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 Sk for the kth position. Using Flygare's equation, we can obtain the molecular diamagnetic shielding term σd as a sum of σd(13CH4) and substituent contributions Dk. The paramagnetic term can therefore be parameterised similarly, and the substituent contributions Pk have been obtained by subtracting Dk(calc) from Sk(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.