Crystallographic Study of the 1,5-Diazabicyclo[3.3.0]octadienedione (9,10-Dioxabimane) System. Crystal Structures and Molecular Confromations of 2,8-Dimethyl-1,5-diazabicyclo[3.3.0]octa-2,7-diene-4,6-dione [syn-(CH3,H)-9,10-dioxabimane], 2,6-Dimethyl-1,5-diazabicyclo[3.3.0]octa-2,6-diene-4,8-dione[anti-(CH3,H)-9,10-dioxabimane] and 2,6- Dimethyl-3,5-dioxo-4,11-diazatricyclo[5.3.1.04,11]undeca-1,6-diene-9,9-dicarbonitrile {μ-[C(CN)2]-syn-(CH2,CH3)-9.10-dioxabimane}/