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961.
A simple screening method was developed to discriminate between synthetic and natural colorants present in foods in order to reduce the use of expensive instruments such as a liquid chromatograph with diode array detection. A rapid flow system was proposed in which samples containing natural and synthetic colorants in an acetic acid medium were passed through a wool/cotton column, where only synthetic colorants were retained and were then eluted with dilute ammonia. Yellow, red and green-blue-brown additives can be monitored at 400, 530 and 610 nm, respectively, with a conventional spectrophotometer. Complete discrimination (no false positives) between natural and synthetic colorants can be obtained for molar concentrations of natural colorants (in the absence of synthetic ones) up to 2000 (yellow), 2000 (red) and 10 000 (brown) times that of the detection limit (DL) of synthetic additives. The reliability of the method was established at five concentrations (between 0.5 and 3 DL) of the synthetic colorants tartrazine (yellow), erythrosin B (red) and brilliant black BN (brown). For a cut-off concentration of 2 DL, the percentage of false negatives ranges from 8 to 12%. Finally, the method was applied to screening several fruit drinks and candies for the determination of synthetic colorants with a sampling frequency of 10 h−1. 相似文献
962.
M. Carmen Carre o Jos L. Garcí a Ruano Miguel A. Toledo Antonio Urbano 《Tetrahedron letters》1994,35(52):9759-9762
The synthesis of the title compounds in enantiomerically pure form is described. Their cycloadditions with cyclopentadiene and 2,3-dimethyl-1,3-butadiene in the presence of ZnBr2 take place on the unsubstituted dienophilic double bond C5-C6 giving access to optically active 4a,5,8,8a-tetrahydronaphthoquinones with a diastereoisomeric excess ranging from 40 to 72%. 相似文献
963.
Bojan VR Fernández EJ Laguna A López-de-Luzuriaga JM Monge M Olmos ME Silvestru C 《Journal of the American Chemical Society》2005,127(33):11564-11565
Dinuclear three-coordinate Au(I) complex [Au2{(Ph2Sb)2O}3](ClO4)2 1 displays an interesting phosphorescent behavior in which a large Stokes' shift is observed. Ab initio calculations show that the main distortion for the first triplet excited state, which is responsible for the luminescence behavior of complex 1, is a Jahn-Teller distortion for only one of the Au(I) centers together with a gold-gold distance shortening. This behavior could be extrapolated to other phosphorescent dinuclear three-coordinate Au(I) complexes. 相似文献
964.
Hernández-Borges J Rodríguez-Delgado MA García-Montelongo FJ Cifuentes A 《Electrophoresis》2004,25(13):2065-2076
A highly sensitive procedure to detect multiple pesticides at trace levels in foods is presented. Initially a comparative study between capillary electrophoresis (CE)-UV and CE-mass spectrometry (MS) is carried out analyzing five pesticides not studied up to now (pyrimethanil, pyrifenox, cyprodinil, cyromazine, and pirimicarb). The comparison between CE-UV and CE-MS is established in terms of separation efficiency, speed of analysis, reproducibility, and sensitivity. A good separation of these compounds is achieved by both techniques using a volatile aqueous buffer containing 0.3 M ammonium acetate/acetic acid at pH 4. Time analysis reproducibility is studied for the same day (n = 5) and three different days (n = 15), showing no significant differences between CE-UV and CE-MS. The study on peak areas reproducibility shows a slightly worse reproducibility for CE-MS compared with CE-UV. The best limit of detection (LOD) that can be achieved for these pesticides using CE-UV was 0.6 microg/mL. CE-MS provides LODs one order of magnitude better than CE-UV. Chemometrics are used to optimize the multiple parameters that play a role in solid-phase microextraction (SPME) and CE-MS analysis (e.g., extraction and desorption times, nebulizer pressure, dry gas flow, dry gas temperature, percentage of organic solvent and acid in the sheath liquid, etc.). The combined use of chemometrics and SPME-CE-MS clearly improves the LODs that can be achieved allowing the detection of pesticides at concentrations down to 15 ng/mL. The usefulness of this approach is demonstrated detecting multiple pesticides in different food samples as grapes and orange juice in a single run. The concentrations detected are below the maximum residue limits (MRLs) permitted for these pesticides in foods corroborating the value of our approach. This work demonstrates, to our knowledge for the first time, the good possibilities of the combined use of SPME-CE-MS and chemometrics. 相似文献
965.
