Quantum chemical studies of methadone.

Quantum chemical calculations were performed on the flexible methadone molecule to test the hypothesis that it structurally mimics the fused ring structure of morphine. In these calculations using the semiempirical, PCILO method, protonated and nonprotonated conformations were considered representative of different types of intramolecular interaction at the morphine receptor. Calculated energies for these conformations were compared to those calculated for protonated and nonprotonated extended chain and crystal structure conformers. Lowest energy conformations showed intramolecular bonding but the resultant molecular geometries were not very morphine-like. A comparison of the structure of methadone to that of meperidine seemed equally as valid.