全文获取类型
收费全文 | 1505篇 |
免费 | 31篇 |
国内免费 | 6篇 |
专业分类
化学 | 1135篇 |
晶体学 | 9篇 |
力学 | 25篇 |
数学 | 177篇 |
物理学 | 196篇 |
出版年
2022年 | 16篇 |
2021年 | 25篇 |
2020年 | 19篇 |
2019年 | 24篇 |
2018年 | 18篇 |
2017年 | 11篇 |
2016年 | 42篇 |
2015年 | 29篇 |
2014年 | 20篇 |
2013年 | 69篇 |
2012年 | 75篇 |
2011年 | 108篇 |
2010年 | 51篇 |
2009年 | 49篇 |
2008年 | 92篇 |
2007年 | 72篇 |
2006年 | 89篇 |
2005年 | 64篇 |
2004年 | 71篇 |
2003年 | 64篇 |
2002年 | 45篇 |
2001年 | 21篇 |
2000年 | 16篇 |
1999年 | 9篇 |
1998年 | 14篇 |
1997年 | 10篇 |
1996年 | 19篇 |
1995年 | 11篇 |
1994年 | 14篇 |
1993年 | 13篇 |
1992年 | 14篇 |
1991年 | 8篇 |
1990年 | 11篇 |
1989年 | 9篇 |
1988年 | 13篇 |
1987年 | 18篇 |
1985年 | 39篇 |
1984年 | 30篇 |
1983年 | 17篇 |
1982年 | 27篇 |
1981年 | 18篇 |
1980年 | 21篇 |
1979年 | 25篇 |
1978年 | 12篇 |
1977年 | 8篇 |
1976年 | 14篇 |
1975年 | 13篇 |
1974年 | 10篇 |
1972年 | 6篇 |
1970年 | 8篇 |
排序方式: 共有1542条查询结果,搜索用时 15 毫秒
161.
Sodium hydrogen telluride opens many epoxides cleanly by an SN2 process to give telluro-alcohols, which by reduction with nickel boride afford alcohols. An intermediate telluro-alcohol was converted to olefin in high yield by treatment with -toluene-sulphonyl chloride in pyridine. Quaternary ammonium salt are also cleaved efficiently by sodium hydrogen telluride. 相似文献
162.
Giuseppe Distefano Maurizio Guerra Francesco Paolo Colonna Derek Jones Giovanni Consiglio Domenico Spinelli 《Chemical physics》1982,72(3):267-279
The shake-up satellites present in the O1s and N1s energy regions of the XPS spectra of the planar 4-N-piperidyl-nitrobenzene (1) and 2-N-piperidyl-5-nitrothiophen (3), and of the sterically hindered 2-N-piperidyl-nitrobenzene (2), 2-N-piperidyl-3-nitrothiophen (4) and 3-N-piperidyl-2-nitrothiophen (5) and, for comparison, of 2-bromo-3-nitrothiophen (6) have been analyzed with the help of CNDO/S CI calculations. The spectra of the planar derivatives closely resemble these of the related compounds previously analysed, the main satellites deriving from the transition between the highest occupied molecular orbital (HOMO, localized at the donor-ring moiety) towards the lowest unoccupied one (LUMO, localized at the nitro group). Calculations indicate that in the ortho derivatives new relaxation processes are allowed upon N1s/NO2 ionization. In 2, where the angle of rotation θ is closer to 90° than 60°, the NR 2 lone-pair orbital relaxes through space towards the core-ionized N/NO2 atom. In 4 and 5, where 45° ? θ ? 60°, both through-ring and through-space relaxations are present. The assignment of the main N1s/NO2 line in 1 is discussed. 相似文献
163.
We analyze convolution semigroups on a regular measure space which satisfies the local doubling property. We assume the kernels are bounded and symmetric with the characteristic small-time, volume-dependent, singularity. Then, using a weak conservation property, we deduce local lower bounds with a comparable singularity.Applications are given to a wide range of subelliptic and strongly elliptic self-adjoint, or near self-adjoint, operators on Lie groups. 相似文献
164.
