全文获取类型
收费全文 | 578篇 |
免费 | 22篇 |
国内免费 | 3篇 |
专业分类
化学 | 364篇 |
晶体学 | 2篇 |
力学 | 23篇 |
数学 | 92篇 |
物理学 | 122篇 |
出版年
2023年 | 4篇 |
2022年 | 9篇 |
2021年 | 9篇 |
2020年 | 17篇 |
2019年 | 10篇 |
2018年 | 11篇 |
2017年 | 11篇 |
2016年 | 15篇 |
2015年 | 12篇 |
2014年 | 17篇 |
2013年 | 38篇 |
2012年 | 53篇 |
2011年 | 61篇 |
2010年 | 25篇 |
2009年 | 19篇 |
2008年 | 45篇 |
2007年 | 26篇 |
2006年 | 29篇 |
2005年 | 34篇 |
2004年 | 18篇 |
2003年 | 9篇 |
2002年 | 12篇 |
2001年 | 7篇 |
2000年 | 7篇 |
1999年 | 12篇 |
1998年 | 6篇 |
1997年 | 2篇 |
1996年 | 4篇 |
1995年 | 3篇 |
1994年 | 4篇 |
1993年 | 4篇 |
1992年 | 5篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 4篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1979年 | 3篇 |
1978年 | 2篇 |
1977年 | 5篇 |
1976年 | 8篇 |
1975年 | 2篇 |
1974年 | 4篇 |
1972年 | 3篇 |
1970年 | 2篇 |
1968年 | 2篇 |
1910年 | 2篇 |
排序方式: 共有603条查询结果,搜索用时 0 毫秒
191.
Jesse Murillo Rina Bhowmick Katie L. M. Harriman Alejandra Gomez-Torres Joshua Wright Robert W. Meulenberg Pere Mir Alejandro Metta-Magaa Muralee Murugesu Bess Vlaisavljevich Skye Fortier 《Chemical science》2021,12(40):13360
Addition of [UI2(THF)3(μ-OMe)]2·THF (2·THF) to THF solutions containing 6 equiv. of K[C14H10] generates the heteroleptic dimeric complexes [K(18-crown-6)(THF)2]2[U(η6-C14H10)(η4-C14H10)(μ-OMe)]2·4THF (118C6·4THF) and {[K(THF)3][U(η6-C14H10)(η4-C14H10)(μ-OMe)]}2 (1THF) upon crystallization of the products in THF in the presence or absence of 18-crown-6, respectively. Both 118C6·4THF and 1THF are thermally stable in the solid-state at room temperature; however, after crystallization, they become insoluble in THF or DME solutions and instead gradually decompose upon standing. X-ray diffraction analysis reveals 118C6·4THF and 1THF to be structurally similar, possessing uranium centres sandwiched between bent anthracenide ligands of mixed tetrahapto and hexahapto ligation modes. Yet, the two complexes are distinguished by the close contact potassium-arenide ion pairing that is seen in 1THF but absent in 118C6·4THF, which is observed to have a significant effect on the electronic characteristics of the two complexes. Structural analysis, SQUID magnetometry data, XANES spectral characterization, and computational analyses are generally consistent with U(iv) formal assignments for the metal centres in both 118C6·4THF and 1THF, though noticeable differences are detected between the two species. For instance, the effective magnetic moment of 1THF (3.74 μB) is significantly lower than that of 118C6·4THF (4.40 μB) at 300 K. Furthermore, the XANES data shows the U LIII-edge absorption energy for 1THF to be 0.9 eV higher than that of 118C6·4THF, suggestive of more oxidized metal centres in the former. Of note, CASSCF calculations on the model complex {[U(η6-C14H10)(η4-C14H10)(μ-OMe)]2}2− (1*) shows highly polarized uranium–arenide interactions defined by π-type bonds where the metal contributions are primarily comprised by the 6d-orbitals (7.3 ± 0.6%) with minor participation from the 5f-orbitals (1.5 ± 0.5%). These unique complexes provide new insights into actinide–arenide bonding interactions and show the sensitivity of the electronic structures of the uranium atoms to coordination sphere effects.Use of Chatt metal-arene protocols with uranium leads to the synthesis of the first well-characterized, unsupported actinide–arenide sandwich complexes. The electronic structures of the actinide centres show a key sensitivity to ion pairing effects. 相似文献
192.
