全文获取类型
收费全文 | 5601篇 |
免费 | 254篇 |
国内免费 | 68篇 |
专业分类
化学 | 3445篇 |
晶体学 | 52篇 |
力学 | 239篇 |
综合类 | 1篇 |
数学 | 1090篇 |
物理学 | 1096篇 |
出版年
2023年 | 48篇 |
2022年 | 45篇 |
2021年 | 72篇 |
2020年 | 90篇 |
2019年 | 97篇 |
2018年 | 162篇 |
2017年 | 126篇 |
2016年 | 189篇 |
2015年 | 188篇 |
2014年 | 220篇 |
2013年 | 450篇 |
2012年 | 471篇 |
2011年 | 463篇 |
2010年 | 302篇 |
2009年 | 232篇 |
2008年 | 351篇 |
2007年 | 378篇 |
2006年 | 297篇 |
2005年 | 297篇 |
2004年 | 240篇 |
2003年 | 188篇 |
2002年 | 176篇 |
2001年 | 55篇 |
2000年 | 56篇 |
1999年 | 42篇 |
1998年 | 29篇 |
1997年 | 29篇 |
1996年 | 54篇 |
1995年 | 39篇 |
1994年 | 35篇 |
1993年 | 26篇 |
1992年 | 28篇 |
1991年 | 20篇 |
1990年 | 21篇 |
1989年 | 18篇 |
1988年 | 10篇 |
1987年 | 12篇 |
1986年 | 11篇 |
1985年 | 31篇 |
1984年 | 25篇 |
1983年 | 14篇 |
1982年 | 23篇 |
1981年 | 17篇 |
1980年 | 18篇 |
1979年 | 21篇 |
1978年 | 23篇 |
1976年 | 18篇 |
1975年 | 13篇 |
1974年 | 15篇 |
1973年 | 15篇 |
排序方式: 共有5923条查询结果,搜索用时 0 毫秒
71.
Linde C Koliaï N Norrby PO Akermark B 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(11):2568-2573
The substrate electronic effects on the selectivity in the catalytic epoxidation of para-substituted cis stilbenes 2a-i were investigated by using (R,R)-[N,N'-bis(3,5-di-tBu-salicylidene)-1,2-cyclohexanediamine]manganese(III) chloride 1 in benzene as the catalyst with iodosobenzene as the terminal oxidant. A Hammett study of the selectivity results reveals a stronger electrophilic character than previously assumed in the (salen)Mn-catalyzed reaction. In general, the best correlations with the experimental values were obtained by using the Hammett sigma + values, which gave rho = -1.37 for the rate of cis-epoxide formation and rho = -0.43 for the rate of the stepwise process leading to the corresponding trans product. The reaction involves two separate pathways as indicated also by the competitive breakdown of the intermediate on the path to trans epoxide for methoxy-substituted substrates. The asynchronicity in the concerted pathway leading to cis epoxide is apparent for 4-methoxy-4'-nitrostilbene, which yields cis epoxide with 75% ee entirely as a result of electronic effects. 相似文献
72.
The [Ru(bipy)(2)(1)](PF(6))(2) (bipy refers to 2,2'-bipyridine) complex, comprising a ruthenium(II) tris(2,2'-bipyridine) luminophore covalently linked to a di[(o-triethyleneglycoxy)phenyl]amine crown ether 1, has been synthesized and fully characterized. The photophysical properties of this metal complex have been examined in solution at ambient temperature. Luminescence from the metal complex is enhanced significantly in the presence of various adventitious cations, including protons. In particular, Li(+) cations bind to the crown ether, as evidenced by (1)H NMR and luminescence spectroscopy. Cation binding serves to decrease the rate of reductive quenching of the triplet state of the metal complex, thereby increasing the extent of luminescence. The solution-phase conformation of [Ru(bipy)(2)(1)](PF(6))(2), with and without encapsulated Li(+), has been examined by 2-D NMR and by molecular dynamics simulations. 相似文献
73.
