Deuterium Isotope Effects on Aromatic 13C Chemical Shifts. V. Nonadditivity of Methyl Substituent Effects on One-Bond Isotope Shifts for Methylpyridine N-Oxides

One- and two-bond deuterium isotope effects (1Δ and 2Δ) on 13C chemical shifts for methylpyridines and their N-oxides were investigated. The 1Δ values for methylpyridines agree with the calculated values, which are based on a simple additive rule of the methyl substituent effects. On the other hand, the additive rule was not satisfied in 1Δ for their N-oxides. This is attributed to a steric interaction between the substituent and the N-oxide group.