Th. Blumenstock K. Jesse F. J. Comes R. Schmidt H. -D. Brauer 《Chemical physics》1989,130(1-3):289-298
Studies on various aromatic endoperoxides (POs) reveal a different photodynamic behavior despite similar excitation conditions and despite similar chromophore structures. Using picosecond laser pump-probe technique it was found that heterocoerdianthrone (HECD), dissolved in dichloromethane, is produced from photocycloreversion of its endoperoxide with a time constant of τ=40±10 ps. Since the lifetime of the photoreactive S3 state is less than 3 ps, a two-step mechanism is expected. Photocleavage of the endoperoxide of anthradichromene (ADCPO) happens in 55±15 ps, whereas the endoperoxides of dimethylhomöocoerdianthrone (HOCDPO) and dimethoxyhomöocoerdianthrone (DMHDPO) photodissociate much faster. For their photodissociation we can state an upper time limit of 5 ps. The results of polarization and solvent dependent experiments demonstrate that the observed rise of signal is directly correlated with the formation of the parent compound in the ground state. The dynamics of the photodissociation of HECPO does not depend on polarity or viscosity of solvent. Therefore, an oxciplex configuration or an intermediate zwitterion cannot be involved in the photocycloreversion of aromatic endoperoxides. The model of a biradical mechanism must be claimed instead. 相似文献
193.
Jesse Ira Deutsch 《Journal of Number Theory》2005,113(1):149-174
The celebrated Four Squares Theorem of Lagrange states that every positive integer is the sum of four squares of integers. Interest in this Theorem has motivated a number of different demonstrations. While some of these demonstrations prove the existence of representations of an integer as a sum of four squares, others also produce the number of such representations. In one of these demonstrations, Hurwitz was able to use a quaternion order to obtain the formula for the number of representations. Recently the author has been able to use certain quaternion orders to demonstrate the universality of other quaternary quadratic forms besides the sum of four squares. In this paper, we develop results analogous to Hurwitz's above mentioned work by delving into the number theory of one of these quaternion orders, and discover an alternate proof of the representation formula for the corresponding quadratic form. 相似文献
194.
The problem of deleting a row from a Q–R factorization (called downdating) using Gram–Schmidt orthogonalization is intimately
connected to using classical iterative methods to solve a least squares problem with the orthogonal factor as the coefficient
matrix. Past approaches to downdating have focused upon accurate computation of the residual of that least squares problem,
then finding a unit vector in the direction of the residual that becomes a new column for the orthogonal factor. It is also
important to compute the solution vector of the related least squares problem accurately, as that vector must be used in the
downdating process to maintain good backward error in the new factorization. Using this observation, new algorithms are proposed.
One of the new algorithms proposed is a modification of one due to Yoo and Park [BIT, 36:161–181, 1996]. That algorithm is
shown to be a Gram–Schmidt procedure.
Also presented are new results that bound the loss of orthogonality after downdating. An error analysis shows that the proposed
algorithms’ behavior in floating point arithmetic is close to their behavior in exact arithmetic. Experiments show that the
changes proposed in this paper can have a dramatic impact upon the accuracy of the downdated Q–R decomposition.
AMS subject classification (2000) 65F20, 65F25 相似文献
195.
Dr. Jesse V. Gavette Dr. Matthias Stoop Prof. Nicholas V. Hud Prof. Ramanarayanan Krishnamurthy 《Angewandte Chemie (International ed. in English)》2016,55(42):13204-13209
The RNA world hypothesis posits that DNA and proteins were later inventions of early life, or the chemistry that gave rise to life. Most scenarios put forth for the emergence of DNA assume a clean separation of RNA and DNA polymer, and a smooth transition between RNA and DNA. However, based on the reality of “clutter” and lack of sophisticated separation/discrimination mechanisms in a protobiological (and/or prebiological) world, heterogeneous RNA–DNA backbone containing chimeric sequences could have been common—and have not been fully considered in models transitioning from an RNA world to an RNA–DNA world. Herein we show that there is a significant decrease in Watson–Crick duplex stability of the heterogeneous backbone chimeric duplexes that would impede base‐pair mediated interactions (and functions). These results point to the difficulties for the transition from one homogeneous system (RNA) to another (RNA/DNA) in an RNA world with a heterogeneous mixture of ribo‐ and deoxyribonucleotides and sequences, while suggesting an alternative scenario of prebiological accumulation and co‐evolution of homogeneous systems (RNA and DNA). 相似文献
196.