Frédérique Malbec René Milcent Patrick Vicart Anne Marie Bure 《Journal of heterocyclic chemistry》1984,21(6):1769-1774
Thirty new 2-substituted-4-amino-5-alkyl or aryl-2,4-dihydro-1,2,4-triazol-3-ones and ten 2-substituted-5-alkyl or aryl-4-(5-nitro-2-furfurylidene)amino-2,4-dihydro-1,2,4-triazol-3-ones were synthesized and characterised by their sharp melting points, elemental analysis, ir and 1H nmr spectra. These new derivatives of 5-nitro-2-furaldehyde were screened for their antibacterial activities. Most of the compounds showed good activity against one test organism, Staphylococcus aureus. For a few compounds, C.M.I. ranged from 4 to 8 μg/ml (higher results than nitrofurantoin). 相似文献
74.
75.
Jean Ravez Annie Perron Jean Pierre Chaminade Paul Hagenmuller Loïc Rivoallan 《Journal of solid state chemistry》1974,10(3):274-281
Two series of phases with tetragonal bronze-like structure and composition BaxLi5?2xT5O15 (T = Nb, Ta) have been isolated in the systems BaNb2O6LiNbO3 and BaTa2O6LiTaO3. All these phases show ferroelectric-paraelectric transitions. The Curie temperature increases with the lithium content. The value of TC for Ba2.03Li0.94Nb5O15 is the highest ever observed for this type of structure: the obtained phases are potentially good materials for the harmonic generation of the 0.53-μm radiation. The optical yield of the niobate Ba2.14Li0.71Nb5O15 is about 2.5 times that of Ba2NaNb5O15 and 250 times that of the K.D.P. The crystallographic and dielectric data of the system Ba2.14Li0.71Nb5O15Ba2.14Li0.71Ta5O15 characterize three domains, which are respectively antiferroelectric, ferroelectric, and paraelectric. The Curie temperature and the optical yield decrease with increasing tantalum content. 相似文献
76.
Isobe H Tanaka T Nakanishi W Lemiègre L Nakamura E 《The Journal of organic chemistry》2005,70(12):4826-4832
The reaction of [60]fullerene with a variety of a secondary aliphatic amines in 20% v/v dimethyl sulfoxide in chlorobenzene under an atmospheric pressure of molecular oxygen allows regioselective introduction of four amino groups and one epoxide group around one pentagon of the fullerene molecule in good to high yield. This new synthesis of tetraaminofullerene expoxide can be carried out with a simple procedure on a multigram scale at room temperature and affords a variety of functionalized fullerene derivatives. Near-infrared analysis of a mixture of [60]fullerene and piperidine in a deaerated dimethyl sulfoxide/chlorobenzene mixture indicated equilibrium formation of [60]fullerene radical anion (C60*-) that persists at least for 2 weeks at room temperature but reacts immediately with molecular oxygen to give the tetraaminofullerene expoxide. The Benesi-Hildebrand analysis of the concentration dependency of the near-infrared absorption indicated that a [C60*- piperidine*+] radical ion pair is formed with an equivalent constant of K = 0.62 +/- 0.02 M(-1) at 25 degrees C. This and other lines of evidence suggest that the oxygenative amination reaction involves C60-mediated reduction of molecular oxygen by the amine. 相似文献
77.
Charbonnière LJ Schurhammer R Mameri S Wipff G Ziessel RF 《Inorganic chemistry》2005,44(20):7151-7160
A new ligand, LC, bis-[(6'-carboxy-2,2'-bipyridine-6-yl)]phenylphosphine oxide, in which the tridentate 6-carboxy-2,2'-bipyridyl arms are directly linked to a phenylphosphine oxide fragment, has been synthesized. The corresponding [Ln.LC]Cl.xH2O complexes (Ln = Eu, x = 4, and Tb, x = 3) were isolated from solutions containing equimolar amounts of LC and hydrated LnCl3 salts and characterized by elemental analysis, mass spectrometry, and infrared spectroscopy. The interactions of the Eu complex with various anions (AMP(2-), ADP3-, ATP,4- HPO4(2-), and NO3-) were studied by titration experiments, using UV-vis, luminescence spectroscopy, and excited-state lifetime measurements. The results are in keeping with strong interactions with the ADP3-, ATP4-, and phosphate anions in TRIS/HCl buffer (0.01 M, pH = 7.0), as revealed by the determination of the conditional stepwise association constants. These values are higher than the one determined for ligand LB, bis[(6'-carboxy-2,2'-bipyridine-6-methyl-yl)]-n-butylamine (Delta log K approximately 1-2). The interaction of complexes [Ln.LB]+ and [Ln.LC]+ with nitrate, monohydrogenophosphate, methyl phosphate (MeP2-), methyldiphosphate (MeDP3-), and methyltriphosphate (MeTP4-) anions was investigated by means of quantum mechanical (QM) calculations. The results, combined with data on the photophysical impact of the sequential competitive binding of anions to the Eu complexes in water, suggest that LB is too flexible to ensure a good coordination pocket, while the molecular structure of ligand LC stabilizes both the formation of the lanthanide complexes and its adducts with ATP. 相似文献
78.
