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排序方式: 共有113条查询结果,搜索用时 46 毫秒
81.
U. Siemeling L. Klling O. Kuhnert B. Neumann A. Stammler H.‐G. Stammler G. Fink E. Kaminski A. Kiefer R. R. Schrock 《无机化学与普通化学杂志》2003,629(5):781-792
Two different types of modified imido and phosphaneiminato ligands are investigated, namely chelate ligands and redox‐functionalised ligands. The first examples of di(organoimido)chromium as well as di(phosphaneiminato)titanium and niobium chelates are described. Furthermore, the first complexes containing redox‐functionalised organoimido ligands are presented, together with the first structurally characterised redox‐functionalised phosphaneiminato complex. Compounds of the type [(RN)2M(CH2Ph)2] (M = Cr, Mo) are used as catalysts for the (co‐)polymerisation of the polar olefins methyl methacrylate, acrylonitrile and vinyl acetate. A range of X‐ray crystal structure determinations provide clear evidence for the quantum‐chemical result that, similar to organoimido complexes, the potential energy well for the angle at the nitrogen atom is very shallow for phosphaneiminato complexes. 相似文献
82.
Sattely ES Cortez GA Moebius DC Schrock RR Hoveyda AH 《Journal of the American Chemical Society》2005,127(23):8526-8533
First, an efficient method for the synthesis of optically enriched N-fused bicyclic structures is reported. Through Mo-catalyzed desymmetrization of readily available achiral polyene substrates, 5,6-, 5,7-, and 5,8-bicyclic amides can be synthesized in up to >98% ee. The effects of catalyst structure, olefin substitution, positioning of Lewis basic functional groups and ring size are examined and discussed in detail. In the second phase of investigations, a catalytic asymmetric method for highly enantioselective (up to 97% ee) synthesis of small- and medium-ring unsaturated cyclic amines is reported; optically enriched products bear a secondary amine or a readily removable Cbz or acetamide unit. Regio- and diastereoselective functionalizations of olefins within the optically enriched amine products have been carried out. Both catalytic asymmetric methods include transformations that lead to the formation of trisubstituted as well as disubstituted cyclic alkenes. The protocols outlined herein afford various cyclic amines of high optical purity; such products are not easily accessed by alternative protocols and can be used in enantioselective total syntheses of biologically active molecules. 相似文献
83.
84.
Blanc F Rendón N Berthoud R Basset JM Copéret C Tonzetich ZJ Schrock RR 《Dalton transactions (Cambridge, England : 2003)》2008,(24):3156-3158
[(triple bond SiO)Mo(triple bond NAr)(=CHCMe2R)(OtBu)], a well-defined silica supported alkene metathesis catalyst precursor, shows a dramatic enhancement of activity and selectivity compared to [Mo(triple bond NAr)(=CHCMe2R)(OtBu)2] and [(triple bond SiO)Mo(triple bond NAr)(=CHCMe2R)(CH2tBu)], respectively. 相似文献
85.
A comparative study of chiral Mo- and Ru-based catalysts to promote enantioselective synthesis of 2,6-disubstituted pyrans and piperidines through asymmetric ring-opening/cross-metathesis (AROM/CM) reactions is presented. These studies demonstrate the critical complementarity that exists between the two classes of chiral catalysts. 相似文献
86.
87.
Christopher R. Schrock & Aihua W. Wood 《advances in applied mathematics and mechanics.》2012,4(1):102-121
Direct Simulation Monte Carlo (DSMC) methods for the Boltzmann
equation employ a point measure approximation to the distribution
function, as simulated particles may possess only a single velocity.
This representation limits the method to converge only weakly to
the solution of the Boltzmann equation. Utilizing kernel density
estimation we have developed a stochastic Boltzmann solver which
possesses strong convergence for bounded and $L^\infty$ solutions
of the Boltzmann equation. This is facilitated by distributing
the velocity of each simulated particle instead of using the
point measure approximation inherent to DSMC. We propose that the
development of a distributional method which incorporates distributed
velocities in collision selection and modeling should improve convergence
and potentially result in a substantial reduction of the variance in
comparison to DSMC methods. Toward this end, we also report initial
findings of modeling collisions distributionally using the
Bhatnagar-Gross-Krook collision operator. 相似文献
88.
89.
Richard R. Schrock 《Macromolecular Symposia》1995,98(1):217-217
Alkylidene complexes of the type Mo(CH-t-Bu)(NAr)(OR)2 (Ar = 2, 6-diisopropylphenyl; OR = O-t-Bu, OCMe2(CF3), OCMe(CF3)2, etc.) can be prepared from a “universal precursor”, Mo(CH-t-Bu)(NAr)(triflate)2(1, 2-dimethoxyethane),1 which can be prepared in three high-yield steps from ammonium dimolybdate.2 These complexes serve as initiators for the ring opening metathesis of norbornenes and substituted norbornadienes,3 or for the polymerization of acetylenes to give polyenes,4 all in a living manner. The organic polymer can be cleaved upon treatment with an aldehyde. Norbornenes that contain a variety of organic or inorganic functionalities can be polymerized to give essentially monodisperse homopolymers and block copolymers. Recently chiral catalysts have been prepared that will polymerize norbornenes and norbornadienes stereoselectively to give all cis isotactic polymers.5 Molybdenum catalysts also have been found that will cyclopolymerize dipropargyl derivatives or substituted phenylacetylenes in a living manner.6 The stereospecific synthesis of all cis, or all trans, tactic polynorbornadienes and polynorbornenes will be discussed in detail, including a proof of tacticity.7 相似文献
90.
Nuria Rendón Dr. Romain Berthoud Dr. Frédéric Blanc Dr. David Gajan Tarun Maishal Dr. Jean‐Marie Basset Dr. Christophe Copéret Dr. Anne Lesage Dr. Lyndon Emsley Prof. Smaranda C. Marinescu Rojendra Singh Dr. Richard R. Schrock Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2009,15(20):5083-5089
Silica supports : Well‐defined silica‐supported Mo–alkylidene complexes containing one alkoxy/aryloxy substituent are selectively obtained by grafting onto silica either well‐defined molecular bis(alkoxy) or alkoxyamido complexes (see scheme). These silica‐supported complexes are highly efficient catalyst precursors for the metathesis of acyclic alkenes. However, they display poor performances in ring‐closing metathesis reactions, possibly due to the relatively large siloxy ligand (silica) inducing a greater rigidity.