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排序方式: 共有1262条查询结果,搜索用时 15 毫秒
941.
A. Paschke Peter Popp Gerrit Schüürmann 《Fresenius' Journal of Analytical Chemistry》1998,360(1):52-57
The critical step in the determination of water solubilitiy (S
w) and octanol-water partition coefficient (K
ow) of hydrophobic organic chemicals by using the generator-column technique and the slow-stirring procedure, respectively,
is the exact quantification of the low water-phase concentrations of the substances under investigation. We have tested the
applicability of solid-phase microextraction (SPME) and gas chromatography with seven chlorinated organic compounds. The substances
cover a S
w range from 500 mg/L to 7 ng/L and a log K
ow range from 3 to 8. The results show that SPME can be a valuable alternative to common preconcentration techniques in the
quantification of hydrophobic organics in pure and octanol-saturated water. The apparent SPME distribution constants K
SPME (obtained with the 100 μm-PDMS fiber for analyte’s partitioning between fiber coating and aqueous sample) do not correlate
directly with octanol/water partition coefficients and thus cannot be recommended as a surrogate parameter for K
ow.
Received: 15 January 1997 / Revised: 2 May 1997 / Accepted: 8 May 1997 相似文献
942.
Concerted as well as stepwise reaction pathways for cyclization of 3-azido-propenal to isoxazole have been investigated by
density functional (B3LYP) and ab initio methods up to CCSD(T)/cc-pVQZ methods. These calculations clearly establish the pathway
with concerted albeit asynchronous nitrogen extrusion and ring closure as the most feasible mechanism. Barriers for cyclization
increase in the order Hartree–Fock<B3LYP<ACPF<CCSD(T). According to the geometrical parameters and the electronic structure
of the TS as evidenced by natural bond order analysis this cyclization can be interpreted as a pseudopericyclic (heteroelectrocyclic)
reaction.
Received: 26 May 2002 / Accepted: 18 June 2002 /
Published online: 14 February 2003 相似文献
943.
Paier J Stockner T Steinreiber A Faber K Fabian WM 《Journal of computer-aided molecular design》2003,17(1):1-11
A 3D QSAR analysis (quantitative structure activity relationships) of a set of 2,2-disubstituted epoxides, substrates for epoxide hydrolases originating from four different organisms, was conducted by CoMFA (comparative molecular field analysis) and CoMSIA (comparative molecular similarity indices analysis), with respect to the enantioselective ring opening to the corresponding vicinal diol. Structural variations of the substrates include alkyl chains of different lengths, unsaturated moieties ((E)- and (Z)-alkenyl, alkinyl, aryl) and electronegative groups (ether oxygens, halogen atoms) at different locations within the 2-substituent group. Generally, all four organisms, namely Rhodococcus ruber NCIMB 11216, Rhodococcus ruber DSM 43338, Rhodococcus ruber DSM 44540 and Rhodococcus ruber DSM 44539, preferentially react with the (S)-enantiomer of the epoxide. Enantioselectivities (enantiomeric ratio, lnE values) show a rather large variation, ranging from almost no (lnE<1) to nearly complete selectivity (lnE>5.3). In addition, the response of the epoxide hydrolases stemming from the four organisms towards structural modifications of the substrate is different. Models for the enantioselectivity (enantiomeric ratio, ln E values) obtained by CoMFA and CoMSIA are of different but reasonable predictive power, e.g., q2
CV=0.701 and r2=0.937 for the CoMFA model of Rhodococcus ruber DSM 43338. Enantiomeric ratios for the test molecules can be well predicted. Plots of steric and electrostatic CoMFA (CoMSIA) fields allow conclusions to be drawn for the choice of the most suitable organism for a specific type of substrate. 相似文献
944.
