Bent polyynes: Ring strain studied by Raman and Infrared Spectroscopies

The structure of sp-carbon chains (polyynes) in solid state has been extensively studied by means of X-ray crystallography. These investigations typically show that a linear polyyne is rarely found and, in some cases, quite dramatic deviations from linearity can be observed. Recently [1] we have shown that polyynes in solution are bent such that inversion symmetry is lost (series of polyynes 1, see Figure 1) and the mutual exclusion principle between IR and Raman spectroscopy is not upheld. In order to asses whether different polyyne curvatures give intensity variations for those bands which violate the mutual exclusion principle, we have synthesized bent polyynes coupling the acetylenic skeleton (in our case 8 carbon atoms in the polyynic chain) with C10, C11, C12 alkyl chains in order to obtain a “ring” system (see Figure 1, systems 3a-c). Different alkyl chain lengths produce different head-tail distances (R) and therefore different curvatures of the polyyne chain. The joint use of Raman and IR spectroscopies in solution allows to reliably probe the bending of the sp chain controlled by the imposed chemical structure. In particular we have proven that both the band positions and the relative intensities in the IR and Raman spectra are curvature sensitive. Furthermore, also the Raman depolarization ratio ρ is curvature sensitive and this fact can be exploited for a purely Raman quantification of chain bending. Density Functional Theory (DFT) calculations have been also carried out on model systems. This allows to nicely interpret and reproduce the experimental observations.