Chapter 4: From Tunnelling Control to Controlling Tunnelling

Quantum mechanical tunnelling provides an important avenue for chemical reactions unable to make an over-the-barrier passage from reactants to products. Tunnelling control of these chemical reactions means that it is neither the barrier height (part of kinetic control) nor the Gibbs energy gain (part of thermodynamic control) but basically the width of the reaction barrier that determines which way a chemical transformation, which could follow at least two distinct pathways, will proceed. Quantum mechanical tunnelling is also important for understanding observed (ro)vibrational spectra of a large number of molecular systems exhibiting the effect of “tunnelling splitting”. For both reaction kinetics and spectroscopy, quantum mechanical tunnelling can be influenced by internal and external means. Internal control of tunnelling means control either via excitations of internal motions (rotations and vibrations) or by isotopic substitutions. In particular, asymmetric isotopic substitution results in an interesting molecular phenomenon called tunnelling switching. External (coherent) control of tunnelling can be achieved by shaped laser pulses. Molecular examples are provided and discussed for each tunnelling case and control scenario.