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971.
Quantitative Prediction of Rate Constants for Aqueous Racemization To Avoid Pointless Stereoselective Syntheses 下载免费PDF全文
Dr. Andrew Ballard Dr. Hiwa O. Ahmad Dr. Stefania Narduolo Lucy Rosa Nikki Chand Dr. David A. Cosgrove Dr. Peter Varkonyi Dr. Nabil Asaad Dr. Simone Tomasi Dr. Niklaas J. Buurma Dr. Andrew G. Leach 《Angewandte Chemie (International ed. in English)》2018,57(4):982-985
Racemization has a large impact upon the biological properties of molecules but the chemical scope of compounds with known rate constants for racemization in aqueous conditions was hitherto limited. To address this remarkable blind spot, we have measured the kinetics for racemization of 28 compounds using circular dichroism and 1H NMR spectroscopy. We show that rate constants for racemization (measured by ourselves and others) correlate well with deprotonation energies from quantum mechanical (QM) and group contribution calculations. Such calculations thus provide predictions of the second‐order rate constants for general‐base‐catalyzed racemization that are usefully accurate. When applied to recent publications describing the stereoselective synthesis of compounds of purported biological value, the calculations reveal that racemization would be sufficiently fast to render these expensive syntheses pointless. 相似文献
972.
The Molecular Structure of gauche‐1,3‐Butadiene: Experimental Establishment of Non‐planarity 下载免费PDF全文
Dr. Joshua H. Baraban Dr. Marie‐Aline Martin‐Drumel P. Bryan Changala Dr. Sandra Eibenberger Dr. Matthew Nava Prof. David Patterson Prof. John F. Stanton Prof. G. Barney Ellison Dr. Michael C. McCarthy 《Angewandte Chemie (International ed. in English)》2018,57(7):1821-1825
The planarity of the second stable conformer of 1,3‐butadiene, the archetypal diene for the Diels–Alder reaction in which a planar conjugated diene and a dienophile combine to form a ring, is not established. The most recent high level calculations predicted the species to adopt a twisted, gauche structure owing to steric interactions between the inner terminal hydrogens rather than a planar, cis structure favored by the conjugation of the double bonds. The structure cis‐1,3‐butadiene is unambiguously confirmed experimentally to indeed be gauche with a substantial dihedral angle of 34°, in excellent agreement with theory. Observation of two tunneling components indicates that the molecule undergoes facile interconversion between two equivalent enantiomeric forms. Comparison of experimentally determined structures for gauche‐ and trans‐butadiene provides an opportunity to examine the effects of conjugation and steric interactions. 相似文献
973.
High‐Temperature and High‐Energy‐Density Dipolar Glass Polymers Based on Sulfonylated Poly(2,6‐dimethyl‐1,4‐phenylene oxide) 下载免费PDF全文
Zhongbo Zhang Dr. David H. Wang Prof. Morton H. Litt Dr. Loon‐Seng Tan Prof. Lei Zhu 《Angewandte Chemie (International ed. in English)》2018,57(6):1528-1531
A new class of high‐temperature dipolar polymers based on sulfonylated poly(2,6‐dimethyl‐1,4‐phenylene oxide) (SO2‐PPO) was synthesized by post‐polymer functionalization. Owing to the efficient rotation of highly polar methylsulfonyl side groups below the glass transition temperature (Tg≈220 °C), the dipolar polarization of these SO2‐PPOs was enhanced, and thus the dielectric constant was high. Consequently, the discharge energy density reached up to 22 J cm?3. Owing to its high Tg , the SO2‐PPO25 sample also exhibited a low dielectric loss. For example, the dissipation factor (tan δ) was 0.003, and the discharge efficiency at 800 MV m?1 was 92 %. Therefore, these dipolar glass polymers are promising for high‐temperature, high‐energy‐density, and low‐loss electrical energy storage applications. 相似文献
974.
Uncatalyzed,Regioselective Oxidation of Saturated Hydrocarbons in an Ambient Corona Discharge 下载免费PDF全文
Stephen T. Ayrton Rhys Jones David S. Douce Mike R. Morris R. Graham Cooks 《Angewandte Chemie (International ed. in English)》2018,57(3):769-773
Atmospheric pressure chemical ionization (APCI) in air or in nitrogen with just traces of oxygen is shown to yield regioselective oxidation, dehydrogenation, and fragmentation of alkanes. Ozone is produced from ambient oxygen in situ and is responsible for the observed ion chemistry, which includes partial oxidation to ketones and C?C cleavage to give aldehydes. The mechanism of oxidation is explored and relationships between ionic species produced from individual alkanes are established. Unusually, dehydrogenation occurs by water loss. Competitive incorporation into the hydrocarbon chain of nitrogen versus oxygen as a mode of ionization is also demonstrated. 相似文献
975.
Fabrication of Defined Polydopamine Nanostructures by DNA Origami‐Templated Polymerization 下载免费PDF全文
Yu Tokura Sean Harvey Chaojian Chen Prof. Dr. Yuzhou Wu Dr. David Y. W. Ng Prof. Dr. Tanja Weil 《Angewandte Chemie (International ed. in English)》2018,57(6):1587-1591
A versatile, bottom‐up approach allows the controlled fabrication of polydopamine (PD) nanostructures on DNA origami. PD is a biosynthetic polymer that has been investigated as an adhesive and promising surface coating material. However, the control of dopamine polymerization is challenged by the multistage‐mediated reaction mechanism and diverse chemical structures in PD. DNA origami decorated with multiple horseradish peroxidase‐mimicking DNAzyme motifs was used to control the shape and size of PD formation with nanometer resolution. These fabricated PD nanostructures can serve as “supramolecular glue” for controlling DNA origami conformations. Facile liberation of the PD nanostructures from the DNA origami templates has been achieved in acidic medium. This presented DNA origami‐controlled polymerization of a highly crosslinked polymer provides a unique access towards anisotropic PD architectures with distinct shapes that were retained even in the absence of the DNA origami template. 相似文献
976.
