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

¹³C-enrichment of furan by custom synthesis followed by modest-pressure synthesis of ¹³C-enriched nanothreads enabled a detailed characterization of the reaction products by a full complement of advanced solid-state NMR techniques, with validation by ab initio calculation of chemical shifts. The ¹³C NMR spectrum was complex, with more than a dozen distinct features, but almost all (> 95%) represented CH moieties are as expected in nanothreads, with only 2–4% CH₂, 0.3% C=O, and 0.3% COO groups, according to spectral editing. Different components were quantified by integration of the fully equilibrated direct-polarization spectrum. Symmetric and asymmetric alkene-containing rings as well as trapped furan were identified by ¹³C-¹³C and ¹H-¹³C NMR. The most intriguing component observed was fully saturated perfect anti furan-derived nanothread segments, with two distinct, sharp peaks, accounting for ca. 10% of the material. The bonding patterns in these periodic structures deduced from DQ/SQ NMR was that of a [4+2] cycloaddition product. While the small number of chemically inequivalent carbon sites eliminated low-symmetry syn/anti threads, the large number of magnetically inequivalent ones (i.e., distinct C-H orientations) in CODEX NMR was incompatible with the high-symmetry syn threads. Anti threads with two chemically and eight magnetically inequivalent sites provide the only consistent fit of the experimental data. These conclusions were convincingly corroborated by quantum-chemical simulations, which showed good agreement of isotropic chemical shifts only for the anti threads. This represents the first molecular-level identification of a specific type of nanothread. The typical length of the perfect, fully saturated thread segments was around 14 bonds and they accordingly constitute small clusters (according to ¹³C and ¹H spin diffusion analyses) which likely reside within an overall hexagonal thread packing along with other, less-perfect or less-saturated brethren. The relatively slow T_1C relaxation confirms the nanometer-scale length of the periodic perfect structure, indicates that the perfect threads are particularly rigid, and enables their selective observation in ¹³C NMR.