QUANTUM-MECHANICAL STUDY OF THE ELECTRONIC-SPECTRA AND STRUCTURE OF SOME 1,2,5-X-DIAZOLE (X=N,O,S) COMPOUNDS

systematic molecular orbital study using the SCF-Pariser-Parr-Pople me thod with;limited configuration interaction (PPP-Cl) between nine sing ly excited states has been conducted on a series of compounds having a 1,2,5-X-diazole ring (where X = O,N,S) condensed to a benzene, naphth alene or anthracene ring. Linderberg's relationship is used to estimat e core-resonance integrals. Attempt has been made to understand the ro le of the diazole ring, particularly the heteroatom X, in deciding the postion and intensity of the electronic transitions. pi-Electronic ch arge densities, bond orders and dipole moments in the electronic groun d and first excited states and ionisation potential and electron affin ities of the molecules in the ground state have been calculated. Exper imental position and intensity of the absorption bands have been expla ined in terms of pi-conjugation of the aromatic rings with the 1,2,5-X -diazole rings, where atom X forms a part of the extended conjugation system through the donation of a lone-pair of electrons. Minor differe nces in the spectra of compounds where heteroatom X is replaced by oxy gen, nitrogen and sulphur atoms has been explained in terms of the dif ferent ionisation potentials of these atoms.