α-Substituent effects on 13C NMR chemical shifts in some aliphatic compounds: Application of principal component analysis (PCA)

The principal component analyses (PCA) were applied in investigations of α-substituent effects on 13C NMR chemical shifts of the total of 169 aliphatic compounds represented by 13 organic classes: (a) carbonyl: acetones, acetophenones, camphors and cyclohexanones; (b) carboxyl: acetic acids, methyl-, ethyl- and phenyl-acetates, ethyl thioacetates, and N,N-diethylacetamides; and (c) unsaturated compounds: cyanides, oximes, and propenes. The effects of twelve α-substituents: F, Cl, Br, I, OMe, OEt, SMe, SEt, NMe2, NEt2, Me and Et were considered. In PCA matrix construction the experimentally 13C NMR data for functional carbon and α-carbon atoms, and theoretically (ab initio) obtained data were used, and two type of PCA results were generated and discussed. In the first PCA score analysis, the division in three compound groups (a–c) was obtained principally because of variations in the lowest unoccupied molecular orbital (LUMO) energies and partial charges on functional carbon atoms. The second PCA results showed division of analyzed compounds in groups of substituents according to heteroatom present (O, N, S, C or H) principally due to the differences in partial charges on α-carbon atoms as detected in the first principal component (PC), as well as, in molecular orbital coefficient of α-carbon atoms according to the second PC loading values. The halogen derivatives were treated as outliers in this PCA. Thus, knowing the structure of the Y–CH2–X compound and calculating few theoretical (ab initio) parameters, four in our case, one could be able to predict the functional and α-carbon NMR chemical shifts with high accuracy based on PCA scores considering two principal component models and PC1 versus PC2 plots.