The Chemical Structure of Carbon Nanothreads Analyzed by

May 29, 2018 - ... selective, and two-dimensional solid-state nuclear magnetic resonance (NMR) experiments complemented by infrared (IR) spectroscopy...
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Article Cite This: J. Am. Chem. Soc. 2018, 140, 7658−7666

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The Chemical Structure of Carbon Nanothreads Analyzed by Advanced Solid-State NMR Pu Duan,† Xiang Li,‡,§ Tao Wang,§,∥ Bo Chen,⊥ Stephen J. Juhl,‡,§ Daniel Koeplinger,‡,§ Vincent H. Crespi,‡,§,∥,# John V. Badding,*,‡,§,∥,# and Klaus Schmidt-Rohr*,† †

Department of Chemistry, Brandeis University, Waltham, Massachusetts 02453, United States Department of Chemistry, §Materials Research Institute, ∥Department of Physics, #Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, United States ⊥ Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States J. Am. Chem. Soc. 2018.140:7658-7666. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/03/19. For personal use only.

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S Supporting Information *

ABSTRACT: Carbon nanothreads are a new type of onedimensional sp3-carbon nanomaterial formed by slow compression and decompression of benzene. We report characterization of the chemical structure of 13C-enriched nanothreads by advanced quantitative, selective, and two-dimensional solidstate nuclear magnetic resonance (NMR) experiments complemented by infrared (IR) spectroscopy. The width of the NMR spectral peaks suggests that the nanothread reaction products are much more organized than amorphous carbon. In addition, there is no evidence from NMR of a second phase such as amorphous mixed sp2/sp3-carbon. Spectral editing reveals that almost all carbon atoms are bonded to one hydrogen atom, unlike in amorphous carbon but as is expected for enumerated nanothread structures. Characterization of the local bonding structure confirms the presence of pure fully saturated “degree-6” carbon nanothreads previously deduced on the basis of crystal packing considerations from diffraction and transmission electron microscopy. These fully saturated threads comprise between 20% and 45% of the sample. Furthermore, 13C−13C spin exchange experiments indicate that the length of the fully saturated regions of the threads exceeds 2.5 nm. Two-dimensional 13C−13C NMR spectra showing bonding between chemically nonequivalent sites rule out enumerated single-site thread structures such as polytwistane or tube (3,0) but are consistent with multisite degree-6 nanothreads. Approximately a third of the carbon is in “degree-4” nanothreads with isolated double bonds. The presence of doubly unsaturated degree-2 benzene polymers can be ruled out on the basis of 13C−13C NMR with spin exchange rate constants tuned by rotational resonance and 1H decoupling. A small fraction of the sample consists of aromatic rings within the threads that link sections with mostly saturated bonding. NMR provides the detailed bonding information necessary to refine solid-state organic synthesis techniques to produce pure degree-6 or degree-4 carbon nanothreads.

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INTRODUCTION

before and after reaction. Large changes in unit-cell dimensions typically break up crystal order and often lead to amorphous products. Benzene would not be expected to undergo a topochemical reaction because large changes in geometry must occur as the van der Waals separations between molecules are replaced by shorter, kinetically stable covalent carbon−carbon bonds. Indeed, the reaction products formed by mechanochemical compression of benzene were described as amorphous9,10 prior to the discovery that control of reaction kinetics by slow compression/decompression1 results in formation of nanothreads that appear to spontaneously self-assemble into single crystal packings.2 Lifting the constraint of topochemical reaction could enable many aromatic molecules to react to form

