Identification of pseudodiatomic behavior in polyatomic bond dissociation: Reaction force analysis.

An interesting uniformity that has been observed for diatomic molecular dissociation has been demonstrated to apply to many single bonds in polyatomic molecules as well. The energy to reach a key point in the bond-breaking process, at which it changes from simply stretching to transition to products, is for most cases a nearly constant fraction of the dissociation energy. The point at which this change occurs corresponds to the minimum of the reaction force F(R) for the dissociation, F(R) being the negative gradient of the potential energy along the reaction coordinate. Thirty nine single bonds were analyzed at the B3PW91/6-31++G(3d,2p) level. Both adiabatic and vertical stretching were considered; those bonds for which these give essentially the same results are labeled "pseudodiatomic."