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991.
The complexes [(DMPP)2M(CH3CN2)]X2 (DMPP = 3,4-dimethyl-1-phenylphosphole; M = Pd, Pt; X = BF4-, NO3-, ClO4-) react with 2 equiv of the dienophiles N,N-dimethylacrylamide (DMAA), 2-vinylpyridine (VyPy), and diphenylvinylphosphine (DPVP) to form bis-[4 + 2] Diels-Alder cycloaddition products. The [M(DMPP)2(DMAA)2]2+ and [M(DMPP)2(VyPy)2]2+ complexes form exclusively as the cis-geometric isomers, whereas for [M(DMPP)2(DPVP)2]2+, both cis- and trans-geometric isomers are formed. The two Diels-Alder cycloadditions occur sequentially, and the absolute configuration of the first reaction influences the absolute configuration of the second. In all cases, recemic mixtures of the (R,R) and (S,S) diastereomers are formed; none of the meso (R,S) diastereomer is observed. New complexes were characterized by elemental analyses, physical properties, infrared spectroscopy, 1H, 1H(31P), 13C(1H), and 31P(1H) NMR spectroscopy, and, in most cases, X-ray crystallography. 相似文献
992.
Thermal decomposition of (C5H5)2Ti(CH3)2 in alkane or benzene solution yields primarily methane, with only traces of ethane. Methane is produced by hydrogen abstraction from both the cyclopentadienyl rings and from the methyl groups, but never from the alkanes or benzene. A scheme of decomposition is suggested involving two paths, each with a two stage formation of methanes. A very much slower ethane formation competes with the second stage. Strong evidence exists for slow hydrogen exchange between the eyelopentadienyl rings and the methyl groups prior to significant production of ethane. Decomposition in diethyl ether yields methane, much of it arising through hydrogen abstraction from the ether. Decomposition in CCl4 and C2Cl4 yields mainly methane but also some methyl chloride and ethane. Both diethyl ether and C2Cl4 almost block methane formation through ring attack. 相似文献
993.
Ab initio, GIAO and IGLO, nuclear shielding calculations are performed on each of the four pyranose ring residues of the tetrasaccharide repeating unit in a single chain of the gellan polysaccharide, [→ 3)-
-Glcp-(1 → 4)-β-
-GlcpA-(1 → 4)-β-
-Glcp-(1 → 4)--
-Rhap(1 n. The results provide an insight into the effects of the changing primary molecular electronic structure on the calculated 13C, 17O and 1H shieldings. In particular, the observed trends in the calculated isotropic (σi) shielding values are rationalised withi the framework of the localised molecular orbital shielding contributions and Mulliken population analyses (MPA). 相似文献
994.
In the presence of enantiopure MTBH(2)(monothiobinaphthol, 2-hydroxy-2[prime or minute]mercapto-1,1[prime or minute]-binaphthyl; 0.2 eq.) quantitative allylation of ArC([double bond]O)Me takes place with impure Sn(CH(2)CH[double bond]CH(2))(4)(prepared from allyl chloride, air-oxidised magnesium and SnCl(4)) to yield tert-homoallylic alcohols in 85-92% ee. In the same process highly purified, or commercial, Sn(CH(2)CH[double bond]CH(2))(4) yields material of only 35-50% ee. The origin of these effects is the presence of small amounts of the compounds, EtSn(CH(2)CH[double bond]CH(2))(3), ClSn(CH(2)CH[double bond]CH(2))(3) ClSnEt(CH(2)CH[double bond]CH(2))(2) in the tetraallyltin sample and the presence of traces of water (which inhibits achiral background reactions). All the triallyl and diallyl species enhance the stereoselectivity in the catalytic allylation reaction, the chlorides more so than the ethyl compound. Hydrolysis of ClSnEt(CH(2)CH[double bond]CH(2))(2) affords crystallographically characterised Sn(4)(mu(3)-O)(mu(2)-Cl)(2)Cl(2)Et(4)(CH(2)CH[double bond]CH(2))(4). Reaction of this latter compound with MTBH(2) leads to the most potent catalyst. 相似文献
995.
