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991.
Hydrogen bonding is a key determinant of much macromolecular structure in nature, but individual donor and acceptor pairs are rarely observed in solution. Their weak interactions result in nanosecond lifetimes and rapid exchange of partners. Reversible encapsulation isolates molecules in very small spaces for milliseconds to hours and allows their characterization by NMR methods. Here we report a competitive study of hydrogen-bonding functions--carboxylic acids, primary amides, and boronic acids--within a multicomponent capsular assembly. The pairwise co-encapsulation of these molecules allows the direct observation of homodimeric boronic acids and their heterodimeric complexes with carboxylic acids and primary amides. The efficiency of boronic acids as hydrogen-bonding partners derives from their adaptable structures rather than from their intrinsic acid/base properties. 相似文献
992.
Zhengzheng Zhou Henry H. Y. Tong Liang Li Fanny L. Y. Shek Yang Lv Ying Zheng 《Crystal Research and Technology》2015,50(7):538-548
This study performed a solid‐state characterization of ursolic acid (UA) crystalline forms, a poorly water‐soluble triterpene with anticancer activity. Two new polymorphs (form I, II), two new solvates (propanol and isopropanol solvates), and a known ethanol solvate were determined and elucidated using a combination of multi‐techniques, including X‐ray single crystal and powder diffraction, Fourier transform infrared spectroscopy (FT‐IR), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). A colorless single crystal of UA was grown from a propanol solution, and its crystalline structure was determined through X‐ray single crystal diffraction. It was determined that the propanol solvate was crystallized in the orthorhombic space group P212121 with unit‐cell parameters a = 7.17200 (8) Å, b = 12.24100 (16) Å, c = 33.8950 (4) Å and Z = 4. The ethanol solvate and propanol solvate were isomorphous crystals. The results of the thermal analysis demonstrate that form I is a meta‐stable form, while form II is a stable form that is monotropically related. 相似文献
993.
Synthesis and Characterization of a Uranium(II) Monoarene Complex Supported by δ Backbonding 下载免费PDF全文
Dr. Henry S. La Pierre Dr. Andreas Scheurer Dr. Frank W. Heinemann Priv.‐Doz. Dr. Wolfgang Hieringer Prof. Dr. Karsten Meyer 《Angewandte Chemie (International ed. in English)》2014,53(28):7158-7162
The low‐temperature (35 °C) reduction of the trivalent uranium monoarene complex [{(Ad,MeArO)3mes}U] ( 1 ), with potassium spheres in the presence of a slight excess of 2.2.2‐cryptand, affords the quantitative conversion of 1 into the uranium(II) monoarene complex [K(2.2.2‐crypt)][((Ad,MeArO)3mes)U] ( 1‐K ). The molecular and electronic structure of 1‐K was established experimentally by single‐crystal X‐ray diffraction, variable‐temperature 1H NMR and X‐band EPR spectroscopy, solution‐state and solid‐state magnetism studies, and optical absorption spectroscopy. The electronic structure of the complex was further investigated by DFT calculations. The complete body of evidence confirms that 1‐K is a uranium(II) monoarene complex with a 5f 4 electronic configuration supported by δ backbonding and that the nearly reversible, room‐temperature reduction observed for 1 at ?2.495 V vs. Fc/Fc+ is principally metal‐centered. 相似文献
994.
Dr. Anukul Jana Dipl.‐Chem. Isabell Omlor Dr. Volker Huch Prof. Dr. Henry S. Rzepa Prof. Dr. David Scheschkewitz 《Angewandte Chemie (International ed. in English)》2014,53(37):9953-9956
Cyclopropylidene is a transient intermediate of the allene–propyne–cyclopropene isomerization. The incorporation of heavier Group 14 elements into the cyclopropylidene scaffold has to date been restricted to the formal replacement of the carbenic carbon atom by a base‐coordinated silicon(II) center. Herein we report the synthesis and characterization of NHC‐coordinated heavier cyclopropylidenes (Si2GeR3X, and Si3R3Br; X=Cl, Mes; R=Tip=2,4,6‐iPr3C6H2) in which the three‐membered ring is exclusively formed by silicon and germanium. In case of the chloro‐substituted Si2Ge‐cyclopropylidene, a stable heavier cycloprop‐1‐yl‐2‐ylidene cation is obtained by NHC‐induced chloride dissociation. 相似文献
995.
Electrophile‐Induced Nucleophilic Substitution of the nido‐Dicarbaundecaborate Anion nido‐7,8‐C2B9H12− by Conjugated Heterodienes 下载免费PDF全文
Dr. René Frank Anup Kumar Adhikari Henry Auer Prof. Dr. Evamarie Hey‐Hawkins 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(5):1440-1446
Substitution of the dicarbaundecaborate anion nido‐7,8‐C2B9H12? ( 1 ) by precise hydride abstraction followed by nucleophilic attack usually leads to symmetric products 10‐R‐nido‐7,8‐C2B9H11. However, thioacetamide (MeC(S)NH2) as nucleophile and acetone/AlCl3 as hydride abstractor gave asymmetric 9‐[MeC(NHiPr)S]‐nido‐7,8‐C2B9H11 ( 2 ), whereas N,N‐dimethylthioacetamide (MeC(S)NMe2) gave the expected symmetric 10‐[MeC(NMe2)S]‐nido‐7,8‐C2B9H11 ( 4 ). For the formation of 2 , acetone and thioacetamide are assumed to give the intermediate MeC(S)N(CMe2) ( 3 ), which then attacks 1 with formation of 2 . Similarly, reaction of acetyliminium chloride [MeC(O)NH(CPh2)]Cl ( 5 ) with 1 in THF gave a mixture of 9‐ and 10‐substituted [MeC(NHCHPh2)O]‐nido‐7,8‐C2B9H11 ( 6 and 7 , respectively). These reactions are the first examples in which compounds (here heterodienes) that unite the functionalities of both hydride acceptor and nucleophilic site react with 1 in a bimolecular fashion. Furthermore, the analogous reaction of 1 and 5 (in an equilibrium mixture with acetyl chloride and benzophenone imine) in MeCN afforded 10‐[MeC(NCPh2)NH]‐nido‐7,8‐C2B9H11 ( 8 ) and MeC(O)NHCHPh2 ( 9 ). 相似文献
996.
