AM1 study of conformational and configurational energy surfaces of 1,3,5,7-tetraazacyclonona-1,2,5,6-tetraene, 1,3,6,8-tetraazacyclodeca-1,2,6,7-tetrene and 1,3,5,7-tetraazacyclodeca-1,2,5,6-tetraene

An investigation employing the AM1 semiempirical SCF-MO method to calculate structure optimization, conformational and configurational interconversion pathways for the two diastereoisomeric forms of 1,3,5,7-tetraazacyclonona- 1,2,5,6-tetraene (2), 1,3,6,8-terraazacyclodeca-1,2,6,7-tetraene (3) and 1, 3,5,7-tetraazacyclodeca-1,2,5,6-tetraene (4) was undertaken. In 2 the (+/-) -isomer is more stable than the meso-isomer by 15.9 kJ mol(-1). Configurati onal isomerization of (+/-)-2 and meso-2 takes place by cis-rotation about a carbodimide (N=C=N) moiety. The calculated energy barrier for this proces s is 57.6 kJ mol(-1). In compound 3, the (+/-)-diastereoisomer is more stab le than the meso-3 isomer by 8 kJ mol(-1). The ground state conformation of (+/-)-3 is the crown (D-2) conformation. Configurational interconversion b etween the (+/-)- and meso-diastereoisomers of 3 requires 50 kJ mol(-1). Th e meso-diastereoisomer of 4 is more stable than the (+/-)-isomer by 8.1 kJ mol(-1). Configurational isomerization of the two diastereoisomers of 4 can take place by cis-rotation and requires about 55 kJ mol(-1). (C) 1999 Else vier Science B.V. All rights reserved.