Theoretical studies of the intramolecular mechanism for the alkoxyphosphazene to alkoxyphosphazane transformation

Semiempirical molecular orbital methods, and non-local density functional methods, have been used to study the minima and transition structures involved in the formation of six-membered ring alkoxycyclophosphazanes. These compounds show a marked conformational preference whereby two methoxy groups are above the ring and one is below (ααβ); the ααα form involving three methoxy groups pointing above the ring has not been observed experimentally. The free-energy calculations predict that the ααα is actually more stable than the ααβ compound, but the transition-structure calculations show that the energy barrier for the formation of the ααα compound is significantly higher than for the ααβ. Calculations involving demethylated compounds indicated that this selectivity arises due to steric effects in the transition structures. Indeed, there is significantly less distortion in the transition structure for the formation of the αβthan for the ααα compound. Density functional calculations, using the Becke 88 exchange and the Lee-Yang-Parr correlation combination of functionals, were in broad agreement with semiempirical PM3 molecular orbital calculations.