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471.
Separation of neutral compounds by microemulsion electrokinetic chromatography: fundamental studies on selectivity 总被引:2,自引:0,他引:2
The selectivity of microemulsion electrokinetic chromatography (MEEKC) was studied utilizing some uncharged model compounds like aromatic amides, steroids, and esters of nicotinic acid. The cosurfactant of the microemulsion was found to be the most important factor affecting the selectivity, and alteration between 6.6% of 1-propanol, 1-butanol, tetrahydrofuran, and 2-ethoxyethanol caused several substantial changes in the migration order. In addition, the nature of the surfactant was found to significantly affect the selectivity. In this case, changes in order of migration was observed by replacement of half the content of sodium dodecyl sulfate (SDS) with either sodium dioctyl sulfosuccinate (SDOSS), 3-(N,N-dimethylmyristylammonio) propanesulfonate (MAPS), polyoxyethylene sorbitan monolaurate (Tween 21), and polyoxyethylene 23 lauryl ether (Brij 35). MEEKC was also accomplished with 3.3% of the anionic surfactant sodium cholate and with the cationic surfactant N-cetyl-N,N,N-trimethylammonium bromide (CTMA). Both provided substantial differences in selectivity as compared to the SDS-based systems. With SDS as surfacant, the concentration was varied within 1.0-4.5%. Minor selectivity changes were observed as the concentration of the surfacant was reduced, but the major effect was a reduction in the total migration time. The organic solvent of the microemulsion droplets was found only to have minor impact on the selectivity. 相似文献
472.
Olsen CA Jørgensen MR Hansen SH Witt M Jaroszewski JW Franzyk H 《Organic letters》2005,7(9):1703-1706
[reaction: see text] A mild resin-immobilization strategy employing a readily prepared trityl bromide resin for anchoring building blocks via a phenol group has been developed. With N(alpha)-Fmoc-Tyr-OPfp as a starter building block, it was possible to prepare asymmetrically substituted hybrids of spider- and wasp-type polyamine toxins using solid-phase peptide synthesis conditions. 相似文献
473.
Gaebert C Mattay J Toubartz M Steenken S Müller B Bally T 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(4):1294-1304
Radical cations were generated from different phenyl-substituted aziridines by pulse radiolysis in aqueous solution containing TlOH.+, N3. or SO4.- as oxidants or in n-butyl chloride, by 60Co gamma radiolysis in Freon matrices at 77 K, and in some cases by flash photolysis in aqueous solution. Depending on the substitution pattern of the aziridines, two different types of radical cations are formed: if the N atom carries a phenyl ring, the aziridine appears to retain its structure after oxidation and the resulting radical cation shows an intense band at 440-480 nm, similar to that of the radical cation of dimethylaniline. Conversely, if the N atom carries an alkyl substituent while a phenyl ring is attached to a C-atom of the aziridine, oxidation results in spontaneous ring opening to yield azomethine ylide radical cations which have broad absorptions in the 500-800 nm range. In aqueous solution the two types of radical cations are quenched by O2 with different rates, whereas in n-butyl chloride, the ring-closed aziridine radical cations are not quenchable by O2. The results of quantum chemical calculations confirm the assignment of these species and allow to rationalize the different effects that phenyl rings have if they are attached in different positions of aziridines. In the pulse radiolysis experiments in aqueous solution, the primary oxidants can also be observed, whereas in n-butyl chloride a transient at 325 nm remains unidentified. In the laser flash experiments, both types of radical cations were also observed. 相似文献
474.
The structures and complete force fields of cis and trans nitrous acid have been calculated with a (7, 3) basis set. The differences between the two stable isomers are reproduced well. The dipole moments, centrifugal distortion constants, vibrational frequencies and isotope shifts also agree satisfactorily with observed values. The fully optimized structures of rotamers corresponding to intermediate values of internal rotation around the N-O bond have been calculated to investigate structural changes during internal rotation. The considerable changes show the strong influence of π electron delocalization in the planar forms. The barrier is calculated to be 8.7 kcal mol−1 in reasonable agreement with experimental values. Both the cis and trans barriers are calculated to be attractive dominant in contradiction to earlier work. 相似文献
475.