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Carbon nanothreads are a novel sp -bonded one-dimensional carbon nanomaterial.1,2 They fill the last entry in a matrix of carbon nanomaterial hybridization (sp2/sp3) and dimensionality (0/1/2/3D).3 Fully saturated “degree-6” nanothreads4,5 may uniquely combine extreme strength, flexibility and resilience,6,7 while partially saturated “degree-4” threads4 may act as novel organic conductors (Figure 1).1,2 In a remarkable solid state chemical reaction at ∼20 GPa, dense high-symmetry hexagonal single crystal packings of carbon nanothreads emerge from within a low-symmetry monoclinic polycrystalline benzene molecular solid, apparently from benzene “degree-0” (Figure 1) molecular stacks.2 Formation of a single-crystal product in the solid state usually requires topochemical reaction8 from single-crystal reactants in which there is near commensuration between the periodicities © 2018 American Chemical Society

Received: April 6, 2018 Published: May 29, 2018 7658

DOI: 10.1021/jacs.8b03733 J. Am. Chem. Soc. 2018, 140, 7658−7666

Article

Journal of the American Chemical Society

although detectable differences in lattice constants are expected in certain cases.2 Uniform “beading” patterns parallel to the nanothread axes and ∼15 nm in length (i.e., ∼100 carbon bond lengths) are observed in TEM images, suggesting that local order can be present over at least this distance.17 Knowledge of the atomic structure of nanothreads is important to understanding both their structure−property relations for applications2,11,13 and the nontopochemical synthesis reaction, and also for improving synthesis protocols to enable the production of large quantities of pure nanothreads of desired structure, hybridization, and composition. Solid-state nuclear magnetic resonance (NMR) spectroscopy is an excellent method for comprehensive structural and quantitative18 compositional analysis of organic materials19−27 and has played a seminal role in elucidating structure−property relations as new carbon nanomaterials such as sp2-bonded nanotubes28 and sp3-bonded nanodiamond29 have emerged. Here we present a systematic and detailed analysis of the chemical and nanometer-scale composition of carbon nanothreads that uses advanced NMR spectroscopic techniques complemented by infrared (IR) spectroscopy. Two-dimensional 13C−13C NMR on organic materials can provide detailed information about bonding and molecular structure.21,22,25−27,30 Here we apply it, combined with spectral editing, to 13C-enriched nanothreads, which enables us to identify 4-atom segments. 30 Specifically, on a sample synthesized mechanochemically from 13C-enriched benzene,2 we quantify the composition in terms of CH,31 CH2,32 and CH3 groups as well as C not bonded to H.33 Alkenes and aromatic carbons are identified through dipolar dephasing as well as 13C and 1H chemical shifts in two-dimensional NMR spectra. We distinguish between degree-2 polymer (a possible intermediate along the addition pathway to degree-4 and degree-6 nanothreads; see Figure 1)4 and degree-4 nanothreads (likely intermediates along the addition pathway to degree-6 nanothreads)4 by 13C−13C correlation experiments with tuning of spin-exchange rate constants using rotational resonance and 1H decoupling that reveal the characteristic sp3−sp2-hybridized carbon ratio in the alkene-containing structures. We use longrange 13C spin diffusion experiments to estimate the length of saturated “diamondoid” degree-6 nanothread segments. Our two-dimensional 13C−13C NMR spectra furthermore enable us to distinguish between degree-6 nanothreads that have a single C site, such as single-site polytwistane (143652)5 or tube (3,0) (123456), and multisite structures. The combined experimental observations provide compelling evidence for the synthesis of organized carbon nanothreads rather than amorphous carbon.

Figure 1. Benzene stack and example nanothreads with degrees of saturation of 2, 4, and 6. Bonds formed between benzene rings are in red. Not all hydrogens are shown.

