CNDO/2 and INDO all-valence-electron calculations on the geometry and properties of some interhalogens
A consistent and non-empirical application has been made of the ‘semi-empirical’ CNDO/2 and INDO theories of Pople et al. to the geometry and bonding of interhalogens. Various alternative configurations have been considered and the ones with the minimum energies are obtained. The theoretical equilibrium geometries are then utilised in predicting molecular dipole moments, orbital energies and ionisation potentials, electronic transition energies, nuclear quadrupole coupling constants, harmonic force constants, etc. No experimental quantities apart from bond lengths have been used in these calculations. The various parameters required are obtained by atomic Hartree–Fock calculations and comparison with ab initio computations. The orbitals used are valence s and p Slater-type AO's with exponents chosen so as to reproduce their SCF values for r.
It is shown that these two methods are more successful in explaining the equilibrium configuration of a given geometric arrangement than in making a proper prediction of relative stabilities of alternative arrangements. Certain trends of behaviour are correctly predicted while the harmonic force constants do not show any trend. In general, the results deteriorate as one goes down the vertical family. The INDO method generally makes numerically better predictions of molecular properties.
Unlike some other theories of the bonding in these molecules, no use has been made of d orbitals. It appears that generally successful predictions can be made for ground-state properties without including them.