全文获取类型
收费全文 | 4227篇 |
免费 | 116篇 |
国内免费 | 17篇 |
专业分类
化学 | 2963篇 |
晶体学 | 42篇 |
力学 | 103篇 |
数学 | 200篇 |
物理学 | 1052篇 |
出版年
2021年 | 42篇 |
2020年 | 40篇 |
2019年 | 45篇 |
2018年 | 33篇 |
2017年 | 32篇 |
2016年 | 75篇 |
2015年 | 70篇 |
2014年 | 86篇 |
2013年 | 194篇 |
2012年 | 216篇 |
2011年 | 258篇 |
2010年 | 130篇 |
2009年 | 117篇 |
2008年 | 208篇 |
2007年 | 198篇 |
2006年 | 218篇 |
2005年 | 189篇 |
2004年 | 199篇 |
2003年 | 180篇 |
2002年 | 189篇 |
2001年 | 133篇 |
2000年 | 113篇 |
1999年 | 72篇 |
1998年 | 54篇 |
1997年 | 55篇 |
1996年 | 59篇 |
1995年 | 47篇 |
1994年 | 50篇 |
1993年 | 52篇 |
1992年 | 62篇 |
1991年 | 39篇 |
1990年 | 41篇 |
1989年 | 47篇 |
1988年 | 41篇 |
1987年 | 42篇 |
1986年 | 39篇 |
1985年 | 86篇 |
1984年 | 77篇 |
1983年 | 25篇 |
1982年 | 39篇 |
1981年 | 38篇 |
1980年 | 38篇 |
1979年 | 64篇 |
1978年 | 48篇 |
1977年 | 53篇 |
1976年 | 48篇 |
1975年 | 32篇 |
1974年 | 24篇 |
1973年 | 19篇 |
1972年 | 15篇 |
排序方式: 共有4360条查询结果,搜索用时 37 毫秒
91.
Mugishima T Tsuda M Kasai Y Ishiyama H Fukushi E Kawabata J Watanabe M Akao K Kobayashi J 《The Journal of organic chemistry》2005,70(23):9430-9435
[Structure: see text]. Citrinadin A (2) is a pentacyclic indolinone alkaloid isolated from the cultured broth of a fungus, Penicillium citrinum, which was separated from a marine red alga. The absolute stereochemistry of the pentacyclic core in 2 and its new congener, citrinadin B (1), was elucidated by analysis of the ROESY spectrum for the chlorohydrin derivative (3) of 1 as well as comparison of the electronic circular dichroism (ECD) spectra for 1 and 2 with those of known spirooxiindole alkaloids. On the other hand, the absolute configuration at C-21 bearing an epoxide ring was assigned as S by comparison of the vibrational circular dichroism (VCD) spectra of 1 with those of model compounds 2S- and 2R-2,3-epoxy-3,3-dimethyl-1-phenylpropan-1-one (4a and 4b, respectively). 相似文献
92.
Ishii K Itoya H Miwa H Fujitsuka M Ito O Kobayashi N 《The journal of physical chemistry. A》2005,109(26):5781-5787
We have investigated the excited-state properties and singlet oxygen ((1)Delta(g)) generation mechanism in phthalocyanines (4M; M = H(2), Mg, or Zn) and in low-symmetry metal-free, magnesium, and zinc tetraazaporphyrins (TAPs), that is, monobenzo-substituted (1M), adjacently dibenzo-substituted (2AdM), oppositely dibenzo-substituted (2OpM), and tribenzo-substituted (3M) TAP derivatives, whose pi conjugated systems were altered by fusing benzo rings. The S(1)(x) and S(1)(y) states (these lowest excited singlet states are degenerate in D(4)(h) symmetry) split in the low-symmetry TAP derivatives. The excited-state energies were quantitatively determined from the electronic absorption spectra. The lowest excited triplet (T(1)(x)) energies were also determined from phosphorescence spectra, while the second lowest excited triplet (T(1)(y)) states were evaluated by using the energy splitting between the T(1)(x) and T(1)(y) states previously reported (Miwa, H.; Ishii, K.; Kobayashi, N. Chem. Eur. J. 2004, 10, 4422-4435). The singlet oxygen quantum yields (Phi(Delta)) are strongly dependent on the pi conjugated system. In particular, while the Phi(Delta) value of 2AdH(2) is smallest in our system, that of 2OpH(2), an isomer of 2AdH(2), is larger than that of 4Zn, in contrast to the heavy atom effect. The relationship between the molecular structure and Phi(Delta) values can be transformed into a relationship between the S(1)(x) --> T(1)(y) intersystem crossing rate constant (k(ISC)) and the energy difference between the S(1)(x) and T(1)(y) states (DeltaE(S)(x)(T)(y)). In each of the Zn, Mg, and metal-free compounds, the Phi(Delta)/tau(F) values (tau(F): fluorescence lifetime), which are related to the k(ISC) values, are proportional to exp(-DeltaE(S)(x)(T)(y)), indicating that singlet oxygen ((1)Delta(g)) is produced via the T(1)(y) state and that the S(1)(x) --> T(1)(y) ISC process follows the energy-gap law. From the viewpoint of photodynamic therapy, our methodology, where the Phi(Delta) value can be controlled by changing the symmetry of pi conjugated systems without heavy elements, appears useful for preparing novel photosensitizers. 相似文献
93.
