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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Predictions of the Fluorine NMR Chemical Shifts of Perfluorinated Carboxylic Acids,CnF2n+1COOH (n ) 6-8)
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Predictions of the Fluorine NMR Chemical Shifts of Perfluorinated Carboxylic Acids,CnF2n+1COOH (n ) 6-8)

机译：全氟化羧酸，CnF2n + 1COOH（n）6-8的氟NMR化学位移的预测

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摘要

Perfluorinated carboxylic acids (PFCAs) are a class of persistent environmental pollutants. Commercially available PFCAs are mixtures of linear and branched isomers, possibly with impurities. Different isomers have different physical and chemical properties and toxicities. However, little is known about the properties and the finer details of the structures of the individual branched isomers. Full geometry optimizations for the linear n-alkane (C6-C27) PFCAs indicated that all have helical structures. The helical angle increases slightly with increasing chain length, from 16.3° in C6F13COOH to 17.0° in C27F55COOH. This study predicts 19F NMR parameters for 69 linear and branched isomers of the perfluoro carboxylic acids C6F13COOH, C7F15COOH, and C8F17COOH. B3LYP-GIAO/6-31++G(d,p)//B3LYP/6-31G(d,p) was used for the NMR calculations with analysis of the chemical shifts by the natural bond orbital method. The predictions of the 19F chemical shifts revealed the differences among the CF3, CF2, and CF groups. In general, the absolute values for the chemical shifts for the CF3 group are smaller than 90 ppm, for the CF larger than 160 ppm, and for the CF2 between 110 and 130 ppm. The chemical shifts of the branched isomers are smaller in magnitude than the linear ones. The decrease is correlated with the steric hindrance of the CF3 groups, the more hindered the CF3, the greater the decrease in the 19F chemical shifts. The predicted 19F chemical shifts are similar to those for analogous perfluoro compounds with other terminal functional groups such as -SO3H or -SO3NH2CH2CH3.
机译：全氟羧酸（PFCA）是一类持久性环境污染物。市售的PFCA是线性和支链异构体的混合物，可能还带有杂质。不同的异构体具有不同的理化性质和毒性。然而，对于各个支链异构体的性质和结构的更详细的了解甚少。线性正构烷烃（C6-C27）PFCA的完整几何优化表明，它们均具有螺旋结构。螺旋角随链长的增加而略有增加，从C6F13COOH中的16.3°到C27F55COOH中的17.0°。这项研究预测了全氟羧酸C6F13COOH，C7F15COOH和C8F17COOH的69个直链和支链异构体的19F NMR参数。使用B3LYP-GIAO / 6-31 ++ G（d，p）// B3LYP / 6-31G（d，p）进行NMR计算，并通过自然键轨道方法分析化学位移。对19F化学位移的预测揭示了CF3，CF2和CF组之间的差异。通常，CF 3基团的化学位移的绝对值小于90 ppm，CF大于160 ppm，CF2的绝对值介于110和130 ppm之间。支链异构体的化学位移的量级小于线性异构体。减少与CF3基团的空间位阻相关，CF3越受阻，19F化学位移的减少越大。预测的19F化学位移类似于具有其他末端官能团（例如-SO3H或-SO3NH2CH2CH3）的类似全氟化合物的化学位移。

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