Ab Initio Study of Conformational and Configurational Properties of 1, 3- Diazacyclohepta-1, 2-diene and 1, 3-Diazacycloocta-1, 2-diene

Ab initio calculations at HF/6-31G* level of theory for geometry optimization and MP2/6-31G*//HF/6-31G* for a single point total energy calculation are reported for the importantenergy-minimum conformations and transition-state geometries of 1, 3-diazacyclohepta-1, 2-diene (2) and 1, 3-diazacycloocta-1, 2-diene (3). The C2 symmetric twist-chair (2-TC)conformation of 2 is calculated to be 7.4 kJ mol-1 more stable than the twist-boat (2-TB, C2)conformer. Interconversion of 2-TC and 2-TB conformations takes place by a relatively highenergy(58.2 kJ mol-1) transition state. The unsymmetrical chair (3-C) conformation of 3 iscalculated to be 6.6 kJ mol-1 more stable than the C2 symmetric twist-boat (3-TB) form. Thetransition state linking 3-C and 3-TB conformations is 40 kJ mol-1 above 3-C. Racemization of2-TC and 3-C conformations occures by cis-rotation and requires 77.9 and 83.5 kJ mol-1,respectively.