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.