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

Abstract

C-enrichment of furan by custom synthesis followed by modest-pressure synthesis of 13 C-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 13 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 2 , 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 13 C-13 C and 1 H-13 C 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 13 C and 1 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 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 13 C NMR.

Solid-State Pathway Control via Reaction-Directing Heteroatoms: Ordered Pyridazine Nanothreads through Selective Cycloaddition

“…26−28 In comparison, nanothreads derived from furan 29 and thiophene 30 indicate enhanced structural order due, in large part, to their localized bonding, which limits potential reaction pathways to [4 + 2] cycloaddition between the carbon atoms in the ring, confirmed by advanced NMR studies of furan nanothreads. 31 If such predictable reactivity could be generalized to more complex nanothread systems, it would represent a major advancement for controlled solid-state synthesis and enable the a priori prediction of polymerized nanothread structures from their precursor structures alone.…”

Photochemically-Mediated Polymerization of Molecular Furan and Pyridine: Synthesis of Nanothreads at Reduced Pressures

“…Furan was recently reported to produce anti-nanothreads during slow compression to 15 GPa. 8,21 In situ synchrotron powder XRD revealed a new diffraction peak, from furan-derived nanothreads, begins to form at 10 GPa which aligns well with the rise in a broad PL background in Raman spectroscopy; taken together, this suggests that the reaction begins around 10 GPa. Furan undergoes several phase changes under pressure that are observable by XRD; the last such transition occurs at circa 4 GPa.…”

“…Prior efforts toward minimizing the pressure required for nanothread formation highlight the roles of temperature and reduced precursor aromaticity, 8,[14][15][16][17]19 as the mechanism of nanothread formation is theorized to be governed by cycloadditions. [20][21][22] Nanothreads derived from molecular furan and pyridine exhibit polymerization pressures lower than those seen for benzene, owing to decreased aromaticity. [3][4][5]8 The role of temperature in the formation of nanothreads has been studied, wherein pyridine held at 14-18 GPa and 400-500 K affords an ordered solid exhibiting a hexagonal six-fold diffraction pattern, suggesting successful 1D polymerization.…”

Mechanistic insights into the pressure-induced polymerization of aryl/perfluoroaryl co-crystals

“…benzene), 15,16 along a stack of molecules in the solid state. These polymers, theorized to form through a series of [4 + 2] cycloadditions, 17,18 have been reported from several monomers, including pyridine, 19,20 furan, 21,22 thiophene, 23 aniline, 24 and azobenzene. 25 Notably, the compression of benzene had been performed for decades prior to nanothread discovery, but primarily lead to amorphous carbon as a result of multiple kinetically favorable reaction pathways opening upon quick collapse of the crystal.…”