Christopher P. Gordon Neal Dalton Nicholas Vandegraaff John Deadman David I. Rhodes Jonathan A. Coates Stephen G. Pyne Renate Griffith John B. Bremner Paul A. Keller 《Tetrahedron》2018,74(12):1253-1268
As of mid-2017, only one structure of the human immunodeficiency virus (HIV) integrase core domain co-crystallised with an active site inhibitor was reported. In this structure (1QS4), integrase is complexed with a diketo-acid based strand-transfer inhibitor (INSTI). This structure has been a preferred platform for the structure-based design of INSTIs despite concerns relating to structural irregularities arising from crystallographic packing effects. A survey of the current pool of 297 reported integrase catalytic core structures indicated that the anatomy of the active site in the complex structure 1QS4 exhibits subtle variations relative to all other structures examined. Consequently, the 1QS4 structure was employed for docking studies. From the docking of twenty-seven allyltyrosine analogues, a 3-point inhibitor binding motif required for activity was established and successfully utilised in the development of a tripeptide displaying an EC50 value of 10 ± 5 μM in HIV infected human T-cells. Additional docking of “in-house” compound libraries unearthed a methyl ester based nitrile derivative displaying an IC50 value of 0.5 μM in a combined 3′-processing and strand-transfer assay. 相似文献
977.
John T. Gupton Alex Shimozono Evan Crawford Joe Ortolani Evan Clark Matt Mahoney Campbell Heese Jeffrey Noble Carlos Perez Mandry Rene Kanters Raymond N. Dominey Emma W. Goldman James A. Sikorski Daniel C. Fisher 《Tetrahedron》2018,74(21):2650-2663
Highly functionalized pyrroles with appropriate regiochemical functionality represent an important class of marine natural products and potential drug candidates. We describe herein a detailed study of the reaction of α-aminoacid esters with vinylogous amides and also β-halovinylaldehydes for the regiospecific synthesis of 2,3,4-trisubstituted and 1,2,3,4-tetrasubstituted pyrroles. Since the vinylogous amides and β-halovinylaldehydes are readily available precursors, rapid access to a wide variety of unsymmetrically substituted pyrroles is accomplished via this methodology. 相似文献
978.
Selective Hydrogen Atom Abstraction through Induced Bond Polarization: Direct α‐Arylation of Alcohols through Photoredox,HAT, and Nickel Catalysis 下载免费PDF全文
Jack Twilton Melodie Christensen Dr. Daniel A. DiRocco Dr. Rebecca T. Ruck Dr. Ian W. Davies Prof. Dr. David W. C. MacMillan 《Angewandte Chemie (International ed. in English)》2018,57(19):5369-5373
The combination of nickel metallaphotoredox catalysis, hydrogen atom transfer catalysis, and a Lewis acid activation mode, has led to the development of an arylation method for the selective functionalization of alcohol α‐hydroxy C?H bonds. This approach employs zinc‐mediated alcohol deprotonation to activate α‐hydroxy C?H bonds while simultaneously suppressing C?O bond formation by inhibiting the formation of nickel alkoxide species. The use of Zn‐based Lewis acids also deactivates other hydridic bonds such as α‐amino and α‐oxy C?H bonds. This approach facilitates rapid access to benzylic alcohols, an important motif in drug discovery. A 3‐step synthesis of the drug Prozac exemplifies the utility of this new method. 相似文献
979.
Glen R. Jones Dr. Athina Anastasaki Richard Whitfield Nikolaos Engelis Evelina Liarou Prof. David M. Haddleton 《Angewandte Chemie (International ed. in English)》2018,57(33):10468-10482
Key advances within the past 10 years have transformed copper‐mediated radical polymerization from a technique which was not very tolerant of protic media into a range of closely related processes capable of controlling the polymerization of a wide range of monomers in pure water at ppm catalyst loadings. This approach has afforded water‐soluble macromolecules of desired molecular weight, architecture, and chemical functionality, with applications ranging from drug delivery to oil processing. In this Review we highlight and critically evaluate the synthetic methods that have been developed to control radical polymerization in water by using copper complexes as well as identify future areas of interest and challenges still to be overcome. 相似文献
980.
Dr. J. Philipp Wagner David C. McDonald II Prof. Dr. Michael A. Duncan 《Angewandte Chemie (International ed. in English)》2018,57(18):5081-5085
The OH+ cation is a well‐known diatomic for which the triplet (3Σ?) ground state is 50.5 kcal mol?1 more stable than its corresponding singlet (1Δ) excited state. However, the singlet forms a strong donor–acceptor bond to argon with a bond energy of 66.4 kcal mol?1 at the CCSDT(Q)/CBS level, making the singlet ArOH+ cation 3.9 kcal mol?1 more stable than the lowest energy triplet complex. Both singlet and triplet isomers of this molecular ion were prepared in a cold molecular beam using different ion sources. Infrared photodissociation spectroscopy in combination with messenger atom tagging shows that the two spin isomers exhibit completely different spectral signatures. The ground state of ArOH+ is the predicted singlet with a covalent Ar?O bond. 相似文献