全文获取类型
收费全文 | 6116篇 |
免费 | 148篇 |
国内免费 | 37篇 |
专业分类
化学 | 4476篇 |
晶体学 | 21篇 |
力学 | 119篇 |
数学 | 694篇 |
物理学 | 991篇 |
出版年
2021年 | 53篇 |
2020年 | 67篇 |
2019年 | 70篇 |
2016年 | 95篇 |
2015年 | 89篇 |
2014年 | 135篇 |
2013年 | 241篇 |
2012年 | 260篇 |
2011年 | 312篇 |
2010年 | 169篇 |
2009年 | 152篇 |
2008年 | 240篇 |
2007年 | 258篇 |
2006年 | 280篇 |
2005年 | 233篇 |
2004年 | 204篇 |
2003年 | 190篇 |
2002年 | 205篇 |
2001年 | 120篇 |
2000年 | 131篇 |
1999年 | 68篇 |
1998年 | 62篇 |
1997年 | 59篇 |
1996年 | 104篇 |
1995年 | 91篇 |
1994年 | 69篇 |
1993年 | 86篇 |
1992年 | 81篇 |
1991年 | 62篇 |
1990年 | 74篇 |
1989年 | 61篇 |
1987年 | 50篇 |
1986年 | 59篇 |
1985年 | 77篇 |
1984年 | 81篇 |
1983年 | 62篇 |
1982年 | 85篇 |
1981年 | 83篇 |
1980年 | 78篇 |
1979年 | 99篇 |
1978年 | 75篇 |
1977年 | 71篇 |
1976年 | 75篇 |
1975年 | 52篇 |
1974年 | 69篇 |
1973年 | 59篇 |
1968年 | 50篇 |
1967年 | 49篇 |
1966年 | 56篇 |
1964年 | 50篇 |
排序方式: 共有6301条查询结果,搜索用时 464 毫秒
951.
Dr. Xiaoyan Sun Prof. Dr. Shaodong Zhou Dr. Lei Yue Dr. Maria Schlangen Prof. Dr. Helmut Schwarz 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(14):3550-3559
Thermal gas-phase reactions of the ruthenium-oxide clusters [RuOx]+ (x=1–3) with methane and dihydrogen have been explored by using FT-ICR mass spectrometry complemented by high-level quantum chemical calculations. For methane activation, as compared to the previously studied [RuO]+/CH4 couple, the higher oxidized Ru systems give rise to completely different product distributions. [RuO2]+ brings about the generations of [Ru,O,C,H2]+/H2O, [Ru,O,C]+/H2/H2O, and [Ru,O,H2]+/CH2O, whereas [RuO3]+ exhibits a higher selectivity and efficiency in producing formaldehyde and syngas (CO+H2). Regarding the reactions with H2, as compared to CH4, both [RuO]+ and [RuO2]+ react similarly inefficiently with oxygen-atom transfer being the main reaction channel; in contrast, [RuO3]+ is inert toward dihydrogen. Theoretical analysis reveals that the reduction of the metal center drives the overall oxidation of methane, whereas the back-bonding orbital interactions between the cluster ions and dihydrogen control the H−H bond activation. Furthermore, the reactivity patterns of [RuOx]+ (x=1–3) with CH4 and H2 have been compared with the previously reported results of Group 8 analogues [OsOx]+/CH4/H2 (x=1–3) and the [FeO]+/H2 system. The electronic origins for their distinctly different reaction behaviors have been addressed. 相似文献
952.
953.
Alexey V. Salin Albert R. Fatkhutdinov Anton V. Il'in Vladimir I. Galkin 《国际化学动力学杂志》2014,46(4):206-215
Quaternization of triphenylphosphine with maleic and cis‐aconitic acids is strongly accelerated by participation of the cis‐carboxyl group in stabilization of the phosphonium zwitterion intermediate by intramolecular hydrogen bonding, in spite of steric hindrance by the acid's reaction center. A similar effect for trans‐isomeric acids is not observed, which can be rationalized on the basis of spatial structures of the generated zwitterions, implying an electrostatic interaction between the phosphonium center and carbonyl oxygen atom. The effect of anchimeric assistance decreases when the intramolecular hydrogen bonding disfavors attack of the phosphine on the sterically less hindered carbon atom of the C=C bond, as observed for cis‐aconitic acid. 相似文献
954.