Sodium hydrogen telluride reduces .-amine -oxides to the corresponding amines in very high yield, but does not reduce sulphoxides. This reagent also reduces nitrones to .-amines at pH6 and to imines at alkaline pH (10–11). 相似文献
165.
Bert Fraser-Reid Deen B. Tulshian Ray Tsang Derek Lowe Vernon G.S. Box 《Tetrahedron letters》1984,25(41):4579-4582
Structurally equivalent derivatives of pyranosides and cyclohexanes undergo the spiro-Claisen rearrangement with different stereochemical results. A rationalization for the course observed with the pyranosides is suggested, which invokes interaction of the oxygen lone pair with a (developing) electron deficient centre at the spiro carbon. 相似文献
166.
By comparison of pKa values derived from 15N and 13C nuclear magnetic resonance (NMR) spectroscopies, the assignment of the 15N resonances of apramycin is completed. 13C NMR spectra appear to provide accurate pKa determinations with this aminoglycoside. Hydroxylation adjacent to one of the basic nitrogens of apramycin appears to change the pKa values of all five amines of the molecule. 相似文献
167.
Geoffrey C. Allen Philip M. Tucker Brian E. Hayden Derek F. Klemperer 《Surface science》1981,102(1):207-226
The initial stages in the oxidation of magnesium, aluminium and magnesium/aluminium alloys have been studied using a photoelectron spectrometer (XPS and exoelectron energy spectra) and the diode technique (surface potentials and volumetric adsorption of oxygen). This paper describes and characterises the exoelectron emission and the visible wavelength photoemission which occurs during oxidation. At low oxygen exposures (< 10 L), exoelectrons with a 4 eV energy spread are emitted in the dark from magnesium and magnesium/aluminium alloy. Aluminium shows no such emission. Another, dark pressure-dependent emission of exoelectrons occurs only on magnesium/aluminium alloy and aluminium at high oxygen exposures. The onset of this emission lies 3 eV higher and it again has a 4 eV spread. When the surface is illuminated, it is possible to distinguish photo/exoelectron emission, which occurs during oxygen uptake, and true photoemission which occurs in vacuo. Both these types of emission, which were only recorded on magnesium, show a resolved triplet. Photoelectric work functions as low as 0.3 eV were observed. A patchy surface evidently develops during oxidation. This process, which may correspond to the nucleation of oxide islands, is slower than the chemisorption of of oxygen. The nature of the low work function patches, which we associate with the electron emissions, is therefore very dependent on the oxygen ambient pressure. A tentative energy scheme is given for such “exopatches” which are neither metal nor oxide and contain a curious energy level that must be situated above the vacuum level of the patch, as well as above the metal Fermi level. The energy with which exoelectrons are emitted derives from the heat of adsorption. 相似文献
168.
The IR and NMR spectra of a range of 2-arylhydrazones of 1,2,3-tricarbonyl compounds have been recorded. Compounds containing different end groups in the 1,2,3-tricarbonyl system can exist in two different tautomeric forms. The influence of electronic and steric factors on the structure of the preferred tautomer is discussed. 相似文献
169.
Ola Bratteli Akitaka Kishimoto Derek W. Robinson 《Communications in Mathematical Physics》1978,61(3):209-238
First we derive stability properties of KMS states and subsequently we derive the KMS condition from stability properties. New results include a convergent perturbation expansion for perturbed KMS states in terms of appropriate truncated functions and stability properties of ground states. Finally we extend the results of Haag, Kastler, Trych-Pohlmeyer by proving that stable states ofL
1-asymptotically abelian systems which satisfy a weak three point cluster property are automatically KMS states. This last theorem gives an almost complete characterization of KMS states, ofL
1-asymptotic abelian systems, by stability and cluster properties (a slight discrepancy can occur for infinite temperature states).Supported during this research by the Norwegian Research Council for Science and Humanities 相似文献
170.