HARDNESS PROFILE AND ACTIVATION HARDNESS FOR ROTATIONAL ISOMERIZATIONPROCESSES .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 ene rgy of rotational isomerization reactions. We present in this paper an extension of that model for obtaining a direct relation between hardn ess and potential energy profiles. The hardness is related to the pote ntial energy through a two-term equation in which the leading term is a linear dependence on the potential energy, and the second term can b e seen as a correction occurring in cases where the critical points fo r hardness and energy profiles do not match. It is shown that the hard ness and energy profiles behave in opposite ways that can be rationali zed in terms of two parameters. A qualitative proof of the maximum har dness principle (PMH), within the frame of the model potential employe d, is given, and its consistency with the Hammond postulate is discuss ed in the light of results for different molecules presenting rotation al isomerization.