Gaining mechanistic insight into chemical reactions is of fundamental importance for the rational design of new synthetic methodology and the reduction of trial-and-error experimentation. Chemical systems can exist in a variety of distinct states, determined by the spatial arrangement of atoms within the system. The energy associated with these states can be represented by a potential energy surface (PES), which illustrates how the energy varies as a function of atomic positions -an example PES is displayed in Figure . The PES includes energy minima, corresponding to stable equilibrium structures, and first-order saddle points, characteristic of transition states (TSs), where energy is maximised in one direction and minimised in all others. Only when reactant species collide with the correct orientation and sufficient energy will a chemical reaction occur.