Investigation of pure inductive effects on benzene ring by 13C NMR chemical shifts: A theoretical study using fictitious nuclear charges of hydrogen atoms (‘H ∗ method’)

Abstract

Using fictitious hydrogen atoms (H ∗) bearing a non-integer nuclear charge Z ∗ enables us to study directly and continuously inductive effects at a given magnitude. We then investigate pure inductive effects on benzene carbon-13 chemical shifts, freely from real substituents. They exhibit a linear dependence on the nuclear charge Z ∗, with a well-known reverse effect for ortho position: this implies two opposite mechanisms, namely the ‘classic’ inductive effect and π-polarization. A comparison with 4-H ∗-substituted bicyclo-[2.2.2]-octanes highlights the predominance of the latter one for the para-substitution. The additivity of inductive effects on chemical shifts is also directly proved by using poly-H ∗-substituted benzenes.