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13 C NMR substituent effects in aliphatic systems: Principal component analysis and correlations with electronegativities
Author(s) -
Zalewski Romuald I.,
Schneider HansJörg,
Buchheit Ulrich
Publication year - 1992
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1260300110
Subject(s) - substituent , chemistry , electronegativity , principal component analysis , chemical shift , scaling , group (periodic table) , stereochemistry , crystallography , computational chemistry , organic chemistry , statistics , mathematics , geometry
Principal component (PC) or factor analysis was applied to 13 C substituent effects (literature SCS values) in α to δ positions of 47 aliphatic frameworks with 4–13 different substituents X. Two PC terms for each substituent X are sufficient to describe 99.3% of all C‐α shifts within < ± 0.6 ppm, even in the largest data matrix containing 13 × 47 = 611 shifts; shifts in β. γ and δ positions require a third PC term for a similar percentage SCS accuracy. Scaling problems with the different order of magnitudes between α to δ SCS variations are avoided by analysing the corresponding data sets separately; this approach, together with the large data base, leads to a dominating first PC term for C‐α which unambiguously correlates with substituent electronegativities (EN), in contrast to an earlier factor analysis. The same result is obtained by direct comparison of α‐SCS with different EN scales, including for the first time group electronegativities. PCs for β to γ SCS indicate only less consistent trends, which again agree partially with results from direct correlations, but not with EN values. The present PC analysis did not include stereochemical descriptors, but furnishes clear distinctions between clusters of (pseudo‐) equatorially or axially substituted compounds. The PC loadings for C‐α also lead to the unambiguous distinction of primary, secondary, tertiary and quaternary carbon atoms.