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首页> 外文期刊>Chemistry: A European journal >Determination of the Relative Stereochemistry of Flexible Organic Compounds by Ab Initio Methods: Conformational Analysis and Boltzmann-Averaged GIAO ~(13)C NMR Chemical Shifts
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Determination of the Relative Stereochemistry of Flexible Organic Compounds by Ab Initio Methods: Conformational Analysis and Boltzmann-Averaged GIAO ~(13)C NMR Chemical Shifts

机译：从头算方法确定柔性有机化合物的相对立体化学：构象分析和玻尔兹曼平均GIAO〜（13）C NMR化学位移

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Ab initio calculations at the Hartree-Fock level with full-geometry optimization using the 6-31G(d) basis set, and GIAO (gauge including atomic orbitals) ~(13)C NMR chemical shifts, are presented here as a support in the study of the stereochemistry of low-polar organic compounds having an open-chain structure. Four linear stereoisomers, fragments of a natural product previously characterized by experimenhtal ~(13)C NMR spectra, which possesses three stereogenic centers, 11 carbon atoms, and 38 atoms in total, were considered. Conformational searches, by empirical force-field molecular dynamics, pointed out the existence of 8-13 relevant conformers per stereoisomer. Thermochemical calculations at the ab initio level in the harmonic approximation of the vibrational modes, allowed the evaluation, at 298.15 K, of the standard Gibbs free energy of the conformers. The ~(13)C NMR chemical shift of a given carbon atom in each stereoisomer was considered as the average chemical shift value of the same atom in the different conformers. The averages were obtained by the Boltzmann distribution, using the relative standard free energies as weighting factors. Computed parameters related to linear correlation plots of experimental ~(13)C chemical shifts versus the corresponding computed average data allowed us to distinguish among the four stereoisomers.
机译：使用6-31G（d）基集和GIAO（包括原子轨道的量规）〜（13）C NMR化学位移的全几何优化在Hartree-Fock级别进行从头算计算，在本文中提供了支持。研究具有开链结构的低极性有机化合物的立体化学。考虑了四个线性立体异构体，这是天然产物的片段，以前是通过实验〜（13）C NMR光谱表征的，它具有三个立体异构中心，总共11个碳原子和38个原子。通过经验力场分子动力学进行构象搜索，指出每个立体异构体存在8-13个相关构象异构体。在振动模态的谐波近似中，从头算起进行热化学计算，可以在298.15 K处评估标准构型的吉布斯自由能。每个立体异构体中给定碳原子的〜（13）C NMR化学位移被视为同一原子在不同构象异构体中的平均化学位移值。使用相对标准自由能作为加权因子，通过玻耳兹曼分布获得平均值。与实验〜（13）C化学位移线性相关图相对应的计算得出的平均数据相关的计算参数使我们能够区分这四种立体异构体。

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