McDaniel ND Coughlin FJ Tinker LL Bernhard S 《Journal of the American Chemical Society》2008,130(1):210-217
A series of bis-phenylpyridine, bis-aquo iridium(III) complexes is herein shown to robustly and efficiently catalyze the oxidation of water to dioxygen in the presence of a sacrificial oxidant. Through substitution on the cyclometalating ligands of these complexes, it is shown that a broad range of oxidation potentials can be achieved within this class of catalyst. Parallel, dynamic monitoring of oxygen evolution, made possible by equipping reaction vessels with pressure-voltage transducers, facilitates correlation of these complexes' ionization potentials with their respective activity toward water oxidation. The importance of these catalysts lies in (A) their ability to oxidize water in a purely aqueous medium, (B) their simplicity of design, (C) their durability, and (D) the ease with which they can be tuned to accommodate the electrochemical needs of photosensitizers in hypothetical photochemical water oxidation and full artificial photosynthetic schemes. 相似文献
197.
Lawrence J Focsan AL Konovalova TA Molnar P Deli J Bowman MK Kispert LD 《The journal of physical chemistry. B》2008,112(17):5449-5457
Carotenoid (Car) radical intermediates formed upon catalytic or photooxidation of lutein (I), 7'-apo-7',7'-dicyano-beta-carotene (II), and lycopene (III) inside Cu(II)-MCM-41 molecular sieves were studied by pulsed electron nuclear double resonance (ENDOR) spectroscopies. The Davies and Mims ENDOR spectra (15-20 K) were simulated using the hyperfine coupling constants predicted by density functional theory (DFT) calculations. The DFT calculations revealed that upon chemical oxidation, carotenoid radical cations (Car*+) are formed, whereas carotenoid neutral radicals (#Car*) are produced by proton loss (indicated by #) from the radical cation. This loss is to first order independent of polarity or hydrogen bonding for carotenoids I, II, or III inside Cu(II)-MCM-41 molecular sieves. 相似文献
198.
Sevan Chanakian Wanyue Peng Vanessa Meschke A. K. M. Ashiquzzaman Shawon Jesse Adamczyk Valeri Petkov Eric Toberer Alexandra Zevalkink 《Angewandte Chemie (International ed. in English)》2023,62(29):e202301176
AMX compounds with the ZrBeSi structure tolerate a vacancy concentration of up to 50 % on the M-site in the planar MX-layers. Here, we investigate the impact of vacancies on the thermal and electronic properties across the full EuCu1−xZn0.5xSb solid solution. The transition from a fully-occupied honeycomb layer (EuCuSb) to one with a quarter of the atoms missing (EuZn0.5Sb) leads to non-linear bond expansion in the honeycomb layer, increasing atomic displacement parameters on the M and Sb-sites, and significant lattice softening. This, combined with a rapid increase in point defect scattering, causes the lattice thermal conductivity to decrease from 3 to 0.5 W mK−1 at 300 K. The effect of vacancies on the electronic properties is more nuanced; we see a small increase in effective mass, large increase in band gap, and decrease in carrier concentration. Ultimately, the maximum zT increases from 0.09 to 0.7 as we go from EuCuSb to EuZn0.5Sb. 相似文献
199.
Dr. Yihui Xie Danni Luo Jesse A. Wiener Dr. Alexander B. Koval Dr. Conrad T. Pfeiffer Prof. Christian E. Schafmeister 《Angewandte Chemie (International ed. in English)》2023,62(27):e202302809
Here, we report a new class of peptidomimetic macrocycles with well-defined three-dimensional structures and low conformational flexibility. They are assembled from fused-ring spiro-ladder oligomers (spiroligomers) by modular solid-phase synthesis. Two-dimensional nuclear magnetic resonance confirms their shape persistency. Triangular macrocycles of tunable sizes assemble into membranes with atomically precise pores, which exhibit size and shape-dependent molecular sieving towards a series of structurally similar compounds. The exceptional structural diversity and stability of spiroligomer-based macrocycles will be explored for more applications. 相似文献
200.
The Fe storage protein ferritin was used as a size-constrained reaction vessel for the photoreduction and reoxidation of complexed Eu, Fe, and Ti precursors for the formation of oxyhydroxide nanoparticles. The resultant materials were characterized by dynamic light scattering, gel electrophoresis, UV-vis spectroscopy, and transmission electron microscopy. The photoreduction and reoxidation process is inspired by biological sequestration mechanisms observed in some marine siderophore systems. 相似文献