Condensed-phase isomerization through tunnelling gateways

Abstract

We observe that the orientational isomerization of CO on a NaCl(100) surface proceeds by thermally-activated tunneling between 19 and 24K. The rate constants of three isotopomers follow an Arrhenius temperature dependence, exhibiting activation energies below the reaction's predicted barrier height and anomalously small prefactors. In addition, the rates depend strongly on isotope, but non-intuitively on mass. A quantum rate theory of condensedphase tunneling qualitatively explains these observations. Vibrationally excited states, accidentally close in energy but localized on opposite sides of the isomerization barrier, provide tunneling gateways between the isomers in a process that can be many orders-of-magnitude faster than rates predicted by commonly used semi-classical models. This suggests heavy-atom condensed-phase tunneling may be more important than currently assumed.

Manipulating tunnelling gateways in condensed phase isomerization

“…Furthermore, a strong correlation is seen between the prefactor and the activation energy, which has previously been shown to be a fingerprint of the gateway tunnelling mechanism. 8 When the identity of the overlayer is changed from CO to N 2 , we see that the tunnelling rates retain their Arrhenius-like behaviour, but the rates are increased by more than an order of magnitude. From Figure 1b, we see that each activation energy is lowered by the influence of the N 2 overlayer, with an average reduction of 55 cm −1 .…”

“…The details of the experimental procedure have been explained elsewhere 8,11,12 ; only a brief description will be given here. A molecular beam setup is used to dose gas layers on an ultrahigh vacuumcleaved NaCl (1 0 0) surface.…”

“…Recently, we reported experimentally observed tunnelling rates for the rotational isomerization of CO adsorbed to NaCl(1 0 0). 8 This reaction involves a metastable 'O-bound' CO -bound with its O-atom at a Na + surface-site -rotating to the lowest energy 'C-bound' structure, where the C-atom is bound to the Na + . 9,10 The reaction proceeds at low temperature by thermally activated tunnelling through a barrier, and remarkably, the lightest isotopologue did not react the fastest.…”

“…Through comparison with quantum chemical calculations, we inferred a 'tunnelling gateway' mechanism involving transitions between a pair of bound (system) states -one localized predominantly on the reactant and one on the product side of the barrier -that come into resonance by the exchange of energy with a phonon of the environment (bath). 8 These resonances are not a simple function of mass, as they depend on the isotope-specific state energies; as the state energies do not change monotonically with mass, this leads to a non-monotonic KIE. However, the KIE is not a complete test of the gateway mechanism -isotopic substitution does not, for example, alter the underlying potential energy surface (PES), nor does it impact the phonon density of states (DoS) of the surrounding medium, both of which determine the resonances possible in the gateway mechanism.…”

Quantum Tunneling in Peroxide O–O Bond Breaking Reaction

“…Later on, more and more tunneling reactions by carbon have been observed for diverse organic systems in the low-temperature processes. − QMT involving nitrogen atom has been reported for the spin crossover isomerization of transient nitrene intermediates in noble gas matrices. − Theoretical studies reveal that the barriers are cusped through spin crossover, leading to significantly reduced barrier widths to allow heavy-atom tunneling . Recent reports show that quantum tunneling also contributes to the rotation of carbon monoxide adsorbed on condensed surfaces. − Here we report a clear-cut example of concerted heavy-atom tunneling by oxygen and beryllium atoms during the spin crossover ring-opening reaction of singlet beryllium peroxide to form triplet linear dioxide in cryogenic Ne matrix.…”