Hydrogen Tunneling in Stoichiometric and Catalytic Reactions involving Transition Metals

Abstract

Hydrogen tunneling is a type of quantum tunneling in which a hydrogen atom passes through a potential barrier without reaching the transition state governed by classical mechanics. The participation of hydrogen tunneling in a chemical reaction (stoichiometric or catalytic) can result in the formation of products that without this phenomenon would be impossible to achieve or would be formed in a very slow way. This concept paper aims to review some of the most representative examples of transition‐metal mediated chemical reactions involving hydrogen tunneling. The experimental tools to determine the possibility of the participation of quantum tunneling in a chemical reaction are presented. In addition, the theoretical methods that have been developed to calculate the effect of quantum tunneling on chemical reactions are discussed. Finally, from a personal perspective, the steps to be taken in order to predict and implement this phenomenon are proposed.

H-Tunneling Exhibiting Unexpectedly Small Primary Kinetic Isotope Effects

“…A dedicated set of experiments targeting the OH-rotamerization QMT of 4a to 4s was performed using light 14 N2 vs heavy 15 N2 isotopic matrices. 55 A slight increase in the half-life time of 4a appears to occur when changing 14 N2 to 15 N2 matrix: 41.4  1.1 min vs 42.5  0.5 min (Fig. S14 and Table S11).…”

The Conundrum of “Pair Sites” in Langmuir–Hinshelwood Reaction Kinetics in Heterogeneous Catalysis

“…Beyond the examples discussed in the preceding sections, we also note that compelling experimental evidence against the necessity of “pair sites” assumed in the earlier heterogeneous catalysis literature can be gleaned from other literature studies reporting specific mechanisms that enable nonadjacent sites to conduct bimolecular reactions . For example, remote, isolated sites have been shown to interact and communicate with each other via long-range transfer of electrons , or tunneling of hydrogen species such as protons, neutral atoms, and hydride ions during acid–base reactions and proton-coupled electron transfer reactions on solid catalysts. − Other mechanisms known to operate in heterogeneous catalysis that enable a long-range interaction between remote catalytic sites, enabled by the interaction of reactants or products with the solid catalyst, include shuttling of protons and hydroxyl ions via surface PO x sites in phosphate-containing materials − and shuttling of protons assisted by water adducts within zeolite pores. , Water-assisted proton shuttling has also been reported for nonzeolite materials, including supported Pd catalysts, mesoporous silica, , etc., suggesting the widespread relevance of such long-distance interaction mechanisms in heterogeneous catalysis. In summary, the examples highlighted herein and the possible underlying catalytic mechanisms that enable nonadjacent sites to function provide strong motivation to be wary of idealized assumptions behind oft-invoked models when faced with contradictory experimental data.…”

“… 86 For example, remote, isolated sites have been shown to interact and communicate with each other via long-range transfer of electrons 87 , 88 or tunneling of hydrogen species such as protons, neutral atoms, and hydride ions during acid–base reactions and proton-coupled electron transfer reactions on solid catalysts. 89 − 94 Other mechanisms known to operate in heterogeneous catalysis that enable a long-range interaction between remote catalytic sites, enabled by the interaction of reactants or products with the solid catalyst, include shuttling of protons and hydroxyl ions via surface PO x sites in phosphate-containing materials 95 − 98 and shuttling of protons assisted by water adducts within zeolite pores. 99 , 100 Water-assisted proton shuttling has also been reported for nonzeolite materials, including supported Pd catalysts, 101 mesoporous silica, 102 , 103 etc., suggesting the widespread relevance of such long-distance interaction mechanisms in heterogeneous catalysis.…”

H-Tunneling Exhibiting Unexpectedly Small Primary Kinetic Isotope Effects

“…1 Hence, QMT manifestations are important in fields like enzymatic and conventional catalytic C−H bond activation. [11][12][13][14][15][16] Primary kinetic isotope effects (KIEs, e.g., kH/kD) are commonly measured to probe QMT in chemical reactions. The semi-classical interpretation of primary H/D KIEs sets an upper limit of 7-10 for one-step hydrogen transfer reactions occurring at room temperature (corresponding to the maximum zero-point vibrational energy (ZPVE) difference).…”