Extended Huckel calculations of carbon chemical shifts with the aid of gauge-invariant atomic orbitals

¹³C shielding constants have been calculated by means of the Extended Hückel Theory using Gauge-Invariant Atomic Orbitals (GIAO) and including a large number of terms arising from the Hamiltonian and orbital expansions. The results indicate that the method can account for the general trends of ¹³C chemical shifts. It is shown that an approximation in which the paramagnetic contribution is calculated, by retaining a determined type of two-centre terms, gives a correct description of the observed effects.