全文获取类型
收费全文 | 214篇 |
免费 | 8篇 |
专业分类
化学 | 183篇 |
晶体学 | 2篇 |
力学 | 2篇 |
数学 | 6篇 |
物理学 | 29篇 |
出版年
2021年 | 3篇 |
2020年 | 3篇 |
2019年 | 3篇 |
2016年 | 7篇 |
2015年 | 6篇 |
2014年 | 5篇 |
2013年 | 12篇 |
2012年 | 5篇 |
2011年 | 11篇 |
2010年 | 5篇 |
2009年 | 4篇 |
2008年 | 14篇 |
2007年 | 5篇 |
2006年 | 9篇 |
2005年 | 6篇 |
2004年 | 8篇 |
2003年 | 9篇 |
2002年 | 1篇 |
2001年 | 5篇 |
2000年 | 4篇 |
1999年 | 4篇 |
1998年 | 4篇 |
1996年 | 4篇 |
1995年 | 3篇 |
1994年 | 1篇 |
1993年 | 6篇 |
1992年 | 4篇 |
1991年 | 4篇 |
1989年 | 4篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 11篇 |
1984年 | 5篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1977年 | 5篇 |
1976年 | 1篇 |
1974年 | 4篇 |
1973年 | 4篇 |
1972年 | 1篇 |
1971年 | 3篇 |
1970年 | 1篇 |
1962年 | 1篇 |
1961年 | 3篇 |
1960年 | 3篇 |
排序方式: 共有222条查询结果,搜索用时 15 毫秒
211.
212.
The aim of this study was to investigate the changes in carbohydrate components and the crystalline structure in hemp bast
fibers by adding ethyl acetate to acetic acid/water pulping processes. It was found that ethyl acetate added to acetic acid/water
process had a positive effect on yield, viscosity and carbohydrate components in pulp. It was assumed that the delignification
ratio increased by adding ethyl acetate to aqueous acetic acid pulping. Xylose content in hemp bast fibers was affected more
negatively in the ethyl acetate/acetic acid/water process than in the acetic acid/water one. Crystallinity and crystallite
size were higher in pulp sample obtained by the acetic acid/water process without ethyl acetate. 相似文献
213.
TeodorSilviu Balaban Andreas Eichhfer MichaelJ. Krische Jean‐Marie Lehn 《Helvetica chimica acta》2006,89(2):333-351
The title compounds 6 and 7 were synthesized in good yield (Schemes 1 and 2), and their mode of assembly was studied both in solution, for the tetrakis(decyloxy) derivative 6 , and in the crystal, for the tetramethoxy analogue 7 . The pyrimidin‐2‐amine moieties of 6 and 7 can engage in three different supramolecular interactions: i) metal ligation via one of the pyrimidine N‐atoms, ii) cooperative double H‐bonding via the NH2 group, and iii) π–π‐stacking interactions. In solution, coordination of the central Zn‐atom within the soluble porphyrinatozinc complex 19 leads to significant changes in the NMR and absorption spectra of 6 . In the absence of metal ligation, the next strongest interaction is H‐bonding which can operate in nonpolar or moderately polar solvents. In these cases, however, no stacking interaction or inclusion compounds could be put into evidence in the case of 6 by absorption, fluorescence, or NMR spectroscopies. The π‐stacking interactions were only observed in the crystal of 7 in conjunction with double H‐bonding. Slightly disordered DMSO molecules are also H‐bonded to the NH2 groups of 7 , perturbing the expected packing. The present study illustrates some of the challenges inherent to directing hierarchical assembly processes in the solid state. 相似文献
214.
Irina C. Covaci Petre Ionita Miron T. Caproiu Radu Socoteanu Titus Constantinescu Alexandru T. Balaban 《Central European Journal of Chemistry》2003,1(1):53-64
Starting from N-hydroxyphthalimide (5) and 1,3-dibromopropane (6) we obtained 1,3-bis(phthalimidooxy)propane (7) which led
to 1,3-bis(aminooxy)propane dihydrochloride (8). From its reaction with picryl chloride or 4-cyano-2,6-dinitrochlorobenzene,
the two title compounds (4b, 4a) were obtained.
