Bent polyynes: Ring geometry studied by Raman and IR spectroscopies

IR and Raman spectroscopy have been used to examine the vibrational characteristics of a series of three macrocyclic tetraynes in comparison to an acyclic analog. By changing the length of the alkyl tether of the macrocycles, varying degrees of bending of the tetrayne moiety can be achieved, and the joint use of IR and Raman spectroscopies provides an avenue to probe the impact of bending on the sp-chain. The spectroscopic data show a general trend toward increasing activation of Raman bands in the IR spectra, and vice versa, as bending of the polyyne chain is increased. DFT calculations provide a detailed rationalization of the experimental observations.