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Linear Relationship between 13C NMR Chemical Shifts and the Bending of sp-Carbon Chains.

Polyynes show a strictly linear relationship between the energy impact and the bending of the polyyne chain. The energy, which is necessary to bend the acetylenic chain, decreases with increasing number of acetylene units. A deviation from linearity in polyynes can be realized in solution by violation of the mutual-exclusion principle between IR and Raman spectra. However, there is still no possibility to measure the extent of the non-linearity in solution. Here, we show that the 13C NMR spectroscopy represents an appropriate tool for this as we found an almost perfect linear relationship between the bending of the alkyne chain and the change of the chemical shift of the outer acetylenic carbon atoms. By using molecular bows in which the alkyne chain can be bent by switching the azobenzene unit, this correlation can be proved experimentally. In the future, this correlation should enable the determination of the extent of the bending and the strain energy in polyynes. Consequently, polyynes could be employed as probes for measuring further molecular forces.