全文获取类型
收费全文 | 230篇 |
免费 | 11篇 |
专业分类
化学 | 188篇 |
力学 | 9篇 |
数学 | 14篇 |
物理学 | 30篇 |
出版年
2022年 | 2篇 |
2021年 | 3篇 |
2020年 | 1篇 |
2019年 | 5篇 |
2018年 | 2篇 |
2016年 | 7篇 |
2015年 | 6篇 |
2014年 | 6篇 |
2013年 | 6篇 |
2012年 | 21篇 |
2011年 | 14篇 |
2010年 | 9篇 |
2009年 | 7篇 |
2008年 | 29篇 |
2007年 | 25篇 |
2006年 | 17篇 |
2005年 | 12篇 |
2004年 | 7篇 |
2003年 | 8篇 |
2002年 | 5篇 |
2001年 | 4篇 |
1998年 | 1篇 |
1996年 | 2篇 |
1995年 | 5篇 |
1994年 | 7篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1982年 | 1篇 |
1981年 | 4篇 |
1980年 | 1篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1975年 | 4篇 |
1973年 | 1篇 |
1917年 | 1篇 |
1893年 | 1篇 |
排序方式: 共有241条查询结果,搜索用时 656 毫秒
61.
Maurette L Godard C Frau S Lepetit C Soleilhavoup M Chauvin R 《Chemistry (Weinheim an der Bergstrasse, Germany)》2001,7(6):1165-1170
A copper-mediated procedure for terminal alkynyl-propargyl coupling has been applied to "skipped" bis-terminal undecatetrayne and 1,4-bis(pseudo)halobut-2-ynes with the aim of preparing ring carbomers of representative strained and loose cycloalkanes, namely [N]pericyclynes. Two unprecedented, cyclic. "skipped" polyynes with CH2 vertices have been isolated as mixtures of diastereoisomers: an isomer 1b and a dimer 2a of [5]pericyclyne 1a. The isomer 1b is a cyclotetrayne with an exocyclic allene function resulting from a unique formal SN process. Its structure has been established by 1H/13C HMQC and HMBC two-dimensional NMR analysis. According to density functional theory calculations, it is about 6 kcalmol(-1) more stable than [5]pericyclyne (1a). Compound 1b can also be regarded as a C13-relaxed [4]pericyclyne, a long sought "skipped" C12 tetrayne. The dimer 2a is a C30 ring that results from a formal SN process. It is a stable ring carbomer of cyclodecane, that is, a [10]pericyclyne, with four CH2 vertices. 相似文献
62.
Lepetit C Brøndsted Nielsen M Diederich F Chauvin R 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(20):5056-5066
Aromaticity enhancement is a possible driving force for the low reduction potentials of buta-1,3-diynediyl-expanded [N]radialenes: this hypothesis is theoretically analyzed for the expanded [3]radialene prototype. This study is undertaken within a more general prospect, namely the evaluation of the variation of aromaticity with endocyclic and peripheral carbomeric expansion of [3]radialene and its mono- and dianions. The structures, denoted as [C-H](6) (h)[C-C](3) (k)carbo-[3]radialene(q) (h=0, 1; k=0, 1, 2; q=0, -1, -2), were optimized in relevant singlet, doublet, or triplet spin states at the B3PW91/6-31G** level. They were found to be all planar. The structural aromaticity was measured through the average bond length d(av) over the [C-C](3) (k)carbo-[3]radialene core, and by the corresponding bond-length equalization parameter sigma(d), related to Krygowski's GEO. The magnetic aromaticity was measured by Schleyer's NICS values at the center of the rings. Regarding the relative variation of NICS and sigma(d), two classes of species can be distinguished according to their endocyclic expansion level. The species with a nonexpanded (k=0) or doubly expanded (k=2) ring constitute the first class: they exhibit D(3h) symmetry and a strong correlation of NICS with sigma(d). The species with a singly expanded ring (k=1) fall far from the correlation line, and constitute the second class. This class distinction is related to the degeneracy scheme of the frontier orbitals of the neutral representative. A finer appraisal of the electron (de)localization is brought by the ELF (Electron Localization Function) analysis of the electron density. It allows for a weighting of relevant resonance forms. Unsubstituted species are well described by the superimposition of two or three resonance forms. For (doublet spin state) monoanionic species, their respective weights are validated by comparison with AIM spin density. The weighted mean, n, of the formal numbers of paired pi(z) electrons in the resonance forms was calculated and compared with the closest even integer of either forms 4m+2 or 4m. A density-based continuous generalization of the orbital-based discrete Hückel rule is then heuristically proposed through an analytical correlation of NICS versus sigma(d), n, and S, the spin of the species. The frontier-orbital-degeneracy pattern of neutral species is discussed with respect to structural and magnetic aromaticity criteria. A decreasing HOMO-LUMO gap versus endocyclic expansion is obtained, but [C-C](3) (1)carbo-[3]radialene possesses the highest HOMO and LUMO energies. Vertical and adiabatic electron affinities of neutral and monoanionic species were also computed and compared with related experimental data. 相似文献
63.
We prove a conjecture of J. Alexander concerning the base point freeness and very ampleness of linear systems on general blowings–up of the projective plane, whose standard form has at most 9 multiple points. 相似文献
64.