The dynamic processes of N(1s) core-hole excitation in gas-phase CH3CN molecule have been studied at both Hartree-Fock and hybrid density-functional theory levels. The vibrational structure is analyzed for fully optimized core-excited states. Frank-Condon factors are obtained using the linear coupling model for various potential surfaces. It is found that the vibrational profile of the N-K absorption can be largely described by a summation of two vibrational progressions: a structure-rich profile of nu(CN) stretching mode and a large envelope of congestioned vibrational levels related to the strong (-C-CN) terminal bending bond. Excellent agreement between theoretical and experimental spectra is obtained. 相似文献
79.
Alvarez CM Alvarez MA García-Vivó D García ME Ruiz MA Saáez D Falvello LR Soler T Herson P 《Dalton transactions (Cambridge, England : 2003)》2004,(24):4168-4179
The new hydride complexes [Mo2Cp2(mu-H)(mu-PHR)(CO)4] having bulky substituents (R = 2,4,6-C(6)H2tBu3= Mes*, R = 2,4,6-C6H2Me3= Mes) have been prepared in good yield by addition of Li[PHR] to the triply bonded [Mo2Cp2(CO)4] and further protonation of the resulting anionic phosphide complex [Mo2Cp2(mu-PHR)(CO)4]-. Protonation of the Mes* compound with either [H(OEt2)2][B{3,5-C6H3(CF3)2}4] or HBF4.OEt2 gives the cationic phosphinidene complex [Mo2Cp2(mu-H)(mu-PMes*)(CO)4]+ in high yield. In contrast, protonation of the analogous hydride compounds with Mes or Cy substituents on phosphorus give the corresponding unsaturated tetracarbonyls [Mo2Cp2(mu-PHR)(CO)4]+, which are unstable at room temperature and display a cis geometry. Decomposition of the latter give the electron-precise pentacarbonyls [Mo2Cp2(mu-PHR)(mu-CO)(CO)4]+, also displaying a cis arrangement of the metal fragments. In the presence of BF4- as external anion, fluoride abstraction competes with carbonylation to yield the neutral fluorophosphide hydrides [Mo2Cp2(mu-H)(mu-PFR)(CO)4]. Similar results were obtained in the protonation reactions of the hydride compounds having a Ph substituent on phosphorus. In that case, using HCl as protonation reagent gave the chloro-complex [Mo2ClCp2(mu-PHPh)(CO)4] in good yield. The structures and dynamic behaviour of the new compounds are analyzed on the basis of solution IR and 1H, 31P, 19F and 13C NMR data as well as the X-ray studies carried out on [Mo2Cp2(mu-H)(mu-PHMes)(CO)4](cis isomer), [Mo2Cp2(mu-H)(mu-PFMes)(CO)4](trans isomer), [Mo2Cp2(mu-PHCy)(mu-CO)(CO)4](BF4) and [Mo2ClCp2(mu-PHPh)(CO)4]. 相似文献
80.
A simple and rapid densitometric method has been developed for the simultaneous determination of amoxycillin trihydrate and clavulanate potassium in pharmaceutical preparations. After extraction of the analytes with distilled water, the extracts were spotted on precoated silica gel plates, which were then eluted with butyl acetate-methanol-glacial acetic acid-water (15 + 7.5 + 7.5 + 3, v/v). Quantitative evaluation was performed by measuring the absorbance reflectance of the analyte spots at lambda = 240 nm. The densitometric method is rapid, selective, precise, and accurate and, thus, can be used for routine analysis of pharmaceutical preparations in quality control laboratories of the pharmaceutical industry. 相似文献