Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

132 EXPERIMENTAL METHODS FOR FAST REACTIONStj~ -+ @ --f 0 must be considered as two-step, and it will be associated with tworelaxation times. Protonation reactions are generally very fast3 3, 37, so the relaxationtime for step ($ --t @ will be short; metalation reactions of magnesium are much~lower~~.~~, so the relaxation time associated with step @ + @ will be comparativelylong. On the other hand, the other intermediate, MgOxH", is present inmuch smaller concentration than MgOx', Ox- and HOx. The steady-state approximationcan be applied to its formation rate, and pathway @ + @ -+ 0 maybe regarded as single-step and thus associated with a single relaxation time. However,these two relaxation processes are coupled, and in this scheme give rise to onlytwo relaxation times-a short one for the protonation and a longer one for the metalations.The first of these was too fast to be measured by the technique used (temperature-jump);the expression for the second illustrates how relaxation terms canbe combined.Path