Effect on Activation Energies

Fig. 15 Predicted cooperative effects on activation energies (in kcal/mol) at the B3LYP/ 6-31G level for model enediynes ( push and pull denote through-space repulsive (steric) and attractive (H-bonding) interactions of ort/zo-substituents with in-plane 71-orbitals of an adjacent acetylene moeity).

It is clear that there are dramatic substituent effects on the product ratio both from homologation and from deuterium substitution. It is particularly notable that the substituent effects on activation energy are additive, that is, deuteration lowers the ratio by a factor of about 25 in both UP and TP. 

A second type of polar effect on activation energies is revealed in comparisons of the reactions of CF3 and CH3 radicals with non-polar molecules. In general, it is found that Eqhs Eqf3 2—3 kcal mole" for attack on non-polar alkanes or silanes. In terms of the Polanyi relations for CH3 and CF3, the difference in bond strengths D(CF3—H) — D(CH3-H) = 2 kcal mole" would account for about 1 kcal mole" difference in the activation energies. The additional 1—2 kcal mole" lowering in as compared with Fchs for tbe same substrate is... 

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