Perpendicular Effects on Transition States. 2. Energy Profiles and Reaction Surfaces

It is shown how the free energy profiles along and perpendicular to the reaction coordinate determine a two-dimensional energy surface for describing substituent effects in organic reactivity. This affords a generalization of the hyperbolic-paraboloid surfaces, extendible to large energy variations. It also shows that the reaction surface and curve-crossing approaches lead to a similar mathematical description of the problem. The various models are compared with experimental results for free energy of activation of proton-transfer reactions between benzoic acids and methanol and of Cope and Claisen rearrangements.