全文获取类型
收费全文 | 2894篇 |
免费 | 100篇 |
国内免费 | 10篇 |
专业分类
化学 | 2198篇 |
晶体学 | 34篇 |
力学 | 20篇 |
数学 | 169篇 |
物理学 | 583篇 |
出版年
2023年 | 14篇 |
2021年 | 28篇 |
2020年 | 31篇 |
2019年 | 38篇 |
2018年 | 29篇 |
2017年 | 26篇 |
2016年 | 61篇 |
2015年 | 62篇 |
2014年 | 67篇 |
2013年 | 133篇 |
2012年 | 133篇 |
2011年 | 183篇 |
2010年 | 98篇 |
2009年 | 83篇 |
2008年 | 171篇 |
2007年 | 179篇 |
2006年 | 153篇 |
2005年 | 163篇 |
2004年 | 175篇 |
2003年 | 152篇 |
2002年 | 152篇 |
2001年 | 61篇 |
2000年 | 59篇 |
1999年 | 30篇 |
1998年 | 40篇 |
1997年 | 22篇 |
1996年 | 39篇 |
1995年 | 28篇 |
1994年 | 27篇 |
1993年 | 21篇 |
1992年 | 21篇 |
1991年 | 32篇 |
1990年 | 31篇 |
1989年 | 22篇 |
1988年 | 23篇 |
1987年 | 12篇 |
1986年 | 28篇 |
1985年 | 33篇 |
1984年 | 36篇 |
1983年 | 19篇 |
1982年 | 25篇 |
1981年 | 32篇 |
1980年 | 31篇 |
1979年 | 20篇 |
1978年 | 20篇 |
1977年 | 31篇 |
1976年 | 13篇 |
1975年 | 16篇 |
1974年 | 14篇 |
1973年 | 18篇 |
排序方式: 共有3004条查询结果,搜索用时 15 毫秒
101.
3-Alkylthio-4-amino-1,6-dihydro-1,2,4-triazin-5(4H)-ones were synthesized by the reduction of 3-thio-4-amino-1,2,4-triazine-3,5(2.H,4H)-diones and successive S-alkylation. The regiospecific alkylation on the N-1 position or the exo amino group leads to a variety of 1,6-dihydro-1,2,4-triazin-5(4H)-one derivatives. An alternative synthesis of 3-thio-4-amino-1,6-dihydro-1,2,4-triazine-3,5(2H,4H)-diones was accomplished through the cyclization of 1-thiocarbohydrazidoacetamide derivatives. 相似文献
102.
4H-[1,3,4]Thiadiazolo[2,3-c][1,2,4]triazin-4-ones were prepared by cyclization of 3-thio-4-amino-1,2,4-tri-azine-3,5(2H,4H)-diones with a variety of acids in the presence of concentrated sulfuric acid. The synthesis appears to be general. 相似文献
103.
Nishibayashi Y Milton MD Inada Y Yoshikawa M Wakiji I Hidai M Uemura S 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(5):1433-1451
The scope and limitations of the ruthenium-catalyzed propargylic substitution reaction of propargylic alcohols with heteroatom-centered nucleophiles are presented. Oxygen-, nitrogen-, and phosphorus-centered nucleophiles such as alcohols, amines, amides, and phosphine oxide are available for this catalytic reaction. Only the thiolate-bridged diruthenium complexes can work as catalysts for this reaction. Results of some stoichiometric and catalytic reactions indicate that the catalytic propargylic substitution reaction proceeds via an allenylidene complex formed in situ, whereby the attack of nucleophiles to the allenylidene C(gamma) atom is a key step. Investigation of the relative rate constants for the reaction of propargylic alcohols with several para-substituted anilines reveals that the attack of anilines on the allenylidene C(gamma) atom is not involved in the rate-determining step and rather the acidity of conjugated anilines of an alkynyl complex, which is formed after the attack of aniline on the C(gamma) atom, is considered to be the most important factor to determine the rate of this catalytic reaction. The key point to promote this catalytic reaction by using the thiolate-bridged diruthenium complexes is considered to be the ease of the ligand exchange step between a vinylidene ligand on the diruthenium complexes and another propargylic alcohol in the catalytic cycle. The reason why only the thiolate-bridged diruthenium complexes promote the ligand exchange step more easily with respect to other monoruthenium complexes in this catalytic reaction should be that one Ru moiety, which is not involved in the allenylidene formation, works as an electron pool or a mobile ligand to another Ru site. The catalytic procedure presented here provides a versatile, direct, and one-step method for propargylic substitution of propargylic alcohols in contrast to the so far well-known stoichiometric and stepwise Nicholas reaction. 相似文献
104.
