全文获取类型
收费全文 | 5427篇 |
免费 | 148篇 |
国内免费 | 18篇 |
专业分类
化学 | 3978篇 |
晶体学 | 17篇 |
力学 | 63篇 |
数学 | 968篇 |
物理学 | 567篇 |
出版年
2020年 | 49篇 |
2019年 | 50篇 |
2018年 | 37篇 |
2016年 | 92篇 |
2015年 | 91篇 |
2014年 | 101篇 |
2013年 | 163篇 |
2012年 | 159篇 |
2011年 | 225篇 |
2010年 | 113篇 |
2009年 | 105篇 |
2008年 | 213篇 |
2007年 | 206篇 |
2006年 | 212篇 |
2005年 | 241篇 |
2004年 | 184篇 |
2003年 | 163篇 |
2002年 | 162篇 |
2001年 | 77篇 |
2000年 | 73篇 |
1999年 | 67篇 |
1998年 | 51篇 |
1997年 | 72篇 |
1996年 | 69篇 |
1995年 | 74篇 |
1994年 | 52篇 |
1993年 | 53篇 |
1992年 | 68篇 |
1991年 | 45篇 |
1990年 | 67篇 |
1989年 | 38篇 |
1988年 | 62篇 |
1987年 | 64篇 |
1986年 | 40篇 |
1985年 | 93篇 |
1984年 | 55篇 |
1983年 | 59篇 |
1982年 | 67篇 |
1981年 | 62篇 |
1980年 | 48篇 |
1979年 | 64篇 |
1978年 | 68篇 |
1977年 | 51篇 |
1976年 | 73篇 |
1975年 | 72篇 |
1974年 | 52篇 |
1973年 | 46篇 |
1972年 | 39篇 |
1970年 | 45篇 |
1966年 | 41篇 |
排序方式: 共有5593条查询结果,搜索用时 312 毫秒
61.
Karl Heinz Dötz Barbara Fügen-Köster Dietmar Neugebauer 《Journal of organometallic chemistry》1979,182(4):489-498
1-Diethylaminopropyne reacts with pentacarbonyl[methoxy(2,2-diphenylethenyl)carbene]chromium in a stereoselective way to give pentacarbonyl-[diethylamino-E-(2-methoxy-1-methyl-4,4-diphenyl-1,3-butadienyl)carbene]-chromium via insertion of the alkyne into the metal-carbene bond. The reaction of the ynamine with pentacarbonyl[methylamino(phenyl)carbene]chromium results in insertion of the alkyne and loss of carbon monoxide to give cis-tetracarbonyl[3-aza-1-methyl-2-phenyl-2-butenyl)diethylaminocarbene]-chromium. Its structure was established by oxidative degradation in an aqueous medium to give 2-benzoyl-N,N-diethylpropanamide and finally confirmed by an X-ray analysis. The new compounds are characterized spectroscopically. 相似文献
62.
Herwig G. Winkler Hagen T. Spath Prof. Dr. Karl Torkar 《Monatshefte für Chemie / Chemical Monthly》1975,106(2):535-545
The phase diagrams of the systems CsN3/Zn(N3)2 and KN3/Zn(N3)2 have been obtained employing the microscopic technique ofL. Kofler andA. Kofler. Within the system CsN3/Zn(N3)2 three eutectics at 148°C, 142°C, and 210°C were found. Besides Cs2Zn(N3)4, melting incongruently in the interval 153°C to 170°C, there exist two further compounds of the most probable composition Cs3Zn2(N3)7 and CsZn2(N3)5, melting congruently at 170°C and 210°C, resp. In the system KN3/Zn(N3)2 there exist two eutectics at 203°C and 172°C and two compounds, one of them, i.e. K2Zn(N3)4, melting congruently at 206°C, the other one, with composition KZn3(N3)7 or KZn4(N3)9, melting incongruently at 210°C.
Mit 8 Abbildungen
Herrn Professor Dr.Heribert Grubitsch zum 70. Geburtstag gewidmet. 相似文献
Mit 8 Abbildungen
Herrn Professor Dr.Heribert Grubitsch zum 70. Geburtstag gewidmet. 相似文献
63.
