5-RING-CYCLOAMIDINES - DEEPLY COLORED HET EROCYCLES WITH UNUSUAL PROPERTIES .2. MOLECULAR-STRUCTURE AND ELECTRONIC-STRUCTURE

The crystal and molecular structure of two N,N '-aryl-5-amino-imidazol e-4-imides were detected by X-ray analysis. The structural parameters are discussed in conjunction with theoretical data obtained by density functional and ab initio quantum theory. The interpretation of the mo lecular structures derived from the X-ray diffraction study is made mo re difficult by statistic disorder or by enclosure of water in the cry stal. The theory predicts a planar 5-amino-imidazole-4-imide parent st ructure with an energy barrier to H-transfer of about 25 kcal/mol (DFT RB3LYP/6-311+G* and ab initio G2(MP2) calculations). The CC bond of the five-membered ring is exceedingly long. CC-bond length elongation is also reported for oxalbisamidine. The structure of the parent compo und is discussed in terms of polymethinic and antiaromatic substructur es. Weinholds's Natural Bond Orbital (NBO) scheme and Natural Resonanc e Theory (NRT) as well as Schleyer's criterion of the Nucleus Independ ent Chemical Shift (NICS) is consulted to reveal the very nature of th e unique bond system.