AM1 study of the equilibrium geometries and racemization mechanisms in open-chain carbodiimides. Conformational and configurational energy surface of 1,3,4,6-tetraazacyclonona-1,2,4,5-tetraene

Abstract

An investigation employing the AM1 semiempirical self-consistent field molecular orbital method to calculate structure optimization, conformational and configurational interconversion pathways for open-chain carbodiimides 1a-h has been undertaken. These calculations show that the stereoisomerization takes place by trans-rotation. The calculated structural parameters and racemization energy barriers are in fairly good agreement with experimental results. For 1,3,4,6-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 conformation, which is 17.9 kJmol −1 less stable than the boat conformation of meso- 2. Configurational interconversion between the meso- and (±)-diastereoisomers of 2 requirs about 57.9 kJ mol −1.