The interpretation of carbon nuclear magnetic resonance shifts

Abstract

¹³ C Chemical shifts, when corrected for the diamagnetic term, become additive for polysubstitution by halogen, or methyl, methoxy-, or phenyl groups; and the branched alkanes, and fused (or linked) arenes, follow the simple rules. The corrected shifts for acyclic (including branched) alkanes and simple cycloalkanes (except cyclopropane) are given by five parameters which are increments for each α-, β-, γ-, and δ-carbon substituent, and a small constant term to include methane. The methylcyclohexane parameters vary with conformation.

The increase in corrected (paramagnetic) shift with substitution is related to the molecular orbital theory. The shift increases as Q_AB, the double bond term, increases.

The corrected shift has a periodic relationship with the atomic number of the ligand X in Me_nX. The corrected shift increases across the period of the ligand X (as its electronegativity increases), but also increases down the group of the ligand (as its electronegativity decreases), except for a small decrease from the first row to the second for the more electronegative ligands.

While increments in corrected shift are additive for saturated ligands, conjugative interaction with multiply bonded substituents reduces the resultant paramagnetic shift.