Carranza J Sletten J Brennan C Lloret F Cano J Julve M 《Dalton transactions (Cambridge, England : 2003)》2004,(23):3997-4005
Three new copper(ii) complexes of formula [Cu(tppz)(NCO)(2)].0.4H(2)O (1), [Cu(2)(tppz)Br(4)](2) and [Cu(3)(tppz)(C(5)O(5))(3)(H(2)O)(3)].7H(2)O (3)[tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine; C(5)O(5)(2-) = croconate, dianion of 4,5-dihydroxycyclopent-4-ene-1,2,3-trione] have been synthesised and structurally characterized by X-ray diffraction methods. The structure of complex is made up of neutral [Cu(tppz)(NCO)(2)] mononuclear units and uncoordinated water molecules. The mononuclear units are grouped by pairs to give a rather short copper-copper distance of 3.9244(4) angstroms. The structure of complex 1 consists of neutral tppz-bridged [Cu(2)(tppz)Br(4)] dinuclear units, the copper-copper separation across tppz being 6.6198(1) angstroms. The dinuclear units are further connected through weak, double out-of-plane Cu-Br...Cu bridges [Br(1)...Cu(1a) 4.0028(17) angstroms] creating tetranuclear entities, the copper-copper separation through this interaction being 4.3299(21) angstroms. The structure of complex 3 is built of neutral [Cu(3)(tppz)(C(5)O(5))(3)(H(2)O)(3)] trinuclear units and uncoordinated water molecules. Tppz and one of the croconate groups act as bridging ligands, the former exhibiting the bis-terdentate coordination mode and the latter adopting an unusual asymmetrical bis-bidentate bridging mode through three adjacent oxygen atoms. The other two croconate groups exhibit the bidentate coordination mode. The intramolecular copper-copper separations are 6.5417(9)(across tppz) and 4.3234(9) angstroms (through bis-bidentate croconato). The magnetic properties of 2 and 3 have been investigated in the temperature range 1.9-300 K. The magnetic behaviour of complex 2 is that of an antiferromagnetically coupled copper(II) dimer (J = -40.9 cm(-1), the Hamiltonian being H = -JS(A).S(B)). In the case of compound , the chi(M) T vs. T plot is typical of an overall antiferromagnetic coupling with a low-lying spin doublet being fully populated at T < 10 K. The values of the intramolecular antiferromagnetic interactions in 3 are -19.9 (across tppz) and -32.9 cm(-1)(through bridging croconato). Density functional type calculations were performed on model dinuclear fragments of 3 in order to analyze the efficiency of the exchange pathways involved and also to substantiate the coupling parameters. 相似文献
966.
967.
Balzani V Clemente-León M Credi A Lowe JN Badjić JD Stoddart JF Williams DJ 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(21):5348-5360
We have investigated the (1)H NMR spectra, the absorption spectra, the fluorescence spectra and decays, and the electrochemical properties of i). a tritopic receptor in which three benzo[24]crown-8 macrorings are fused onto a triphenylene core, ii). a trifurcated trication wherein three dibenzylammonium ions are linked 1,3,5 to a central benzenoid core, and iii). their 1:1 adduct which constitutes a triply-threaded, two-component supramolecular bundle. X-Ray crystallography has established the precise geometry of this paucivalent recognition motif in the solid state. In addition to [N(+)-H...O] hydrogen bonding and [C-H...O] interactions between the NH(2) (+) centers on the three dibenzylammonium ion containing arms of the trication and the three crown ether rings in the tritopic receptor, there is a stabilizing [pi...pi] stacking interaction between the two aromatic cores. Mass spectrometry and (1)H NMR spectroscopy have confirmed the integrity of the 1:1 adduct beyond the solid state, provided the solvents are relatively apolar (e.g., chloroform and acetonitrile). The intense fluorescence emissions of the two recognition components are quenched upon association with the concomitant appearance of a lower energy, broad fluorescence band originating from the pi-pi stacking in the 1:1 adduct of the aromatic cores in the two matching components. Titration experiments, including Job plots, establish the 1:1 stoichiometry of the adduct, an observation which is also confirmed by electrochemical experiments. The electrochemical results show that, both in the tritopic receptor and in the superbundle itself, the first oxidation process is associated with the hexaalkoxytriphenylene core. The successive oxidation processes of the peripheral dioxybenzene units are affected by charge-transfer interactions in the tritopic receptor, whereas, in the superbundle, such units are not interacting. In acetonitrile solution, dethreading/rethreading of the 1:1 adduct can be controlled quantitatively by addition of base and acid. Dethreading and rethreading is also observed by (1)H NMR spectroscopy when dimethylsulfoxide is added to a solution of the 1:1 adduct in equal volumes of acetonitrile and chloroform. A trifurcated trication where methyl groups are located on the para positions of the three dibenzylammonium ions, which are linked 1,3,5 to the neutral benzenoid core, has been employed to demonstrate that dethreading of the 1:1 adduct involves doubly-threaded and singly-threaded species, that is, the paucivalent site is dismembered in a sequence of logical steps involving stable intermediates. This molecular recognition system is a rare example of a supramolecular entity based on a cooperative binding motif that can be switched on and off by chemical means. 相似文献
968.
