共查询到20条相似文献,搜索用时 15 毫秒
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Two isomeric benzenoid hydrocarbons – tribenzo[b,n,pqr]perylene and tribenzo[b,k,pqr]perylene played a crucial role in the formulation of the Clar aromatic sextet theory. The basic assumption of this theory is that tribenzo[b,n,pqr]perylene is more stable than tribenzo[b,k,pqr]perylene because the former has five, whereas the latter only four aromatic sextets. We now approach this stability problem
from a different direction. By means of a recently developed molecular-orbital-based method it is possible to estimate the
energy effects of individual cycles, as well as pairs, triplets, etc. of cycles in polycyclic conjugated molecules. From these energy-effects one can better understand which structural details
are responsible for the thermodynamic stability of the underlying molecule. In particular, it is possible to rationalize (in
a quantitative manner) the causes of differences in the thermodynamic stability of isomers. Our analysis corroborates the
conclusion of Clar theory, but points out a number of hitherto overlooked structure-stability connections. 相似文献
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Two isomeric benzenoid hydrocarbons – tribenzo[b,n,pqr]perylene and tribenzo[b,k,pqr]perylene played a crucial role in the formulation of the Clar aromatic sextet theory. The basic assumption of this theory is that tribenzo[b,n,pqr]perylene is more stable than tribenzo[b,k,pqr]perylene because the former has five, whereas the latter only four aromatic sextets. We now approach this stability problem
from a different direction. By means of a recently developed molecular-orbital-based method it is possible to estimate the
energy effects of individual cycles, as well as pairs, triplets, etc. of cycles in polycyclic conjugated molecules. From these energy-effects one can better understand which structural details
are responsible for the thermodynamic stability of the underlying molecule. In particular, it is possible to rationalize (in
a quantitative manner) the causes of differences in the thermodynamic stability of isomers. Our analysis corroborates the
conclusion of Clar theory, but points out a number of hitherto overlooked structure-stability connections.
Correspondence: Ivan Gutman, Faculty of Science, University of Kragujevac, P.O. Box 60, 34000 Kragujevac, Serbia. 相似文献
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Ab Initio Study of Conformational Properties of ( Z, Z)-, ( E, Z)-, and ( E, E)-Cyclonona-1,5-dienes
Issa Yavari Rahebeh Amiri Mina Haghdadi 《Monatshefte für Chemie / Chemical Monthly》2004,44(5):937-950
Ab initio calculations at the HF/6-31G* level of theory for geometry optimization and MP2/6-31G*//HF/6-31G* for a single point total energy calculation are reported for the important energy-minimum conformations and transition-state geometries of (Z,Z)-, (E,Z)-, and (E,E)-cyclonona-1,5-dienes. The C2 symmetric chair conformation of (Z,Z)-cyclonona-1,5-diene is calculated to be the most stable form; the calculated energy barrier for ring inversion of the chair conformation via the Cs symmetric boat-chair geometry is 58.3kJmol–1. Interconversion between chair and twist-boat-chair (C1) conformations takes place via the twist (C1) as intermediate. The unsymmetrical twist conformation of (E,Z)-cyclonona-1,5-diene is the most stable form. Ring inversion of this conformation takes place via the unsymmetrical chair and boat-chair geometries. The calculated strain energy for this process is 63.5kJmol–1. The interconversion between twist and the boat-chair conformations can take place by swiveling of the trans double bond with respect to the cis double bond and requires 115.6kJmol–1. The most stable conformation of (E,E)-cyclonona-1,5-diene is the C2 symmetric twist-boat conformation of the crossed family, which is 5.3kJmol–1 more stable than the Cs symmetric chair–chair geometry of the parallel family. Interconversion of the crossed and parallel families can take place by swiveling of one of the double bonds and requires 142.0kJmol–1. 相似文献
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Ab Initio Study of Conformational Properties of ( Z, Z)-, ( E, Z)-, and ( E, E)-Cyclonona-1,5-dienes
Issa Yavari Rahebeh Amiri Mina Haghdadi 《Monatshefte für Chemie / Chemical Monthly》2004,135(8):937-950
