1H chemical shifts in NMR.: Part 18.: Ring currents and π-electron effects in hetero-aromatics
1H chemical shifts in NMR.: Part 18.: Ring currents and π-electron effects in hetero-aromatics
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DOI：
10.1039/b201789j
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发表时间：
2002-01-01
期刊：
影响因子：
--
通讯作者：
Reid, M
中科院分区：
文献类型：
--
作者：
Abraham, RJ;Reid, M
本文通过对文献和CDCl_3溶液中的NMR谱的归属，得到了一些杂环化合物和相关化合物的H ~（-1）化学位移。这些化合物包括呋喃、吡咯、噻吩、恶唑、咪唑和噻唑、各种甲基和4，5-二氢衍生物、苯并衍生物苯并呋喃、吲哚和苯并噻吩以及相关化合物乙烯基甲基醚、苯酚、苯甲醚、苯胺、乙烯基甲基硫醚和苯硫酚。并对吡啶、嘧啶、吡嗪、哒嗪、喹啉、异喹啉等六元杂环化合物及其甲基衍生物进行了研究。在这些分子中的1 H化学位移进行了分析的环电流和π电子效应与模型（CHARGE 7 h）的计算的两个键和三键的电子效应。该模型给出了这些化合物中质子化学位移的第一个全面的计算。对于所考虑的数据集（215个质子化学位移），范围为δ = 1.9至9.4 ppm，观测位移与计算位移的均方根误差为0.096 ppm。该模型还允许在程序中计算的单独的相互作用方面的化学位移的解释。这表明环电流和π电子密度对H-1化学位移的影响很大。甲基取代对化学位移有很大的影响，这是由于甲基化合物中的π电子密度增加。发现呋喃、吡咯和噻吩的环电流与苯环电流相等，但氮杂氮的引入使环电流降低约10%。在五元和六元杂环中均为10%。在二氮杂苯中，该效应是累积的。
The H-1 chemical shifts of a number of hetero-aromatics and related compounds were obtained by the assignment of the NMR spectra in CDCl3 solution or from the literature. These included furan, pyrrole, thiophene, oxazole, imidazole and thiazole, various methyl and 4,5-dihydro derivatives, the benzo derivatives benzofuran, indole and benzothiophene plus the related compounds vinyl methyl ether, phenol, anisole, aniline, vinyl methyl sulfide and thiophenol. The six membered hetero-aromatics pyridine, pyrimidine, pyrazine, pyridazine, quinoline and isoquinoline and a number of their methyl derivatives were also investigated. The 1 H chemical shifts in these molecules were analysed in terms of the ring currents and pi-electron effects together with a model (CHARGE7h) for the calculation of the two-bond and three-bond electronic effects. This model gives the first comprehensive calculation of the proton chemical shifts in these compounds. For the data set considered (215 proton chemical shifts) ranging from delta = 1.9 to 9.4 ppm the rms error of observed vs. calculated shifts was 0.096 ppm. The model also allows the interpretation of the chemical shifts in terms of the separate interactions calculated in the programme. This showed the large effects of the ring currents and pi-electron densities on the H-1 chemical shifts. Methyl substitution has a large effect on the chemical shifts which is due to increased pi-electron densities in the methyl compounds. The ring currents in furan, pyrrole and thiophene were found to be equal to the benzene ring current, but the introduction of an aza nitrogen decreased the ring current by ca. 10% in both the five and six-membered heterocyclics. The effect was cumulative in the diazabenzenes.