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911.
Paci P Zvinevich Y Tanimura S Wyslouzil BE Zahniser M Shorter J Nelson D McManus B 《The Journal of chemical physics》2004,121(20):9964-9970
We used a tunable diode laser absorption spectrometer to follow the condensation of D(2)O in a supersonic Laval nozzle. We measured both the concentration of the condensible vapor and the spectroscopic temperature as a function of position and compared the results to those inferred from static pressure measurements. Upstream and in the early stages of condensation, the quantitative agreement between the different experimental techniques is good. Far downstream, the spectroscopic results predict a lower gas phase concentration, a higher condensate mass fraction, and a higher temperature than the pressure measurements. The difference between the two measurement techniques is consistent with a slight compression of the boundary layers along the nozzle walls during condensation. 相似文献
912.
Raman spectroscopy is used to study the complexes of 4,4′-dinitrobiphenyl with biphenyl, 4-hydroxybiphenyl, 4-bromobiphenyl and p-terphenyl, which crystallize in a highly unusual geometry. Their phonon spectra at 125 K and 18 K are compared and the effect of isotopic substitution of biphenyl on the phonon spectra of its complex is examined. Internal vibrations of the components in the crystalline complex are compared with those observed in the pure crystals of the components. The results from both phonon and intramolecular vibration studies show that these complexes form in fixed stoichiometries, are governed by geometrical factors, and are stabilized primarily by van der Waals interaction, although other kinds of interactions may provide additional stabilization. The 4,4′-dinitrobiphenyl molecule as well as biphenyl and p-terphenyl are centrosymmetric and remain so when the complexes are cooled from room temperature to 18 K. For biphenyl complex, this conclusion is supported by the observed IR spectra which show mutual exclusion between IR-active and Raman active vibrations. Crystal splitting is observed on the 410 cm?1 vibration of 4,4′-dinitrobiphenyl. This splitting is attributed to the presence of more than one 4,4′-dinitrobiphenyl molecules in the complex unit. 相似文献
913.
The gram-negative bacterium Campylobacter jejuni has a general N-linked glycosylation pathway encoded by the pgl gene cluster. One of the proteins in this cluster, PgIB, is thought to be the oligosaccharyl transferase due to its significant homology to Stt3p, a subunit of the yeast oligosaccharyl transferase complex. PgIB has been shown to be involved in catalyzing the transfer of an undecaprenyl-linked heptasaccharide to the asparagine side chain of proteins at the Asn-X-Ser/Thr motif. Using a synthetic disaccharide glycan donor (GaINAc-alpha1,3-bacillosamine-pyrophosphate-undecaprenyl) and a peptide acceptor substrate (KDFNVSKA), we can observe the oligosaccharyl transferase activity of PgIB in vitro. Furthermore, the preparation of additional undecaprenyl-linked glycan variants reveals the ability of PgIB to transfer a wide variety of saccharides. With the demonstration of PgIB activity in vitro, fundamental questions surrounding the mechanism of N-linked glycosylation can now be addressed. 相似文献
914.
Barbara Kwiatek Marek K. Kalinowski 《Monatshefte für Chemie / Chemical Monthly》1987,118(6-7):733-740
An attempt has been made to estimate the effect of the subsequent ion pairs formation on the value of theHammett reaction constant measured polarographically. To this purpose, the reduction ofpara-substituted nitrobenzenes and 2-substituted 9,10-anthraquinones was studied at a dropping mercury electrode in N,N-dimethylformamide. The ion pairs reaction constant of substituted nitrobenzene radical anions with tetraethylammonium cations is equal to –1.49 V/ unit. The corresponding reaction constant for the semiquinone—Li+ series was estimated to be –0.90 V/ unit. The method of calculation was based on the additivity rule withinHammett's treatment.
