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801.
HF, MP2, and B3LYP calculations with different basis sets have been used in the computation of the stabilization energies of C(3)H(7)X isomers, where X is F, Cl, and Br. The experimental stabilization energies of the structural isomers of C(3)H(7)Cl and C(3)H(7)Br have been reproduced via B3LYP calculations. However, the calculated stabilization energies of fluoropropane isomers from their reported enthalpies of formation have been reproduced in all methods of calculations in present work. The experimental relative stabilities of the gauche conformers of 1-fluoro-, 1-chloro-, and 1-bromopropanes have been also reproduced via some of the used calculations in the present work. The effect of the geminal interactions on X atomic charges and on the C-X and C-C bond lengths in halopropane isomers are also discussed. 相似文献
802.
《Tetrahedron letters》2004,45(14):2943-2945
Oxidation of lincomycin with H2O2 in alkaline media leads to N-oxides, besides the conversion of thiomethyl group into sulfoxides and sulfones. NH4OH favors formation of the S-isomer; both R- and S-isomers of the N-oxide are formed in the presence of NaOH. Addition of acetonitrile markedly accelerates the reaction. 相似文献
803.
Zhao L Yang W Luo Y Zhai T Zhang G Yao J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(12):3773-3778
Organic nanotubes of various diameters were fabricated from the isomeric molecule dibenzoylmethane (DBM) by using an immersing technique with ordered porous alumina membrane as the template. The ratio of the enol isomers of DBM increased as the diameters of the nanotubes decreased. In addition, although almost no fluorescence could be detected for the DBM monomer, a striking enhancement in the fluorescence emission intensity of the nanotubes was observed as the diameters decreased. This is due to the increased ratio of the enol isomers. 相似文献
804.
Transition Metal Complexes of P-rich Phosphanes and Silylphosphanes. VII Carbonyl Complexes of the Heptaphosphane(3) iPr2(Me3Si)P7 From the reaction of iPr2(Me3Si)P7 1 with one equivalent of Cr(CO)5THF the yellow products iPr2(H)P7[Cr(CO)5] 2 and iPr2(Me3Si)P7[Cr(CO)5] 3 were isolated by column chromatography on silicagel. The P? H group in 2 results from a cleavage of the P? SiMe3 bond during chromatography. The Cr(CO)5 group in 2 is linked to an iPr? Pe atom, in 3 to the Me3Si? Pe atom of the P7 skeleton. The substituents do not force the formation of a single isomer; nevertheless 3 ist considerably favoured as compared to 2 . From the reaction of 1 with 2 equivalents of Cr(CO)5THF the yellow iPr2(H)P7[Cr(CO)5]2 4 was isolated bearing one Cr(CO)5 group at an iPr? Pe atom, the other one at a Pb atom of the P7 skeleton. Compound 3 yields with Cr(CO)4NBD the red iPr2(Me3Si)P7[Cr(CO)5][Cr(CO)4] 5 . Three isomers of 5 appear. Two Pe atoms of 5 are bridged by the Cr(CO)4 group, the third Pe atom is linked to the Cr(CO)5 ligand. iPr2(H)P7[Fe(CO)4] was isolated from the reaction of 1 with Fe2(CO)9. 31P NMR and MS data are reported. 相似文献
805.
Rita Kakkar 《International journal of quantum chemistry》2003,94(2):93-104
The B3LYP density functional has been used to calculate the reaction paths for the interconversion of the three stable isomers of C3H4, namely, propyne, allene, and cyclopropene, and the results compared with experimental data. It is found that the trans nonplanar isomer of vinylmethylene plays a central role in the interconversions. Other thermochemical parameters, like the entropies of reaction and activation, have also been calculated. The rate constants of the various competing reactions have been calculated using the Rice–Ramsperger–Kassel–Marcus theory. Kinetic isotope effects for the reactions have also been investigated to gain an insight into their mechanisms. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003 相似文献
806.
The separation and determination of nitrophenol isomers by high-performance capillary zone electrophoresis 总被引:1,自引:0,他引:1
o-Nitrophenol, m-nitrophenol, and p-nitrophenol could well be separated by capillary zone electrophoresis (CZE) by only adjusting the run buffer with methanol. Efficiency up to 105 theoretical plates per meter was achieved. The effects of several important factors were investigated to find optimum conditions. The linear range, regression equation, and the recovery were given. This method possessed the advantages of simplicity, rapidity, and good reproducibility; it can be developed for the separation of practical samples in environment analysis. 相似文献
807.
