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31.
Maurice R. Battaglia 《Chemical physics letters》1978,54(1):124-127
Measurements of the Kerr effect and Cotton—Mouton effect of a series of non-dipolar, axially-symmetric, diamagnetic molecules have been made at 25°C and 632.8 nm. The static orientational correlation functions for these liquids were drawn from depolarised Rayleigh scattering studies and the derived magnetic anisotropies are in good agreement with literature data. 相似文献
32.
A novel supramolecular strategy to prepare bidentate ligands via the assembly of functionalised monomeric ligands on a dimeric zinc(II) porphyrin template is presented; the assembled bidentate ligands show chelating behaviour and their rhodium complexes display enhanced selectivity in the hydroformylation compared to the non-template analogue. 相似文献
33.
In addition to halide oxidation, the vanadium haloperoxidases are capable of oxidizing sulfides to sulfoxides. Four vanadium complexes with tripodal amine ligands, K[VO(O(2))(heida)] (1), VO(2)(bpg) (2), K[VO(2)(ada)] (3), and K(2)[VO(O(2))(nta)] (4), previously shown to perform bromide oxidation (Colpas, G. J.; Hamstra, B. J.; Kampf, J. W.; Pecoraro, V. L. J. Am. Chem. Soc. 1996, 118, 3469-3477), have now been shown to oxidize aryl alkyl sulfides to the corresponding sulfoxides. The oxidation was observed by the disappearance of thioanisole's ultraviolet absorption at 290 nm, by the change in the aromatic region of the (1)H NMR spectrum of the sulfides, and by changes in the complexes' (51)V NMR spectra. The amount of methyl phenyl sulfide oxidized in 3 h was 1000 equiv (per metal complex). The oxidation product is almost exclusively sulfoxide, with very little sulfone (less than 3% over a 3 h experiment) formed. This is consistent with an electrophilic oxidation mechanism, as had been proposed for oxidation of bromide by 1-4. The rate was found to be first order in substrate concentration, similar to the rate law observed for bromide oxidation. Unlike the bromide oxidation, the equivalent of acid required for peroxovanadium complex activation is not consumed. The complexes 1-4 are not reactive with styrene or cyclooctene. The relevance of these reactions to the mechanism of the vanadium haloperoxidases and, more generally, peroxovanadium oxygenation of sulfides will be discussed. 相似文献
34.
Leatherman MD Svejda SA Johnson LK Brookhart M 《Journal of the American Chemical Society》2003,125(10):3068-3081
The synthesis of a series of (alpha-diimine)NiR(2) (R = Et, (n)Pr) complexes via Grignard alkylation of the corresponding (alpha-diimine)NiBr(2) precursors is presented. Protonation of these species by the oxonium acid [H(OEt(2))(2)](+)[BAr'(4)](-) at low temperatures yields cationic Ni(II) beta-agostic alkyl complexes which model relevant intermediates present in nickel-catalyzed olefin polymerization reactions. The highly dynamic nature of these agostic alkyl cations is quantitatively addressed using NMR line broadening techniques. Trapping of these complexes with ethylene provides cationic Ni alkyl ethylene species, which are used to determine rates of ethylene insertion into primary and secondary carbon centers. The Ni agostic alkyl cations are also trapped by CH(3)CN and Me(2)S to yield Ni(R)(L)(+) (L = CH(3)CN, Me(2)S) complexes, and the dynamic behavior of these species in the presence of varied [L] is discussed. The kinetic data obtained from these experiments are used to present an overall picture of the ethylene polymerization mechanism for (alpha-diimine)Ni catalysts, including effects of reaction temperature and ethylene pressure on catalyst activity, polyethylene branching, and polymer architecture. Detailed comparisons of these systems to the previously presented analogous palladium catalysts are made. 相似文献
35.
Carlier J Arscott S Thomy V Camart JC Cren-Olivé C Le Gac S 《Journal of chromatography. A》2005,1071(1-2):213-222
We report here on an integrated microfabricated device dedicated to the preparation of biological samples prior to their on-line analysis by electrospray ionization-mass spectrometry (ESI-MS). This microfluidic device is fabricated using the negative photoresist SU-8 by microtechnology techniques. The device includes a chromatographic module plus an ESI interface for MS. The chromatographic module is dedicated to sample purification and is based on a polymer monolithic phase which includes hydrophobic moieties. The ESI interface is integrated onto the chip and is based on a capillary slot. We present here the integration of these different modules onto a single system that is fabricated via a SU-8-based microtechnology route. We present also their testing for the purification of peptide samples. This started with a partial integration step with the combination of at least two of the modules (microsystem + monolith; microsystem + nib) and their test before the fabrication and testing of fully integrated microsystems. 相似文献
36.