Peter Q. Miraglia Bilge Yilmaz Juliusz Warzywoda Albert Sacco Jr. 《Journal of Crystal Growth》2004,270(3-4):674-684
Morphological, surface and crystallographic analyses of titanosilicate ETS-4 products, with diverse habits ranging from spherulitic particles composed of submicron crystallites to large single crystals, are presented. Pole figures revealed that crystal surfaces with a-, b- and c- axes corresponded to 110, 010 and 001 directions, respectively. Thus, technologically important 8-membered ring pores and titania chains in ETS-4 run along the b-axis of single crystals and terminate at the smallest crystal face. Height of the spiral growth steps observed on 1 0 0 and 0 0 1 surfaces corresponded to the interplanar spacings associated with their crystallographic orientation, and is equivalent to the thickness of building units that form the ETS-4 framework. Data suggest that the more viscous synthesis mixtures, with a large driving force for growth, increased the two- and three-dimensional nucleation, while limiting the transport of nutrients to the growth surface. These conditions increase the tendency for stacking fault formation on 1 0 0 surfaces and small angle branching, which eventually results in spherulitic growth. The growth of high quality ETS-4 single crystals (from less viscous synthesis mixtures) occurred at lower surface nucleation rates. Data suggest that these high quality, large crystals grew due to one-dimensional nucleation at spiral hillocks, and indicate that under these conditions un-faulted growth is preferred. 相似文献
955.
Albert S. SHABAEV Azamat A. ZHANSITOV Zhanna I. KURDANOVA Leana Kh. KUCHMENOVA Svetlana Yu. KHASHIROVA 《色谱》2018,36(4):395-399
Poly(ether ether ketone)(PEEK)was synthesized via polycondensation of hydroquinone with 4,4′-difluorobenzophenone at 320℃for 5 h.Thermal and thermo-oxidative degradation of PEEK was studied over a wide range of temperatures.In an inert medium,decomposition of the polymer occurred in one stage,with the formation of a coke residue accounting for approximately 50%of the original polymer mass.In air,the mass loss curve exhibited two distinct stages.The first stage involved breakdown of the main polymer chain,the speed of which indicated a radical chain failure mechanism.In the second stage,the rate of mass loss clearly decreased,indicating a transition from the radical chain failure mechanism to simple combustion reactions(wherein the polymer combusted completely).To further investigate the nature of the processes occurring during the pyrolysis of PEEK,the investigations were carried out using gas chromatograph under isothermal conditions.It was concluded that during thermal degradation,the decomposition of the polymer starts with the rupture of ketone and ether bonds and proceeds to destruction of the benzene ring at higher temperatures,which is accompanied by the formation of H2O and CH4.Above 500℃,the polymer degradation further involved thermohydrolysis.The thermo-oxidation of PEEK,which was accompanied mainly by the formation ofH2,was noticeable beginning at 325℃.The total yield of the latter indicated oxidation of fragments of the benzene ring. 相似文献
956.
957.
Thomas O. Schrader Xiuwen Zhu Michelle Kasem Sufang Li Chunyan Liu Albert Ren Chunrui Wu Graeme Semple 《Tetrahedron letters》2018,59(21):2030-2033
Asymmetric syntheses of N-protected (R)-4-halo-6,6a,7,8,9,10-hexahydro-5H-pyrazino[1,2-a][1,n]naphthyridines, advanced intermediates for the synthesis of highly potent and selective 5-HT2C agonists, are described. The key transformation involves ring opening of N-protected bicyclic sulfamidate (R)-hexahydro-3H-pyrazino[1,2-c][1,2,3]oxathiazine 1,1-dioxide with (4-halo-2-fluoropyridin-3-yl)lithiums or (3-bromo-5-fluoropyridin-4-yl)lithium. In situ hydrolyses of the resultant sulfamic acids and subsequent intramolecular nucleophilic aromatic substitutions (SNAr) produce the enantiopure tricycles. The two step procedure represents new methodology for the stereoselective syntheses of tetrahydronaphthyridines. 相似文献
958.
