Theoretical prediction of the vibrational spectrum of naphthalene in the first excited singlet state

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Abstract
Complete harmonic force fields have been calculated for the ground state (S 0) and the first excited singlet state (S 1) of naphthalene using the multiconfiguration self‐consistent field (MCSCF) approach. Identical calculations were performed for benzene to test the methodology with already available theoretical and empirical force fields. Two different basis sets were applied (STO‐3G and near double‐zeta) and all π‐orbitals included in the active space. The geometries of ground and excited states were separately optimized. Following the ideas of Pulay, the force constants were scaled before calculating frequencies and normal modes. For the ground states the influence of correlation is discussed by comparison with Pulay’s results. Except for special vibrations where correlation effects turn out to be important, the use of Pulay’s scaling factors leads to a satisfactory description of the in‐plane‐vibrations. In the case of benzene the calculated frequency shifts between S 0 and S 1 are in complete qualitative agreement with experimental observations. In the case of naphthalene the new theoretical results suggest several revisions of earlier empirical assignments.