全文获取类型
收费全文 | 2069篇 |
免费 | 35篇 |
国内免费 | 14篇 |
专业分类
化学 | 1152篇 |
晶体学 | 8篇 |
力学 | 81篇 |
数学 | 215篇 |
物理学 | 662篇 |
出版年
2022年 | 13篇 |
2021年 | 15篇 |
2020年 | 19篇 |
2019年 | 23篇 |
2018年 | 13篇 |
2017年 | 15篇 |
2016年 | 20篇 |
2014年 | 25篇 |
2013年 | 83篇 |
2012年 | 70篇 |
2011年 | 92篇 |
2010年 | 49篇 |
2009年 | 28篇 |
2008年 | 74篇 |
2007年 | 81篇 |
2006年 | 94篇 |
2005年 | 82篇 |
2004年 | 73篇 |
2003年 | 74篇 |
2002年 | 58篇 |
2001年 | 62篇 |
2000年 | 61篇 |
1999年 | 38篇 |
1998年 | 33篇 |
1997年 | 36篇 |
1996年 | 37篇 |
1995年 | 41篇 |
1994年 | 46篇 |
1993年 | 59篇 |
1992年 | 35篇 |
1991年 | 35篇 |
1990年 | 37篇 |
1989年 | 35篇 |
1988年 | 23篇 |
1987年 | 31篇 |
1986年 | 19篇 |
1985年 | 35篇 |
1984年 | 23篇 |
1983年 | 32篇 |
1982年 | 24篇 |
1981年 | 24篇 |
1980年 | 27篇 |
1979年 | 19篇 |
1978年 | 24篇 |
1977年 | 35篇 |
1975年 | 30篇 |
1974年 | 23篇 |
1973年 | 18篇 |
1972年 | 24篇 |
1966年 | 12篇 |
排序方式: 共有2118条查询结果,搜索用时 13 毫秒
951.
952.
953.
Harris C. Levey 《Zeitschrift für Angewandte Mathematik und Physik (ZAMP)》1960,11(2):152-157
Zusammenfassung Ein ebener Flüssigkeitsstrahl von der Weited wird normal gegen eine Wand geleitet, die von der Quelleh Einheiten entfernt ist. Man nimmt gewöhnlich an, dass, wenn das Verhältnish/d gross ist, die Auswirkung des Hindernisses nicht zur Quelle des Strahles zurückwirkt, so dass im besonderen wichtige Eigenschaften des Strahls, wie sein Impuls und die Massenströmung, dürch den Widerstand unberührt bleiben.An einem einfachen Modell, in dem der Strahl, bestehend aus einer inkompressiblen und reibungslosen Flüssigkeit, von einem Schlitz in der Wand einer unendlichen Kammer ausströmt, zeigen wir, dass, soweit diese Haupteigenschaften betroffen sind, der Strahl durch das Hindernis beinahe ungestört bleibt, selbst für Werte der Verhältniszahlh/d, die nahe an der Einheit liegen. 相似文献
954.
Pandey AS Harris TV Giles LJ Peters JW Szilagyi RK 《Journal of the American Chemical Society》2008,130(13):4533-4540
An X-ray crystallographic refinement of the H-cluster of [FeFe]-hydrogenase from Clostridium pasteurianum has been carried out to close-to atomic resolution and is the highest resolution [FeFe]-hydrogenase presented to date. The 1.39 A, anisotropically refined [FeFe]-hydrogenase structure provides a basis for examining the outstanding issue of the composition of the unique nonprotein dithiolate ligand of the H-cluster. In addition to influencing the electronic structure of the H-cluster, the composition of the ligand has mechanistic implications due to the potential of the bridge-head gamma-group participating in proton transfer during catalysis. In this work, sequential density functional theory optimizations of the dithiolate ligand embedded in a 3.5-3.9 A protein environment provide an unbiased approach to examining the most likely composition of the ligand. Structural, conformational, and energetic considerations indicate a preference for dithiomethylether as an H-cluster ligand and strongly disfavor the dithiomethylammonium as a catalytic base for hydrogen production. 相似文献
955.
