engleski

Interaction-induced absorption in infrared spectra of diphenylacetylene and diphenyldiacetylene

In infrared spectra of diphenyldiacetylene (DPDA) an unusual phenomenon has been observed going from solid to solution. In the solid state of DPDA in the region of triple bond stretching only one band has been observed at ~ 2150 cm-1 which describes an IR allowed antisymmetrical stretching of the acetylenic bridge. On the contrary, in solution and in melt, an extra IR band appears at ~2220 cm-1. A band at the same position has also been observed in Raman spectra and assigned to the symmetrical stretching of the acetylenic bridge. Likewise, in IR spectra of melted and solvated diphenylacetylene (DPA) a band occurs at ~2219 cm-1 which is a consequence of the analogous normal mode. In order to explain this behaviour, IR spectra of melted DPA and DPDA at different temperatures and IR spectra of DPA and DPDA in various solvents and concentration ranges have been recorded. Using solvents with different characteristics like polarities or donor/acceptor number, it has been noticed that the forbidden band exhibits different behaviour than the allowed bands. First, while in polar solvents the forbidden band decreases, in nonpolar solvents it increases with an increase of concentration. Second, in solutions where concentration of DPA and DPDA has been kept constant but a molar ratio of polar and nonpolar solvent has been changed, the forbidden band showed greater increase with an increase of polarity of mixture than allowed bands. No significant wavenumber shift of the discussed band has been noticed. Since the average value of the square of the internal field is different from zero in seemingly isotropic surroundings, it affects the integrated absorption intensity of the observed IR forbidden band, which is proportional to it. However, the major reason for appearing of the forbidden band is great polarizability change during symmetrical stretching of the acetylenic

IR; interaction-induced transition; polarizability

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