全文获取类型
收费全文 | 3924篇 |
免费 | 218篇 |
国内免费 | 2篇 |
专业分类
化学 | 3255篇 |
晶体学 | 23篇 |
力学 | 40篇 |
数学 | 486篇 |
物理学 | 340篇 |
出版年
2024年 | 4篇 |
2023年 | 48篇 |
2022年 | 28篇 |
2021年 | 59篇 |
2020年 | 72篇 |
2019年 | 79篇 |
2018年 | 88篇 |
2017年 | 85篇 |
2016年 | 169篇 |
2015年 | 165篇 |
2014年 | 152篇 |
2013年 | 248篇 |
2012年 | 319篇 |
2011年 | 351篇 |
2010年 | 199篇 |
2009年 | 154篇 |
2008年 | 297篇 |
2007年 | 295篇 |
2006年 | 264篇 |
2005年 | 220篇 |
2004年 | 158篇 |
2003年 | 172篇 |
2002年 | 122篇 |
2001年 | 44篇 |
2000年 | 32篇 |
1999年 | 25篇 |
1998年 | 13篇 |
1997年 | 27篇 |
1996年 | 40篇 |
1995年 | 14篇 |
1994年 | 14篇 |
1993年 | 15篇 |
1992年 | 17篇 |
1991年 | 12篇 |
1990年 | 7篇 |
1989年 | 6篇 |
1988年 | 8篇 |
1987年 | 9篇 |
1986年 | 11篇 |
1985年 | 15篇 |
1984年 | 15篇 |
1983年 | 8篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1980年 | 10篇 |
1978年 | 4篇 |
1977年 | 4篇 |
1976年 | 6篇 |
1974年 | 5篇 |
1973年 | 7篇 |
排序方式: 共有4144条查询结果,搜索用时 15 毫秒
91.
Maria I. Colombo Sebastián A. Testero Silvina C. Pellegrinet María L. Bohn Edmundo A. Rúveda 《The Chemical Educator》2001,6(6):350-352
A comprehensive laboratory experiment suitable for advanced undergraduate organic chemistry students has been designed. The experiment is based on the dienone-phenol rearrangement reaction of the sesquiterpene santonin to give -desmotroposantonin acetate. It challenges students to solve an earlier controversial stereochemical problem. The students carry out the reaction and analyze spectroscopic data to determine the stereochemistry of the starting material and the product. In addition, they perform simple molecular modeling calculations, which enable them to rationalize the stereochemical outcome of the transformation and discuss the mechanism of the dienone-phenol rearrangement and related rearrangements reported in the literature. 相似文献
92.
Tandori J Miksovska J Valerio-Lepiniec M Schiffer M Maróti P Hanson DK Sebban P 《Photochemistry and photobiology》2002,75(2):126-133
Flash-induced absorbance spectroscopy was used to analyze the proton uptake and electron transfer properties of photosynthetic reaction centers (RC) of Rhodobacter capsulatus that have been genetically modified near the primary quinone electron acceptor (Q(A)). M246Ala and M247Ala, which are symmetry-related to the positions of two acidic groups, L212Glu and L213Asp, in the secondary quinone electron acceptor (QB) protein environment, have been mutated to Glu and Asp, respectively. The pH dependence of the stoichiometry of proton uptake upon formation of the P+Q(A)- (H+/P+Q(A)-) and PQ(A) (H+/Q(A)-) (P is the primary electron donor, a noncovalently linked bacteriochlorophyll dimer) states have been measured in the M246Ala --> Glu and the M247Ala --> Asp mutant RC, in the M246Ala-M247Ala --> Glu-Asp double mutant and in the wild type (WT). Our results show that the introduction of an acidic group (Glu or Asp) in the QA protein region induces notable additional proton uptake over a large pH region (approximately 6-9), which reflects a delocalized response of the protein to the formation of Q(A)-. This may indicate the existence of a widely spread proton reservoir in the cytoplasmic region of the protein. Interestingly, the pH titration curves of the proton release caused by the formation of P+ (H+/P+: difference between H+/P+Q(A)- and H+/PQ(A)- curves) are nearly superimposable in the WT and the M246Ala --> Glu mutant RC, but substantial additional proton release is detected between pH 7 and 9 in the M247Ala --> Asp mutant RC. This effect can be accounted for by an increased proton release by the P+ environment in the M247Ala --> Asp mutant. The M247Ala --> Asp mutation reveals the existence of an energetic and conformational coupling between donor and acceptor sides of the RC at a distance of nearly 30A. 相似文献
93.