1H-NMR and 13C-NMR spectra are presented. For comparison with the analogous N-methoxy-2,6-dinitro-4-R-anilines 1a, 1b (R=CN or R=NO2), wer report the hydrophobic characteristics (by RPTLC), electronic spectra for the neutral compounds and their anions, pK
a
values, and the behavior towards oxidizers (DPPH, PbO2, Pb(CH3COO)4, KMnO4 and Ag2O); DPPH converts compounds 1a, 1b and 4a, 4b into betainic structures 2a, 2b respectively. 相似文献
215.
Alexandru T. Balaban Cornelia Uncuţ/a Mihai Elian Filip Chiraleu Antonie Dinculescu 《Tetrahedron》1984,40(13):2547-2553
Pairwise chemical shift nonequivalence of the 2,6-methyl and 3,5-protons in 1H NMR spectra, as well as of the 2,6-methyl, 2,6-ring and 3,5-ring carbons in 13C NMR spectra, was observed for N-alkyl-2,4,6-trimethylpyridinium salts 2. Dynamic NMR spectroscopy demonstrates appreciably higher activation free energies ΔG# for rotation around the N(sp2)-C(sp3 bond than ΔG# for the analogously substituted mesityl derivatives, in agreement with the shorter N-C bond distance than for the C-C bond. 相似文献
216.
Mircea V. Diudea Ovidiu Minailiuc Alexandru T. Balaban 《Journal of computational chemistry》1991,12(5):527-535
New local graph invariants, “regressive vertex degrees” (which are slightly augmented vertex degrees) are introduced on the basis of decreasing contributions of more remote vertexes to the classical vertex degrees. Several such invariants are proposed (BR, ER, SR) where t (either t = 1 or t = 2) is an operator expressing the attenuation with increasing topological distance, according to formula (1) or (2). With the aid of these new local invariants, new topological indices (global graph invariants), Y (namely BY, EY or SY) are introduced and exemplified. Their ability to express the branching and to order alkanes is investigated. An appendix gives some recursive relationships for computing these indices. 相似文献
217.
Tadeusz Balaban 《Communications in Mathematical Physics》1995,167(1):103-154
A class of low temperature lattice classical spin models with a symmetry groupO(N) is considered, including the classical Heisenberg model. In this paper a renormalization group approach in a small field approximation is formulated and studied, with a goal to prove the so-called spin wave picture displaying massless behavior of the models.The work has been partially supported by the NSF Grant DMS-9102639 相似文献
218.
Aletras AH Ding S Balaban RS Wen H 《Journal of magnetic resonance (San Diego, Calif. : 1997)》1999,137(1):247-252
Displacement encoding with stimulated echoes (DENSE) was developed for high-resolution myocardial displacement mapping. Pixel phase is modulated by myocardial displacement and data spatial resolution is limited only by pixel size. 2D displacement vector maps were generated for the systolic action in canines with 0.94 x 1.9 mm nominal in-plane resolution and 2.3 mm/pi displacement encoding. A radial strain of 0.208 was measured across the free left ventricular wall over 105 ms during systole. DENSE displacement maps require small first-order gradient moments for encoding. DENSE magnitude images exhibit black-blood contrast which allows for better myocardial definition and reduced motion-related artifacts. 相似文献
219.
Two new types of real (i.e., noninteger) local vertex invariants (LOVIs), denoted by ci and c′i and called distance-enhanced exponential connectivities, are defined via eqs. (1)–(3) and (1′)–(3′), respectively. Only the case when the exponent z equals 1 in eqs. (3) and (3′) is discussed in detail. Both these LOVIs span the range from 0–1, but their dependence upon topology is fairly different, as seen from Table II, where ci and c′i values for all heptane and octane isomers are displayed. From these LOVIs, by simple summation over all graph vertices two new topological indexes (TIs), denoted by XC and XC′, respectively, are obtained. Their intermolecular ordering of all alkanes with four to nine carbon atoms is discussed. On their basis, correlations with boiling points and critical pressures of alkanes are presented. © 1993 John Wiley & Sons, Inc. 相似文献
220.