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排序方式: 共有211条查询结果,搜索用时 203 毫秒
91.
Waseeq Ahmad Siddiqui Saeed Ahmad Muhammad Ilyas Tariq Hamid Latif Siddiqui Masood Parvez 《Acta Crystallographica. Section C, Structural Chemistry》2008,64(1):o4-o6
The structures of N‐(2‐chlorophenyl)‐4‐hydroxy‐2‐methyl‐2H‐1,2‐benzothiazine‐3‐carboxamide 1,1‐dioxide and N‐(4‐chlorophenyl)‐4‐hydroxy‐2‐methyl‐2H‐1,2‐benzothiazine‐3‐carboxamide 1,1‐dioxide, both C16H13ClN2O4S, are stabilized by extensive intramolecular hydrogen bonds. The 4‐chloro derivative forms dimeric pairs of molecules lying about inversion centres as a result of intermolecular N—H...O hydrogen bonds, forming 14‐membered rings representing an R22(14) motif; the 2‐chloro derivative is devoid of any such intermolecular hydrogen bonds. The heterocyclic thiazine rings in both structures adopt half‐chair conformations. 相似文献
92.
Ahmad Z Mehmood S Fatima I Malik A Ifzal R Afza N Iqbal L Latif M Nizami TA 《Magnetic resonance in chemistry : MRC》2008,46(1):94-98
Salsolins A (1) and B (2), the new triterpenes, have been isolated from the chloroform soluble fraction of Salsola baryosma along with 2alpha,3beta,23,24-tetrahydroxyurs-12-en-28-oic acid (3) reported for the first time from this species. Their structures have been assigned from 1H and 13C NMR spectra, DEPT and by 2D COSY, NOESY, HMQC and HMBC experiments. The compounds 1-3 showed significant antioxidant activity. 相似文献
93.
Magda A. Barsy Eman A. El Rady Fawi M. Abd El Latif 《Journal of heterocyclic chemistry》2008,45(3):773-778
94.
Methyl-monomethoxy conduritol-B and methyl-dimethoxy conduritol-B were synthesized starting from 2-methylbenzo-1,4-quinone. Bromination of 2-methylbenzo-1,4-quinone was followed by the reduction of the carbonyl groups with NaBH4 to give a dioldibromo compound. Methyl-dimethoxy conduritol-B was synthesized from the reaction of the dioldibromo compound with CH3ONa, followed by acetylation with Ac2O-pyridine to obtain methyl-dimethoxy diacetate. On the other hand, acetylation of the methyl-dioldibromo compound followed by reaction with LiOH gave a monoepoxide compound stereoselectively. The reaction of the epoxide with H+/Ac2O afforded the monobromo triacetate. Controlled reaction of monobromo-triacetate with CH3ONa in MeOH furnished the desired new methyl-monomethoxy conduritol-B. The structures of all synthesized compounds were characterized by spectroscopic methods. 相似文献
95.
Curve fitting is an interesting and important subject in mathematics and engineering. It has been studied extensively and a number of approaches, mostly based on polynomials and piecewise polynomials, have been employed. In the usual setting, some data points are given and one wants to find a polynomial function with the minimum violations measured by a norm in the given data points. In these approaches, norms are applied to aggregate all violations as a scalar. 相似文献
96.
Aya O. EI-Badarany Shaimaa M. Helal Mohamed S. AbdelLatif 《Journal of Nonlinear Modeling and Analysis》2020,2(3):375-391
In this paper, the invariant subspace method is utilized to obtain
some new exact solutions for the time fractional thin-film equation. The frac-
tional derivative in the considered equation is given in Remain-Liouville and
Caputo senses. Some new invariant subspaces have been obtained that are not
reported in the literature before. 相似文献
97.
Five flavonoids, namely (-)-epicatechin-3-O-β-glucopyranoside (1), 5-hydroxy-3-(4-hydroxylphenyl)pyrano[3,2-g]chromene-4(8H)-one (2), 6-(p-hydroxybenzyl)taxifolin-7-O-β-D-glucoside (tricuspid) (3), quercetin-3-O-α-glucopyranosyl-(1 → 2)-β-D-glucopyranoside (4) and (-)-epicatechin(2-(3,4-dihydroxyphenyl)-3,4-dihydro-2H-chromene-3,5,7-triol (5), were isolated from the leaves of mango (Mangifera indica L.). Antifungal activity of these compounds was evaluated against five fungal species, namely Alternaria alternata (Fr.) Keissler, Aspergillus fumigatus Fresenius, Aspergillus niger van Tieghem, Macrophomina phaseolina (Tassi) Goid. and Penicillium citrii. Six concentrations, namely 100, 300, 500, 700, 900 and 1000 ppm of each of the five flavonoids were employed by means of the poisoned medium technique. All concentrations of the five test flavonoids significantly suppressed fungal growth. However, the specificity of different test compounds was evident against different fungal species. In general, antifungal activity of the flavonoids was gradually increased by increasing their concentrations. The highest concentration (of 1000 ppm) of compounds 1-5 reduced the growth of different target fungal species by 63-97%, 56-96%, 76-99%, 76-98% and 82-96%, respectively. 相似文献
98.