María Angeles Monge Enrique Gutirrez Puebla María Del Mar Ramos Gallego Carmen Pardo Jos Elguero 《Journal of heterocyclic chemistry》1992,29(2):499-502
The fluoroborate of 1H-2-methylbenzotriazolium monohydrate crystallizes in the Pbca space group [Z = 8, a = 16.734(3), b = 19.153(9), c = 6.937(1) Å], The hydrogen bond network between the three molecules, 2MeBzTr, HBF4 and H2O, corresponds to a situation intermediate between the fluoroborate of a protonated benzotriazole (BF4?.2MeBzTrH+) and a neutral benzotriazole hydrogen-bonded to fluoroboric acid (HBF4,2MeBzTr). The behaviour of the title compound in solution is also intermediate between these two extreme situations. Thus, the present compound is one of the rare examples of a proton solvated only by very weak hydrogen bond acceptors. 相似文献
94.
Vicente J González-Herrero P Pérez-Cadenas M Jones PG Bautista D 《Inorganic chemistry》2005,44(20):7200-7213
Platinum(II) complexes with (fluoren-9-ylidene)methanedithiolato and its 2,7-di-tert-butyl- and 2,7-dimethoxy-substituted analogues were obtained by reacting different chloroplatinum(II) precursors with the piperidinium dithioates (pipH)[(2,7-R2C12H6)CHCS2] [R = H (1a), t-Bu (1b), or OMe (1c)] in the presence of piperidine. The anionic complexes Q2[Pt{S(2)C=C(C12H6R(2)-2,7)}2] [R = H, (Pr(4)N)(2)2a; R = t-Bu, (Pr4N)(2)2b, (Et4N)(2)2b; R = OMe, (Pr4N)(2)2c] were prepared from PtCl(2), piperidine, the corresponding QCl salt, and 1a-c in molar ratio 1:2:2:2. In the absence of QCl, the complexes (pipH)(2)2b and [Pt(pip)(4)]2b were isolated depending on the PtCl(2):pip molar ratio. The neutral complexes [Pt{S2C=C(C12H6R(2)-2,7)L(2)] [L = PPh(3), R = H (3a), t-Bu (3b), OMe (3c); L = PEt(3), R = H (4a), t-Bu (4b), OMe (4c); L(2) = dbbpy, R = H (5a), t-Bu (5b), OMe (5c) (dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridyl)] were similarly prepared from the corresponding precursors [PtCl2L2] and 1a-c in the presence of piperidine. Oxidation of Q(2)2b with [FeCp2]PF6 afforded the mixed Pt(II)-Pt(IV) complex Q2[Pt2{S2C=C[C12H6(t-Bu)(2)-2,7]}4] (Q(2)6, Q = Et4N+, Pr4N+). The protonation of (Pr4N)(2)2b with 2 equiv of triflic acid gave the neutral dithioato complex [Pt2{S2CCH[C12H6(t-Bu)(2)-2,7]}4] (7). The same reaction in 1:1 molar ratio gave the mixed dithiolato/dithioato complex Pr4N[Pt{S2C=C[C12H6(t-Bu)(2)-2,7]}{S2CCH[C12H6(t-Bu)(2)-2,7]}] (Pr(4)N8) while the corresponding DMANH+ salt was obtained by treating 7 with 2 equiv of 1,8-bis(dimethylamino)naphthalene (DMAN). The crystal structures of 3b and 5c.CH2Cl2 have been solved by X-ray crystallography. All the platinum complexes are photoluminescent at 77 K in CH2Cl2 or KBr matrix, except for Q(2)6. Compounds 5a-c and Q8 show room-temperature luminescence in fluid solution. The electronic absorption and emission spectra of the dithiolato complexes reveal charge-transfer absorption and emission energies which are significantly lower than those of analogous platinum complexes with previously described 1,1-ethylenedithiolato ligands and in most cases compare well to those of 1,2-dithiolene complexes. 相似文献
95.
