Tunneling heavy particle transfer

In a typical case, the barrier widths in heavy-particle tunneling reactions correspond to transfer distances that are much smaller than that for hydrogen transfer and are not usually realized at van der Waals interreactant spacings in solids. Therefore, chemical conversions associated with heavy-particle tunneling are rare, often occurring in exoergic reactions where d is much smaller than the geometric transfer distance. A few examples of these reactions are cited in Section 9.2. 

When the mass of the tunneling particle is extremely small, it tunnels in the one-dimensional static barrier. With increasing mass, the contribution from the intermolecular vibrations also increases, and this leads to a weaker mass dependence of k, than that predicted by the onedimensional theory. That is why the strong isotope H/D effect is observed along with a weak k m) dependence for heavy transferred particles, as illustrated in fig. 18. It is this circumstance that makes the transfer of heavy reactants (with masses m < 20-30) possible. 

Hydrogen-atom or proton-transfer transfers between heavy atoms often lead to vibrationally adiabatic paths with two maxima. Formally, the resolution of this problem should be made in the fi amework of the canonically unified theory [5], In practice, given the approximate nature of our treatment, it suffices to use TST with AFj equal to the value for the maximum point. For these cases, the tunnelling correction of the particles wifli energies between that of the highest of the two maxima and the minimum between tiiese is calculated for the highest barrier only. 

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