Perfect and Defective ¹³C-Furan-Derived Nanothreads from Modest-Pressure Synthesis Analyzed by ¹³C NMR

The molecular structure of nanothreads produced by the slow compression of ¹³C₄-furan was studied by advanced solid-state NMR. Spectral editing showed that >95% of carbon atoms were bonded to one hydrogen (C-H) and that there were 2-4% CH₂, 0.6% C═O, and <0.3% CH₃ groups. Alkenes accounted for 18% of the CH moieties, while trapped, unreacted furan made up 7%. Two-dimensional (2D) ¹³C-¹³C and ¹H-¹³C NMR identified 12% of all carbon in asymmetric O-CH═CH-CH-CH- and 24% in symmetric O-CH-CH═CH-CH- rings. While the former represented defects or chain ends, some of the latter appeared to form repeating thread segments. Around 10% of carbon atoms were found in highly ordered, fully saturated nanothread segments. Unusually slow ¹³C spin-exchange with sites outside the perfect thread segments documented a length of at least 14 bonds; the small width of the perfect-thread signals also implied a fairly long, regular structure. Carbons in the perfect threads underwent relatively slow spin-lattice relaxation, indicating slow spin exchange with other threads and smaller amplitude motions. Through partial inversion recovery, the signals of the perfect threads were observed and analyzed selectively. Previously considered syn-threads with four different C-H bond orientations were ruled out by centerband-only detection of exchange NMR, which was, on the contrary, consistent with anti-threads. The observed ¹³C chemical shifts were matched well by quantum-chemical calculations for anti-threads but not for more complex structures like syn/anti-threads. These observations represent the first direct determination of the atomic-level structure of fully saturated nanothreads.