The hydrothermal reaction of Ln(2)O(3) (Ln = Dy and Ho), Cu(OAc)(2).2H(2)O, and oxydiacetic acid in the approximate mole ratio of 1:3:8 resulted in the formation of two new members of the isostructural series of polymers formulated as [(Cu(3)Ln(2)(oda)(6)(H(2)O)(6)).12H(2)O](n), crystallizing in the hexagonal crystal system, space group P6/mcc (No. 192). Temperature-dependent magnetic susceptibilities and EPR spectra are reported for the heterometallic compounds Cu-Dy 1, Cu-Ho 2, Cu-Er 3, and Cu-Y 4. The results are discussed in terms of the structure of the compounds, the electronic properties of the lanthanide ions, and the exchange interactions between the magnetic ions. 相似文献
96.
del Mar Conejo M Fernández R del Río D Carmona E Monge A Ruiz C Márquez AM Sanz JF 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(18):4452-4461
The beryllocenes [Be(C(5)Me(4)H)(2)] (1), [Be(C(5)Me(5))(2)] (2), and [Be(C(5)Me(5))(C(5)Me(4)H)] (3) have been prepared from BeCl(2) and the appropriate KCp' reagent in toluene/diethyl ether solvent mixtures. The synthesis of 1 is facile (20 degrees C, overnight), but generation of decamethylberyllocene 2 demands high temperatures (ca. 115 degrees C) and extended reaction times (3-4 days). The mixed-ring beryllocene 3 is obtained when the known [(eta(5)-C(5)Me(5))BeCl] is allowed to react with K[C(5)Me(4)H], once more under somewhat forcing conditions (115 degrees C, 36 h). The structures of the three metallocenes have been determined by low-temperature X-ray studies. Both 1 and 3 present eta5/eta1 geometries of the slip-sandwich type, whereas 2 exhibits an almost regular, ferrocene-like, sandwich structure. In the mixed-ring compound 3, C(5)Me(5) is centrally bound to beryllium and the eta(1)-C(5)Me(4)H ring bonds to the metal through the unique CH carbon atom. This is also the binding mode of the eta(1)-ring of 1. To analyze the nature of the bonding in these molecules, theoretical calculations at different levels of theory have been performed on compounds 2 and 3, and a comparison with the bonding in [Be(C(5)H(5))(2)] has been made. As for the latter molecule, energy differences between the eta5/eta5 and the eta5/eta1 structures of 2 are very small, being of the order of a few kcal mol(-1). Constrained space orbital variations (CSOV) calculations show that the covalent character in the bonding is larger for [Be(C(5)Me(5))(2)] than for [Be(C(5)H(5))(2)] due to larger charge delocalization and to increased polarizability of the C(5)Me(5) fragment. 相似文献
97.
