Deuterium isotope effects on C-13 NMR chemical shifts reflect the smaller steric size of CD3 compared to CH3 groups

[GRAPHICS] A CD3 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 algebraic ally smaller than the effect of a CH3 group, in agreement with the notion o f shorter C-D relative to C-H bonds. Hence, the deuterium isotope effect of CD3 upon delta (C-A) is shielding when the substituent effect is deshieldi ng, and vice versa.