Trilisa M. Perrine Rajat K. Chaudhuri Karl F. Freed 《International journal of quantum chemistry》2005,105(1):18-33
Simple and quadratic Padé resummation methods are applied to high‐order series from multireference many‐body perturbation theory (MR‐MBPT) calculations using various partitioning schemes (Møller–Plesset, Epstein–Nesbet, and forced degeneracy) to determine their efficacy in resumming slowly convergent or divergent series. The calculations are performed for the ground and low‐lying excited states of (i) CH2, (ii) BeH2 at three geometries, and (iii) Be, for which full configuration interaction (CI) calculations are available for comparison. The 49 perturbation series that are analyzed include those with oscillatory and monotonic divergence and convergence, including divergences that arise from either frontdoor or backdoor intruder states. Both the simple and quadratic Padé approximations are found to speed the convergence of slowly convergent or divergent series. However, the quadratic Padé method generally outperforms the simple Padé resummation. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 相似文献
64.
3,4-Dihydro-5(1H)-pyrromethenones are easier attacked at themeso-position by electrophiles than 5(1H)-pyrromethenones. This is demonstrated both by aMannich-type-substitution or deuterium-exchange-experiments and by the addition of O-, S-, and N-Nucleophiles to the exocyclic double bond of the model-dihydropyrromethenone (Z)-1 under very mild reaction conditions. Applying these results to the chemistry of 2,3-dihydro-bilatrienes-abc, their chemical characteristics—especially their tautomeric behavior and their dominant C-5-selectivity towards electrophiles—become better understandable. 相似文献
65.
Eva BeckerChristian Slugovc Eva RübaChristina Standfest-Hauser Kurt MereiterRoland Schmid Karl Kirchner 《Journal of organometallic chemistry》2002,649(1):55-63
The synthesis and some reactions of the Ru(II) and Ru(IV) half-sandwich complexes [RuCp(EPh3)(CH3CN)2]+ (E=P, As, Sb, Bi) and [RuCp(EPh3)(η3-C3H5)Br]+ have been investigated. The chemistry of this class of compounds is characterized by a competitive coordination of EPh3 either via a RuE or a η6-arene bond, where the latter is favored when the former is weaker, that is in going down the series. Thus in the case of Bi, the starting material [RuCp(CH3CN)3]+ does not react with BiPh3 to give [RuCp(BiPh3)(CH3CN)2]+ but instead gives only the η6-arene species [RuCp(η6-PhBiPh2)]+ and [(RuCp)2(μ-η6,η6-Ph2BiPh)]2+. Similarly, the EPh3 ligand can be replaced by an aromatic solvent or an arene substrate. Thus, the catalytic performance of [RuCp(EPh3)(CH3CN)2]+ for the isomerization of allyl-phenyl ethers to the corresponding 1-propenyl ethers is best with E=P, while the conversion drops significantly using the As and Sb derivatives. By the same token, only [RuCp(PPh3)(CH3CN)2]+ is stable in a non-aromatic solvent, whereas both [RuCp(AsPh3)(CH3CN)2]+ and [RuCp(SbPh3)(CH3CN)2]+ rearrange upon warming to [RuCp(η6-PhEPh2)]+ and related compounds. In addition, the potential of [RuCp(EPh3)(CH3CN)2]+ as precatalysts for the transfer hydrogenation of acetophenone and cyclohexanone has been investigated. Again aromatic substrates are clearly less suited than non-aromatic ones due to facile η6-arene coordination leading to catalyst's deactivation. 相似文献
66.
alpha-(N-Carbamoylalkyl)cuprates couple with enol triflates derived from carbocyclic and heterocyclic (i.e., piperidinones) beta-keto esters. Product yields are higher with the alkyl(cyano)cuprates [i.e., RCu(CN)Li, 56-93%] than with the dialkylcuprate reagents (i.e., R(2)CuLi.LiCN). An enol nonaflate works as well as the corresponding enol triflate. A facile synthetic route to gamma-amino alpha,beta-enoates not readily prepared from gamma-keto-alpha,beta-enoates is thus established. The gamma-amino-alpha,beta-enoates, available via N-Boc deprotection, can be cyclized to annulated pyrrolin-2-ones. 相似文献
67.
