Thermally averaged constant, unimolecular

However, as pointed out above, the commonly proposed free radical mechanism is not entirely consistent with the observed behavior of H-donor solvents and coal. Further, a thermally promoted C-C or C-0 bond-scission is inconsistent with our observations in the -PrOH work at 335°C. As also mentioned, a major fraction of the coal was converted in this system to a product with a number-average molecular weight of less than 500. If we consider that the rate constant for the unimolecular scission of the central bond in bibenzyl is expressed (5) as... 

It has been demonstrated on several occasions that mai kinetic quantities such as the rate constants of thermal unimolecular reactions or product yields and Stem-Volmer plots in chemical or photochemical activation, depend only slightly on the particular collision model, as long as situations with the same average energy transfimed per collision are considered. Often, particularly good agreement between various models is observed when is chosen as the basis for such comparisons (see below). 

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