MOLECULAR-STRUCTURE AND CONFORMATIONAL DYNAMICS OF ,9A,16,18A-TETRAZADIBENZO[A,L]PENTACENE-9,18-DIONE

The X-ray study of 9a,16,18a-tetreazadibenzo[a,l]pentacene-9,18-dione (1), a model compound for a novel class of thermostable polyheteroaryl enes, polyquinoquinazolones, has been carried out. The nonsymmetric fl attened structure of the molecule observed is a result of intermolecul ar interactions. It was established using quantum-chemical calculation s by the semiempirical AM1 method that the annelation of the 1,6-dihyd ropyrimidinone ring by aromatic cycles results in increasing the confo rmational flexibility of the dihydrocycle due to weakening the conjuga tion between the carbonyl group and the remaining x-system of the mole cule. It was shown by X-ray study and quantum-chemical calculations th at protonation of 1 results in a substantial change in the molecular s tructure due to the large contribution of the 1,4-dihydro tautomeric f orm to the structure of the 1,6-dihydropyrimidinone ring. A tendency f or the conformational flexibility of the heterocycle to increase upon protonation was revealed.