Dr. Robert Lizatović Marvin Assent Arjan Barendregt Jonathan Dahlin Dr. Anna Bille Katharina Satzinger Dagnija Tupina Prof. Albert J. R. Heck Dr. Stefan Wennmalm Dr. Ingemar André 《Angewandte Chemie (International ed. in English)》2018,57(35):11334-11338
Protein‐based encapsulation systems have a wide spectrum of applications in targeted delivery of cargo molecules and for chemical transformations in confined spaces. By engineering affinity between cargo and container proteins it has been possible to enable the efficient and specific encapsulation of target molecules. Missing in current approaches is the ability to turn off the interaction after encapsulation to enable the cargo to freely diffuse in the lumen of the container. Separation between cargo and container is desirable in drug delivery applications and in the use of capsids as catalytic nanoparticles. We describe an encapsulation system based on the hepatitis B virus capsid in which an engineered high‐affinity interaction between cargo and capsid proteins can be modulated by Ca2+. Cargo proteins are loaded into capsids in the presence of Ca2+, while ligand removal triggers unbinding inside the container. We observe that confinement leads to hindered rotation of cargo inside the capsid. Application of the designed container for catalysis was also demonstrated by encapsulation of an enzyme with β‐glucosidase activity. 相似文献
959.
Tatiana Yu Samgina Sergey V. Kovalev Miriam D. Tolpina Polonca Trebse Gregor Torkar Albert T. Lebedev 《Journal of the American Society for Mass Spectrometry》2018,29(5):842-852
Our scientific interests involve de novo sequencing of non-tryptic natural amphibian skin peptides including those with intramolecular S–S bond by means of exclusively mass spectrometry. Reliable discrimination of the isomeric leucine/isoleucine residues during peptide sequencing by means of mass spectrometry represents a bottleneck in the workflow for complete automation of the primary structure elucidation of these compounds. MS3 is capable of solving the problem. Earlier we demonstrated the advanced efficiency of ETD-HCD method to discriminate Leu/Ile in individual peptides by consecutive application of ETD to the polyprotonated peptides followed by HCD applied to the manually selected primary z-ions with the targeted isomeric residues at their N-termini and registration of the characteristic w-ions. Later this approach was extended to deal with several (4–7) broad band mass ranges, without special isolation of the primary z-ions. The present paper demonstrates an advanced version of this method when EThcD is applied in the whole mass range to a complex mixture of natural non-tryptic peptides without their separation and intermediate isolation of the targeted z-ions. The proposed EThcD method showed over 81% efficiency for the large natural peptides with intact disulfide ring, while the interfering process of radical site migration is suppressed. Due to higher speed and sensitivity, the proposed EThcD approach facilitates the analytical procedure and allows for the automation of the entire experiment and data processing. Moreover, in some cases it gives a chance to establish the nature of the residues in the intact intramolecular disulfide loops. 相似文献
960.
Dr. Shu‐Xian Hu M. Sc. Wan‐Lu Li M. Sc. Jun‐Bo Lu Dr. Junwei Lucas Bao Dr. Haoyu S. Yu Prof. Dr. Donald G. Truhlar Dr. John K. Gibson Dr. Joaquim Marçalo Prof. Dr. Mingfei Zhou Prof. Dr. Sebastian Riedel Prof. Dr. W. H. Eugen Schwarz Prof. Dr. Jun Li 《Angewandte Chemie (International ed. in English)》2018,57(12):3242-3245
The concept of oxidation state ( OS ) is based on the concept of Lewis electron pairs, in which the bonding electrons are assigned to the more electronegative element. This approach is useful for keeping track of the electrons, predicting chemical trends, and guiding syntheses. Experimental and quantum‐chemical results reveal a limit near +8 for the highest OS in stable neutral chemical substances under ambient conditions. OS =+9 was observed for the isolated [IrO4]+ cation in vacuum. The prediction of OS =+10 for isolated [PtO4]2+ cations is confirmed computationally for low temperatures only, but hasn't yet been experimentally verified. For high OS species, oxidation of the ligands, for example, of O?2 with formation of .O?1 and O?O bonds, and partial reduction of the metal center may be favorable, possibly leading to non‐Lewis type structures. 相似文献