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排序方式: 共有195条查询结果,搜索用时 31 毫秒
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Chromogenic benzo-12-crown-4, benzo-13-crown-4 and benzo-14-crown-4 ethers have been compared with monoaza-12-crown-4 and monoazal-13-crown-4 ethers. Of these compounds, monoaza-13-crown-4 exhibited the best analytical characteristics toward the selective determination of lithium. K(ex)Li/K(ex)Na was 525 for the monoaza-crown-4 compound. 相似文献
3.
The 13C spectra of 5,6,9,10-tetradehydrocyclodeca[1,2,3,4-def]-benzo [7,8]biphenylene, 1, and 5,6,9,10-tetradehydrocyclodeca [1,2,3,4-def]-naphtho [2,3-7,8]biphenylene, 2, are reported as are those of a number of simpler acetylenic hydrocarbons used as spectral references. Most of the shifts can be assigned unambiguously. The acetylenic shift assignments were verified by ortho-proton, sp-carbon (1H(1)-13Csp(3)) decoupling experiments. A simple additive shift correlation is found for the hydrocarbons containing unstrained acetylenic groups. However, significant discrepencies are found for the 13C shifts for the strained hydrocarbons 1, 2, 1,2-bis(phenylethynyl)-benzene, 12, and 2,3-bis(phenylethynyl)-naphthalene, 13. The discrepencies are particularily large for carbons near the triple bonds and are attributed to a combination of strain, rehybridization, and other proximity effects related to the interaction between the ortho-substituted acetylenic carbons. 相似文献
4.
It is demonstrated that a previously developed topological expression for the total π-electron energy of aromatic molecules provides a good qualitative account of localization energies. The logarithm of the ratio of the algebraic structure counts for the ground state and the localized reaction intermediate is the principle energy determining factor. This expression fails for hydrocarbons with unusually small HOMO-LUMO separations. An alternative topological expression that includes a correction for this situation provides an excellent non-empirical explanation for the successful empirical use of Dewar reactivity numbers in correlating exact localization energies. The present analysis provides an explanation for the success of the traditional resonance structure counting technique in predicting relative rates of aromatic substitution. The analysis develop applied only to alternant hydrocarbons. 相似文献
5.
Katritzky AR Xu YJ Vakulenko AV Wilcox AL Bley KR 《The Journal of organic chemistry》2003,68(23):9100-9104
Molecules were prepared with substituted nitrobenzyl groups covalently bonded to N-(4-hydroxy-3-methoxybenzyl)acetamide (2) by ether or carbonate linkages. These compounds decomposed under irradiation at 363 nm. Those with carbonate linkages decomposed at slower rates than those with ether linkages. Molecules with dimethoxy-substituted benzyl groups decomposed more slowly than monomethoxy-substituted benzyl groups due to the electronic characteristics of the benzylic carbon. 相似文献
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H. E. Miller H. Rsler A. Wohlpart H. Wyler M. E. Wilcox H. Frohofer T. J. Mabry Andr S. Dreiding 《Helvetica chimica acta》1968,51(6):1470-1474
During studies on the biogenesis of betalains (I) in cactus fruits (Opuntia sp.). DL -dopa-1-[14C] and -2-[14C] were incorporated into betanin (III) which was obtained radiopure after crystallization. The specific activity remained constant after conversion to betanidin (IV) and to a neobetanidin derivative (IX). Reaction of radiobetanin with proline afforded indicaxanthin (V) carrying more than 90% of the radioactivity. Dopa (VI) is thus an efficient precursor for betalamic acid (VIII) but not for cyclodopa (VII). Decarboxylation of radiobetanidin and radioindicaxanthin showed that the carboxyl group of dopa remained a carboxyl group in the biotransformation to betalamic acid. It is concluded that the aromatic ring of dopa is cleaved and that re-cyclization involving the nitrogen generates the dihydropyridine moiety. Under the same conditions mevalonic acid, aspartic acid and phenylalanine showed low incorporations. Studies with beet seedlings and DL -dopa-1-[14C], -2-[14C] and DL -tyrosine-1-[14C] afforded similar results but with low incorporations. 相似文献
9.
The dependence of Hückel π-electron energies, Eπ, on the basic graph theoretical parameters N (the number of vertices), ν (the number of edges) and ASC (the algebraic structure count) is explored. The form with the ASC enters Eπ is established and an equation for Eπ is developed. It is shown how the early and apparent success of the (resonance) theory rested on the fortunate fact that all Kekulé structures for benzenoid hydrocarbons and acyclic polyenes have the same parity. The significance of ASC in determining chemical stability and reactivity is dicussed briefly. 相似文献
10.
[structure: see text] [structure: see text] [structure: see text] Several Precipiton phosphines were prepared and employed in the Staudinger reaction and in the reduction of secondary ozonides. Both amines and aldehdyes were obtained in good to excellent yields and purities. After use of the phosphine, isomerization and precipitation of the spent phosphorus reagent were induced by exposure to visible light in the presence of erythrosin B, a triplet sensitizer. Products were isolated by simple filtration. The use of the triplet sensitizer has the added advantage of eliminating [2 + 2] cycloaddition reactions between trans-Precipitons. 相似文献