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991.
Summary The enantiomers of diphenylalanine (DPA) were well separated by chiral HPLC and NMR spectroscopy on the chiral stationary
phase (CSP) derived from (18-crown-6)-2,3,11,12-tetracarboxylic acid (18-C-6-TA). The chromatographic parameters such as separation
factors and retention times were greatly influenced by the mobile phase conditions. The (+)-18-C-6-TA used in the CSP was
also employed as a chiral solvating agent for the enantiodiscrimination of the DPA enantiomers by NMR spectroscopy. The proton
of the DPA analyte showing the chemical shift nonequivalences was used in determining the enantiomeric composition of the
analyte. 相似文献
992.
993.
Si-Geun Lee Young-Whan Park Dae-Woo Nam Hyosun Lee Kwang Ho Song 《Journal of organometallic chemistry》2004,689(16):2586-2592
Methylene-bridged ansa-metallocene complexes bearing substituents on the cyclopentadienyl (Cp) and fluorenyl (Flu) moieties, namely methylene[9-(2,7-di-tert-butyl)fluorenyl(2-(1,3-dimethylcyclopentadienyl))]zirconium dichloride (1a) and its analogue, methylene[(9-(2,7-di-tert-butyl)fluorenyl(2-(1-methyl-3-phenyl)cyclopentadienyl))]zirconium dichloride (2a), have been prepared from (2,7-di-tert-butyl)-9-prop-2-ynyl-9H-fluorene (2). This procedure includes the use of 3-bromo-1-propyne which affords the methylene bridging unit by way of an intermolecular Pauson-Khand reaction in which norbornadiene and a pendant alkyne cyclize to form a ring that later becomes a substituted cyclopentadienyl group. Ethylene-norbornene (E-N) copolymerization was then carried out using these new complexes (1a and 1b) in the presence of methylaluminoxane (MAO) as a cocatalyst; these activities can be compared to that of isopropylene[9-fluorenyl-cyclopentadienyl]zirconium dichloride (3a). The activity of catalyst 1a was comparable to that of 3a but much higher than that of 2a. In addition, 1a shows higher norbornene insertion performance, and gives an E-N copolymer with a higher glass transition temperature (Tg) than 2a under identical conditions; both 1a and 2a give a lower Tg polymer than 3a does. 相似文献
994.
We report the details of the first total synthesis of erythromycin B using two different strategies for the end game. The first of these follows a classical approach in which the desosamine and cladinose residues are sequentially appended to a macrocyclic lactone, which was formed by cyclization of a seco acid derivative, to give a bis-glycosylated macrolide intermediate that is converted into erythromycin B. The second strategy features an abiotic approach in which a seco acid bearing a desosamine residue is cyclized to give a monoglycosylated macrocyclic lactone that is then transformed into erythromycin B via a sequence of steps involving refunctionalizations and a glycosylation to introduce the cladinose moiety. Attempts to prepare a bis-glycosylated seco acid by de novo synthesis were unsuccessful. The syntheses of the key seco acid intermediates feature the oxidative transformation of a furan containing C(3)-C(10) to provide a dioxabicyclo[3.3.1]nonenone that served as a template on which to create the stereocenters at C(6) and C(8). A stereoselective aldol reaction was used to establish the C(11)-C(15) segment, and a stereoselective crotylation was implemented to introduce the propionate subunit comprising C(1)-C(2). 相似文献
995.
We synthesized Leu‐Arg‐Pro‐Val‐Ala‐Ala‐Glu, the peptide contained in lactoferrin (Lf), to identify the angiotensin converting enzyme (ACE) inhibition. In an attempt to know the structure‐activity relationship of this peptide, we replaced Pro (the third amino acid residues from N‐terminal) or Val (the fourth amino acid residues from N‐terminal) with Ala (neutral amino acid), Glu (acidic amino acid) or Lys (basic amino acid) to produce six peptides. From the in vitro ACE inhibition (IC50) of these synthesized peptides, the original peptide (Leu‐Arg‐Pro‐Val‐Ala‐Ala‐Glu) showed higher ACE inhibition than the replaced six peptides. Thus, replacement of Pro at the third amino acid residues or Val at the fourth position with Ala, Glu or Lys revealed the ACE inhibition to be lower than the original form of Leu‐Arg‐Pro‐Val‐Ala‐Ala‐Glu. Otherwise, we added one peptide at the C‐terminal of Leu‐Arg‐Pro‐Val‐Ala‐Ala‐Glu and found both products with an addition of Val (Leu‐Arg‐Pro‐Val‐Ala‐Ala‐Glu‐Val) or Ile (Leu‐Arg‐Pro‐Val‐Ala‐Ala‐Glu‐Ile) showing a lower ACE inhibition than the original one. The ACE inhibitions produced by both replaced peptides were without significance. Also, deletion of the last peptide at the C‐terminal (Leu‐Arg‐Pro‐Val‐Ala‐Ala) failed to produce a marked change of ACE inhibition as compared to the original one. These results suggest that Pro and Val are essential in the peptide for inhibition of ACE activity. 相似文献
996.
