A noteworthy feature of bond dissociation/formation reactions

The reaction force F(R c ) of a process is the negative derivative of the potential energy V (R c ) of the system with respect to the intrinsic reaction coordinate R c . For the dissociation of a bond, F(R c ) has a negative minimum at a point R c = α, while for the formation of the same bond, F(R c ) has a maximum at R c = α. These extrema divide the respective processes into two phases. For the dissociation, these correspond to an initial stretching of the XY bond (structural effect) followed by separation into the fragments X and Y (electronic effects); for bond formation, the electronic phase is first (attractive interaction and gradual bond formation) and then the structural (relaxation to equilibrium separation). The noteworthy feature is that the key point R c = α always is such that V (α), relative to the minimum of V (R c ), is about 27% of the dissociation energy. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007