The relaxed force constant (RFC), which is the inverse of the diagonal elements of the compliance matrix, can be used as a measure of bond strength, as has recurrently been suggested in ordinary research-based literature provided that the bonds make a part of the complete basis set in valence internal coordinates. Herein, the RFC values for heterocyclic systems such as azaborine and diazaborine are reported including benzene as well as borazine. The use of RFC as a bond strength parameter has been reinvestigated. The nature of chemical bonding in these prototype systems are analyzed in depth by quantum theory of atoms in molecules (QTAIM). The bond strengths calculated by QTAIM are in accordance with the calculated RFC values. This study not only reveals the mixed ionic and covalent bonding in heterocycles but also advocates the applicability of RFC as a bond strength parameter for a variety of systems.
