全文获取类型
收费全文 | 142篇 |
免费 | 19篇 |
国内免费 | 1篇 |
专业分类
化学 | 121篇 |
力学 | 7篇 |
数学 | 7篇 |
物理学 | 27篇 |
出版年
2024年 | 1篇 |
2022年 | 3篇 |
2021年 | 3篇 |
2020年 | 2篇 |
2019年 | 3篇 |
2018年 | 3篇 |
2016年 | 11篇 |
2015年 | 7篇 |
2014年 | 6篇 |
2013年 | 11篇 |
2012年 | 18篇 |
2011年 | 10篇 |
2010年 | 10篇 |
2009年 | 4篇 |
2008年 | 10篇 |
2007年 | 9篇 |
2006年 | 8篇 |
2005年 | 2篇 |
2004年 | 1篇 |
2003年 | 8篇 |
2002年 | 5篇 |
2001年 | 2篇 |
2000年 | 4篇 |
1999年 | 6篇 |
1998年 | 1篇 |
1996年 | 2篇 |
1995年 | 4篇 |
1994年 | 4篇 |
1974年 | 1篇 |
1932年 | 2篇 |
1923年 | 1篇 |
排序方式: 共有162条查询结果,搜索用时 15 毫秒
11.
Zoia V. Voitenko Oleksandr I. Halaev Sergey V. Kolotilov Christine Lepetit Bruno Donnadieu Remi Chauvin 《Tetrahedron》2010,66(41):8214-8222
Base- and acid-catalyzed nucleophilic addition of 11H-isoindolo[2,1-a]quinazoline-5-one to aromatic aldehydes and maleimides was investigated. The aldol adducts and condensation products were obtained stereoselectively. Main diastereomers of the Michael adducts were isolated in 74-89% yield, and converted by N-methylation to new stable α-substituted isoindole derivatives, for which 6-methylisoindolo[2,1-a]]quinazoline-5-one stands as the unsubstituted reference. The stability of the latter was monitored in moist aerated CDCl3 solution, and one of the oxidative hydrolysis product was characterized by X-ray diffraction analysis as the corresponding N-arylphthalimide. The reactivity of the unsubstituted 6-methylisoindolo[2,1-a]]quinazoline-5-one was also investigated with acetylenic Michael acceptors. Fully conjugated isoindole derivatives possessing an original pull-push-pull structure were obtained. The conformations and molecular orbitals of the dibenzoylacetylene adduct were studied at the DFT level of theory. Its static quadratic hyperpolarizabilty β0 was also calculated at the ZINDO level. 相似文献
12.
Shen NH Massaouti M Gokkavas M Manceau JM Ozbay E Kafesaki M Koschny T Tzortzakis S Soukoulis CM 《Physical review letters》2011,106(3):037403
We experimentally demonstrate, for the first time, an optically implemented blueshift tunable metamaterial in the terahertz (THz) regime. The design implies two potential resonance states, and the photoconductive semiconductor (silicon) settled in the critical region plays the role of intermediary for switching the resonator from mode 1 to mode 2. The observed tuning range of the fabricated device is as high as 26% (from 0.76 THz to 0.96 THz) through optical control to silicon. The realization of broadband blueshift tunable metamaterial offers opportunities for achieving switchable metamaterials with simultaneous redshift and blueshift tunability and cascade tunable devices. Our experimental approach is compatible with semiconductor technologies and can be used for other applications in the THz regime. 相似文献
13.
14.
Remigiusz Zurawinski Christine Lepetit Yves Canac Laure Vendier Marian Mikolajczyk Remi Chauvin 《Tetrahedron: Asymmetry》2010,21(13-14):1777-1787
The synthesis and structure of Rh(I) and Pd(II) complexes of chiral P,C-chelating phosphino-(α-sulfinylalkyl)phosphonium ylide ligands with a trisubstituted asymmetric ylidic center P+–C1R(S1(O)p-Tol)–M (R = alkyl group) have been investigated, and compared to those of the analogous disubstituted ylide complexes (R = H). Reaction of the ethyl onium ylide of o-bis(diphenylphosphino)benzene with (?)-menthyl-(S)-p-tolylsulfinate afforded the corresponding racemic erythro phosphino-(α-sulfinylethyl)phosphonium in 90% de (R = Me). The racemization process is interpreted by a Berry-like pseudorotation mechanism driven by the steric repulsion between the α-methyl substituent and the bulky menthyloxy S-substituent or sulfur lone pair in the intermediate ylide-sulfinyl adduct. The ylide of phosphino-(α-sulfinylethyl)phosphonium reacts with [Rh(cod)2][PF6] and PdCl2(MeCN)2 to afford the corresponding P,C1-chelated threo-Rh(I) and erythro-Pd(II) mononuclear complexes in 70% yield and total diastereoselectivity. These respective complexes act as efficient catalytic precursors for the hydrogenation of (Z)-α-acetamidocinnamic acid and allylic substitution of 3-acetoxy-1,3-diphenyl-1-propene with sodium dimethyl malonate. The bonding features of the erythro-Pd(II) complex exhibiting a sulfinyl O?Pd interaction are studied theoretically at the DFT level using ELF and MESP analyses. The η2-P,C haptomeric form of the ylide ligand is estimated to compete at 19% with the η1-C haptomeric form dominating at 81%. 相似文献
15.
16.
