Deuterium Isotope Effects as a Probe for CC Hyperconjugation

In several aromatic compounds bearing a perdeuterated tert‐butyl the 4Δ‐deuterium isotope effects on their 13C‐chemical shifts are shown to correlate linearly with σ bond orders. The sign of this effect changes when the deuterated tert‐butyl group is replaced by a deuterated methyl group and thus is indicative of CC hyperconjugation. By comparison with substituent‐induced shifts of a tert‐butyl and a methyl group, the Taft σI and σR constants for the deuterated tert‐butyl and methyl group were determined.