new types of nanothreads with heteroatoms in their carbon backbone or substitution of the exterior hydrogens (e.g., by halogens or other functional groups).2 For example, we have recently shown that pyridine, which has a different solid-state crystal structure than benzene, forms carbon-nitride nanothread crystals.2,11 Thus, the “soft” room-temperature solid-state12 organic synthesis employed for nanothreads allows for incorporation of much more nitrogen into crystalline carbon nanomaterials than the high temperature conditions typically used for synthesis of carbon nanotubes, fullerenes, diamond, and nanodiamond.11,13 Reaction pressures decrease upon decreasing reactant aromaticity, increasing temperature, and photochemical irradiation,14 suggesting a route to nanothread synthesis at the pressures of several GPa used for synthesis of >106 kg/yr of diamond at modest cost.15 Control of chemical structure for a wide range of properties may thus be possible; for example, incorporation of nitrogen into nanothreads allows for tuning of their photoluminescence11 and bandgaps and likely improves their solubility or dispersion in protic solvents.16 Moreover, in contrast to sp2-bonded nanomaterials such as nanotubes and graphene, functionalizing or crosslinking the exterior of degree-6 nanothreads does not disrupt or weaken their carbon backbone.15 X-ray and electron diffraction experiments reveal symmetric two-dimensional spot patterns that provide compelling evidence for hexagonal single crystal packings hundreds of microns across of carbon (and carbon nitride)11 nanothreads that are spaced ∼6.5 Å apart, in good agreement with the spacing predicted by modeling of degree-6 nanothreads.2 The threads and their hexagonal packing are evident in transmission electron microscopy (TEM) imaging as well.1,17 However, although NMR spectroscopy has revealed that nanothreads consist primarily (75−80%) of sp3-bonded carbon,1 the details of the chemical structure along their length are not yet understood. Fifty different degree-6 nanothread structures with 6 sp3-carbons per benzene formula unit (Figure 1) have been enumerated by theory.15 There are also other classes of reaction products, specifically “singly” unsaturated degree-4 nanothreads, which have 4 sp3-carbons per benzene formula unit (Figure 1), and “doubly” unsaturated degree-2 polymers, which have 2 sp3-carbons per benzene formula unit (Figure 1).4,5 Many of the pure degree-6 and pure degree-4 nanothread structures have approximately the same diameter and thus cannot be easily distinguished from each other or from mixtures of both degrees merely by geometric packing considerations,

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EXPERIMENTAL SECTION

Synthesis. As reported previously,2 we compressed polycrystalline mixtures9,10,34 of 13C enriched benzene in solid phases I and II to 23 GPa in a Paris-Edinburgh press over 8 h at 2−3 GPa/h from 14 to 19 GPa and 0.6−1.2 GPa/h from 19 to 23 GPa, held them at pressure for 1 h, and released them to ambient pressure over 6−8 h at the same rates in the same pressure ranges as for compression. NMR. Solid-state NMR experiments were performed on a Bruker Avance DSX400 spectrometer at a 13C resonance frequency of 100 MHz, using a Bruker double-resonance 4 mm magic-angle spinning (MAS) probehead. The ∼1 mg sample was center-packed between cylindrical glass and hollow KelF spacers. 1H and 13C 90° pulse lengths were 4.2 μs, and 1H TPPM decoupling was applied at |γB1|/2π = 60 kHz. A quantitative 13C NMR spectrum was measured using multiCP at 14 kHz MAS, with a Hahn spin echo generated by an 180° pulse with EXORCYCLE phase cycling35 applied one rotation period after the 7659

DOI: 10.1021/jacs.8b03733 J. Am. Chem. Soc. 2018, 140, 7658−7666

Article

Journal of the American Chemical Society end of cross-polarization (CP). The relative intensities of the two main peaks were verified by direct polarization with 60 s recycle delays. Minimal (3%) and not significantly (57% intensity. The minimal remaining signal imposes an upper limit of 0.3% on the concentration of rotating methyl groups. Spectral editing by three-spin coherence selection,32 see Figure 2c, shows that the shoulder near 27 ppm is at least partially from CH2 or immobile CH3 groups. This is consistent with IR bands of CH2 or CH3 groups observed between 1350 and 1500 cm−1 and near 2900 cm−1 (Figure S1). Assignment to CH2 is possible since chemical shift estimations for methylenes in a carbon nanothread environment, see Figure S2, yield chemical shifts between 15 and 30 ppm. The CH2 groups appear to be incorporated into an alkyl environment, given that their shortrange cross peaks are observed to alkyl CH at 42 ppm, see Figure 3. The methylenes or immobilized methyl groups together account for ≤4% of all C. Since the starting material, benzene, contains only CH groups, the identification of CH2/CH3 groups in nanothreads suggests that hydrogen transfer must occur during the high-

