Abstract
The bimolecular rate constants for tritium exchange between (i) Ph2PH and MeOH*, (ii) Ph2PD and MeOD*, (iii) Ph2PH and Me3CSH*, and (iv) Ph2PD and Me3CSD* in MeCN solution were measured. In systems (i) and (ii) the HT/DT kinetic isotope effect depends on the direction of exchange and varies from 3.0 to 1.7 and from 4.8 to 2.4 at 323 and 250 K, respectively. In systems (iii) and (iv) the kinetic isotope effect is independent of the direction of exchange and changes from 2.2 at 323 K to 2.8 at 250 K. The rate constants for deuterium exchange between Ph2PH and MeOH or Me3CSH were measured in C6H6, C2Cl4, (CH2)4O, C5H5N and MeCN solutions. The reactions involve (i) desolvation of MeOH or Me3CSH (ii) double hydrogen transfer in the four-centre cyclic complex, which is found to be the rate limiting-step. The contribution of tunnelling to hydrogen transfer is not significant.
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