Aliev AE Mann SE Rahman AS McMillan PF Corà F Iuga D Hughes CE Harris KD 《The journal of physical chemistry. A》2011,115(44):12201-12211
High-resolution solid-state (2)H MAS NMR studies of the α and γ polymorphs of fully deuterated glycine (glycine-d(5)) are reported. Analysis of spinning sideband patterns is used to determine the (2)H quadrupole interaction parameters, and is shown to yield good agreement with the corresponding parameters determined from single-crystal (2)H NMR measurements (the maximum deviation in quadrupole coupling constants determined from these two approaches is only 1%). From analysis of simulated (2)H MAS NMR sideband patterns as a function of reorientational jump frequency (κ) for the -N(+)D(3) group in glycine-d(5), the experimentally observed differences in the (2)H MAS NMR spectrum for the -N(+)D(3) deutrons in the α and γ polymorphs is attributed to differences in the rate of reorientation of the -N(+)D(3) group. These simulations show severe broadening of the (2)H MAS NMR signal in the intermediate motion regime, suggesting that deuterons undergoing reorientational motions at rates in the range κ ≈ 10(4)-10(6) s(-1) are likely to be undetectable in (2)H MAS NMR measurements for materials with natural isotopic abundances. The (1)H NMR chemical shifts for the α and γ polymorphs of glycine have been determined from the (2)H MAS NMR results, taking into account the known second-order shift. Further quantum mechanical calculations of (2)H quadrupole interaction parameters and (1)H chemical shifts reveal the structural dependence of these parameters in the two polymorphs and suggest that the existence of two short intermolecular C-H···O contacts for one of the H atoms of the >CH(2) group in the α polymorph have a significant influence on the (2)H quadrupole coupling and (1)H chemical shift for this site. 相似文献
956.
Lomont JP Nguyen SC Schlegel JP Zoerb MC Hill AD Harris CB 《Journal of the American Chemical Society》2012,134(6):3120-3126
We report the observation of a solvent-dependent spin state equilibrium in the 16-electron photoproduct CpCo(CO). Time-resolved infrared spectroscopy has been used to observe the concurrent formation of two distinct solvated monocarbonyl photoproducts, both of which arise from the same triplet CpCo(CO) precursor. Experiments in different solvent environments, combined with electronic structure theory calculations, allow us to assign the two solvated photoproducts to singlet and triplet CpCo(CO)(solvent) complexes. These results add to our previous picture of triplet reactivity for 16-electron organometallic photoproducts, in which triplets were not believed to interact strongly with solvent molecules. In the case of this photoproduct, it appears that spin crossover does not present a significant barrier to reactivity, and relative thermodynamic stabilities determine the spin state of the CpCo(CO) photoproduct in solution on the picosecond time scale. While the existence of transition metal complexes with two thermally accessible spin states is well-known, this is, to our knowledge, the first observation of a transient photoproduct that exhibits an equilibrium between two stable spin states, and also the first observed case in which a solvent has been able to coordinate as a token ligand to two spin states of the same photoproduct. 相似文献
957.
Jiani Ma Adam C. Rea Dr. Huiying An Dr. Chensheng Ma Dr. Xiangguo Guan Dr. Ming‐De Li Tao Su Chi Shun Yeung Kyle T. Harris Dr. Yue Zhu Jameil L. Nganga Dr. Olesya D. Fedoryak Dr. Timothy M. Dore Dr. David Lee Phillips 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(22):6854-6865
Photoremovable protecting groups (PPGs) when conjugated to biological effectors forming “caged compounds” are a powerful means to regulate the action of physiologically active messengers in vivo through 1‐photon excitation (1PE) and 2‐photon excitation (2PE). Understanding the photodeprotection mechanism is important for their physiological use. We compared the quantum efficiencies and product outcomes in different solvent and pH conditions for the photolysis reactions of (8‐chloro‐7‐hydroxyquinolin‐2‐yl)methyl acetate (CHQ‐OAc) and (8‐bromo‐7‐hydroxyquinolin‐2‐yl)methyl acetate (BHQ‐OAc), representatives of the quinoline class of phototriggers for biological use, and conducted nanosecond time‐resolved spectroscopic studies using transient emission (ns‐EM), transient absorption (ns‐TA), transient resonance Raman (ns‐TR2), and time‐resolved resonance Raman (ns‐TR3) spectroscopies. The results indicate differences in the photochemical mechanisms and product outcomes, and reveal that the triplet excited state is most likely on the pathway to the product and that dehalogenation competes with release of acetate from BHQ‐OAc, but not CHQ‐OAc. A high fluorescence quantum yield and a more efficient excited‐state proton transfer (ESPT) in CHQ‐OAc compared to BHQ‐OAc explain the lower quantum efficiency of CHQ‐OAc relative to BHQ‐OAc. 相似文献
958.
