Host matrices diffusion coefficients

The high sensitivity and selectivity of the EPR response enables diamagnetic systems to be doped with very low concentrations of paramagnetic ions, the fate of which can be followed during the progress of a reaction. The criteria [347] for the use of such tracer ions are that they should give a distinct EPR spectrum, occupy a single coordination site and have the same valency as, and a similar diffusion coefficient to, the host matrix ion. Kinetic data are usually obtained by comparison with standard materials. 

As an alternative, this chapter describes methods for predicting small-molecule diffusivity that are based on transition-state theory (TST) [82-84] and kinetic Monte Carlo (KMQ [85,86]. These methods capitalize on the proposed penetrant jump mechanism. TST was described in Chapter 1 and is typically used to estimate the rates of chemical reactions from first principles here we use TST to calculate the rate of characteristic jumps for each penetrant in a host polymer matrix. The collection of jump rates can be combined with the penetrant jump topology and KMC to obtain the penetrant diffusion coefficient. Other results obtainable from these simulations are physical aspects related to the jump mechanism the sizes and shapes of voids accessible to penetrant molecules [87], enthalpic and entropic contributions to the penetrant jump rate [88,89], the extent and characteristics of chain motions that accompany each jump [90], and the shape and structure of the jump network itself [91]. 

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