Principal component analysis in studies of substituent‐induced chemical shifts of 1,4‐disubstituted benzenes

Substituent effects on 1H and 13C NMR chemical shifts in α‐monosubstituted ethyl acetates: principal component analysis and 1H chemical shift calculations

“…It is well documented that the chemical shifts are strongly dependent on inductive and mesomeric effects of the substituent, which are influenced by charge alterations. 23,25,26 Therefore, the theoretical parameters obtained for the most stable conformers of ethyl acetates utilizing Gaussain 98 at the 6-31g (d,p) level, such as partial charges on˛-H,˛-C and carbonyl carbon (C-f), hardness and electronegativity, were included as variables in the data matrix. We have utilized the electronegativity parameter, taken as the midpoint between the LUMO and HOMO energies, and the hardness parameter of molecules, representing half of the energy gap between the same orbitals.…”

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

“…3), as one of the starting PCA variables, the great similarities among carboxylic compounds are again obvious (1-4, 6) but there is no evident heteroatom separation. The camphors (11) and cyclohexanones (13) presented different behavior compared to acetones (10) and acetophenones (12), which seems to be more similar to ethyl thioacetates (5) and oximes (9) and these four compound classes are quite close to the uniform group of carboxyl compounds. Together with the most compact (1-4 and 6) carboxyl compounds are situated the propenes (7).…”

“…The interactions between the Y substituent and the X functional group have been studied for years in our laboratory by NMR, IR and rotational isomerism and the differences between observed and expected chemical shifts have been attributed to various substituent effects, all of these were treated by classical regression analysis [7][8][9], and also by PCA in last decade [10][11][12][13][14][15].…”

“…1) compounds which were the object of our investigations. Often, the nonadditivity of their 13 C NMR chemical shifts, manifested by variation in substituent-induced chemical shift (SCS) values, are explained as the consequence of mutual substituent interactions which modify the amount of delocalized charge by the substituent Y [7][8][9].…”