全文获取类型
收费全文 | 1650篇 |
免费 | 41篇 |
国内免费 | 13篇 |
专业分类
化学 | 1334篇 |
晶体学 | 17篇 |
力学 | 22篇 |
数学 | 69篇 |
物理学 | 262篇 |
出版年
2023年 | 10篇 |
2022年 | 7篇 |
2021年 | 12篇 |
2020年 | 18篇 |
2019年 | 15篇 |
2018年 | 14篇 |
2016年 | 27篇 |
2015年 | 20篇 |
2014年 | 44篇 |
2013年 | 90篇 |
2012年 | 79篇 |
2011年 | 103篇 |
2010年 | 54篇 |
2009年 | 49篇 |
2008年 | 93篇 |
2007年 | 106篇 |
2006年 | 124篇 |
2005年 | 110篇 |
2004年 | 98篇 |
2003年 | 93篇 |
2002年 | 104篇 |
2001年 | 27篇 |
2000年 | 25篇 |
1999年 | 28篇 |
1998年 | 18篇 |
1997年 | 18篇 |
1996年 | 19篇 |
1995年 | 28篇 |
1994年 | 18篇 |
1993年 | 10篇 |
1992年 | 4篇 |
1991年 | 11篇 |
1990年 | 7篇 |
1989年 | 7篇 |
1988年 | 10篇 |
1987年 | 11篇 |
1986年 | 11篇 |
1985年 | 18篇 |
1984年 | 17篇 |
1983年 | 8篇 |
1982年 | 14篇 |
1981年 | 13篇 |
1980年 | 14篇 |
1979年 | 17篇 |
1978年 | 18篇 |
1977年 | 11篇 |
1976年 | 5篇 |
1975年 | 5篇 |
1974年 | 18篇 |
1973年 | 9篇 |
排序方式: 共有1704条查询结果,搜索用时 46 毫秒
71.
Uehara K Hikichi S Inagaki A Akita M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(9):2788-2809
A series of dinuclear complexes, [Tp(R)M--M'L(n)] [Tp(iPr(2) )M--Co(CO)(4) (1; M=Ni, Co, Fe, Mn); Tp(#)M--Co(CO)(4) (1'; M=Ni, Co); Tp(#)Ni--RuCp(CO)(2) (3')] (Tp(iPr(2) )=hydrotris(3,5-diisopropylpyrazolyl)borato; Tp(#) (Tp(Me(2),4-Br))=hydrotris(3,5-dimethyl-4-bromopyrazolyl)borato), has been prepared by treatment of the cationic complexes [Tp(iPr(2) )M(NCMe)(3)]PF(6) or the halo complexes [Tp(#)M--X] with the appropriate metalates. Spectroscopic and crystallographic characterization of 1-3' reveals that the tetrahedral, high-spin Tp(R)M fragment and the coordinatively saturated carbonyl-metal fragment (M'L(n)) are connected only by a metal-metal interaction and, thus, the dinuclear complexes belong to a unique class of xenophilic complexes. The metal-metal interaction in the xenophilic complexes is polarized, as revealed by their nu(CO) vibrations and structural features, which fall between those of reference complexes: covalently bonded species [R--M'L(n)] and ionic species [M'L(n)](-). Unrestricted DFT calculations for the model complexes [Tp(H(2) )Ni--Co(CO)(4)], [Tp(H(2) )Ni--Co(CO)(3)(PH(3))], and [Tp(H(2) )Ni--RuCp(CO)(2)] prove that the two metal centers are held together not by covalent interactions, but by electrostatic attractions. In other words, the obtained xenophilic complexes can be regarded as carbonylmetalates, in which the cationic counterpart interacts with the metal center rather than the oxygen atom of the carbonyl ligand. The xenophilic complexes show divergent reactivity dependent on the properties of donor molecules. Hard (N and O donors) and soft donors (P and C donors) attack the Tp(R)M part and the ML(n) moiety, respectively. The selectivity has been interpreted in terms of the hard-soft theory, and the reactions of the high-spin species 1-3' with singlet donor molecules should involve a spin-crossover process. 相似文献
72.