Humblot V Lorenzo MO Baddeley CJ Haq S Raval R 《Journal of the American Chemical Society》2004,126(20):6460-6469
A detailed comparison of tartaric acid (HOOC-CHOH-CHOH-COOH) and succinic acid (HOOC-CH(2)-CH(2)-COOH) molecules on a Cu(110) surface is presented with a view to elucidate how the two-dimensional chirality exhibited by such robust, chemisorbed systems is affected when both OH groups of the former molecule are replaced with H groups, a stereochemical change that leaves the metal-bonding functionalities of the molecule untouched but destroys both chiral centers. It is found that this change does not significantly affect the thermodynamically preferred chemical forms that are adopted, namely the doubly deprotonated bicarboxylate at low coverages (theta = (1)/(6) ML) and the singly deprotonated monocarboxylate at higher coverage. However, the kinetics of phase formation are significantly affected so that the conditions required for self-assembling pertinent two-dimensional chiral phases alter substantially. For both molecules, two-dimensional assembly is found to depend strongly on the nature of the local adsorption motif created, with each motif essentially acting as a "synthon" for the supramolecular assembly. In this respect, it seems that molecule-metal bonding interactions define the general self-assembly structure. The presence/absence of the OH groups, instead, cause a subtler, second-order effect on the finer details of the self-assembled structure. Finally, the creation of chirality in the achiral succinate system is shown to arise from adsorption-induced asymmetrization, inducing point chirality via molecular distortion and/or metal reconstruction of the local adsorption unit. This chiral adsorption unit is then responsible for creating chiral supramolecular through-space and through-metal interactions that propagate a chiral organization. However, the achirality of the succinate ensures that nucleation points of either chirality are equally created, producing a racemic conglomerate of coexisting mirror domains. It is in this aspect that the uniquely aligned OH groups of the rigid bitartrate system wield the greatest effect, by favoring one distortion/reconstruction for the (R,R)-bitartrate and its mirror image distortion/reconstruction for the (S,S)-enantiomer, creating surfaces that are globally chiral on the macroscopic scale. So overall, the OH groups do not dictate the general nature of the assembly but are critical as chiral propagators, breaking the degeneracy and thus promoting asymmetry to chirality. 相似文献
37.
Maurice M. Bursey Donald J. Harvan Carol E. Parker Thomas A. Darden J. Ronald Hass 《Journal of mass spectrometry : JMS》1983,18(12):530-533
The methylnitrenium, ethylnitrenium and dimethylnitrenium ions are prepared by charge reversal collisional activation (CR CA) of the corresponding negative ions; their collisional activation mass spectra are shown to support the assigned structures. MINDO/3 energies are used to evaluate relative energies of [CH4N]+ and [C2H6N]+ isomers, and to determine whether unstable forms rearrange spontaneously to stable ones. As in other examples, charge reversal here generates cations that do not exist in an energy well, but their transient existence is established because their fragmentation is more rapid than their rearrangement to a more stable form. 相似文献
38.
Kim W McMillan RA Snyder JP Conticello VP 《Journal of the American Chemical Society》2005,127(51):18121-18132
Stereoelectronic effects have been identified as contributing factors to the conformational stability of collagen-mimetic peptide sequences. To assess the relevance of these factors within other protein structural contexts, three polypeptide sequences were prepared in which the sequences were derived from the canonical repeat unit (Val-Pro-Gly-Val-Gly) of the protein material elastin. These elastin-mimetic polypeptides, elastin-1, elastin-2, and elastin-3, incorporate (2S)-proline, (2S,4S)-4-fluoroproline, and (2S,4R)-4-fluoroproline, respectively, at the second position of the elastin repeat. Calorimetric and spectroscopic investigations of these three polypeptides indicate that the incorporation of the substituted proline residues had a dramatic effect upon the self-assembly of the corresponding elastin peptide. The presence of (2S,4R)-4-fluoroproline in elastin-3 lowered the temperature of the phase transition and increased the type II beta-turn population with respect to the parent polypeptide, while the presence of (2S,4S)-4-fluoroproline in elastin-2 had the opposite effect. These results suggest that stereoelectronic effects could either enhance or hinder the self-assembly of elastin-mimetic polypeptides, depending on the influence of the proline analogue on the energetics of the beta-turn conformation that develops within the pentapeptide structural repeats above the phase transition. Density functional theory (DFT) was employed to model three possible turn types (betaI-, betaII-, and inverse gamma-turns) derived from model peptide segments (MeCO-Xaa-Gly-NHMe) (Xaa = Pro, 4S-F-Pro, or 4R-F-Pro) corresponding to the turn-forming residues of the elastin repeat unit (Val-Pro-Gly-Val-Gly). The results of the these calculations suggested a similar outcome to the experimental data for the elastin-mimetic polypeptides, in that type II beta-turn structures were stabilized for peptide segments containing (2S,4R)-fluoroproline and destabilized for segments containing (2S,4S)-fluoroproline relative to the canonical proline residue. 相似文献
39.
We present an extensive investigation of the dependence of the scattering intensity difference of right and left circularly polarized light observed in vibrational Raman optical activity (VROA) on the choice of basis set and exchange-correlation functional. These dependencies are investigated for five molecules for which accurate experimental data are available: (S)-methyloxirane, (R)-epichlorhydrin, (S)-glycidol, (M)-spiro[2,2]pentane-1,4-diene, and (M)-sigma-[4]-helicene. Calculations are presented using the SVWN exchange-correlation functional (LDA), the BLYP exchange-correlation functional, and the B3LYP hybrid functional, using six different basis sets: the cc-pVDZ, cc-pVTZ, aug-cc-pVDZ, aug-cc-pVTZ, Sadlej's polarized basis set, and a minimal VROA basis set recently proposed by Zuber and Hug. It is demonstrated that results from pure gradient-corrected and hybrid functionals are comparable and that the aug-cc-pVDZ and aug-cc-pVTZ basis sets yield similar results. Furthermore, the combination of the small basis set by Zuber and Hug with an accurate force field represents the best compromise between computational accuracy and computational efficiency. 相似文献
40.
Benoít Rigo Etienne Tullier Didier Barbry Daniel Couturier Vincent Warin Jocelyne Lamiot Franois Baert 《Journal of heterocyclic chemistry》1990,27(5):1383-1386
3,5-Dihydrobenz[f]indolizin-3-one was prepared by a novel dehydration reaction involving the heating of 1,2,3,5,10,10a-hexahydro[f]indolizine-3,10-dione with polyphosphoric acid. The structure of this new compound was established by X-ray crystallography, by nmr spectroscopy and by reduction to the known products 1,2,3,5-tetrahydrobenz[f]indolizin-3-one and 1,2,3,5,10,10a-hexahydrobenz[f]indolizin-3-one. 相似文献