Abstract In 13C NMR, the chemical shift of the carbons is greatly affected by the stereostructure. Among the atoms of the same group, compared with molecules containingrelativelysmall electronegative atoms (such as Br/I), large electronegative atoms (such as F/Cl) will increase the chemical shift of the α-carbon to which it is connected, and decrease that of its β- and γ-carbon. Most textbooks are too abstract on their explanation,which makes if difficult for students to understand. This paper explained in detail the influence of substituents on the chemical shifts of β- and γ-carbons through the steric electronic effects and steric hindrance effects. Among these effects, the γ-gauche effect is the most widely used that is often applied for the determination of the configuration of stereoisomers. The large electronegative atom helps to increase the ratio of the gauche conformation of the molecules. Through steric repulsion, the electron cloud of the C—H bond at the γ-position moves to the carbon site, resulting in the deshielding of hydrogen and shielding of carbon in the σ-bond, so that the corresponding hydrogen chemical shift in the γ-position C—H bond increases, while the carbon chemical shift decreases. This kind of spatial repulsion is more valuable in ring systems, which therefore can be widely used for the structural analysis of isomers. In addition, this paper also shares the data from literatures and our group to help readers on their teaching and scientific research.
CHEN Ning, XU Jia-Xi. γ-Gauche Effect and Its Application in the Determination of Stereoisomers[J]. Chinese Journal of Chemical Education, 2022, 43(18): 119-124.
