ADIABATIC SEMIEMPIRICAL PARAMETRIC METHOD FOR COMPUTING ELECTRONIC-VIBRATIONAL SPECTRA OF COMPLEX-MOLECULES .2. ACENES

Calculations of the excited state structure and absorption and emissio n spectra of acenes (benzene, naphthalene, anthracene and tetracene) h ave been performed using the devised parametric method. A relatively s mall structural group H->C= is used as the primary fragment for acenes , as well as for polyenes and diphenylpolyenes. Within the framework o f the given method, the simplest ''first approximation model'', which uses only two common parameters for all the groups (delta H-rs(tau)/de lta q(i)(b) = 0.08 a.u. and delta(2)H(rs)(tau)/(delta q(i)(b))(2) = 0. 3 a.u.), maintains the accuracy of the calculation to approximately 20 cm(-1) for the excited state frequencies and 0.1 for the relative int ensities of the vibrational fundamentals. The major peculiarities of t he electronic spectra and the fine effects (such as mirror symmetry br eakdown) are reproduced in the calculations. Thus quantitative compute r simulation of the molecular structural properties, sophisticated tre atment of the spectral data and fitting of the model parameters are ma de possible by the use of this semi-empirical parametric approach, tog ether with the special data bank of molecular parameters. (C) 1997 Els evier Science B.V.