STUDY OF THE RELATIONSHIP BETWEEN DECOMPOSITION ENERGIES OF VARIOUS HETEROCYCLES DERIVED BY USING THE PM3 METHOD

Experimental and circumstantial data were employed to assess the relia bility of PM3-calculated energies of heterocycle decompositions which result in a biradical and a molecule with a closed shell. Calculated d ecomposition energies (E(D)) reproduced the relationship between exper imental E(D) for oxazole, imidazole and thiazole although the calculat ed E(D) values were lower than the experimental ones. In the case of t he heterocycles (HCs) containing carbonyl groups (maleimide, oxazinone , etc.) the variations in E(D) values calculated for distinct HCs and different decomposition pathways of the same HC correlated in many cas es with variations in the stabilities of the biradicals formed during the HC decompositions. The differences in relative stabilities of the resulting molecules with a dosed shell, the variations in the capacity for decomposition between five- and six-membered HCs, and the differe nces in the heats of formation for isomeric HCs were also found to aff ect the E(D) values for HCs containing carbonyl groups. E(D) values fo r six-membered HCs containing only nitrogen atoms as heteroatoms were higher than those for other HCs considered but lower than that for ben zene. A decrease in E(D) with an increase in the number of nitrogen at oms in such HCs seemed to be caused by the known capacity of nitrogen atoms to draw the electron density off an aromatic ring thereby weaken ing the bond strengths of the ring.