Constraining Carbon Nanothread Structures by Experimental and Calculated Nuclear Magnetic Resonance Spectra

A one-dimensional
(1D) sp3 carbon nanomaterial with
high lateral packing order, known as carbon nanothreads, has recently
been synthesized by slowly compressing and decompressing crystalline
solid benzene at high pressure. The atomic structure of an individual
nanothread has not yet been determined experimentally. We have calculated
the 13C nuclear magnetic resonance (NMR) chemical shifts,
chemical shielding tensors, and anisotropies of several axially ordered
and disordered partially saturated and fully saturated nanothreads
within density functional theory and systematically compared the results
with experimental solid-state NMR data to assist in identifying the
structures of the synthesized nanothreads. In the fully saturated
threads, every carbon atom in each progenitor benzene molecule has
bonded to a neighboring molecule (i.e., 6 bonds per molecule, a so-called
“degree-6” nanothread), while the partially saturated
threads examined retain a single double bond per benzene ring (“degree-4”).
The most-parsimonious theoretical fit to the experimental 1D solid-state
NMR spectrum, constrained by the measured chemical shift anisotropies
and key features of two-dimensional NMR spectra, suggests a certain
combination of degree-4 and degree-6 nanothreads as plausible components
of this 1D sp3 carbon nanomaterial, with intriguing hints
of a [4 + 2] cycloaddition pathway toward nanothread formation from
benzene columns in the progenitor molecular crystal, based on the
presence of nanothreads IV-7, IV-8, and square polymer in the minimal
fit.