The interpretation of carbon nuclear magnetic resonance shifts. Part 2. The alkane (empirical) shielding parameters, and an absolute scale for carbon shielding

Abstract

Grant has shown that the nuclear magnetic shielding of ¹³C in the alkanes can be predicted as a sum of σ(¹³CH₄) and substituent parameters S_k for the kth position. Using Flygare's equation, we can obtain the molecular diamagnetic shielding term σ_d as a sum of σ_d(¹³CH₄) and substituent contributions D_k. The paramagnetic term can therefore be parameterised similarly, and the substituent contributions P_k have been obtained by subtracting D_k(calc) from S_k(obs). The unusual fall-off with distance of Grant's substituent parameters is interpreted as the difference between the smooth fall-off of the diamagnetic parameters with distance (as r^–1), and the less regular fall-off of the paramagnetic parameters, because of the dependence on the conformation of the γ, δ, etc., substituents. The variation with chain length of the n-alkyl substituent parameters can be interpreted similarly. Absolute shielding terms for ¹³C are derived from Ozier, Crapo, and Ramsey's molecular beam resonance measurements of the spinrotation constant for ¹³CO.