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531.
Michal W. Majchrzak Antoni Koteko Joseph B. Lambert 《Magnetic resonance in chemistry : MRC》1983,21(9):539-543
A new bicyclic system, 3,7-diazabicyclo[4.1.0]heptane, has been prepared from 3-(ethoxycarbonyl)-7,3-oxazabicyclo[4.1.0]heptane by reaction with sodium azide and reduction of the resulting tosyloxy azide with lithium aluminum hydride. The molecule can exist in four stereoisomeric half-chairs, depending on the configuration of the two nitrogen atoms. Half-chair ring reversal and piperidine nitrogen inversion are fast on the NMR time scale at all observed temperatures. Inversion of the secondary aziridine nitrogen becomes slow as the temperature is lowered (Tc= ?10°C). Complete analysis of the 1H spectrum was possible with the 1,5,5-trideuteriated analog. At slow exchange, two aziridine invertomers are present with an exo/endo ratio of approximately 0.7 in toluene-d8, 0.7 in CH2Cl2 and 1.7 in CHCl3/CH2Cl2. The free energy of activation for nitrogen inversion is 13.2 kcal mol?1 at ?10°C in CHCl3/CH2Cl2. 相似文献
532.
Influence of the initiator and additional hydrophobic copolymer on the morphology of thermosensitive poly(N-isopropylacrylamide) (pNIPAM) microspheres, and their presumed application for the stabilization of biologically active molecules
were evaluated in this study. Three different types of pNIPAM were synthesized, applying various components: PN1 is a polymer
with terminal anionic groups resulting from potassium persulfate initiator; PN2 was synthesized with a 2,2′-azobis(2-methylpropionamidine)
dihydrochloride initiator introducing cationic amidine terminal groups; in the PN3 polymer, anionic terminals were implemented,
however, increased hydrophobicity was maintained using N-tert-butyl functional groups. Turbidity measurements of the obtained dispersions confirmed specific thermosensitivity of synthesized
microspheres in the range of 32–33°C. The polymerization course was proved by infrared spectroscopy and 1H NMR assessments, whereas the size of the synthesized microspheres, expressed as planar area, was evaluated by dynamic light
scattering (DLS), scanning electron microscopy (SEM) and optical microscopy (OM). The respective surface patterns of the freeze-dried
microspheres were evaluated by SEM. Planar area of the synthesized macromolecules was in the range between 0.41–3.22 μm, depending
on the substrates composition and the method applied for the measurements. The assessments performed in the dry stage gave
higher values of the diameter and planar area of the observed microspheres. The measured diameter and planar area increased
in the following order for the PN3 microspheres: DLS, OM, SEM. In the case of PN1 and PN2, the observed diameters were positioned
as: DLS, SEM, OM. These differences were assigned both to varied intramolecular hydrophobic-hydrophilic interactions of the
polymer chains and to the environment, i.e. low pressure in the SEM conditions and aqueous solvent in the DLS measurements.
The observed gaps in the freeze-dried PN2 polymer resulted in an attempt to evaluate the application of this polymer for mechanical
stabilization of certain macromolecules or nanocrystals in the size range between 10 nm and 20 nm. 相似文献
533.
534.
535.
The thermal cyclodimerization of acrolein with methyl vinyl ketone gives selectively only one substituted dihydropyran. The rate is only slightly sensitive to the nature of the solvent; however a slight decrease of the reaction rate with increasing solvent polarity was observed. Effects of high pressure give strong support for a one-step mechanism and the formation of a transition state more compact than the adduct. 相似文献
536.
Antoni K. Wisor Piotr Kus Leszek Czuchajowski 《Monatshefte für Chemie / Chemical Monthly》1983,114(11):1213-1222
The UV spectra of the title compounds were interpreted as a result ofPPP-CI-1 calculations of excitation energies and oscillator strengths for the singlet—singlet transitions. A good agreement of the theoretical transitions with the experimental spectrum was found. A strong transannular effect was characteristic for thesyn isomers, the effect decreased in the order:syn-ring anthracenophane,syn-ring naphtalenophane,anti-ring naphthalenophane,anti-ring anthracenophane. Also the influence of a pseudo-substituent effect was found and discussed. 相似文献
537.
