | 9篇 |
1983年 | 6篇 |
1982年 | 10篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1976年 | 4篇 |
1974年 | 3篇 |
1973年 | 5篇 |
排序方式: 共有2614条查询结果,搜索用时 15 毫秒
41.
Caignan GA Deshmukh R Wilks A Zeng Y Huang HW Moënne-Loccoz P Bunce RA Eastman MA Rivera M 《Journal of the American Chemical Society》2002,124(50):14879-14892
The origin of the unusual regioselectivity of heme oxygenation, i.e. the oxidation of heme to delta-biliverdin (70%) and beta-biliverdin (30%), that is exhibited by heme oxygenase from Pseudomonas aeruginosa (pa-HO) has been studied by (1)H NMR, (13)C NMR, and resonance Raman spectroscopies. Whereas resonance Raman indicates that the heme-iron ligation in pa-HO is homologous to that observed in previously studied alpha-hydroxylating heme oxygenases, the NMR spectroscopic studies suggest that the heme in this enzyme is seated in a manner that is distinct from that observed for all other alpha-hydroxylating heme oxygenase enzymes for which a structure is known. In pa-HO, the heme is rotated in-plane approximately 110 degrees, so the delta-meso-carbon of the major orientational isomer is located within the HO-fold in the place where the alpha-hydroxylating enzymes typically place the alpha-meso-carbon. The unusual heme seating displayed by pa-HO places the heme propionates so that these groups point in the direction of the solvent-exposed heme edge and appears to originate in large part from the absence of stabilizing interactions between the polypeptide and the heme propionates, which are typically found in alpha-hydroxylating heme oxygenase enzymes. These interactions typically involve Lys-16 and Tyr-112, in Neisseriae meningitidis HO, and Lys-16 and Tyr-134, in human and rat HO-1. The corresponding residues in pa-HO are Asn-19 and Phe-117, respectively. In agreement with this hypothesis, we found that the Asn-19 Lys/Phe-117 Tyr double mutant of pa-HO exists as a mixture of molecules exhibiting two distinct heme seatings; one seating is identical to that exhibited by wild-type pa-HO, whereas the alternative seating is very similar to that typical of alpha-hydroxylating heme oxygenase enzymes and is related to the wild-type seating by approximately 110 degrees in-plane rotation of the heme. Furthermore, each of these heme seatings in the pa-HO double mutant gives rise to a subset of two heme isomeric orientations that are related to each other by 180 degrees rotation about the alpha-gamma-meso-axis. The coexistence of these molecules in solution, in the proportions suggested by the corresponding area under the peaks in the (1)H NMR spectrum, explains the unusual regioselectivity of heme oxygenation observed with the double mutant, which we found produces alpha- (55%), delta- (35%), and beta-biliverdin (10%). Alpha-biliverdin is obtained by oxidation of the heme seated similar to that of alpha-hydroxylating enzymes, whereas beta- and delta-biliverdin are formed from the oxidation of heme seated as in wild-type pa-HO. 相似文献
42.
A. Moór 《Periodica Mathematica Hungarica》1982,13(2):129-135
In this note necessary and sufficient conditions are determined for Weyl—tsukispaces to have a birecurrent metric, i.e.,
m
k
g
ij
=
km
g
ij
. It is proved that in this space the metric tensor is an eigen-tensor. The special caseP
j
i
= (x)
j
i
is examined and we prove that in this case the recurrent metric tensor is likewise birecurrent. 相似文献
43.
