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

Abstract

<p><sup>13</sup>C-enrichment of furan by custom synthesis followed by modest-pressure synthesis of <sup>13</sup>C-enriched nanothreads enabled a detailed characterization of the reaction products by a full complement of advanced solid-state NMR techniques, with validation by <i>ab initio</i> calculation of chemical shifts. The <sup>13</sup>C 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% CH<sub>2</sub>, 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 <sup>13</sup>C-<sup>13</sup>C and <sup>1</sup>H-<sup>13</sup>C NMR. The most intriguing component observed was fully saturated perfect <i>anti</i> 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 <i>syn/anti</i> threads, the large number of magnetically inequivalent ones (<i>i.e.,</i> distinct C-H orientations) in CODEX NMR was incompatible with the high-symmetry <i>syn</i> threads. <i>Anti</i> 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 <i>anti</i> 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 <sup>13</sup>C and <sup>1</sup>H spin diffusion analyses) which likely reside within an overall hexagonal thread packing along with other, less-perfect or less-saturated brethren. The relatively slow <i>T</i><sub>1C</sub> 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 <sup>13</sup>C NMR. </p>