全文获取类型
收费全文 | 94篇 |
免费 | 1篇 |
专业分类
化学 | 67篇 |
晶体学 | 10篇 |
力学 | 1篇 |
数学 | 2篇 |
物理学 | 15篇 |
出版年
2022年 | 1篇 |
2021年 | 5篇 |
2020年 | 1篇 |
2018年 | 1篇 |
2017年 | 2篇 |
2016年 | 5篇 |
2014年 | 2篇 |
2013年 | 8篇 |
2011年 | 1篇 |
2010年 | 2篇 |
2009年 | 2篇 |
2008年 | 2篇 |
2007年 | 5篇 |
2006年 | 4篇 |
2005年 | 1篇 |
2004年 | 3篇 |
2002年 | 1篇 |
2000年 | 2篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1997年 | 1篇 |
1996年 | 3篇 |
1995年 | 5篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1991年 | 2篇 |
1989年 | 2篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1984年 | 3篇 |
1981年 | 2篇 |
1980年 | 5篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有95条查询结果,搜索用时 13 毫秒
81.
The 6-nitro- and 8-nitro- groups in the tetrazolo[1,5-a]pyridine molecule exhibit completely different influences on the tetrazole–azide equilibrium. Introduction of the methyl-, nitro-, azido groups or a bromine atom in positions 5, 6, 7 or 8 of the nitrotetrazolopyridine produce changes in the equilibrium constants. Based on the IR spectra taken in the solid state, the tetrazole structure was assigned for almost all the compounds studied. Only one of them, 2,6-diazido-3-nitropyridine, exists in the diazido-form in the solid. The 1H, 13C, 15N, and 17O NMR spectral parameters (coupling constants, chemical shifts) as well as ab initio molecular orbital calculations were used to describe the tetrazole–azide tautomerism in solutions. The differences in the NMR parameters between the neutral compound (6,8-dinitrotetrazolo[1,5-a]pyridine) and its σ-adducts are also included as data for distinguishing between both molecules. 相似文献
82.
L. Stefaniak M. Witanowski B. Kamieski G. A. Webb 《Magnetic resonance in chemistry : MRC》1980,13(4):274-276
15N NMR studies on some signly labelled sydnones, N-acetylsydonimines and their hydrochlorides, as well as those on some sydnonimine hydrochlorides, show that in each case protonation takes place at the exocyclic moiety, \documentclass{article}\pagestyle{empty}\begin{document}$ - {\rm O}^ \ominus - \mathop {\rm N}\limits^ \ominus {\rm COCH}_3 \,{\rm or}\, - \mathop {{\rm NH}}\limits^ \ominus $\end{document}, respectively. Complete assignments of the nitrogen chemical shifts are possible for the labelled compounds, including the isomeric structures of N-alkyl–N-cyanomethyl–N–nitrosoamines to which, unstable, free sydnonimines are converted. 相似文献
83.
L. Stefaniak John D. Roberts M. Witanowski B. T. Hamdi G. A. Webb 《Magnetic resonance in chemistry : MRC》1984,22(4):209-214
15N NMR chemical shift data are presented for 14 azolopyridines, together with the results of INDO/S-SOS calculations of nitrogen shieldings. Previous 14N NMR results for some of these compounds are reinterpreted. The 14N data and their assignments are shown to be reliable for the indolizine nitrogen atom from arguments based on relative line widths. The pyridine-type nitrogens are more reliably assigned from the 15N spectra combined with the results of the INDO/S-SOS calculations for individual molecules. A combination of 14N and 15N NMR spectra, together with the shielding calculations, provides a basis for unambiguous assignments of all the various nitrogen environments considered. 相似文献
84.
Lech Stefaniak 《Tetrahedron》1977,33(19):2571-2575
14N NMR spectra indicate that the sydnones, their hydrochlorides, the N-acetylsydnonimines, their hydrochlorides and the sydnonimine hydrochlorides studied exist as such in solution. The sydnonimines themselves are involved in a more complicated equilibrium; neutral solutions contain only the corresponding isomeric alkyl-cyanomethyl-nitrosoamine molecules which upon acidification are turned into cyclic cations, representing true sydnonimine structure, protonated at the exocyclic N atom. 相似文献
85.
Protonation and deprotonation of the title compounds, was studied by means of 15N NMR. The shieldings of the ring nitrogen atoms are found to be very sensitive to changes in the amount of protonation. In contrast the 15N shieldings of the cyano and amino groups are found to be relatively insensitive to protonation effects and are unsuitable for estimating the degree of protonation occurring. 相似文献
86.
JPC – Journal of Planar Chromatography – Modern TLC - The main objective of the study was to find a relationship between the structure and lipophilicity of metalloporphyrins with... 相似文献
87.
M. Witanowski L. Stefaniak B. Kamieski G. A. Webb 《Magnetic resonance in chemistry : MRC》1980,14(4):305-308
The applicability of nitrogen chemical shifts to the localization of N-oxide groups in azine ring systems is exemplified by 1,2,4-triazine systems. The average excitation energy approximation in the theory of magnetic shielding gives a good linear correlation with the nitrogen shieldings in azine N-oxides. 相似文献
88.
B. Kamienski W. Schilf J. Sitkowski L. Stefaniak G. A. Webb 《Journal of chemical crystallography》1989,19(6):1003-1008
Carbon and nitrogen NMR results are reported for a collection of furoxans and related compounds, taken on DMSO solutions. The15N chemical shifts and those of neighboring13C nuclei have characteristic values for the structures studied. Relative14N signal widths are used to identify the nitro group resonances. Valence tautomerism at room temperature is reported for one compound. The INADEQUATE technique is used to determine some1J(13C-13C) data. 相似文献
89.
J. Goworek W. Stefaniak 《Colloids and surfaces. A, Physicochemical and engineering aspects》1993,80(2-3):251-256
The total pore volumes and the core size distributions were calculated for the silica gel Si-100 and chemically modified silica gels RP-8 and RP-18 on the basis of the thermal desorption data of various liquids. The results obtained are compared with those from the nitrogen adsorption/desorption method. Good consistency between the parameters obtained by using different methods is observed in the case of unmodified silica gel. The presence of long alkyl chains on the silica surface strongly influences the pore diameter calculated from thermogravimetric data. 相似文献
90.