The influence of monomer structure on the thermodynamic properties of lattice model polymer blends is investigated through Monte Carlo computations. The model of lattice polymers with monomer structure has been used extensively in the context of the lattice cluster theory (LCT), a thermodynamic theory for polymer mixtures in the liquid state. The Monte Carlo computations provide the first unequivocal test of the accuracy of the LCT predictions for binary mixtures of polymers with structured monomers. Four types of monomer structures are analyzed, corresponding to to the monomers of polyethylene, polypropylene, polyethylethylene, and polyisobutylene (PIB). Most computations use chains with M=12 and 24 beads and the total volume fraction of the beads is phi=0.6. Both structurally symmetric and asymmetric blends are investigated. For the symmetric case, the predictions of the LCT for the energies of mixing and the liquid-liquid coexistence curves are in qualitative agreement with the Monte Carlo computations, except for the PIB/PIB symmetric blend. For structurally asymmetric blends, the LCT does not capture contributions to the energy of mixing arising solely from structural differences between the components. Computational estimates of the nonideal entropy of mixing indicate that the LCT also underestimates the entropic cost of mixing chains with different structures, thus explaining some discrepancies between the theoretical and the Monte Carlo liquid--liquid coexistence curves. 相似文献
68.
Non-K-region -quinones of polycyclic aromatic hydrocarbons are prepared in four steps from cyclic ketones via dehydrogenation of tetrahydrodiols with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. 相似文献
69.
Karl Grubmayr Ulrike Gabriella Wagner 《Monatshefte für Chemie / Chemical Monthly》1988,119(6-7):793-812
1H NMR-spectroscopic investigations of the acid catalyzed addition of methanol to dihydrodipyrrinones (Z)-2, (E)-2, and4 show the C-protonation of their enamide parts to be the first and rate determining step forming the key intermediate, the N-acyl-immonium ion N+. Its ability to add nucleophiles diastereoselectively can be used to prepare the adductsl-3 andl-5. Exclusive formation of thelike-isomer can be explained by a stereoelectronically favoured approach of the nucleophile and by the thermodynamically favoured arrangement of the bulky ring substituents. Both explanations are based on low temperature X-ray crystal structure determinations: in the first place, the orientation of the added nucleophile could be found to be nearly parallel to the -plane of the lactam unit and quasi-axial with respect to theenvelope-like conformation of the five-ring lactam; in the second place, the relative orientation of the methoxycarbonyl-metyl-group at C-3 and the pyrrolylmethyl-substituent at C-4 could be found to be atrans-quasi-diequatorial one. 相似文献
70.
All-electron ab initio Hartree–Fock (HF ), valence configuration interaction (CI ), and multiconfiguration self-consistent-field (CASSCF ) calculations have been applied to investigate the electronic states of the CrC molecule. The molecule is predicted as having four low-lying electronic states, 3∑?, 5∑?, 7∑?, and 9∑?, separated by an energy gap of 0.55 eV from the next higher-lying state, 1∑?, which is followed by the states 5Π and 7Π. The four lowest-lying electronic states are due to the coupling of the angular momenta of the 6Sg Cr+ ion with those of the 4Su C? anion. The chemical bond in the 3∑? ground state can be viewed as a quadruple bond composed of two σ and two π bonds. One σ bond is due to the formation of a molecular orbital that is doubly occupied. The remaining bonds, i.e., one σ and two π bonds, arise from valence-bond couplings. The π bonds originate from the valence-bond couplings of the electrons in the C 2pπ orbitals with those in the Cr 3dπ orbitals. The σ bond originates from the valence-bond coupling of the C 2pσ electron with an electron in the Cr 4s, 4p hybrid that is polarized away from the C atom. 相似文献