Cai X Sakamoto M Fujitsuka M Majima T 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(22):6471-6477
The lifetimes of benzophenone in the higher triplet excited state (BP(T(n))) and several BP derivatives in the T(n) states were measured directly to be tau(T(n))=37+/-7 ps and 20-33 ps, respectively, by using the nanosecond-picosecond (ns-ps) two-color/two-laser flash photolysis method. Based on the direct measurements of tau(T(n)) of BP(T(n)), the triplet energy transfer (TET) from BP(T(n)) to quenchers (Q), such as carbon tetrachloride (CCl4), benzene (Bz), and p-dichlorbenzene (DCB), was investigated. The fast TET from BP(T(n)) to Q can be attributed to the lifetime-dependent quenching process, according to the Ware theoretical model of the bimolecular energy transfer reaction. The contribution of the lifetime-dependent term on k(TET) was 27, 60, and 86% for CCl4, Bz, and DCB as the Q of BP(T(n)), respectively, indicating that the TET from BP(T(n)) to Q is influenced not only by tau(T(n)), but also by the size of Q. 相似文献
73.
The reaction of C6Cl5Ni(PPhMe2)2Cl with CHRCR′CH2MgX (X = Br or Cl) yields π-allylnickel compounds, (π-CHRCR′CH2)Ni(PPhMe2)C6Cl5 (Ia, R = R′ = H; Ib, R = H, R′ = CH3; Ic, R = CH3, R′ = H), which are stable in the solid state below ca. 150°C and are fairly stable in solution in the absence of oxygen. The π-allyl group was found by PMR spectroscopy to be rigid even in the presence of an excess of PPhMe2, P(OEt)3 or P(OMe)3. 相似文献
74.
Ab initio molecular orbital calculations have been performed to investigate the structures and quantum effects of the proton motion in NH(3):HCl:(H(2)O)(n) (n = 0-3) clusters using a MP2/aug-cc-pVDZ level of theory. Three new stable structures and one transition-state structure are investigated for these clusters. The detailed analyses of the intermolecular interactions suggest that three-body interactions play an important role to determine the relative stability in each size of cluster. The quantum effects of the proton motion result in frequency shifts for proton-stretching modes. Our one-dimensional and two-dimensional models fairly closely reproduce the experimental proton-stretching vibrational frequency of the NH(3):HCl cluster. The most stable isomer for n = 1 has a proton-transfer structure, which is weakened by the quantum effects of the proton motion. 相似文献
75.
Abe M Adam W Hara M Hattori M Majima T Nojima M Tachibana K Tojo S 《Journal of the American Chemical Society》2002,124(23):6540-6541
Photodenitrogenation of the diazenes 4 affords exclusively the housanes 5 through intramolecular cyclization of the spectrally detected and characterized singlet diradicals 3. The lifetime of singlet diradical 3, determined by transient absorption measurements, depends on the Y and Z substituents at the para position of the phenyl ring and has the following order: Y, Z = OMe, OMe > OMe, CN > CN, CN > OMe, H > Cl, Cl approximately CN, H approximately Me, Me > H, H. This unprecedented substituent effect reveals stabilization of the singlet 2,2-dimethoxycyclopentane-1,3-diyl diradicals 3 through radical, zwitterionic, pi-bonding, and hyperconjugative structures. 相似文献
76.
Heng-dao Quan Hui-e Yang Masanori Tamura Akira Sekiya 《Journal of fluorine chemistry》2004,125(7):1169-1172
A novel fluorination reagent and catalyst, SbF5/PAF (porous aluminum fluoride), was prepared by impregnating SbCl5 into PAF and then treating with anhydrous hydrogen fluoride. The prepared reagent had an excellent catalytic activity in halogen-exchange, and also improved the properties of SbF5, such as hydroscopicity, corrosion, and toxicity. SbF5/PAF was successfully used in organic synthesis as a fluorinating reagent, and a fixed bed catalyst for F/Cl exchange. 相似文献
77.
