Vibrational and NMR properties of polyynes

The bond-stretching phonon modes of linear polyynes with hydrogen atom termination at the both ends are calculated as a function of chain length within the density functional theory. The frequency of one of two particular Raman active phonon modes monotonically decreases with the increase of polyyne chain length while that of the other one shows an oscillating behavior, consistent with previous Raman measurements. The relative Raman intensity of the two phonon modes are evaluated by optimized geometries for ground states and excited states. We also present a nuclear magnetic resonance (NMR) calculation for spin–spin coupling constants as a function of distance between hydrogen and carbon-13 nuclei and, within carbon-13 nuclei, up to the polyyne center of symmetry. We compare the calculated results with recent NMR experiments.