Deuterium Isotope Effects on ¹³C NMR Chemical Shifts Reflect the Smaller Steric Size of CD₃ Compared to CH₃ Groups

Abstract

A CD(3) group close in space to (but many bonds distant from) a carbon atom A causes a substituent effect on the chemical shift of C(A) that is algebraically smaller than the effect of a CH(3) group, in agreement with the notion of shorter C-D relative to C-H bonds. Hence, the deuterium isotope effect of CD(3) upon delta(C(A)) is shielding when the substituent effect is deshielding, and vice versa.