Conformational analysis / Hexahydrotetrazine / Tetrahydrodioxadiazine The condensation product 3a of hydrazine sulfate, formaldehyde, and hydrogen peroxide, for which the tricyclic 7 : 6: 7-ring structure 3a was previously established for the solid state by X-ray analysis, exists as a mixture of conformations 3a and 3b in solution. AM l calculations show the two 7 : 6 : 7-tricyclic conformers 3% and 3b to be of approximately equal energy, and substantially lower in energy than the three isomeric 6 : 6 : 6-perhydroheteroanthracene conformers 2% 2b, and 2c. By contrat, analogous MNDO calculations predicted that the perhydroanthracene analog conformers 2 should be more stable than 3 CAHd2) < AHd3)].
AM 1 calculations of model compounds, containing various subunits of the tricyclic systems, demonstrate that three significant factors are responsible for the relative energies: a) The 1,3-1one pair repulsions in the 6 : 6 : 6 system. b) The ring strain of the central tetra-aza unit in the 7 : 6 : 7 system. c) The advantageous energetic consequences of the anomeric effect in the stable conformation of the 7 : 6 : 7 system.