Hong‐Ze Gao Zhong‐Min Su Chun‐Sheng Qin Ri‐Gen Mo Yu‐He Kan 《International journal of quantum chemistry》2004,97(6):992-1001
Bis(2‐methyl‐8‐quinolinolato)aluminum(III) hydroxide complex (AlMq2OH) is used in organic light‐emitting diodes (OLEDs) as an electron transport material and emitting layer. By means of ab initio Hartree–Fock (HF) and density functional theory (DFT) B3LYP methods, the structure of AlMq2OH was optimized. The frontier molecular orbital characteristics and energy levels of AlMq2OH have been analyzed systematically to study the electronic transition mechanism in AlMq2OH. For comparison and calibration, bis(8‐quinolinolato)aluminum(III) hydroxide complex (Alq2OH) has also been examined with these methods using the same basis sets. The lowest singlet excited state (S1) of AlMq2OH has been studied by the singles configuration interaction (CIS) method and time‐dependent DFT (TD‐DFT) using a hybrid functional, B3‐LYP, and the 6‐31G* basis set. The lowest singlet electronic transition (S0 → S1) of AlMq2OH is π → π* electronic transitions and primarily localized on the different quinolate ligands. The emission of AlMq2OH is due to the electron transitions from a phenoxide donor to a pyridyl acceptor from another quinolate ligand including C → C and O → N transference. Two possible electron transfer pathways are presented, one by carbon, oxygen, and nitrogen atoms and the other via metal cation Al3+. The comparison between the CIS‐optimized excited‐state structure with the HF ground‐state structure indicates that the geometric shift is mainly confined to the one quinolate and these changes can be easily understood in terms of the nodal patterns of the highest occupied and lowest unoccupied molecular orbitals. On the basis of the CIS‐optimized structure of the excited state, TD‐B3‐LYP calculations predict an emission wavelength of 499.78 nm. An absorption wavelength at 380.79 nm on the optimized structure of B3LYP/6‐31G* was predicted. They are comparable to AlMq2OH 485 and 390 nm observed experimentally for photoluminescence and UV‐vis absorption spectra of AlMq2OH solid thin film on quartz, respectively. Lending theoretical corroboration to recent experimental observations and supposition, the reasons for the blue‐shift of AlMq2OH were revealed. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2004 相似文献
44.
A method for direct de termination of total in organic arsenic (III+V), arsenic (III) and dimethylarsinate (DMA) in sea water was developed by combining continuous‐flow selective hydride generation and inductively coupled plasma mass spectrometry (ICP‐MS) is presented. The principle underlying selective hydride generation is based on proper control of the reaction conditions for achieving separation of the respective arsenic species. The effects of pH and composition of reaction media on mutual interference between the arsenic species were investigated in detail. The results indicate that the appropriate media for the selective determination of total in organic arsenic, DMA and As(III) are 6 M HNO3, acetate buffer at pH = 4.63 and citrate buffer at pH = 6.54, respectively. The concentrations of total inorganic arsenic species, As(III+V), and As(III) were respectively deter mined and that of As(V) was obtained by the difference between them. As to the concentration of DMA, it was obtained after correction from the interference caused by As(III) and As(V). By following the established procedure, the detection lim its (as based on 3‐sigma criterion) for As(III+V), As(III) and DMA were 0.050, 0.009, and 0.002 ng/mL, respectively. There liability of the pro posed method was evaluated in terms of precision and spike addition. The results indicated that the precision of better than 3% and spike recovery of 95 to 105% for all the arsenic species tested in the natural sea water samples can be obtained. 相似文献
45.
Cα-Tetrasubstituted α-amino acids are ubiquitous and unique structural units in bioactive natural products and pharmaceutical compounds. The asymmetric synthesis of these molecules has attracted a lot of attention, but a more efficient method is still greatly desired. Here we describe the first sequential four-step acylation reaction for the efficient synthesis of chiral Cα-tetrasubstituted α-amino acid derivatives from simple N-acylated amino acids via an auto-tandem catalysis using a single nucleophilic catalyst. The synthetic efficiency is improved via a direct enantioselective C-acylation; the methodology affords the corresponding Cα-tetrasubstituted α-amino acid derivatives with excellent enantioselectivities (up to 99% ee). This step-economic, one-pot, and auto-tandem strategy provides facile access to important chiral building blocks, such as peptides, serines, and oxazolines, which are often used in medicinal and synthetic chemistry.The first four-step sequential reaction for the synthesis of Cα-tetrasubstituted chiral α-amino acid derivatives via auto-tandem catalysis has been developed. 相似文献
46.
47.
<正>永远的老师我与何泽慧老师的直接接触始于三十多年前——20世纪70年代,我已经记不清是在哪一年了。1974年我因高能物理上马从甘肃的长庆油田调到高能物理研究所,开始从事接近我专业学科的科研工作。那时我已30岁,从事宇宙线学科的科研工作才刚刚开始,我满腔热情,寻求着一个尽快切入的方式,以弥补岁月带给我的延迟。 相似文献
48.
49.
从本征模的角度阐明了频率搅拌技术的混响原理,并根据激励信号的频域特征,系统研究了频率搅拌方式下获取实时均匀场与统计均匀场的三种有效实现途径。选取线性扫频这一搅拌方式为主要研究对象,检验了混响室频率搅拌下的最低可用频率、场均匀性与统计特性三项主要技术指标。试验结果表明:线性扫频搅拌方式下,混响室的最低可用频域与机械搅拌方式基本一致;场均匀性满足IEC 61000-4-21标准规定的容差要求;场值波动规律符合理论分布模型。 相似文献
50.