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RESULTS AND DISCUSSION In this section, we first compare the one-dimensional 13C NMR spectrum of carbon nanothreads with that of amorphous polymers and carbon materials. Then we discuss the various moieties, in particular sp2-hybridized carbons, identified by spectral editing and two-dimensional NMR and estimate their proximities in the nanothread structures based on 13C spin diffusion. Finally, we quantify the component moieties and combine them in an illustrative schematic model. Nanothreads vs H-Rich Amorphous Carbon. The quantitative solid-state 13C NMR spectrum of the 13C-enriched sample (Figure 2a) exhibits two bands, a large one near 40 ppm characteristic of sp3-hybridized carbon, and a smaller and broader one near 130 ppm characteristic of sp2-hybridized carbon, the latter accounting for 28% of the total intensity. The spectral pattern resembles the one previously reported,1 which also exhibited the shoulders near 145 and 25 ppm, indicating that nanothread synthesis is reproducible. CH-selection by dipolar DEPT, see Figure 2b, confirms that most carbon atoms in the material are bonded to one hydrogen atom, as expected. While the CH band in the NMR spectrum of nanothreads is broad compared to that of a specific site in an amorphous polymer (see Figure 2b), the spread of frequencies is far less pronounced than in the shell of nanodiamond24 or in amorphous carbons.19,40 Hydrogen-rich tetrahedral amorphous carbon (ta-C:H)40 or diamond-like amorphous carbon films,19 specifically those made from cyclohexane chemically vapor 7660

DOI: 10.1021/jacs.8b03733 J. Am. Chem. Soc. 2018, 140, 7658−7666

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sp2-Carbons. The smaller peak in the 13C NMR spectrum of Figure 2a, near 130 ppm, is characteristic of sp2-hybridized carbon, which is found in both degree-4 nanothreads and degree-2 polymers. It is therefore important to differentiate between sp2-carbon in degree-4 nanothreads and in other structures or impurities. We will systematically show that there are at least three different types of sp2-hybridized carbon present, one of which is associated with degree-4 nanothreads. Furthermore, we will demonstrate that most of this sp2 carbon is linked by one or several covalent bonds to the sp3-carbon nanothreads and thus there is no minority phase of aromatic sp2-carbon. Figure 2d shows a 13C NMR spectrum detected after 1H decoupling has been gated off for 68 μs. The dipolar fields of 1 H spins dephase the magnetization of immobile CHn groups, leaving only the peaks of carbons not bonded to H and of mobile segments, which are not present in any of enumerated nanothread structures and are minority constituents of the sample, as evidenced by their much smaller areas relative to the spectrum of all carbons in Figure 2a. Alkenes. Cross peaks between CH resonating at 130 ppm and sp3−CH at 40 ppm in the DQ/SQ (double quantum/ single quantum) spectrum (Figure 4) can be unambiguously

Figure 3. Alkyl regions of 13C−13C exchange spectra after (a) minimal 0.1 ms and (b) 10 ms spin exchange, with exchange signals of CH− CH highlighted by dotted and of CH−CHn>2 by dashed squares. MAS frequency: 14 kHz.

