cis- and trans, 1,3,5,7-tetraazadecalin (TAD) (2 and 4) and several N- Me (7-9) and N-acyl (10-12) substituted derivatives in the cis and trans series were prepared and characterized. The X-ray structures of 4 and 12 were analyzed and the conformational equilibria of 2 and 11 were evaluated by VT- NMR. A QM (ab initio) and MM computational study was carried out, to provide relative stabilities and geometrical parameters, which were compared with experiment. The MM2-AE (modified for the anomeric effect in N-C-N containing molecules) and MM3 force fields are useful tools for these systems. The results were interpreted in the light of stereoelectronic effects in such fused 1,3-diazane systems.