Hindmarsh JP Hollingsworth KG Wilson DI Johns ML 《Journal of colloid and interface science》2004,275(1):165-171
Various nuclear magnetic resonance (NMR) techniques were used to monitor the freezing behaviour of suspended 2-mm-diameter drops. The drops were composed of hydrocarbon oils emulsified in either water or water/sucrose mixtures. As such they were good model systems for the study of spray freezing, sharing structural similarities with potential products such as ice cream. In particular, simple 1H NMR spectroscopy was used to monitor and individually quantify the freezing or solidification behaviour of the various constituent species of the drops. In addition, the effect of freezing on the emulsion droplet size distribution (and hence emulsion stability) was also measured based on NMR self-diffusion measurements. The effect of freeze/thaw cycling was also similarly studied. The nucleation temperature of the emulsion droplets was found to depend on the emulsion droplet size distribution: the smaller the droplets, the lower the nucleation temperature. Emulsion droplet sizing indicated that oil-in-sucrose-solution emulsions were more stable, showing minimal coalescence, whereas oil-in-water emulsions showed significant coalescence during freezing and freeze/thaw cycling. 相似文献
996.
Summary Further work on the micellar properties of ethoxylated anionic and cationic surfactants is reported. Micellar molecular weights in water obtained from light scattering and corrected for charge effects, increased with increase in hydrocarbon chain length but decreased with increase in ethoxy group number. Micellar size of the ethoxylated surfactants was smaller than usual for paraffin chain salts. Micellar weights decreased 18% as the temperature increased from 25 to 45 °C. Addition of 0.1M. electrolyte increased micellar weight four fold. Analysis of the hydrodynamic data showed the ethoxylated micelles were hydrated with two molecules of water per ethoxy group. The hydrocarbon chain had little effect on hydration.
With 7 figures and 2 tables 相似文献
Zusammenfassung Es werden weitere Ergebnisse über micellare Eigenschaften äthoxylierter anionischer und kationischer oberflächenaktiver Stoffe berichtet. Micellare Molekulargewichte wurden ermittelt durch Lichtstreuung und für Ladungseffekte korrigiert. Diese nahmen mit der Länge der Kohlenwasserstoffkette zu, mit der Zahl der Äthoxy-Gruppen dagegen ab. Die micellare Größe der äthoxylierten oberflächenaktiven Stoffe war geringer als üblich bei Paraffinkettensalzen. Das Micellargewicht ging bei einem Temperaturanstieg von 25° auf 45° um 18% zurück. Wenn 0.1M. Elektrolyt hinzukam, steigerte sich das Micellargewicht um das Vierfache. Eine Analyse der hydrodynamischen Daten zeigte, daß die äthoxylierten Micellen einen Hydratgehalt von zwei Wassermolekülen per Äthoxy-Gruppe hatten. Die Kohlenwasserstoffkette hatte kaum Auswirkungen auf die Hydration.
With 7 figures and 2 tables 相似文献
997.
998.
The electrochemical response of two-dimensional networks of pristine single-wall carbon nanotubes (SWNTs) has been investigated. SWNTs were grown by catalyzed chemical vapor deposition on an insulating SiO2 substrate, and then electrically contacted by lithographically defined Au electrodes. Subsequent insulation of the contact electrodes enabled the electrochemical properties of the SWNT network to be isolated and directly studied for the first time. The electrochemical activity of the SWNT network was found to be strongly dependent on the applied potential. For the same SWNT electrode, the limiting current for the oxidation of 5 mM Fe(phen)32+ was found to be much greater than expected based on the signal for the reduction of 5 mM Ru(NH3)63+. Simultaneous conductance and electrochemical measurements demonstrated decreasing conductance as the potential was scanned negative (versus Ag/AgCl) with the minimum conductance at around the reduction potential for Ru(NH3)63+. These results are consistent with the presence of both metallic and semiconducting SWNTs in the SWNT network electrode. Moreover, these results show that through appropriate choice of mediator and electrode potential, metallic SWNTs can be electrochemically addressed independently of semiconducting SWNTs. 相似文献
999.
Jana D. Wilson Tammy D. Fulmer Lisette P. Dasher Charles F. Beam 《Journal of heterocyclic chemistry》1980,17(2):389-391
C(α),N-Dilithiophenylhydrazones were prepared from phenylhydrazones in an excess of lithium diisopropylamide and condensed with diethyl carbonate followed by an acid cyclization to give 2-pyrazolin-5-ones. 相似文献
1000.
Let (x,t)y (x,t),x[0, 1],t[0,T], be the solution of the diffusion equation in one spatial variable corresponding to zero initial conditions and boundary controluL
2(0,T). GivenfL
2(0, 1), it is not possible, in general, to find a controlu such thaty(·,T)=f. We extend the space of controls in such a manner thatL
2(0,T) can be considered to be a subset of a new spaceS of control elements; this space contains elements which do provide a solution to the problem of moments associated with the problem of makingy(·,T)=f inL
2(0, 1). We show then that the action of the elements ofS can be approximated by that of control functions inL
2(0,T) in a suitable manner. 相似文献