Progression of combination bands in the infrared spectra of polyenes

The infrared spectra of polyenes of different chain length (and conjugation length) have been studied. A progression of weak bands in the frequency range 1900-1700 cm−1 is observed in all molecules studied. The spectra are compared with that of trans-polyacetylene. The observed bands are interpreted as binary combinations of CH out-of-plane modes which correspond to two-phonon states at well-defined k values of the dispersion curves of trans-polyacetylene, considered as a one-dimensional crystal. The physical relevance of these observations is presented and discussed.