Raphaël Tripier Marcel Hollenstein Mourad Elhabiri Anne‐Sophie Chauvin Gaël Zucchi Claude Piguet Jean‐ClaudeG. Bünzli 《Helvetica chimica acta》2002,85(7):1915-1929
Bis(2‐{6‐(diethylcarbamoyl)‐4‐[(4‐isothiocyanatophenyl)ethynyl]pyridin‐2′‐yl}‐1‐ethylbenzimidazol‐5‐yl)methane ( L G ) reacts with trivalent lanthanide ions in acetonitrile to yield triple‐stranded dimetallic helicates [Ln2( L G )3]6+. 1H‐NMR Data point to the helicates being the only species formed under stoichiometric conditions and having a time‐averaged D3 symmetry on the NMR time scale. The photophysical properties of L G and its helicates are discussed with respect to the closely related ligands L B , L E , and their complexes, two ligands devoid of the isothiocyanatophenylethynyl substituent. The quantum yield of the ligand fluorescence is three times smaller compared to L E , while that of the EuIII‐centered luminescence (1.1%) is three times larger. On the other hand, the luminescence of TbIII is not sensitized by L G . This is explained in terms of energy differences between the singlet and triplet states on one hand, and between the 0‐phonon transition of the triplet state and the excited metal ion states on the other. This work demonstrates that bulky substituents in the 4‐position of the pyridine ring do not prevent the formation of triple‐stranded helicates, opening the way for luminescent probes that can easily be coupled to biological materials. 相似文献
65.
Substituted glyoxals with protected ketogroup were obtained by reaction of α-chloro-α-(aryl- or alkyl-thio)ketones with sodium methylate in methanol. 相似文献
66.
α-Ketoaldehydes with protected ketogroup 2 or protected aldehydrogroup 3 are obtained by reaction of thiophenol or methanol with α-chloro-α-(phenylthio)aldehydes 7. 相似文献
67.
Structural Chemistry - The present computational study complements experimental efforts to describe and characterize carbo-benzene derivatives as paradigms of aromatic carbo-mers. A long-lasting... 相似文献
68.
Anastasia Burmistrova Valérie Gabelica Anne-Sophie Duwez Edwin De Pauw 《Journal of the American Society for Mass Spectrometry》2013,24(11):1777-1786
Electrospray ionization (ESI) soft desolvation is widely used to investigate fragile species such as nucleic acids. Tandem mass spectrometry (MS/MS) gives access to the gas phase energetics of the intermolecular interactions in the absence of solvent, by following the dissociation of mass-selected ions. Ion mobility mass spectrometry (IMS) provides indications on the tridimensional oligonucleotide structure by attributing a collision cross section (CCS) to the studied ion. Electrosprayed duplexes longer than eight bases pairs retain their helical structure in a solvent-free environment. However, the question of conformational changes under activation in MS/MS studies remains open. The objective of this study is to probe binding energetics and characterize the unfolding steps occurring prior to oligonucleotide duplex dissociation. Comparing the evolution of CCS with collision energy and breakdown curves, we characterize dissociation pathways involved in CID-activated DNA duplex separation into single strands, and we demonstrate here the existence of stable dissociation intermediates. At fixed duplex length, dissociation pathways were found to depend on the percentage of GC base pairs and on their position in the duplex. Our results show that pure GC sequences undergo a gradual compaction until reaching the dissociation intermediate: A-helix. Mixed AT-GC sequences were found to present at least two conformers: a classic B-helix and an extended structure where the GC tract is a B-helix and the AT tract(s) fray. The dissociation in single strands takes place from both conformers when the AT base pairs are enclosed between two GC tracts or only from the extended conformer when the AT tract is situated at the end(s) of the sequence. Figure
? 相似文献
69.
Functional carbo‐Butadienes: Nonaromatic Conjugation Effects through a 14‐Carbon, 24‐π‐Electron Backbone
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Arnaud Rives Dr. Valérie Maraval Dr. Nathalie Saffon‐Merceron Prof. Remi Chauvin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(2):483-492
A systematic study of carbo‐butadiene motifs not embedded in an aromatic carbo‐benzene ring is described. Dibutatrienylacetylene (DBA) targets R1?C(R)?C?C?C(Ph)?C≡C?C(Ph)?C?C?C(R)?R2 are devised, in which R is C≡CSiiPr3 and R1 and R2 are R, H, or 4‐X‐C6H4, with the latter including three known representatives (X: H, NMe2, or NH2). The synthesis method is based on the SnCl2‐mediated reduction of pentaynediols prepared by early or late divergent strategies; the latter allows access to a OMe–NO2 push–pull diaryl‐DBA. If R1 and R2 are H, an over‐reduced dialkynylbutatriene (DAB) with two allenyl caps was isolated instead of the unsubstituted DBA. If R1=R2=R, the tetraalkynyl‐DBA target was obtained, along with an over‐reduced DBA product with a 12‐membered 1,2‐alkylidene‐1H2,2H2‐carbo‐cyclobutadiene ring. X‐ray crystallography shows that all of the acyclic DBAs adopt a planar trans–transoid–trans configuration. The maximum UV/Vis absorption wavelength is found to vary consistently with the overall π‐conjugation extent and, more intriguingly, with the π‐donor character of the aryl X substituents, which varies consistently with the first (reversible) reduction potential and first (irreversible) oxidation peak, as determined by voltammetry. 相似文献
70.
Remi Chauvin 《Journal of mathematical chemistry》1994,16(1):269-283
The search for a definition of distances over sets of skeletal analogs (identified to G-Hilbert spaces of vector ligand parameters) is initiated from the algebraic formulation of the constant of stereogenic pairing equilibria (pairing product). A basic definition equation is devised from thermodynamical speculations. The equation is proved to have always a single potential distance solution Dp as soon as the pairing product is discriminating. The equation of Dp is constructed in order to satisfy three consistency requirements: completeG-invariance (arbitrary orientations selected to describe skeletal analogs do not affect the value of Dp); extension properties (Dp coincides with two standard completelyG-invariant distances or with the Euclidean distance in borderline cases); all the distance properties except, perhaps, the triangular inequality. The latter point remains challenging in general, and is computationally verified in some examples. 相似文献