HARDNESS PROFILE AND ACTIVATION HARDNESS FOR ROTATIONAL ISOMERIZATION PROCESSES .2. THE MAXIMUM HARDNESS PRINCIPLE

In a recent paper [J. Phys. Chem. 1995, 99, 5325] we presented a model characterizing the activation hardness in terms of the activation energy of rotational isomerization reactions. We present in this paper an extension of that model for obtaining a direct relation between hardness and potential energy profiles. The hardness is related to the potential energy through a two-term equation in which the leading term is a linear dependence on the potential energy, and the second term can be seen as a correction occurring in cases where the critical points for hardness and energy profiles do not match. It is shown that the hardness and energy profiles behave in opposite ways that can be rationalized in terms of two parameters. A qualitative proof of the maximum hardness principle (PMH), within the frame of the model potential employed, is given, and its consistency with the Hammond postulate is discussed in the light of results for different molecules presenting rotational isomerization.