ESR. and ENDOR. spectra are reported for the radical anion of the 2-methyl derivative (III) of 1,6-methano[10]annulene (I). Comparison of the hyperfine data for I·? and III·? with those for the radical anions of naphthalene (II) and its 1-methyl derivative (IV), respectively, confirms the suggestion that – despite the vastly different α-proton coupling constants – the π-spin distributions should be similar in the two series. This result provides further support for the view that the π-spin distribution, in particular, and the cyclic π-delocalisation, in general, are not seriously perturbed by moderate deviations of the π-perimeter from planarity. 相似文献
945.
Fabian Gerson Joanna Jachimowicz Ichiro Murata Kazuhiro Nakasuji Kagetoshi Yamamoto 《Helvetica chimica acta》1975,58(8):2473-2483
The radical anion and the radical cation of azuleno[1,2,3-cd]phenalene (III) have been investigated by ESR. spectroscopy, along with the radical anion of 2-phenylazulene (IV). Also studied has been the neutral radical obtained by one-electron reduction of cyclohepta[cd]phenalenium-cation (VI⊕). Assignment of the proton coupling constants for the radical ions III. ·?, III ·⊕ and IV·⊕, and the radical VI · is supported by comparison with the ESR. spectra of specifically deuteriated derivatives III-d5 ·?, III-d5 ·⊕, IV-d2 ·? and VI-d1′. The experimental results are in full accord with qualitative topological arguments and predictions of HMO models. Whereas the radical anion III ·? exhibits α-spin distribution similar to that of IV ·?the corresponding radical cation III ·⊕ and the neutral radical VI · are related in this respect to phenalenyl (V·). It is noteworthy that oxidation of III by conc. H2SO4 yields a paramagnetic species (IIIa ·⊕) which has a similar – but not an identical – structure as the radical cation III ·⊕ produced from III with AlCl3 in CH3NO2. 相似文献
946.
Fabian Gerson Xue-Zhi Qin Thomas Bally Jean-Nicolas Aebischer 《Helvetica chimica acta》1988,71(5):1069-1072
The hitherto unknown radical cation of cyclobutene ( 2 ) has been generated in a CFCl3 matrix by γ rays at 77 K. The coupling constants, as determined from the ESR spectrum of 2 +, are 2.80 and 1.11 mT for the four CH2 and the two CH = protons, respectively. Photo-induced ring opening of 2 + yields a radical cation which exhibits the same ESR and ENDOR spectra as those observed upon direct ionization of s-trans-buta-1, 3-diene (s-trans -1 ). The radical cation s-trans -1 +, should, therefore, be the final product of this conversion. 相似文献
947.
Synthesis of 1,3‐Amino Alcohols by Hydroxy‐Directed Aziridination and Aziridine Hydrosilylation 下载免费PDF全文
Dr. Yong‐Qiang Zhang Fabian Bohle Robin Bleith Dr. Gregor Schnakenburg Prof. Dr. Stefan Grimme Prof. Dr. Andreas Gansäuer 《Angewandte Chemie (International ed. in English)》2018,57(41):13528-13532
We describe an approach to N‐tosyl 1,3‐amino alcohols that consists of a diastereoselective aziridination reaction of acyclic allylic alcohols and an unprecedented regioselective hydrosilylation of α‐hydroxy aziridines. The products contain up to three contiguous stereocenters. Computational studies outline key aspects of the aziridination mechanism, which is different and more intricate than anticipated. 相似文献
948.
E. E. Paschke B. A. Bidlingmeyer J. G. Bergmann 《Journal of polymer science. Part A, Polymer chemistry》1977,15(4):983-989
A new solvent system comprising a mixture of nitrobenzene and tetrachloroethane has been developed for determining the molecular size distribution of poly(ethylene terephthalate) (PET) by gel-permeation chromatography (GPC). Unlike hot m-cresol, which until now has been the standard GPC solvent for PET, the nitrobenzene–tetrachloroethane mixture can be used at room temperature and thus minimizes polymer degradation and distribution equilibration. Results with this new system reveal reproducibly discernible differences between PET samples prepared by melt and solid-state polymerization, differences not previously noted in GPC reports based on the use of m-cresol. 相似文献
949.
950.