Time scale hypothesis

Other evidence for the model of molecular stretching comes from the qualitative relationship between onset and molecular time scale [7,8]. The solution viscosity, temperature, and molecular weight all change onset in the way predicted by the time scale hypothesis. The magnitude of the change is not exactly as predicted, but the experiments did not attempt to control the variables previously listed. Concentrations were much larger than the individual molecule limit which also means that the time scale would depend upon concentration. Since the direction of the change is correct, the time scale relationship to onset must be correct also. 

It has been suggested that a sensitive test of the diffusion model would be found in the evolution of the eh yield (Schwarz, 1969). Early measurements by Hunt and Thomas (1967) and by Thomas and Bensasson (1967) revealed -6% decay within the first 10 ns and 15% decay in 50 ns. The diffusion theory of Schwarz predicts a very substantial decay ( 30%) in the first nanoseconds for instantaneous energy deposition. Schwarz (1969) tried to mitigate the situation by first integrating over pulse duration (-4.2 ns) and then over the detector response time (-1.2 ns). This improved the agreement between theory and experiment somewhat, but a hypothesis of no decay in this time scale would also agree with experiment. Thus, it was decided that a crucial test of the diffusion theory would... 

Quantum yields for the formation of 141 from 138 in TFE-MeCN were estimated by transient absorption actinometry (Table l).62 The data refer to solvated carbocations (141) since ion pairs (140) are too short-lived for detection on the ns time scale. The modest to poor yields of 141 could be due to predominant ion-pair recombination (140 -> 142), or to parallel protonation (139 — 140) and insertion (139 — 142). Picosecond LFP studies on photoheterolyses of A CH-X in MeCN revealed that the ratio of collapse to escape (k /ki) for [Ar2CH+ X-] is slightly affected by p-substituents (H, Me, OMe) and by X (Cl, Br).66 In contrast, 4>M1 was found to increase by a factor of 17 as p-H (138d) was replaced with p-OMe (138a).62 Hence the ion-pair hypothesis seems difficult to reconcile with the effect of p-substituents on unless the strong nucleophile RO in 140 behaves differently from the weakly nucleophilic halide ions. 

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