Koji Nakabayashi Masaki Kawano Prof. Dr. Tatsuhisa Kato Prof. Dr. Ko Furukawa Dr. Shin‐ichi Ohkoshi Prof. Dr. Toshiya Hozumi Dr. Makoto Fujita Prof. Dr. 《化学:亚洲杂志》2007,2(1):164-170
We show a new approach to manipulating the through‐space spin–spin interaction by utilizing the confined cavity of a self‐assembled M6L4 coordination cage. The coordination cage readily encapsulates stable organic radicals in solution, which brings the spin centers of the radicals closer to each other. In sharp contrast to the fact that the radical in solution in the absence of the cage is in a doublet state, in the presence of the cage through‐space spin–spin interaction is induced through cage‐encapsulation effects in solution as well as in the solid state, resulting in the triplet state of the complex. These results were confirmed by ESR spectroscopy and X‐ray crystallography. The quantity of triplet species generated by encapsulation in the cage increases with increasing affinity of the radicals to the cage. We estimated the affinity between several types of guests and the cage in solution by cyclic voltammetry. We also demonstrate that the through‐space interaction of organic radicals within the self‐assembled coordination cage can be controlled by external stimuli such as heat or pH. 相似文献
105.
An improved spectrophotometric determination of uranium(VI) is proposed using 4-(2-pyridylazo)resorcinol(PAR) in the presence of benzyldimethylstearyltrimethylammonium chloride(BSTAC) as a cationic surfactant. The calibration graph is linear in the range of 0.3-60 microg/10 ml uranium(VI), measuring the absorbance at 550 nm. The reproducibility for 19.0 microg/10 ml uranium(VI) is 0.57%. The third-derivative method using the third-derivative distance (d(3)A/dlambda(3)) among lambda(1) 530 nm, lambda(3) 594 nm and lambda(2) 565 nm was also investigated. 相似文献
106.
Oka H Suzuki M Harada K Iwaya M Fujii K Goto T Ito Y Matsumoto H Ito Y 《Journal of chromatography. A》2002,946(1-2):157-162
pH-Zone-refining counter-current chromatography was successfully applied to the separation of the main components of Food Color Red No. 106 (R-106, acid red, Color Index No. 45100). A 300-mg quantity of sample was separated using the following two-phase solvent system: n-butanol-water, 40 mM sulfuric acid in organic stationary phase and 30 mM ammonia in aqueous mobile phase. The obtained fractions were analyzed by high-performance liquid chromatography and fast atom bombardment mass spectrometry. The separation yielded 261.9 mg of main component of acid red with purity of 99.9%. 相似文献
107.
Shinsaku Fujita 《Theoretical chemistry accounts》1990,78(1):45-63
Summary A coset representation (G(/G
i
)), which is defined algebraically by a coset decomposition of a finite groupG by its subgroupG
i
, is shown to be a method for the decomposition of a regular body into its point group orbits. This proof also shows that each member of theG(/G
i
) orbit belongs to theG
i
site-symmetry. In addition, a general equation concerning the multiplicities of such coset representations is derived and shown to involve Brester's equations and thek-value equations of framework groups as special cases. The relationship of the coset representation and the site-symmetry affords a general procedure for obtaining symmetry adapted functions. 相似文献
108.
The Bose-Einstein condensation of free relativistic particles [=(M
2
c
4
+c
2
p
2
)
1/2
–Mc
2
] is studied rigorously. For massless bosons (=cp), the condensation transition of third (second) order occurs in2 (3) dimensions (D). The molar heat capacity follows the T
2
(T
3
) law below the condensation temperature Tc [k
B
Tc=(2
2
c
2
n/1.645)
1/2
[(
2
3
c
3
n/1.202)
1/3
], reaches4.38 (10.8) R at T=Tc, and approaches the high-temperature-limit value2 (3) R with no jump (a jump equal to6.75R) in2 (3)D. For finite-mass (M) bosons, the phase transition occurs only in3D with the condensation temperature Tc always smaller than that of the corresponding nonrelativistic bosons [=(2M)
–1
p
2
]. If the mass M is reduced to zero, the condensation temperature Tc grows monotonically and reaches eventually that of massless relativistic bosons. This mass-dependence of Tc is therefore distinct from the case of nonrelativistic bosons, where Tc grows to infinity as M 0. A brief discussion is given for a possible connection with the normal-to-super transition of the independently moving Cooper pairs (bosons). 相似文献
109.
110.