Thermal Valence Isomerization of SPs

Variation of the structure of the heterocyclic nucleus and benzopyran moieties of the SP molecule has no substantial effect on the free energy of activation of the SP. 

No correlation exists between the CSpi,o-0(l)bond length in SP14-SP17 and the free energy of activation. 

The lack of this correlation is apparently due to the fact that the Cspiro-0(1) bond length in the ground transition state (the region of thermochromic cleavage)40 

---