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.