SEMIEMPIRICAL PARAMETRIC METHOD IN THE THEORY OF VIBRONIC SPECTRA OF POLYATOMIC-MOLECULES

A semiempirical parametric method for calculating the vibrational stru cture of the electronic spectra of polyatomic molecules is developed; the method is based on the adiabatic molecular model and uses a single parametric system for all excited states. Within the model approach, simplified analytical expressions for potential surface variation duri ng molecular excitation are derived; the expressions include the princ ipal terms according to the order of magnitude. The first and second d erivatives of Coulomb and resonance one-electron integrals with respec t to natural coordinates in a basis set of hybrid atomic orbitals are used as parameters. It is shown that the parameters possess distinct l ocality, are transferable in molecular series and may be easily ranked according to absolute values; describing a molecular model requires f ew most significant parameters. Excitation-induced variations of poten tial surfaces and absorption spectra of some molecules (butadiene, hex atriene, octatetraene) are calculated using only two parameters, which are the same for all molecules. The results of calculations are in go od agreement with the experimental data.