MNDO calculations applied to structural changes in 1,2,4-oxadiazole substituents

MNDO calculations on several substituted 1,2,4-oxadiazole compounds are discussed. Changes in geometry, atomic charge distribution, rotational barriers and molecular orbital energies are examined. A correlation exists between the thermal stability of the heterocycle and the excess electronic charge accumulated inside the ring as well as its distribution. For the case of phenylated derivatives, the nonplanar conformation is energetically more stable than the corresponding planar one, which might explain the relative decrease in thermal stability of phenylated derivatives. It is observed that as the planar conformation starts being populated, a greater amount of electrons are donated to the 1,2, 4-oxadiazole ring.