Abstract
Through-space and through-bonds interactions between two nonbonding orbitals are shown to be first-order and second-order perturbations of nonbonding orbitals by the framework sigma orbitals. Through-space interactions for diamines and triplet diradicals are always destabilizing, but through-bonds interactions are stabilizing owing to their coupling with through-space effect. Anomalies in level ordering and magnitudes of energy splitting can be rationalized with the present approach. Orbtial interactions predicted with the PMO method were found to be consistent with those obtained by STO-3G calculations and photoelectron spectroscopy.