Unusual frequency increases of the CC stretching mode of polyenes and of the Kekulé mode of benzene in S1 states: application of a simple valence bond model

Abstract

An unusually high frequency shift of the totally symmetric CC stretching mode of various polyenes to their lowest excited singlet (S₁) states from the ground (S₀) state has been investigated by a model calculation. Comparison with the high frequency shift of the ν₁₄(Kekulé) mode of benzene to the S₁ state reveals a similarity in the frequency increases of the two types of vibrational mode in the S₁ state. Interaction force constants of the carbon–carbon bond stretchings have been modelled by means of a simple valence bond approach. In both cases the effective vibronic coupling between the S₁ and S₀ states through the modes has been connected with the interaction force constants and transition moments, both estimated by a valence bond method.