To provide theoretical insight into the structures and properties of Sc3N@C80, which has been isolated in high yield and purity as a new stable endohedral metallofullerene, density functional calculations are carried out for the Sc3?nLanN@C80 (n=0–3) series. Because of electron transfer from Sc3N to C80, the electronic structure of Sc3N@C80 is formally described as (Sc3N)6+C$_{80}^{6-}$. The encapsulated Sc3N cluster takes a planar structure with long Sc–Sc distances and is highly stabilized inside the Ih cage of C80, which rotates rapidly. As the number of La atoms increases, the Sc3?nLanN cluster is forced to maintain a pyramidal structure in Sc3?nLanN@C80. In addition, the C80 cage takes an open‐shell electronic structure due to an increase in the number of electrons transferring from Sc3?nLanN. These make the endohedral structure less stable and more reactive. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1353–1358, 2001 相似文献
94.
Tatsuo SuzukiMasaharu Inui Seijiro HosokawaSusumu Kobayashi 《Tetrahedron letters》2003,44(18):3713-3716
Ketene N,O-acetals were prepared stereoselectively and submitted to a Lewis acid-mediated 1,3-rearrangement to afford C-alkylated products. The reactions proceeded in a stereoselective manner to construct a chiral quaternary carbon in high selectivity. The stereochemistry of the quaternary center was found to be opposite to that obtained by an anionic direct dienolate alkylation. 相似文献
95.
Beker H Boggild H Boissevain J Cherney M Dodd J Esumi S Fabjan CW Fields DE Franz A Hansen KH Holzer EB Humanic TJ Jacak BV Jayanti R Kalechofsky H Kobayashi T Kvatadze R Lee YY Leltchouk M Lörstad B Maeda N Medvedev A Miake Y Miyabayashi A Murray M Nagamiya S Nishimura S Noteboom E Pandey SU Piuz F Polychronakos V Potekhin M Poulard G Sakaguchi A Sarabura M Shigaki K Simon-Gillo J Sondheim W Sugitate T Sullivan JP Sumi Y van Hecke H Willis WJ Wolf K Xu N 《Physical review letters》1995,74(17):3340-3343
96.
97.
98.
99.
100.
Tadashi Kon Tetsuro Kobayashi Shoichi Kitamura Keiichiro Nakamura Shunji Adachi 《Zeitschrift fur Physik C Particles and Fields》1994,61(2):239-246
In the framework of the minimal supersymmetric standard model (MSSM) and theR-parity breaking model (RBM), we investigate production processes of the scalar top (stop) at HERA energies. These models are characterized by the possible existence of the light stop whose mass is lighter than those of the top quark and the other squarks. It is shown that in the MSSM the stop pair production via boson-gluon fusion gives a sizeable cross section and the most serious background $ep \to ec\bar cX$ could be suppressed by appropriate kinematical cuts. We also show that in the RBM the stop is singly produced in the neutral current processes and we have a clear signal as a sharp peak in the Bjorken parameterx distribution of the scattered electron. 相似文献