The geometry of the transition state showed an essentially linear geometry of both central carbon atoms, best represented as formal η 2 coordination of a diarylacetylene to the migrating hydrogen atom. In accordance with Hammond's postulate, the more favourable reactions (substrates A17 and A18) showed an earlier transition state, 25 with the C-H distances reflecting the position of the transition state across all substrates. The corresponding plot showed a well-defined curve in a driving force range of around 25 kcal mol -1 (Figure , circles). 26 The relationship evidences a large, thermodynamically independent contribution to the energy barriers of 12.5 kcal mol - 1 . This is presumably linked to the change in hybridisation and consequent weakening of the C-H bond. In addition, the curvature of the plot, defined by θ, is markedly visible, suggesting a strong coupling of the energy of the transition state to that of the connecting minima. This would stem from the geometric proximity between the TS and the minima and an ensuing strong impact of the same factors on the three stationary points. Finally, the Eeq of 7.7 kcal mol -1 hints at a detectable degree of asymmetry in influence, showing how a hydrogen migration from a variable aryl-substituted sp 2 carbon to an unsubstituted phenyl-substituted sp 2 carbon is not perfectly identical to its reverse reaction. This results in the Emin of the reverse reaction (Emin -Eeq) being only 4.8 kcal mol -1 , showing that the thermodynamic dependence of 1,2-hydride shifts depends less on what substitution the product carbocation has and more on the substitution of the reactant carbocation.