Studies on ¹³C 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°.

Abstract

Substituent effects on the ¹³C 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 CCl₄ solution. Regression analyses of ¹³C and ¹H chemical shifts in CCl₄ 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 ¹³C 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 ¹³C and ¹H chemical shifts.