Summary High-performance liquid chromatography was applied for the separation and determination of the Δ2 and Δ3 isomers of 7-aminodeacetoxycephalosporanic acid (7·ADCA) and cephalexin monohydrate in their mixtures. Separation was performed
on a column containing bonded octadecyl silica phase. The effects of pH (in the range of 3.0–7.6) on the investigated compounds
were studied. Two organic modifiers, acetonitrile and methanol, were used to improve and accelerate separation.
The applied procedure was very reliable for quantitative determination. Excellent correlation with the data of microbiological
assay was found. The procedure was very convenient even when other impurities were present in the sample. This is an advantage
with respect to microbiological assays. 相似文献
808.
Mercury chalcogenides HgE (E=O, S, Se, etc.) are described in the literature to possess rather stable bonds with bond dissociation energies between 53 and 30 kcal mol(-1), which is actually difficult to understand in view of the closed-shell electron configuration of the Hg atom in its ground state (...4f(14)5d(10)6s(2)). Based on relativistically corrected many body perturbation theory and coupled-cluster theory [IORAmm/MP4, Feenberg-scaled IORAmm/MP4, IORAmm/CCSD(T)] in connection with IORAmm/B3LYP theory and a [17s14p9d5f]/aug-cc-pVTZ basis set, it is shown that the covalent HgE bond is rather weak (2-7 kcal mol(-1)), the ground state of HgE is a triplet rather than a singlet state, and that the experimental bond dissociation energies have been obtained for dimers (or mixtures of monomers, dimers, and even trimers) Hg2E2 rather than true monomers. The dimers possess association energies of more than 100 kcal mol(-1) due to electrostatic forces between the monomer units. The covalent bond between Hg and E is in so far peculiar as it requires a charge transfer from Hg to E (depending on the electronegativity of E) for the creation of a single bond, which is supported by electrostatic forces. However, a bonding between Hg and E is reduced by strong lone pair-lone pair repulsion to a couple of kcal mol(-1). Since a triplet configuration possesses somewhat lower destabilizing lone pair energies, the triplet state is more stable. In the dimer, there is a Hg-Hg pi bond of bond order 0.66 without any a support. Weak covalent Hg-O interactions are supported by electrostatic bonding. The results for the mercury chalcogenides suggests that all experimental dissociation energies for group-12 chalcogenides have to be revised because of erroneous measurements. 相似文献
809.
Summary Capillary zone electrophoresis has been used for the enantiomeric separation of racemic ortho-fenfluramine and meta-fenfluramine
employing a phosphate buffer at pH 2.5 added with cyclodextrins. The cyclodextrin type and concentration strongly influenced
the chiral resolution. The uncharged β-cyclodextrin polymer gave enantiomeric resolution of both ortho and meta isomers, while
γ-cyclodextrin was a good chiral selector for only ortho-fenfluramine; heptakis-2,3,6-tri-O-methyl-β-cyclodextrin permitted
base line separation of meta-fenfluramine enantiomers but only partial resolution of racemic ortho-fenfluramine. The optimized
electrophoretic method was applied to the quantitative analysis of 1-meta-fenfluramine (minor component in the mixture) and
d-meta-fenfluramine in a commercial pharmaceutical formulation. Good reproducibility for migration time and corrected peak
areas (R.S.D. <0.8% and <1.2%, respectively) was achieved and the presence of the minor component of the mixture was found
to be in accord to previous determinations performed by other analytical methods. 相似文献
810.
Yu. N. Belokoń Yu. N. Bubnov T. D. Churkina A. A. Chesnokov K. A. Kochetkov N. Yu. Kuznetsov V. N. Khrustalev N. I. Raevskii 《Russian Chemical Bulletin》1997,46(5):982-989
The stereochemistry of addition of Br2 toE-andZ-(R)-N-cinnamoyl-4-phenyloxazolidin-2-ones was studied. It was established that both theE-andZ-isomers give only two out of four possible diastereoisomers in the presence of Lewis acids (BPr3 or AlBr3). The absolute configurations of the diastereoisomers [(2S', 3R') and (2R', 3S') of the side chain] were established by X-ray structural analysis. The stereochemistry observed is a consequence of the stepwise
bromination and the absence of bridging bromine atoms along the reaction coordinate. In the case of theZ-isomer, the diastereoselectivity of the reaction was high, whereas it is low in the case of theE-isomer. It was suggested that at the first stage of addition of Br2 at the C=C bond, the attack of the Br+ cation occurs at the α position, and the second stage of transfer of Br− occurs with the participation of boron or aluminum complexes in the intermediate state of the bromination reaction. This
hypothesis as well as the results of calculations of the initial conformations of the substrates provide an explanation of
the regularities observed.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1022–1028, May, 1997. 相似文献