Summary. Ab initio calculations at the HF/6-31G* level of theory for geometry optimization and MP2/6-31G*//HF/6-31G* for a single point total energy calculation are reported for the important energy-minimum conformations and transition-state geometries of (Z,Z)-, (E,Z)-, and (E,E)-cyclonona-1,5-dienes. The C2 symmetric chair conformation of (Z,Z)-cyclonona-1,5-diene is calculated to be the most stable form; the calculated energy barrier for ring inversion of the chair conformation via the Cs symmetric boat-chair geometry is 58.3kJmol–1. Interconversion between chair and twist-boat-chair (C1) conformations takes place via the twist (C1) as intermediate. The unsymmetrical twist conformation of (E,Z)-cyclonona-1,5-diene is the most stable form. Ring inversion of this conformation takes place via the unsymmetrical chair and boat-chair geometries. The calculated strain energy for this process is 63.5kJmol–1. The interconversion between twist and the boat-chair conformations can take place by swiveling of the trans double bond with respect to the cis double bond and requires 115.6kJmol–1. The most stable conformation of (E,E)-cyclonona-1,5-diene is the C2 symmetric twist-boat conformation of the crossed family, which is 5.3kJmol–1 more stable than the Cs symmetric chair–chair geometry of the parallel family. Interconversion of the crossed and parallel families can take place by swiveling of one of the double bonds and requires 142.0kJmol–1. 相似文献
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Optimal configurations of functionalized fullerenes C
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X
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for n = 2, 4, 6, 8 have been determined by application of three topological stability measures. It has been found that the optimal
configurations with higher n contain optimal configurations with lower values of n. Two stability measures prefer configurations in which functional groups Xs are crowded while the third one points to configurations with isolated pairs of Xs. 相似文献
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The cardenolide glycosides glucocorotoxigenin, scorpioside, and coronillobioside and a new compound which has been called glucocoroglaucigenin have been isolated from the seeds ofCoronilla glauca L. for the first time. The new glycoside has the structure of 3-(-D-glucopyranosyloxy)-14,19-dihydroxy-5-card-20(22)-enolide. This glycoside has also been isolated from the seeds ofC. scorpioides.Khar'kov Pharmaceutical Institute. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 676–679, September–October, 1985. 相似文献
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冻地银莲花中三萜皂苷的HPLC/MSn分析 总被引:1,自引:0,他引:1
药用植物冻地银莲花(Anemone rupestris ssp. gelida)的全草中含有丰富的三萜皂苷, 经HPLC-MSn分析, 发现其总皂苷中共含有10余个三萜皂苷成分. 除由总离子流色谱图给出各皂苷成分的相对含量外, 从每个色谱峰的正负离子ESI-MS获得相应皂苷的分子量, 进而由多级质谱给出糖链连接的信息, 结合文献报道, 将11个主要三萜皂苷成分分别鉴定为革叶常春藤皂苷F (1), 牡丹草皂苷D (2), 刺楸皂苷B (3), 革叶常春藤皂苷E (4), 红毛七皂苷D (5), 常春藤皂苷B(6), 刺五加皂苷C3 (7), 牡丹草皂苷B (8), 刺楸皂苷A (9), 木通皂苷D (10)和齐墩果酸-3-鼠李糖基-(12)-[葡萄糖基-(14)]-阿拉伯糖苷(11). 其中微量成分2, 5, 7, 10和11为首次从冻地银莲花中分离鉴定. 最后通过与标准品的HPLC保留时间对照证实了他们的存在. 相似文献
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D. T. Asilbekova S. D. Gusakova A. I. Glushenkova A. R. Azizkhodzhaev E. M. Érkkenova M. Sakhibaeva 《Chemistry of Natural Compounds》1994,29(5):574-577
A lower content of chlorophylls, phospholipids, and linolenic acid and a higher content of rutin, carotenoids, and xanthophylls in the biomass ofRuta graveolens L. obtainedin vitro has been found as compared with these indices from the epigeal part of the plant grownin vivo.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Republic of Uzbekistan, Tashkent. Institute of Botany, Academy of Sciences of the Republic of Uzbekistan, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 651–655, September–October, 1993. 相似文献
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R. U. Umarova V. A. Maslennikova N. K. Abubakirov 《Chemistry of Natural Compounds》1978,13(6):692-693
Summary It has been shown that the unsaponifiable fraction of the oil ofErysimum cuspidatum contains -sitosterol and campesterol, and the oil ofErysimum diffusum contains -sitosterol.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 6, pp. 821–823, November–December, 1977. 相似文献
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