Carina Hey Pui Cheung Jianchao Xu Chi Lung Lee Yanfeng Zhang Ruohan Wei Donald Bierer Xuhui Huang Xuechen Li 《Chemical science》2021,12(20):7091
Herein, we report the development of a facile synthetic strategy for constructing diverse peptide structural architectures via chemoselective peptide ligation. The key advancement involved is to utilize the benzofuran moiety as the peptide salicylaldehyde ester surrogate, and Dap–Ser/Lys–Ser dipeptide as the hydroxyl amino functionality, which could be successfully introduced at the side chain of peptides enabling peptide ligation. With this method, the side chain-to-side chain cyclic peptide, branched/bridged peptides, tailed cyclic peptides and multi-cyclic peptides have been designed and successfully synthesized with native peptidic linkages at the ligation sites. This strategy has provided an alternative strategic opportunity for synthetic peptide development. It also serves as an inspiration for the structural design of PPI inhibitors with new modalities.Methods of introducing peptide salicylaldehyde esters and hydroxyl amine functionality into the peptide side chain have been developed. Diverse peptide structural motifs were constructed via ligation with native amide linkages at the ligation sites. 相似文献
997.
998.
Phosphoinositide-based signaling processes are crucially important in intracellular signal transduction events. Inositol phosphate analogues have been useful in probing the structure-activity relationships between inositol phosphates and biomacromolecules, and in studying biological functions of newly found inositol phosphates. Thus, a systematic and ready access to inositol stereoisomers is highly desirable. And practical and convenient syntheses of conduritols and related compounds are also important because of their biological activities and their synthetic utilities in the preparation of other bioactive molecules. We herein report the first syntheses of all possible diastereomers of conduritol and various derivatives of eight inositol stereoisomers in high enantiopurity from myo-inositol, which involve efficient enzymatic resolution of the intermediates conduritol B and C derivatives, followed by oxidation-reduction or the Mitsunobu reaction, and cis-dihydroxylation in stereo- and regioselective manners. 相似文献
999.
Treatment of GaCl3 with one equiv of Li[NC4H3(CH2NMe2)‐2] (n = 1, 2, 3) in diethyl ether at ?78 °C yields GaCl3‐n[NC4H3(CH2NMe2)‐2]n (n = 1, 1 ; n = 2, 2 ; n = 3, 3 ). Compound 1 reacts with two equiv of RLi to afford GaR2[NC4H3(CH2NMe2)‐2] ( 4a, R=Me; 4b, R=Bu ) via transmetallation. Reacting 2 with one equiv of RLi in diethyl ether, 3 and 4 are formed via ligand redistribution. Variable temperature 1H NMR spectroscopic experiments reveal that the five‐coordinate gallium compound 3 is fluxional and results in a coalescence temperature at 5 °C, at which ΔG≠ is calculated at ca. 10.4 Kcal/mole. All the new compounds have been characterized by 1H and 13C NMR spectroscopy and the structures of compounds 3 and 4a have also been determined by X‐ray crystallography. 相似文献
1000.
Helen S. J. Lee Richard E. Kendrick Daphne Vince-Prue 《Photochemistry and photobiology》1985,42(6):625-632
Abstract— An examination has been made of the involvement of phytochrome in the circadian rhythm of flowering in Pharbitis nil Chois. The peak position of Pfr absorption changes with time after a red light pulse. The shortest absorption wavelength of Pfr occurs at the same time as flowering is inhibited by red light in dark grown, red light pretreated plants. Pelletable and supernatant phytochrome show a similar trend with lowest values found at the time of flower inhibition. Neither phototransformation kinetics nor intermediates of phytochrome which accumulate in white light show such a relationship to the circadian rhythm found in flowering of dark grown P. nil. 相似文献