Zur Additivität derHammet'schen Reaktionskonstanten. Paar-Effekte bei der polarographischen Reduktion von Nitrobenzol und 9,10-Anthrachinonderivaten
Zusammenfassung Es wurde eine Abschätzung des Effekts der nachfolgenden Ionenpaarbildung auf den polarographisch bestimmten Wert derHammet'schen Reaktionskonstanten versucht. Dazu wurde die Reduktion vonpara-substituierten Nitrobenzolen und 2-substituierten 9,10-Anthrachinonen an der tropfenden Quecksilberelektrode in N,N-Dimethylformamid untersucht. Die Ionenpaar-Reaktionskonstanten von substituierten Nitrobenzol-Radikalanionen mit Tetraethylammoniumkationen ist gleich –1.49 V/-Einheiten. Die ents prechende Reaktionskonstante für die Semichinon-Li+ Serie wurde mit –0.9 V/-Einheiten abgeschätzt. Die Berechnungsmethode basiert auf der Additivitätsregel derHammet'schen Reaktionskonstanten.相似文献
915.
Barbara Odell Stephen J. Hammond Richard Osborne Michael W. Goosey 《Journal of computer-aided molecular design》1996,10(2):89-99
Summary Proctolin (Arg-Tyr-Leu-Pro-Thr) was the first insect neuropeptide to be chemically characterised. It plays an essential role in insect neurophysiology and is involved in muscular contraction and neuromodulation. Elements of secondary structure in solution have been studied by comparing data obtained from NMR and molecular dynamics simulations. Different secondary structural requirements are associated with agonist and antagonist activities. A favoured conformation of proctolin has an inverse -turn, comprising an intramolecular hydrogen bond near the C-terminal end between Thr NH and Leu CO. Antagonists have a more compact structure resembling a paperclip loop, containing an intramolecular hydrogen bond between Tyr NH and Pro CO, possibly stabilised by a salt bridge between the N- and C-terminal groups. A cyclic analogue retains antagonist activity and resembles a -bulge loop, also comprising intramolecular hydrogen bonds between Tyr NH and Pro CO and Thr CO. These models may offer feasible starting points for designing novel compounds with proctolinergic activity. 相似文献
916.
A swollen lamellar phase, based on sodium dodecyl sulfate, pentanol and NaBr brine, was examined by means of NMR spectroscopy of isotopes with quadrupolar characteristics, namely 2H, 17O, 23Na and 81Br, present at natural abundance. Such a dilute lyotropic liquid crystal was capable of inducing a preferential orientation of the cobalt coordination compounds [Co(en)3]3+ and Co(acac)3, the 59Co NMR signals of which displayed quadrupolar splittings. Moreover, it imparted alignment in the magnetic field to the organocobaloxime [ClCD2Co(Hdmg)2H2O], as shown by the quadrupolar splitting of the deuterium of the axial ligand. 相似文献
917.
Barbara Matuszczak Thierry Langer Kurt Mereiter 《Journal of heterocyclic chemistry》1998,35(1):113-115
The preparation of N-acetylpyrazolyl-quinoxalines and results of semiempirical MO calculations are presented. The structure determination of the isomers 3a, b was achieved by X-ray analysis. 相似文献
918.
The thermal decomposition of the nickel(II) complexes of imidazolidine-2-thione (Imdt), Ni(Imdt)4X2 (X = Cl and Br), have been studied in air and in argon by means of TG and DTG, as well as in nitrogen by DTA. Decomposition schemes have been proposed for the complexes in air and inert atmospheres. End-products have been identified for the reactions in air by means of X-ray powder diffraction. Quantitative DTA has been used to derive reaction enthalpies (ΔHR) from the melting and decomposition endotherms of Imdt and the complexes in nitrogen. The factors which govern the reaction enthalpies of these complexes have been discussed. 相似文献
919.
920.
Peracetylated glycals were treated with terminal alkyne derivatives under Lewis acid catalysis to result in the formation of the pure α- or β- anomerically substituted hex-2-enopyranosides carrying 2(E)-halogenated olefins. 相似文献