Quantum Mechanical Tunneling Is Essential to Understanding Chemical Reactivity

Tunneling is always present but often hidden underneath thermal reactivity. It plays out at low temperatures and can then significantly affect reactivity and selectivity of a chemical reaction. While common for light atoms, in particular, hydrogen, tunneling also occurs for heavier elements when energy barriers are low and narrow, the latter factor being more important. Tunneling control of chemical reactions constitutes, next to kinetic and thermodynamic control, the third paradigm of chemical reactivity. Quantum mechanical tunneling is a consequence of the wave nature of particles that implies that a particle can penetrate a potential energy barrier even though there is insufficient energy to overcome it. This has serious consequences for chemical reactions, a fact that has, however, not been appreciated fully. Only in the last few years has it become clear how important tunneling is for understanding the rates and selectivities of chemical reactions. This review emphasizes the role that tunneling plays in reactions of exemplary small molecules, where the effects are most readily discerned. Once fully understood, the control and application of tunneling in chemical synthesis, ideally stimulated externally through catalysis or energy transfer, will open fascinating new ways of conducting chemical reactions. Quantum mechanical tunneling is a consequence of the wave nature of particles that implies that a particle can penetrate a potential energy barrier even though there is insufficient energy to overcome it. This has serious consequences for chemical reactions, a fact that has, however, not been appreciated fully. Only in the last few years has it become clear how important tunneling is for understanding the rates and selectivities of chemical reactions. This review emphasizes the role that tunneling plays in reactions of exemplary small molecules, where the effects are most readily discerned. Once fully understood, the control and application of tunneling in chemical synthesis, ideally stimulated externally through catalysis or energy transfer, will open fascinating new ways of conducting chemical reactions. the homolytic cleavage of a particular bond in a ground state molecule into the ground electronic states of the ensuing constituent molecular fragments. a molecular system with two conceivable tunneling products produces one preferentially over the other, depending on isotopic composition. proportional change in the reaction rate of a chemical reaction when one of the atoms in the reactants is replaced by one of its isotopes. description of the potential energies of molecules depending on their structure (i.e., the coordinates of their atoms). a process in which a particle penetrates a finite potential energy barrier even though it possesses insufficient energy to overcome it. This is due to the quantum nature of particles that states that there is a non-zero probability of finding the particle on the other side of the barrier. describes the reaction rates of elementary chemical reactions on the basis of a chemical equilibrium between reactants and transition states.