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611.
Avalos M Babiano R Cabanillas A Cintas P Higes FJ Jiménez JL Palacios JC 《The Journal of organic chemistry》1996,61(11):3738-3748
1,3-Dipolar cycloaddition of 1,3-thiazolium-4-olates, readily prepared from thioureido derivatives, and trans-beta-nitrostyrene at room temperature in methylene chloride (48 h) resulted in two readily separable diastereomeric racemic 4,5-dihydrothiophenes via transient cycloadducts that underwent rearrangement under these reaction conditions. Using chiral carbohydrate-derived nitroalkenes, two diastereomeric dihydrothiophenes were obtained, showing that regiospecificity and facial selectivity were involved in these cycloadditions. (1)H NMR data and trapping experiments with isolated initial cycloadducts indicated that the cycloadditions were reversible and accounted for observed adduct and final product ratios. Single-crystal X-ray determinations established the structures of critical intermediates and products, and PM3 semiempirical MO calculations provide a rationalization for both the reactivity of the thiazolium-4-olates and the regioselectivity observed in the cycloadditions. 相似文献
612.
Efraim Reyes 《Tetrahedron letters》2005,46(39):6605-6609
The ability of amino acids to catalyze dynamic kinetic asymmetric transformation (DYKAT) with excellent stereoselectivity is presented. The novel DYKAT process was applied in the proline-catalyzed one-step de novo synthesis of triketide- and deoxysugars, which were formed with excellent diastereoselectivity and up to 99% ee. The de novo synthesis is accomplished via an enzyme-like DYKAT process, which includes continuous amino acid-mediated racemization of the acceptor β-hydroxy aldehyde in combination with amino acid-catalyzed direct selective aldol addition. 相似文献
613.
Marisela Reyes Daniel Mercades Bernardo Fontal Trino Suárez Fernando Bellandi Ricardo R. Contreras Isolda Romero Yuraima Fonseca Pedro Cancines 《Reaction Kinetics and Catalysis Letters》2007,90(2):347-354
The chlorocarbonyl bis-[butylphenyl (meta-sulfonate-phenyl)phosphine] rhodium (I) complex shows catalytic hydroformylation
activity in toluene/water biphasic medium for 1-hexene, cyclohexene, 2,3-dimethyl-2-butene and 2-methyl-2-pentene, their binary
mixtures and a real Venezuelan naphtha, under standardized reaction conditions (1000 psi of syngas (1:1 H2/CO), 100°C, substrate/catalyst molar ratio (600:1) and 4 h reaction time), obtaining high percent conversion to oxygenated
products. 相似文献
614.
VíctorManuel Gutirrez‐García Gloria Reyes‐Rangel Omar Muoz‐Muiz Eusebio Juaristi 《Helvetica chimica acta》2002,85(12):4189-4199
Inexpensive acryloyl chloride was converted in 91% overall yield to two derivatives of β‐alanine, (R,R,R)‐ 6 and (R,R,S)‐ 6 , containing two chiral auxiliaries. C‐Alkylation of (R,R,R)‐ and (R,R,S)‐ 6 via a dianion derivative, was performed by direct metallation with 2.2 equiv. of lithium hexamethyldisilazane (LHMDS) in THF at ?78°. C‐Alkylation of (R,R,S)‐ 6 ‐Li2 (‘matched' pair of chiral auxiliaries) afforded the mono‐alkylated products 8 – 11 in 29–96% yield and 54–95% stereoselectivity. Employment of LiCl as an additive generally increased stereoselectivities, whereas the effect of HMPA as a cosolvent was erratic. Chemical correlation of the major diastereoisomer from the alkylation reactions with (S)‐α‐alkyl‐β‐alanine ( 12 – 15 ) showed that addition of the electrophile preferentially takes place on the enolate's Si‐face. This conclusion is also supported by molecular‐modeling studies (ab initio HF/3‐21G), which indicate that the lowest‐energy conformation for (R,R,S)‐ 6 ‐Li2 presents the more sterically hindered Re‐face of the enolate. The theoretical studies also predict a determining role for N? Li? O chelation in (R,R,S)‐ 6 ‐Li2, giving rise to an interesting ‘ion‐triplet' configuration for the dilithium dianion. 相似文献
615.
The addition of [NBu4]Br or [NBu4][BH4] to solutions of [Pt4(mu-PPh2)4(C6F5)4(CO)2] yields the complexes [NBu4]2[Pt4(mu-PPh2)4(mu-X)2(C6F5)4] (X=Br, H,) in which the two CO groups have been replaced by two anionic, bridging X ligands. The total valence electron counts are 64 and 60, respectively; thus, complex does not require Pt-Pt bonds, while two metal-metal bonds are present in, as their NMR spectra confirm. Also, the NMR spectra indicate a nonsymmetrical "Pt(mu-PPh2)2Pt(mu-PPh2)(mu-X)Pt(mu-PPh2)(mu-X)Pt" disposition for and. Treatment of with HX (X=Cl, Br) yields the complexes [NBu4]2[Pt4(mu-PPh2)4(mu-H)2(C6F5)3X] (X=Cl, Br,). These complexes react with [Ag(OClO 3)PPh3] with displacement of the halide and formation of [NBu4][Pt4(mu-PPh2)4(mu-H)2(C6F5)3PPh3]. Complexes maintain the same basic skeleton as, with two Pt-Pt bonds. Complex is, however, an isomer of the symmetric [NBu4]2[{(C6F5)2Pt(mu-PPh2)2Pt(mu-Br)}2], which has been prepared by a metathetical process from the well-known [NBu4]2[{(C6F5)2Pt(mu-PPh2)2Pt(mu-Cl)}2]. The comparison of the X-ray structures of and confirms the different disposition of the bridging ligands, and their main structural differences seem to be related to the size of Br- and its position in the skeleton. 相似文献
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