4-Substituted pyridine-2,6-dicarboxylic acids, (E)-dipicH2, and 4-substituted pyridine-2,6-bis(thiocarboxylic) acids, (E)-pdtcH2, (E = OCnH2n + 1, SCnH2n + 1) have been synthesized and used as O,N,O- and S,N,S-pincer ligands with palladium. In the fourth coordination site the complexes bear 4-decyloxy-4'-stilbazole (L1), 4-decyloxy-N-(4-pyridylmethylene)anilines (L2), decyl 4-pyridinecarboxylate (L3), 4-(4'-decyloxyphenyl)pyridinecarboxylate (L4), 4-(3',4',5'-tridecyloxybenzyl)pyridinecarboxylate (L5), 4-isocyano-1-decyloxybenzene (L6), or 4-isocyano-4'-decyloxybiphenyl (L7). Thermotropic mesomorphism is observed for the (E)-dipic complexes with L5 and n = 12, which display columnar phases. The complexes with S,N,S-pincers show an important depression in the melting point compared to their O,N,O homologues and this change gives rise to mesomorphic materials (SC). However, in the case of L5 the mesomorphic behavior observed for the O,N,O derivative is lost in its S,N,S analogue. The alkylsulfanyl compounds exhibit lower transition temperatures and wider mesophase ranges than their alkoxy analogues. 相似文献
969.
Santana MD García G Julve M Lloret F Pérez J Liu M Sanz F Cano J López G 《Inorganic chemistry》2004,43(6):2132-2140
The preparation, spectroscopic characterization, and magnetic study of three new oxamidate-bridged nickel(II) dinuclear complexes of formulas ([Ni(Me3[12]aneN3)]2(mu-oa))(PF6)2 (1), ([Ni(Me3[12]aneN3)]2(mu-dmoa))(PF6)2 (2), and ([Ni(Me3[12]aneN3)]2(mu-dpoa))(PF6)2 (3) (Me3[12]aneN3 = 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene, oa = oxamidate, dmoa = N,N'-dimethyloxamidate, and dpoa = N,N'-diphenyloxamidate) are reported. The crystal structures of two of them (1 and 3) have been determined. 1 and 3 crystallize in the monoclinic system, space group P2(1)/c, with Z = 2 and a = 7.901(4) A, b = 13.597(6) A, c = 17.565(10) A, and beta = 96.46(4) degrees for 1 and a = 13.854(3) A, b = 17.469(4) A, c = 12.543(3) A, and beta = 116.22(3) degrees for 3. The structures of 1 and 3 consist of dinuclear ([Ni(Me3[12]aneN3)]2(mu-oa))2+ and ([Ni(Me3[12]aneN3)]2(mu-dpoa))2+ cations and hexafluorophosphate anions. Each nickel in 1-3 is five-coordinate, and the substitution of the hydrogen atom of the amidate nitrogen of 1 by a methyl (2) or a phenyl (3) group causes a significant modification of the stereochemistry of the nickel(II) ions from square pyramidal toward trigonal bipyramidal (tau values of 0.12 and 0.48 for 1 and 3, respectively). The NOESY spectrum of 3 has allowed us to achieve the assignment of the phenyl protons of the N,N'-diphenyloxamidate. The value of magnetic coupling between the two nickel(II) ions across the oxamidate bridge [J = -57.0 (oa, 1), -38.0 (dmoa, 2) and -30.5 cm(-1) (dpoa, 3)] is very sensitive to this stereochemical change, and its variation is explained on the basis of orbital considerations. DFT type calculations have been performed to analyze and substantiate the trend of the magnetic coupling in 1-3. 相似文献
970.
The reactive uptake kinetics of ClONO(2) on pure and doped water-ice surfaces have been studied using a coated wall flow tube reactor coupled to an electron impact mass spectrometer. Experiments have been conducted on frozen film ice surfaces in the temperature range 208-228 K with P((ClONO)(2)) < or = 10(-6) Torr. The uptake coefficient (gamma) of ClONO(2) on pure ice was time dependent with a maximum value of gamma(max) approximately 0.1. On HNO(3)-doped ice at 218 K the gamma(max) was 0.02. HOCl formation was detected in both experiments. On HCl-doped ice, uptake was gas-phase diffusion limited (gamma > 0.1) and gas-phase Cl(2) was formed. The uptake of HCl on ice continuously doped with HNO(3) was reversible such that there was no net uptake of HCl once the equilibrium surface coverage was established. The data were well described by a single site 2-species competitive Langmuir adsorption isotherm. The surface coverage of HCl on HNO(3)-doped ice was an order of magnitude lower than on bare ice for a given temperature and P(HCl). ClONO(2) uptake on this HCl/HNO(3)-doped ice was studied as a function of P(HCl). gamma(max) was no longer gas-phase diffusion limited and was found to be linearly dependent on the surface concentration of HCl. Under conditions of low HCl surface concentration, hydrolysis of ClONO(2) and reaction with HCl were competing such that both Cl(2) and HOCl were formed. A numerical model was used to simulate the experimental results and to aid in the parametrization of ClONO(2) reactivity on cirrus ice clouds in the upper troposphere. 相似文献