Polyynes are an intriguing class of molecules consisting of chains of sp-hybridized carbon atoms with formally alternating triple and single carbon–carbon bonds. Such a structure satisfies the valency of every carbon without any substituents, and thus polyynes are effectively a one-dimensional conjugated system. They are, therefore, of interest as molecular wires and as optically active materials. Theoretical analysis suggests that as the length of the molecule increases, the alternation in bond length should decrease with the structure becoming more cumulene-like, that is, with repeated double bonds.

Now, Rik Tykwinski and co-workers from the University of Alberta, Canada have provided experimental evidence of the nature of bond length alternation in polyynes1. They prepared a series of polyynes with between two and twelve alkyne units. The stability of these compounds is problematic, and it was necessary to prepare them with tertiary butyl end-caps — bulky groups that improve stability without exerting a significant effect on the structure.

The team studied the structure of these compounds by using X-ray crystallography, and found that the difference in length between the triple bonds and single bonds does indeed decrease as the length of the molecule increases. Furthermore, this trend in changing bond lengths suggests that all the bonds will never be equal — even in an infinitely long polyyne.