SEMIEMPIRICAL CALCULATION OF FORCE-CONSTANTS OF STRETCHING VIBRATIONS IN EXCITED-STATES OF RETINAL AND ITS RELATED-COMPOUNDS

The excited states of pi -electrons in hexaene, retinal, retinal Schiff-base or protonated retinal Schiff-base are considered by our LCAO-ASMO-SCF-CI methods, under the assumption that the polyene chain of the molecule has all-trans form. And as for each excited state, the total energy of the whole system consisting of pi -electrons, atomic cores and SIGMA -bonds is calculated as a function of the terminal bond length, by fixing the other bond lengths at their equilibrium values. On this basis, the relation between force constant of stretching vibration and pi -electron bond-order in each excited state is semiempirically determined, according to such parametrization for the energy of SIGMA -bonds that can give a good account of the force constants of stretching vibrations used for the normal-coordinate analysis of retinal.