Studies on ^<13>C Magnetic Resonance Spectroscopy. XVII. Elucidation of Substituent-Induced Chemical Shifts of Substituted n-Alkane Series by Means of a Novel Substituent Entropy Constant σ_<s°>.

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Substituent effects on the ^<13>C and ^1H nuclear magnetic resonance (NMR) chemical shifts of aliphatic compounds have been studied, A good agreement was found between chemical shifts in the gas phase and those in CCl_4 solution. Regression analyses of ^<13>C and ^1H chemical shifts in CCl_4 solution and in the gas phase were carried out by using σ_i, σ_S°and σ_<S°MA> as descriptors for α-substituted butane and propane derivatives. Linear combinations of σ_i, σ^+_<S°> and/or (σ^+_<S°>)^2,where the superscript"+"indicates an electron-donating substituent, are necessary for all ^<13>C and α-^1H chemical shifts of propane derivatives, but those of the β-and γ-positions are correlated with σ_i, Except for the β-position signals, a term expressed as σ_<S°MA> is needed to take account of the magnetic anisotropic effect of Cl, Br, I and CN groups on the ^<13>C and ^1H chemical shifts.
1984-06-25