Matloob Ahmad Hamid Latif Siddiqui Saeed Ahmad Graham John Tizzard Masood Parvez 《Journal of chemical crystallography》2010,40(12):1188-1194
Abstract
The crystal structures of four derivatives of pyrazolo[4,3-c][1,2]benzothiazine-2,4-dihydro-3,4-dimethyl-,5,5-dioxide containing benzothiazine dioxide and pyrazolo fused rings that have been synthesized were determined. (C11 H11 N3 O2 S): Mr = 249.29, monoclinic, P21/c, a = 8.3070(3), b = 13.6331(5), c = 10.1661(3) ?, β = 106.924(2)°, V = 1101.45(7) ?3, Z = 4. (C12 H13 N3 O2 S): Mr = 263.31, monoclinic, P21/n, a = 9.729(3), b = 11.224(4), c = 11.436(3) ?, β = 98.85(2)°, V = 1233.9(7) ?3, Z = 4. (C14 H15 N3 O4 S): Mr = 321.35, monoclinic, P21/c, a = 9.2534(3), b = 19.3920(7), c = 7.9489(2) ?, β = 95.323(2)°, V = 1420.21(8) ?3, Z = 4. (C14 H15 N3 O3 S): Mr = 305.35, monoclinic, P21 /c, a = 13.816(7), b = 7.464(3), c = 14.674(8) ?, β = 109.05(3)°, V = 1430.3(12) ?3, Z = 4. The heterocyclic thiazine rings adopt half-chair conformations, and the mean-planes of the phenyl and pyrazolo rings lie between 10.43(11) and 15.93(16)° with respect to each other. In each case, the geometry about thiazine N-atom is trigonal pyramidal and only the first compound containing a donor N-atom shows classical hydrogen bonds. However, non-classical H-bonding of the type C–H···O is observed in all structures. 相似文献99.
The cis- and trans-azobenzenes are known as photochromic isomers with the trans- converting into the cis-form and vice versa upon irradiation with specific wavelengths. We have quantum chemically investigated the cis- and trans-forms of substituted azobenzene diradicals, with two nitronyl nitroxides, imino nitoxides, or verdazyls at para positions and serving as monoradical centers, to determine whether they can exhibit a photoassisted magnetic crossover. Geometries of both substituted and unsubstituted molecules have been optimized by density functional (DF) method UB3LYP using the 6-311G(d,p) basis set. Optimization of the geometry of the cis isomers has required special care. Single point singlet, triplet, and broken symmetry calculations have been done using 6-311++G(3df, 3pd) basis set. The magnetic exchange coupling constants have been estimated from the broken symmetry calculations. Absorption wavelengths have been estimated for both substituted and unsubstituted species from time-dependent DF treatment using restricted spin-polarized methodology RB3LYP and 6-311++G(3df, 3pd) basis set. From the similarity in the calculated absorption wavelengths for the unsubstituted and substituted azobenzenes, and the increased oscillator strengths (f) for the substituted species, we predict that the diradical isomers would be strongly photochromic. From our triplet state and broken symmetry calculations, we predict that both the cis- and the trans-diradicals are antiferromagnetically coupled. This prediction is consistent with the spin alternation rule, and the possibility of a magnetic crossover is nonexistent for these species. 相似文献
100.
S.S. Chong H. Ahmad M.Z. Zulkifli A.A. Latif W.Y. Chong S.W. Harun 《Optics Communications》2012,285(6):1326-1330
A Dual-Wavelength Semiconductor Optical Amplifier (DW-SOA) based fiber ring laser with synchronous wavelength tunability is proposed and experimentally demonstrated. The SOA gain medium strongly suppresses mode competition, thus allowing stable dual-wavelength laser oscillation. The wavelength spacing of the two lasers can be tuned synchronously using a modified hybrid-tuning package incorporating a pair of Fiber Bragg Gratings (FBGs). The DW-SOA demonstrates a laser output with a wavelength spacing of between 0.10 and 8.30 nm (wavelength shift inequality of 0.08 to 0.75 nm). The relationship between the applied strain and wavelength shift of the two tuning modes is also analyzed. 相似文献