Energy of termination

The activation energy of chain termination depends on the kind of termination reaction involved. With termination by recombination of two polymer free radicals, no mass is transferred, and, so, the activation energy is low, but is not zero since a reversal of spin occurs in free radical recombination. Poly(styrene) free radicals are almost exclusively terminated by recombination, which explains the experimentally found low activation energy. In contrast, poly (vinyl acetate) free radicals are mostly terminated by disproportionation. In this case, mass is transferred and the activation energy of termination is correspondingly high. 

The metal-olefin bond energy data are scarce. Such energies for group 8 transition metals are usually about 20-30% lower than bond energies of terminal carbonyl groups. " The metal-olefin bond energies for some iron, nickel, and rhodium complexes are given in Table 6.12. 

Computation of the energy of terminally blocked amino acids indicates that only limited regions of the (, ) Ramachandran map are allowed conformations. The knowledge of these conformations narrows the choice of starting points. However, the number of combinations of these energy minima also increases with the number of amino acids in the polypeptide chain and the problem becomes rapidly insoluble, even for small peptides. Some additional criteria might be employed to reduce the number of possibiUties. 

The dissociation energy of the terminal C—H bond m propane is exactly the same as that of ethane The resulting free radical is primary (RCH2) m both cases... 

Table 6.3 lists the activation energies for termination (these are overall values, not identified as to mode) of several different radicals. The rate constants for termination at 60°C are also given. We shall see in Sec. 6.6 how these constants are determined. 

Note that the final energy of the products in an IRC calculation may not equal tlie sum of the energies of the isolated molecules. An IRC terminates when the energy reaches a minimum for the molecular complex, a level which is slightly above the sum of the isolated product molecules. 

In practice it is only necessary to do the total energy calculation for the block with some convenient termination at the origin of x and to calculate the excess energy rex(O) for this case. The true excess energy can then be obtained by adding the excess energy of the correct terminating piece. An example should make the procedure clear. 

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