Bond Order as a Tool for Molecular Structure and Reactivity

The concept of bond order can be derived exclusively from the properties o.f the density matrix. After definition of a density operator in the basis of a self-consistent-field (SCF) or configuration interaction (CI) wavefunction which is built from molecular · orbitals (MO's), the MO's are expanded in orthogonalized atomic orbitals (OAO's) and the density matrix is constructed over OAO's. The density matrix is partitioned in dia<tomic parts, for which the eigenvalues are determined. The sum of positive eigenvalues weighted with appropriate weighting factors, which are derived with a projection technique, constitutes the bond orders between atomic pairs. In a variety of applications to ground and excited state equilibria and reactions the usefulness of this property is demonstrated. Special attention is focussed on aromatic systems which can be classified with this property.