Franck−Condon Simulation of the S₁ → S₀ Spectrum of Phenol

In this paper, a Franck−Condon simulation of the S 1 → S 0 transition of phenol is given including all normal modes. The geometries of phenol in its S 0 and S 1 states are obtained from CASSCF calculations. The calculated scaled harmonic frequencies are in good agreement with the experimental values. To calculate the Franck−Condon factors, Duschinsky rotations between the S 0 and S 1 states are taken into account. A very strong Duschinsky rotation is observed between modes 1 and 18a and modes 9b, 14, and the OH-bending vibration. To get good agreement between experimental and theoretical intensities, the calculated geometry of the S 1 state is fitted by reducing the C−O bond length and elongating the molecule along mode 6a. Thus, the most significant changes in geometry in the S 1 state can be deduced from the experimentally observed intensity pattern. The program developed to calculate the Franck−Condon factors is described and tested for the well-known spectrum of the benzene molecule. It is shown how the use of a hash table reduces storage space which is necessary for a multidimensional Franck−Condon analysis of large molecules.