This value of the predicted Raman frequency of infinite carbyne (1900 cm -1 ) is slightly higher than that deduced by Gao et al. from the Meier fit of a series of shorter pyridine-terminated polyynes (ν∞ = 1886 cm -1 ) . The frequency for the [5]rotaxane C68•(Mb)4 happens to equal the asymptotic limit, which illustrates that the Raman frequencies have already converged to that of carbyne. Carbyne chains inside double-walled carbon nanotube have lower Raman frequencies (1770-1850 cm -1 ; refs. ), and in this case the presence of the carbon nanotube shifts the resonance to lower frequency by about 100 cm -1 (ref. ). gives an l∞ = 503 nm and v∞ = 1900 cm -1 .