Abdellah I Boggio-Pasqua M Canac Y Lepetit C Duhayon C Chauvin R 《Chemistry (Weinheim an der Bergstrasse, Germany)》2011,17(18):5110-5115
The configurational stability of biaryl motifs is addressed for the 1-naphthyl-N-benzimidazolyl motif substituted by a single diphenylphosphinyl group at the 2-position. The atropoenantiomers of the N-methylated cation H-MIOP, a less sterically locked analogue of the neutral H-MOP ligand, were resolved by enantiospecific cleavage of the N(2)C-P bond of the resolved enantiomers of BIMIONAP. The latter were obtained by enantiospecific N-methylation of the previously resolved enantiomers of neutral BIMINAP. PdCl(2) complexes of the P,C-chelating N-heterocyclic carbene (NHC)-phosphine ligands derived from (R)- and (S)-H-MIOP were prepared by two enantiospecific routes: by N(2)C-P bond cleavage from the (R)- and (S)-BIMIONAP-PdCl(2) complexes, or by simultaneous coordination of the P and C atoms of the in situ generated free NHC-phosphine. The enantiomerization pathways of H-MOP, H-MIOP, and corresponding NHC-phosphine have been investigated at the B3PW91/6-31G(d,p) level of theory. The calculated enantiomerization barriers of H-MOP and H-MIOP in acetonitrile are equal to 176.0 and 146.4 kJ mol(-1), respectively, and are mainly determined by the distortion of the naphthalene and/or benzimidazole motifs in the transition state. Beyond the stability of their optical rotation at room temperature, the respective calculated Oki's racemization temperatures of 334 and 225 °C allowed us to consider both ligands as configurationally stable. 相似文献
17.
The influence of the formal electrostatic interaction on the cis/trans coordination mode at a PdCl(2) center is investigated in a family of isostructural flexible diphosphine ligands Ph(2)P-X-C(6)H(4)-Y-PPh(2), where X and Y stand for neutral or cationic N,C-imidazolylene linkers. While the neutral and monocationic diphosphine spontaneously behave as classical cis-chelating ligands, only the dicationic diphosphine, where the electrostatic repulsion between the formal positive charges specifically takes place, is observed to behave as a trans-chelating ligand. The crucial role of electrostatics is analyzed on the basis of (31)P NMR data in solution and X-ray diffraction data in the crystal state. Comparative theoretical studies of the cis- and trans-chelated complexes, including EDA, static (31)P NMR, MESP, and AIM analyses, have been undertaken on the basis of DFT calculations in the gas phase or in the acetonitrile continuum. Whereas the cis-coordination mode is shown to be thermodynamically favored for the neutral ligand, the trans-coordination mode is found to be preferred for the dicationic homologue. The stereochemical preference is thus shown to be parallel to the expected effect of the formal electrostatic interaction. The results open perspectives for control of the cis- and trans-chelating behavior of flexible bidentate ligands by more or less reversible charge transfer at the periphery of the coordination sphere of a metallic center. 相似文献
18.
Shishkin OV Zubatyuk RI Dyakonenko VV Lepetit C Chauvin R 《Physical chemistry chemical physics : PCCP》2011,13(15):6837-6848
Model complexes of hexaethynylhexamethoxy[6]pericyclyne (HM6P) molecules, with or without dichloromethane (DCM) guests, have been calculated at the M05-2X/6-311G(d,p) DFT level. Analysis of nonbonding interactions shows that the cohesion of the supramolecular tubular assemblies previously observed in the crystal state, relies mainly on C-H···O hydrogen bonds between axial ethynyl and equatorial methoxy substituents of stacked HM6P C(18) macrocycles in a cyclohexane-like chair conformation. The intrinsic binding energy of one HM6P molecule with two neighbors is calculated to be more than 40 kcal mol(-1). The inner channel of the stacks (of ca. 8 ? diameter) are suitable for hosting DCM molecules. Using the Atoms In Molecules (AIM) theory, the interaction between DCM molecules and surrounding triple bonds is analyzed in terms of σ-hole-directed C-Cl···π halogen bonds. A σ-hole-directed Cl···Cl interaction between two chlorine atoms of different DCM molecules is also evidenced. 相似文献
19.
20.
Arabidopsis thaliana shoot tips were successfully cryopreserved using encapsulation-dehydration cryopreservation methods. Between one and seven shoot tips were encapsulated within 4 mm calcium-alginate beads. Beads were formed in the presence of 2 M glycerol + 0.4 M sucrose. The time required to make 10 beads, each containing five shoot tips (4 min), was less than the time required to make 50 beads containing one shoot tip (12 min). Shoot tip regrowth after cryoexposure was between 60 and 68%, with one to seven shoot tips per bead. Using five Arabidopsis shoot tips per bead, alginate beads were formed either in the presence of 2 M glycerol + 0.4 M sucrose or 0.5 M sucrose. Beads formed in the presence of glycerol were immediately air-dried to moisture contents between 0.21 to 0.26 g H2O/g FW (0.27 to 0.38 g H2O/g DW). Alginate beads formed in 0.5 M sucrose were incubated in solutions of 0.5, 0.75, and 1 M sucrose for one day each prior to air-dehydration, achieving moisture contents of 0.19 to 0.21 g H2O/g FW (0.23 to 0.27 g H2O/g DW). Shoot tip regrowth after cryoexposure was between 42 and 65%, with no significant differences among treatments. We successfully reduced the amount of time needed for shoot tip processing for Arabidopsis by encapsulating five shoot tips per alginate bead and by using a glycerol-encapsulation method, without lowering shoot tip regrowth levels after cryopreservation. 相似文献