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Abstract: 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. [Copyright &y& Elsevier]

1H NMR data show that concentration increase shifts the conformational equilibrium of cis-3-N,N-dimethylaminocyclohexanol (1) (cis-3-DACH) from the 1aa conformer, stabilized by an intramolecular hydrogen bond (IAHB), to the 1ee conformer [43% (0.01 M) to 70% (0.40 m), in CCl4], which can form intermolecular hydrogen bonds (IEHB). The percentage of 1ee conformer also increases with the solvent basicity from 36% in C6D12 to 89% in DMSO. The conformational equilibrium of the trans isomer (trans-3-DACH) is also dependent on concentration, since 1ae increases from 77% (0.05 M) to 84% (0.40 M) in CCl4 but not with the solvent polarity. The occurrence of an IAHB in cis-3-DACH was confirmed by the study of a model compound, cis-3-N,N-dimethylamino-1-methoxycyclohexane (2) (cis-3-DAMCH), lacking an OH group and presenting a single conformer 2ee (∼95%). The corresponding trans isomer (trans-3-DAMCH) behaves similarly to trans-3-DACH, since the 2ae conformer occurs as ∼83%, in the studied solvents. The PCMODEL program gave very good coupling constant values for the qualitative analysis of energy changes in the study of concentration and solvent effects, since the energy values obtained for cis and trans isomers of 3-DACH were in good agreement with the theoretically calculated [B3LYP/6–311+g(d,p) level] values. The averaged calculated energy of the IAHB, for conformer 1aa of cis-3-DACH, with optimization of the reference structure and at several levels of theory, is 5.74 kcal mol-1 (1 kcal = 4.184 kJ). Copyright © 2005 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: Principal component analysis is applied to α-monosubstituted phenyl acetates (Y–CH2CO2Ph) where the observed chemical shifts for the α-carbon atom, the carbonyl carbon, and the α-hydrogen atoms are correlated with theoretically derived molecular properties. The effects on 1H and 13C NMR chemical shifts of 11 α-substituents (F, Cl, Br, I, OMe, OEt, SEt, NMe2, NEt2, Me, and Et) were investigated. A strong grouping of the same heteroatom substituents is observed, showing the chemical shift dependence on the type of substituent. [Copyright &y& Elsevier]