ADIABATIC SEMIEMPIRICAL PARAMETRIC METHOD FOR COMPUTING ELECTRONIC-VIBRATIONAL SPECTRA OF COMPLEX-MOLECULES .3. AZINES

Calculations of the excited state structure and absorption and emissio n spectra of azines (pyridine, pyrimidine, pyrazine and their deutero- substituted derivatives) were performed using the parametric method de scribed previously. It is shown that the fragmentary approach for mole cular model construction is suitable for heteroaromatic compounds with relatively small structural groups of the form [GRAPHICS] and [GRAPHI CS] taken as primary fragments. Compared with H->C=, the ''first appro ximation model'' of [GRAPHICS] has an additional parameter of the a-ty pe that accounts for the peculiarities of the electronic density redis tribution of n-pi transitions and affects primarily the changes in va lence angles on excitation of the molecule. The major peculiarities of the vibrational structure of the experimental electronic spectra are well reproduced in the calculations and the parameters of the adiabati c molecular model obtained correlate well with the data estimated by o ther methods. (C) 1997 Elsevier Science B.V.