Theoretical study of the force fields of the three lowest singlet electronic states of linear polyenes

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Abstract
The potential energy surfaces of all trans hexatriene and octatetraene are investigated within the harmonic approximation in the diabatic and adiabatic representations for the 1A−g, 2A−g, and 1B+u electronic states by an extended Pople–Pariser–Parr (PPP/CI) model. The effect of excitation and of vibronic coupling on the molecular force fields of the three states is examined. While electronic excitation affects only diagonal force constants of local oscillators, vibronic coupling changes drastically the couplings between local oscillators. The calculations reproduce well the observed increase of the frequency of the in-phase ag C=C stretch upon excitation to the 2A−g state.