Stepwise HHH-transfer

The case of a stepwise triple proton transfer [25] is illustrated in Fig. 6.15. It can take place along different pathways via at least two metastable intermediates. For example, if hydrons 1, 2 and 3 are transferred one after the other the reac- 

The derivation of expressions for the multiple kinetic isotope effects of the triple hydrogen transfer case is analogous to the HH-transfer but more tedious. Therefore, the reader is referrred to refs. [25] and [26]. The main results are included in Table 6.2. As in the case of the HH-transfer, the kinetic isotope effects derived for the stepwise transfers are valid in the presence of turmeling and are independent of the tunneling model used. By contrast, the kinetic isotope effects of the single barrier reaction are affected by tunneling. 

When the propagation is the rate-limiting step (Fig. 6.16(c)) a similar phenomenon occurs. In the HDD reaction there are pathways where H is transferred both in the first and the final rate-limiting steps, e.g. the sequence 000 — 100 — 101 

The rate constants are independent of the fractionation factor of the dissociation. 

---