Calculations of the 13C nuclear screening tensors for coumarin and some methoxycoumarins

INDO calculations are reported of the ¹³C nuclear screening tensors in coumarin and some of its methoxy derivatives. Calculations based upon a linear combination of gauge dependent atomic orbitals and those using an uncoupled Hartree–Fock approach are described. Comparison with the results of less sophisticated calculations shows that changes in the excitation energies, bond orders and the 〈r^?3〉_2p term are together responsible for determining the ¹³C chemical shifts.