In this type of plot (Figure 1), each axis represents a unique reaction coordinate, the corners represent local minima along the potential surface such as reactants, products or intermediates and the energy axis projects vertically out of the page. Changing a single reaction parameter can change the height of one or more of the corners of the plot. These changes are transmitted across the surface such that the position of the transition state (the saddle point) is altered.
Consider a generic example in which the initial transition state along a concerted pathway is represented by a black dot on a red diagonal (Figure 1). Changing the height of the corners can have two effects on the position of the transition state: it can move along the diagonal, reflecting a change in the Gibbs free energy of the reaction (ΔG°), or perpendicular to it, reflecting a change in the energy of competing pathways. Thus, in accordance with the Hammond postulate, the transition state moves along the diagonal towards the corner that is raised in energy (a Hammond effect) and perpendicular to the diagonal towards the corner that is lowered (an anti-Hammond effect). In this example, R is raised in energy and I(2) is lowered in energy. The transition state moves accordingly and the vector sum of both movements gives the real change in its position.