Fontes M Verdaguer X Solà L Pericàs MA Riera A 《The Journal of organic chemistry》2004,69(7):2532-2543
Here we report the use of 2-piperidino-1,2,2-triphenylethanol (5) as an outstanding catalyst for the ligand-catalyzed arylation of aldehydes. The use of 5 and a 2/1 mixture of Et(2)Zn/Ph(2)Zn provided the corresponding chiral diarylcarbinols with enantiomeric excess of up to 99% ee. The effect of temperature on the reaction enantioselectivity was studied and the inversion temperature (T(inv)) was determined to be 10 degrees C for reaction with p-tolylaldehyde. Most remarkably, lowering the amount of catalyst (5) to 0.5 mol % still afforded excellent levels of enantiocontrol (93.7% ee). Kinetics of the catalyzed and uncatalyzed arylation of aldehydes was studied by means of in situ FT-IR. The background uncatalyzed addition rates to p-tolylaldehyde when using pure Ph(2)Zn and Et(2)Zn/Ph(2)Zn (2/1) suggest that in the latter case a mixed zinc species forms (EtPhZn) minimizing the undesired nonselective addition. Formation of EtPhZn was modeled at the DFT calculation level. A four-center TS (TS-V) corresponding to the Et/Ph scrambling was localized along with two dimers (D-IV and D-VI). The model supports the hypothesis that Et/Ph exchange is a kinetically facile process. Gas evolution experiments during the formation of the active catalyst showed that the formation of an active site with a ONZn-Et (10) moiety is kinetically favored over ONZn-Ph (11). Finally, the phenyl transfer to benzaldehyde was modeled at the PM3(tm) level through anti and syn 5/4/4 tricyclic TS structures for both 10 and 11. The model could correctly predict the sense and selectivity of the overall process and predicted that 11 should be more selective than 10. 相似文献
538.
Borysik AJ Read P Little DR Bateman RH Radford SE Ashcroft AE 《Rapid communications in mass spectrometry : RCM》2004,18(19):2229-2234
An investigation into the use of high-field asymmetric waveform ion mobility spectrometry (FAIMS) coupled to electrospray ionisation mass spectrometry (ESI-MS) for the differentiation of co-populated protein conformers has been conducted on the amyloidogenic protein beta(2)-microglobulin (beta(2)m). Accumulation of beta(2)m in vivo can result in the deposition of insoluble fibrils whose formation is thought to originate from partially folded protein conformers; hence, the folding properties of beta(2)m are of significant interest. We have analysed beta(2)m using ESI-FAIMS-MS under a range of pH conditions and have studied the effect of the ion mobility spectrometry parameters on the behaviour of the various protein conformers. The data show that different protein conformers can be detected and analysed by ESI-FAIMS-MS, the results being consistent with observations of pH denaturation obtained using complementary biophysical techniques. A variant of beta(2)m with different folding characteristics has been analysed for comparison, and the distinctions observed in the data sets for the two proteins are consistent with their folding behaviour. ESI-FAIMS-MS offers significant opportunities for the study of the conformational properties of proteins and thus may present valuable insights into the roles that different conformers play in diseases related to protein folding. 相似文献
539.
Ballester P Capó M Costa A Deyà PM Gomila R Decken A Deslongchamps G 《The Journal of organic chemistry》2002,67(25):8832-8841
A series of tripodal amidopyridine receptors capable of selective recognition of methylmethanetriacetic acid (MMTA) in organic solvents is described. Intramolecular hydrogen-bonding groups, built into some of the receptors, were designed as preorganization devices. Binding was studied by NMR titration, variable temperature NMR experiments, 2D-NMR, isothermal titration calorimetry, and single-crystal X-ray crystallography. The results reveal that a balancing act between inter- and intramolecular hydrogen-bonding interactions in the complexes governs both the dynamics and the geometry of binding. Receptor 1b (without intramolecular hydrogen-bonding groups) features a simple symmetric MMTA binding geometry with optimal enthalpic interactions. In sharp contrast, receptor 1a (with intramolecular hydrogen-bonding groups) reveals a temperature-dependent dual binding mode where MMTA can bind in two completely different geometries. The two solution binding geometries of 1a.MMTA were unraveled by NMR experiments and correlated to the X-ray structures. 相似文献
540.
Solà J Riera A Verdaguer X Maestro MA 《Journal of the American Chemical Society》2005,127(39):13629-13633
A unique methine moiety attached to three heteroatoms (O, P, S) and contained in the PuPHOS and CamPHOS ligands serves as a strong hydrogen-bond donor. Nonclassical hydrogen bonding of this methine with an amido-carbonyl acceptor provides a completely diastereoselective ligand exchange process between an alkyne dicobalthexacarbonyl complex and a phosphine ligand. This weak contact has been studied by means of X-ray analysis, 1H NMR, and quantum mechanical calculations and revealed that the present interaction falls in the range of strong C-H...O=C bonds. The hydrogen-bond bias obtained in the ligand exchange process has been exploited in the asymmetric intermolecular Pauson-Khand reaction to yield the corresponding cyclization adducts in up to 94% ee. 相似文献