Perfect and Defective13C-Furan-Derived Nanothreads from Modest-Pressure Synthesis Analyzed by13C NMR

Abstract

The molecular structure of nanothreads produced by the slow compression of13C4-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% CH3groups. Alkenes accounted for 18% of the CH moieties, while trapped, unreacted furan made up 7%. Two-dimensional (2D)13C-13C and1H-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 slow13C 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 consideredsyn-threads with four different C—H bond orientations were ruled out by centerband-only detection of exchange NMR, which was, on the contrary, consistent withanti-threads. The observed13C chemical shifts were matched well by quantum-chemical calculations foranti-threads but not for more complex structures likesyn/anti-threads. These observations represent the first direct determination of the atomic-level structure of fully saturated nanothreads.