Principal component analysis of carbon-13 substituent-induced chemical shifts of some unsaturated compounds

Principal component analysis (PCA) is applied to 32 disubstituted unsaturated compounds (Y–CH₂–X): cyanides, oximes and propenes; bearing 12 -substituents: F, Cl, Br, I, OMe, OEt, SMe, SEt, NMe₂, NEt₂, Me, and Et. The experimental ¹³C chemical shifts for the -carbon and functional carbon atoms are correlated with theoretically derived molecular properties, i.e. partial charges, electronegativity, hardness, dipole moments and the nuclear repulsion energies. In the first PCA, the clustering of these three classes of organic compounds occurred mostly because of the chemical shifts and partial charges, and also of the dipole moments, hardness and electronegativity parameters as confirmed by loading graph. A strong grouping is observed in the second PCA, showing the chemical shift dependence on the type of heteroatom substituents. Therefore, sulfur, nitrogen, oxygen and neutral groups exhibit four types of C-13 SCS influences, indicating that the heteroatom (Y) properties play a significant role on the effects on chemical shifts. The -halogenated compounds represent a very heterogeneous group due to possible orbital interactions between the functional group and the substituent. The third PCA shows the grouping of F, Cl, Br and I derivatives, confirming the second PCA results that same halogen presents the same or very similar effects on the chemical shifts.