Minimum energy for tunneling

AH -r -r represents the minimum energy for tunneling to occur as described above and is assumed to be isotope independent. Note that a similar effect on the Arrhenius curves may be obtained by using more complex barrier shapes [10]. 

Note that, at low temperatures, a very small minimum energy for tunneling to occur is found, which refers to the reactive complex. This value could, therefore, be determined in addition to the values of AH and AS of the pre-equilibrium. In other cases, as discussed later, only the sum of AH -i- can be determined. The barrier for the transfer is similar to that found for TTAA. The difference between the barriers for H and D is substantially large, of the order of that found for porphyrin. In addition, a contribution for heavy atom tunneling is observed. 

Our result for RbaCeo differs from a recent determination by scanning tunneling microscopy [26] (A—77 K), possibly because NMR relaxation probes the minimum quasiparticle excitation energy, while tunneling probes the maximum in the quasiparticle density of states, or because of differences between surface and bulk properties. Our NMR relaxation data for Rb3C5o clearly deviate from an Arrhenius law below 8 K. At these tem-... 

The double line represents the minimum energy path (MEP), which is the reaction path assumed by TST. The single line represents the instanton trajectory for proton tunneling and the dashed line the instanton trajectory for deuteron tunneling. The heavier deuteron tunnels closer to the MEP, where the barrier is lower. These distinct instanton paths are the reason for the lowering of the KIE by the promoting vibration that we mentioned earlier. 

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