An API 3000 triple-quadrupole instrument and a QSTAR Pulsar quadrupole time-of-flight (TOF) mass spectrometer were compared for the determination of phosphopeptides by precursor ion scanning in both the positive and negative nanoelectrospray ionization modes. The limits of detection for synthetic phosphopeptides were similar (500 amol microl(-1)) for both types of instruments when monitoring precursors of -79 Da (PO(3)(-)). However, the quadrupole TOF system was approximately fivefold more sensitive (1 fmol microl(-1)) than the triple-quadrupole instrument (5 fmol microl(-1)) when monitoring precursors of 216 Da (immonium ion of phosphotyrosine). The recently introduced Q(2)-pulsing function, which enhances the transmission of fragment ions of a selected m/z window from the collision cell into the TOF part, improved the sensitivity of precursor ion scans on a quadrupole TOF instrument. The selectivity of precursor ion scans is much better on quadrupole TOF systems than on triple quadrupoles because the high resolving power of the reflectron-TOF mass analyzer permits high-accuracy fragment ion selection at no expense of sensitivity. This minimizes interferences from other peptide fragment ions (a-, b-, and y- type) of the same nominal mass but with sufficient differences in their exact masses. As a result, the characteristic immonium ion of phosphotyrosine at m/z 216.043 can be utilized for the selective detection of tyrosine phosphorylated peptides. Our data suggest that, in addition to their superior performance for peptide sequencing, quadrupole TOF instruments also offer a very viable alternative to triple quadrupoles for precursor ion scanning, thus combining high sensitivity and selectivity for both MS and MS/MS experiments in one instrument. 相似文献
476.
Robin B. English Raymond J. Haines Nick D. Steen 《Journal of organometallic chemistry》1982,238(3):C34-C36
Reaction of [{Rh(μ-Cl)(CO)2}2] with PPh2H in CO-saturated ethanol yields [Rh3(μ-PPh2)3 (CO)6 (PPh2H)], a red trinuclear cluster of rhodium containing a near-planar six-membered Rh3P3 ring; this compound reversibly undergoes elimination of CO and PPh2H to afford [Rh3(μ-PPh2)3(CO)5]. 相似文献
477.
Background
The binding of ligands to clusters of complement-type repeat (CR)-domains in proteins of the low-density lipoprotein receptor (LDLR) family is dependent on Ca2+ ions. One reason for this cation requirement was identified from the crystal structure data for a CR-domain from the prototypic LDLR, which showed the burial of a Ca2+ ion as a necessity for correct folding and stabilization of this protein module. Additional Ca2+ binding data to other CR-domains from both LDLR and the LDLR-related protein (LRP) have suggested the presence of a conserved Ca2+ cage within CR-domains from this family of receptors that function in endocytosis and signalling.Results
We have previously described the binding of several ligands to a fragment comprising the fifth and the sixth CR-domain (CR56) from LRP, as well as qualitatively described the binding of Ca2+ ions to this CR-domain pair. In the present study we have applied the rate dialysis method to measure the affinity for Ca2+, and show that CR56 binds 2 Ca2+ ions with an average affinity of KD = 10.6 microM, and there is no indication of additional Ca2+ binding sites within this receptor fragment.Conclusions
Both CR-domains of CR56 bind a single Ca2+ ion with an affinity of 10.6 microM within the range of affinities demonstrated for several other CR-domains.478.
The elimination of small neutral fragments from acetyl-, formyl- and ethoxycarbonyl- phenylhydrazines with formation of [C6H8N2]+? ions has been studied. Evidence is obtained from deuterium labelling and from metastable peak intensity ratios, to show that ketene loss from both acetylphenylhydrazines is accompanied (or preceded) by hydrogen transfer to the acylated nitrogen atom to give ions structurally analogous to the phenylhydrazine molecular ion. The decomposing [C6H8N2]+? ions formed from formyl- and ethoxycarbonylphenylhydrazines are also suggested to have a phenylhydrazine-like structure. In the molecular ion of phenylhydrazine interchange occurs between the two ortho hydrogen atoms and two of the three hydrazine hydrogens prior to decomposition; labelling data suggest that the N-1 hydrogen does not participate in the interchange process. 相似文献
479.
Herman W. Zappey Steen Ingemann Nico M. M. Nibbering 《Journal of the American Society for Mass Spectrometry》1992,3(5):515-517
The methoxy cation, CH30+, formed by collision-induced charge reversal of methoxr anions with a kinetic energy of 8 keY, has been differentiated from the isomenric CH2OH+ ion by performing low kinetic energy ion-molecule reactions In the radiofrequency-only quadrupole of a reverse-geometry double-focusing quadrupole hybrid mass spectrometer. The methoxy cation reacts with CH3SH, CH3?CH=CH2, (CH3)2O, and CH3CH2Cl by electron transfer, whereas the CH2OH+ ion reacts by proton transfer with these substrates 相似文献
480.
Steen Hansen 《Rheologica Acta》2008,47(2):169-178
The relaxation spectrum is estimated from dynamic experiments using Bayesian analysis and a new regularization constraint.
In the Bayesian framework, a probability can be calculated for each estimate of the spectrum. This offers several advantages;
(1) an optimal estimate of the relaxation spectrum may be calculated as the mean of a large number of estimates, and (2) reliable
errors for the optimal estimate can be provided using the deviation of all estimates from the mean. Furthermore, the Bayesian
approach (3) gives an estimate of the overall noise level of the experiment, which is usually an important but unknown parameter
for the calculation of relaxation spectra from dynamic experiments by indirect methods (determining the regularization parameter),
and finally, (4) the information content in a given set of experimental data can be quantified. The validity of the Bayesian
approach is demonstrated using simulated data. 相似文献