Fox DM Maupin PH Harris RH Gilman JW Eldred DV Katsoulis D Trulove PC De Long HC 《Langmuir : the ACS journal of surfaces and colloids》2007,23(14):7707-7714
Recent studies on organically modified clays (OMCs) have reported enhanced thermal stabilities when using imidazolium-based surfactants over the typical ammonium-based surfactants. Other studies have shown that polyhedral oligomeric silsesquioxanes (POSS) also improve the thermal properties of composites containing these macromers. In an attempt to utilize the beneficial properties of both imidazolium surfactants and POSS macromers, a dual nanocomposite approach to prepare OMCs was used. In this study, the preparation of a new POSS-imidazolium surfactant and its use as an organic modifier for montmorillonite are reported. The purity, solubility, and thermal characteristics of the POSS-imidazolium chloride were evaluated. In addition, several OMCs were prepared by exchanging the Na+ with POSS imidazolium cations equivalent to 100%, 95%, 40%, 20%, and 5% of the cation exchange capacity of the clay. The subsequent OMCs were characterized using thermal analysis techniques (DSC, SDT, and TGA) as well as 29Si NMR to determine the POSS content in the clay interlayer both before and after thermal oxidation degradation. Results indicate the following: (1) the solvent choice changes the efficiency of the ion-exchange reaction of the clay; (2) self-assembled crystalline POSS domains are present in the clay interlayer; (3) the d-spacing of the exchanged clay is large (3.6 nm), accommodating a bilayer structure of the POSS-imidazolium; and (4) the prepared POSS-imidazolium exchanged clays exhibit higher thermal stabilities than any previously prepared imidazolium or ammonium exchanged montmorillonite. 相似文献
959.
Portieri A Harris RK Fletton RA Lancaster RW Threlfall TL 《Magnetic resonance in chemistry : MRC》2004,42(3):313-320
We recorded both carbon-13 and nitrogen-15 NMR spectra of the three solid forms of sulfanilamide most commonly known. This study led to an interpretation of the solid-state effects seen in cross-polarization magic angle spinning spectra. Relaxation times for the different forms were measured. These show different behaviour for the three forms, arising from mobility variations. To obtain information on local environments, static spectra and spinning sideband manifolds were recorded and analysed for the 15N resonances, using isotopically enriched samples. Shielding asymmetries and anisotropies for the two nitrogen nuclei were obtained, showing very different behaviour for the two sites. Shielding calculations were carried out for both 13C and 15N nuclei, and the results are discussed in relation to the experimental values. 相似文献
960.
Wang G Zhang B Wayment JR Harris JM White HS 《Journal of the American Chemical Society》2006,128(23):7679-7686
Electrostatic-gated transport in chemically modified glass nanopore electrodes with orifice radii as small as 15 nm is reported. A single conical-shaped nanopore in glass, with a approximately 1 microm radius Pt disk located at the pore base, is prepared by etching the exposed surface of a glass-sealed Pt nanodisk. The electrochemical response of the nanopore electrode corresponds to diffusion of redox-active species through the nanopore orifice to the Pt microdisk. Silanization of the exterior glass surface with Cl(Me)(2)Si(CH(2))(3)CN and the interior pore surface with EtO(Me)(2)Si(CH(2))(3)NH(2) introduces pH-dependent ion selectivity at the pore orifice, a consequence of the electrostatic interactions between the redox ions and protonated surface amines. Nanopore electrodes with very small pore orifice radii (< approximately 50 nm) display anion permselectively at pH < 4, as demonstrated by electrochemical measurement of transport through the pore orifice. Ion selective transport vanishes at pH > 6 or when the pore radius is significantly larger than the Debye screening length, consistent with the observed ion selectivity resulting from electrostatic interactions. The ability to introduce different surface functionalities to the interior and exterior surfaces of glass nanopores is demonstrated using fluorescence microscopy to monitor the localized covalent attachment of 5- (and 6)-carboxytetramethylrhodamine succinimidyl ester to interior pore surfaces previously silanized with EtO(Me)(2)Si(CH(2))(3)NH(2). 相似文献