Simons electrochemical fluorination (ECF) of 1,4-dimethyl-1,4-homopiperazine, methyl 4-ethylhomopiperazin-1-ylacetate and 1,4-bis(methoxycarbonylmethyl)-1,4-homopiperazine was studied. For comparison, ECF of three piperazines with a N-(methoxycarbonylmethyl) group(s) was also studied. ECF of 1,4-dimethyl-1,4-homopiperazine gave a low yield of corresponding perfluoro(1,4-dimethyl-1,4-homopiperazine) together with perfluoro(2,6-diaza-2,6-dimethylheptane) as the major product. Corresponding perfluoro(homopiperazines) with mono- and/or di-(fluorocarbonyldifluoromethyl) groups [CF2C(O)F] at the 1- and/or 4-position were formed in low yields from methyl 4-ethylhomopiperazin-1-ylacetate and 1,4-bis(methoxycarbonylmethyl)-1,4-homopiperazine, respectively. These new seven-membered perfluoro(1,4-dialkyl-1,4-homopiperazines) were accompanied by the formation of mono- and/or di-basic linear perfluoroacid fluorides resulting from the CC bond scission at the 2- and 3-positions of the ring. From mono- and/or di-N-(methoxycarbonylmethyl)-substituted piperazines, corresponding perfluoropeperazines having the acid fluoride group(s) were formed in low yields. 相似文献
78.
Miki Murata Hiyoruki Yamasaki Tsukasa Ueta Masayuki Nagata Masanori Ishikura Shinji Watanabe Yuzuru Masuda 《Tetrahedron》2007,63(19):4087-4094
The general and efficient silylation of aryl halides has been developed utilizing triethoxysilane and a rhodium catalyst. The substrate scope is broad and includes ortho-, meta-, and para-substituted electron-rich and -deficient aryl iodides. In addition, the silylation of aryl bromides and fluoroalkanesulfonates proceeded in the presence of tetra-n-butylammonium iodide. 相似文献
79.
Shiro Kobayashi Hiroshi Uyama 《Journal of polymer science. Part A, Polymer chemistry》2002,40(2):192-209
Topics concerning the cationic ring‐opening polymerization of cyclic imino ethers and functional material production based on the resulting polymers are reviewed. Cyclic imino ethers are readily subjected to isomerization polymerization via cationic initiators. Mechanistic studies have provided a new concept, electrophilic polymerization. Double isomerization polymerization and no‐catalyst alternating copolymerization are interesting examples that show characteristics of the ring opening of cyclic imino ethers. The living polymerization of these monomers affords precisely controlled polymeric materials. Through the use of the unique properties of the product polymers, various functional polymeric materials, such as polymeric nonionic surfactants, compatibilizers, hydrogels, stabilizers for dispersion polymerization, biocatalyst modifiers, and supramolecular assemblies, have been developed. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 192–209, 2002 相似文献
80.
Ogata M Kaneya D Shin-Ya K Li L Abe Y Katoh H Seki S Seki Y Gonda R Urano S Endo T 《Chemical & pharmaceutical bulletin》2005,53(9):1167-1170
Many researchers have stated that eugenol might inhibit lipid peroxidation at the stage of initiation, propagation, or both, and many attempts have been made to elucidate the mechanism of its antioxidant activity. Nevertheless, details of its mechanism are still obscure. This study was carried out to investigate the trapping effect of eugenol on hydroxyl radical generated from L-3,4-dihydroxyphenylalanine (DOPA) in MiliQ water and the generation mechanism of the hydroxyl radical by this system which uses no metallic factor. This was studied by adding L-DOPA and 5,5-dimethyl-1-pyrroline N-oxide (DMPO) to phosphate buffered saline (PBS) or MiliQ water, and the generation of hydroxyl radical was detected on an ESR spectrum. By this method, the effect of antioxidants was detected as a modification of ESR spectra. We found that the eugenol trapped hydroxyl radicals directly, because it had no iron chelating action, did not trap L-DOPA semiquinone radical and inhibited hydroxyl radicals with or without iron ion. 相似文献