Andrés R Galakhov MV Gómez-Sal MP Martín A Mena M del Carmen Morales-Varela M Santamaría C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(4):805-811
The photochemical treatment of mu(3)-alkylidyne complexes [[TiCp*(mu-O)](3)(mu(3)-CR)] (R=H (1), Me (2), Cp*=eta(5)-C(5)Me(5)) with the amines (2,6-Me(2)C(6)H(3))NH(2), Et(2)NH, and Ph(2)NH and the imine Ph(2)C=NH leads to the partial hydrogenation of the alkylidyne moiety that is supported on the organometallic oxide, [Ti(3)Cp*O(3)], and the formation of new oxoderivatives [[TiCp*(3)(mu-CHR)(R'NR")] (R"=2,6-Me(2)C(6)H(3), R'=H, R=H (3), Me (4); R'=R"=Et, R=H (5), Me (6); R'=R"=Ph, R=H (7), Me (8)) and [[TiCp*(mu-O)](3)(mu-CHR)(N=CPh(2))] (R=H (9), R=Me (10)), respectively. A sequential transfer hydrogenation process occurs when complex 1 is treated with tBuNH(2), which initially gives the mu-methylene [[TiCp*(mu-O)](3)(mu-CH(2))(HNtBu)] (11) complex and finally, the alkyl derivative [[TiCp*(mu-O)](3)(mu-NtBu)Me] (12). Furthermore, irradiation of solutions of the mu(3)-alkylidyne complexes 1 or 2 in the presence of diamines o-C(6)H(4)(NH(2))(2) and H(2)NCH(2)CH(2)NH(2) (en) affords [[TiCp*(mu-O)](3)(mu(3)-eta(2)-NC(6)H(4)NH)] (13) and [[TiCp*(mu-O)](3)(mu(3)-eta(2)-NC(2)H(4)NH)] (14) by either methane or ethane elimination, respectively. In the reaction of 1 with en, an intermediate complex [[TiCp*(mu-O)](3)(mu-CH(2))(NHCH(2)CH(2)NH(2))] (15) is detected by (1)H NMR spectroscopy. Thermal treatment of the complexes 4-10 quantitatively regenerates the starting mu(3)-alkylidyne compounds and the amine R'(2)NH or the imine Ph(2)C=NH; however, heating of solutions of 3 or 4 in [D(6)]benzene or a equimolecular mixture of both at 170 degrees C produces methane, ethane, or both, and the complex [[TiCp*(mu-O)](3)[mu(3)-eta(2)-NC(6)H(3)(Me)CH(2)]] (16). The molecular structure of 8 has been established by single-crystal X-ray analysis. 相似文献
98.
[reaction: see text] Enantiomerically pure gamma-fluoroalkyl beta-amino sulfones are readily synthesized in three steps starting from fluorinated imidoyl chlorides and arylmethyl sulfones. A complementary two-step sequence starting from chiral fluorinated beta-amino sulfoxides has also been developed. To illustrate the application of this procedure, a new method for the synthesis of alpha-fluoroalkyl allylic amines in optically pure form involving a Julia methylenation-desulfonylation reaction is presented. 相似文献
99.
Summary Magnetic susceptibilities of the biacetyldihydrazone (BdH) complexes [M(BdH)3](NO3)2 (M = CoII, NiII, CuII or ZnII), [Fe(BdH)3](NO3)3, [M(BdH)3](Ni(dto)2] (M = CoII, NiII or ZnII; dto = dithiooxalate), [(BdH)2Cu(dto)Ni(dto)] and [Fe(BdH)3]2[Ni(dto)2]3 have been studied in the 4.2–295 K range. ZnII complexes are diamagnetic, and complexes of NiII, CuII and FeIII obey the Curie-Weiss law. The CoII complexes behave anomalously and the results are interpreted in terms of a high spinlow spin equilibrium. 相似文献
100.
A new flow injection (FI) system for the determination of Pb(II) at trace level with a preconcentration step and spectrophotometric detection is proposed. It is based on preconcentration of lead ions on chitosan and dithizone-lead complex formation in aqueous medium (pH 9). The chemicals and FIA variables influencing the performance of the system were optimized and applied to the determination of lead in natural, well, and drinking water samples. It is a simple, highly sensitive, and low cost alternative methodology. The method provided a linear rage between 25 and 250 μg l−1, a detection limit of 5.0 ng ml−1 and a sample throughput of 15 h−1. The obtained results of spiked samples are in good agreement between the proposed method and ICP-AES. 相似文献