AMI STUDY OF THE EQUILIBRIUM GEOMETRIES AND RACEMIZATION MECHANISMS IN OPEN-CHAIN CARBODIIMIDES - CONFORMATIONAL AND CONFIGURATIONAL ENERGYSURFACE OF 1,3,4,6-TETRAAZACYCLONONA-1,2,4,5-TETRAENE

An investigation employing the AM1 semiempirical self-consistent field molecular orbital method to calculate structure optimization, conform ational and configurational interconversion pathways for open-chain ca rbodiimides la-h has been undertaken. These calculations show that the stereoisomerization rakes place by trans-rotation. The calculated str uctural parameters and racemization energy barriers are in fairly good agreement with experimental results. For 1,3,3,6-tetraazacyclonona-1, 2,4,5-tetraene (2), the meso-diastereoisomer is calculated to be 17.9 kJ mol(-1) more stable than the (+/-)-isomer. The boat conformation of meso-2 is 8.3 kJ mol(-1) more stable than the chair form. The ground- state conformation of (+/-)-2 is the axial symmetrical twist-boat conf ormation, which is 17.9 kJ mol(-1) less stable than the boat conformat ion of meso-2. Configurational interconversion between the meso- and ( +/-)-diastereoisomers of 2 requirs about 57.9 kJ mol(-1). (C) 1998 Els evier Science B.V.