Beyond the transition state

How reactants transform into products is the central question of all chemical change – and a complex one. A full analysis of even a simple reaction has to take into account not only all the degrees of freedom of the molecules concerned but also how they are influenced by their environment.

It’s surprising in retrospect that the old approach – traditional transition-state theory – did as well as it did. It sought to collapse this high-dimensional and stochastic process into a one-dimensional problem where the reactants change along a single ‘reaction coordinate’, first energetically uphill to the transition state and then downhill to the products. Those who learnt this view might recall some vague unease at the arbitrariness of both the transition state – why this arrangement of atoms? – and the reaction coordinate, which never seemed to get defined.

The ability to simulate reactions from quantum first principles has somewhat negated that arbitrariness: we can now insert all the ingredients into a numerical model and see what happens. One problem then, however, is how to collapse the high-dimensional model into a comprehensible view of the reaction pathway.