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')

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 n-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. (c) 2007 Elsevier B.V. All rights reserved.