A Remarkable Solvent Effect on the Nuclearity of Neutral Titanium(IV)‐Based Helicate Assemblies 下载免费PDF全文
David Michael Weekes Carine Diebold Dr. Pierre Mobian Dr. Clarisse Huguenard Dr. Lionel Allouche Prof. Marc Henry 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(17):5092-5101
The spontaneous self‐assembly of a neutral circular trinuclear TiIV‐based helicate is described through the reaction of titanium(IV) isopropoxide with a rationally designed tetraphenolic ligand. The trimeric ring helicate was obtained after diffusion of n‐pentane into a solution with dichloromethane. The circular helicate has been characterized by using single‐crystal X‐ray diffraction study, 13C CP‐MAS NMR and 1H NMR DOSY solution spectroscopic, and positive electrospray ionization mass‐spectrometric analysis. These analytical data were compared with those obtained from a previously reported double‐stranded helicate that crystallizes in toluene. The trimeric ring was unstable in a pure solution with dichloromethane and transformed into the double‐stranded helicate. Thermodynamic analysis by means of the PACHA software revealed that formation of the double‐stranded helicates was characterized by ΔH(toluene)=?30 kJ mol?1 and ΔS(toluene)=+357 J K?1 mol?1, whereas these values were ΔH(CH2Cl2)=?75 kJ mol?1 and ΔS(CH2Cl2)=?37 J K?1 mol?1 for the ring helicate. The transformation of the ring helicate into the double‐stranded helicate was a strongly endothermic process characterized by ΔH(CH2Cl2)=+127 kJ mol?1 and ΔH(n‐pentane)=+644 kJ mol?1 associated with a large positive entropy change ΔS=+1115 J K?1?mol?1. Consequently, the instability of the ring helicate in pure dichloromethane was attributed to the rather high dielectric constant and dipole moment of dichloromethane relative to n‐pentane. Suggestions for increasing the stability of the ring helicate are given. 相似文献
997.
Seid M. Adem Elisabeth Mansfield John P. Keogh Henry K. Hall Jr. Craig A. Aspinwall 《Analytica chimica acta》2013
Phosphorylcholine (PC) based phospholipid bilayers have proven useful as capillary coating materials due to their inherent resistance to non-specific protein adsorption. The primary limitation of this important class of capillary coatings remains the limited long-term chemical and physical stability of the coatings. Recently, a method for increasing phospholipid coating stability in fused silica capillaries via utilization of polymerized, synthetic phospholipids was reported. Here, we expand upon these studies by investigating polymerized lipid bilayer capillary coatings with respect to separation performance including run-to-run, day-to-day and column-to-column reproducibility and long-term stability. In addition, the effects of pH and capillary inner diameter on polymerized phospholipid coated capillaries were investigated to identify optimized coating conditions. The coatings are stabilized for protein separations across a wide range of pH values (4.0–9.3), a unique property for capillary coating materials. Additionally, smaller inner diameter capillaries (≤50 μm) were found to yield marked enhancements in coating stability and reproducibility compared to wider bore capillaries, demonstrating the importance of capillary size for separations employing polymerized phospholipid coatings. 相似文献
998.
Wendy Zhong Bruce Hilton Gary Martin Lijun Wang Shiuhang Henry Yip 《Journal of heterocyclic chemistry》2013,50(2):281-287
High resolution MS, 1D, and 2D NMR were used to determine the structure of a unique cationic impurity generated during the synthesis of Preladenant? H/D exchange experiments were performed by both MS and NMR to confirm the acidic nature of the cationic dihydroimidazole proton. The presence of three exchangeable protons was established by MS experiments and the disappearance of the C11 proton in 1H‐NMR spectrum on equilibration with D2O confirmed the acidic nature of the cationic dihydroimidazole proton. A piperazine ring contraction mechanism is proposed for the formation of the cationic dihydroimidazole. 相似文献
999.
William C. McKee Jay Agarwal Henry F. Schaefer Paul von R. Schleyer 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2014,126(30):8009-8012
C(CH3)5+ is the first reported example of a five‐coordinate carbon atom bound only to separate (that is, monodentate) carbon ligands. This species illustrate the limits of carbon bonding, exhibiting Lewis‐violating “electron‐deficient bonds” between the hypercoordinate carbon and its methyl groups. Though not kinetically persistent under standard laboratory conditions, its dissociation activation barriers may permit C(CH3)5+ fleeting existence near 0 K. 相似文献
1000.
A simple variational method is used to determine the modal properties of single-mode couplers and Y junctions when the cores are close together or coalesce. The results complement the well-known perturbation expressions for large core separation. Together the two approximation methods provide accurate approximations over the complete range of separation from infinite separation to complete coalescence. 相似文献