Carbon-13 nuclear magnetic resonance spectroscopy: VIII—aliphatic hydrocarbon derivatives

Carbon-13 NMR chemical shifts of several series of aliphatic hydrocarbon derivatives–-substituted methanes, ethanes, isopropanes, n-propanes and n-butanes–-were found to have a linear relationship with σ-electron densities (Q^σ) calculated by the method of σ-included ω-HMO. A plot of the ¹³C NMR chemical shift of a given carbon in a substituted propane versus that of the corresponding carbon in a substituted butane showed a good linearity with a slope of unity. The values of the ¹³C chemical shifts of the n-butyl derivatives converged rapidly to a constant value as the distance from the substituent increased. Accordingly, the value for the δ-carbon was found to be constant regardless of the substituent. These results show that the ¹³C NMR chemical shifts of aliphatic hydrocarbon derivatives are mainly dependent on inductive effects. The convergence shown by the experimental results is supported by the calculated results of the Q^σ values of the n-butyl derivatives.