pressure polymerization of benzenes. The extra hydrogens in CH2/CH3 groups likely come from aromatic substitution and sigmatropic H-shifts (see the analysis of aromatic linkers below). Multiple Sites in Degree-6 Nanothreads. The broadening of the 40-ppm nanothread signal is inhomogeneous according to the elongated diagonal ridge in the short-time 2D exchange spectrum in Figure 3a. This means that many CH sites with different (though unresolved) chemical shifts coexist in the material. The sharp peak at 129 ppm in Figure 2 confirms that this is not a problem of poor magnetic-field homogeneity (“shim”) or susceptibility effects. Furthermore, broadening due to unpaired electrons can be excluded in carbon nanothreads by analyzing the T1H and T1C relaxation. Figure S3 shows that both relaxation processes are relatively slow and have exponential character, unlike the characteristically fast, nonexponential relaxation24 when unpaired electrons are significant. Off-diagonal intensity connecting different frequencies within the band near 40 ppm is observed in a 2D 13C−13C spectrum with 10 ms of spin exchange, see Figure 3b vs 3a. This indicates the proximity of, and in fact direct bonding between, sp3-CH sites with significantly different chemical shifts. In other words, the nanothreads have multiple (>6) chemically inequivalent sites, unlike single-site models such as polytwistane (143652) or tube (3,0) (123456). A mixture of multiple single-site degree-6 nanothreads, while consistent with broadening in 1D NMR spectra and in pair-distribution functions,1 does not account for the 2D NMR pattern since it would produce peaks only on the diagonal. The multisite degree-6 carbon nanothreads recently enumerated5 can account for the quasicontinuous distribution of 13C chemical shifts observed, with different thread structures appearing in distinct columns and/or as changes in thread structure along a given column. Quantumchemical calculations within density functional theory can further identify nanothread structures consistent with the observed positions and widths of the 13C NMR signals and indicate that a disordered degree-6 thread as well as certain degree-4 and -6 threads that represent natural termination points of polymer formation are the most plausible candidate structures.41

Figure 4. Sheared DQ/SQ (double quantum/single quantum) 13 C−13C spectrum showing cross peaks of directly bonded carbons. Alkene- and aromatic-carbon cross peaks are labeled. MAS frequency: 14 kHz.

attributed to alkenes, which are present in both degree-2 polymers and degree-4 nanothreads.4 The bonding of both carbons to hydrogen is clearly shown by dipolar dephasing, as no significant signal of nonprotonated C remains near 130 ppm in the dipolar-dephased spectrum (Figure 2d). Therefore, the (130 ppm, 40 ppm) cross peak cannot be assigned to an alkyllinked aromatic ring because an aromatic carbon bonded to an alkyl site cannot be bonded to H (see the structures shown near the cross peaks in Figure 4). Note that alkyl-linked nonprotonated aromatic carbons are instead observed at 145 ppm (see below). The 1H chemical shift of 5.5 ppm associated with the 130-ppm carbon in a 1H−13C spectrum (Figure S4) is distinct from aromatic 1H at >6.2 ppm and thus confirms the assignment of the 130-ppm of C−H resonance to alkenes. The 1 H chemical shift below 6 ppm also indicates that the double 7661

DOI: 10.1021/jacs.8b03733 J. Am. Chem. Soc. 2018, 140, 7658−7666

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Journal of the American Chemical Society

out the presence of degree-2 polymers. Thus, if there are any degree-2 polymers formed along the pathway to nanothreads (possibly by “para polymerization” of a diradical)4 they do not survive in the final reaction products. Degree-2 polymers were calculated to have the highest energies among structures with degree of saturation from 0 to 6,4 which might be why they were not observed in the product sample. Degree-4 Nanothreads vs Dispersed Cyclohexadienes. The analysis so far ruled out a sequence of multiple cyclohexadiene rings as found in degree-2 polymer. A careful analysis of the 13C−13C spin exchange data shown in Figure 5 makes it possible to also rule out individual cyclohexadiene rings linked to sp3-carbon neighbors, which is more challenging since the eventual sp3:sp2 intensity ratio may be close to the 4:2 value of degree-4 nanothreads. By optimizing the spin dynamics, the exchange processes for alkene and diene structures can be made distinct, see Figure 5a. Adjusting the spinning frequency to rotational resonance between sp2- and sp3-hybridized C speeds up exchange from sp2- to sp3hybridized carbons. The rotational resonance condition is fulfilled when the spinning frequency is equal to the chemicalshift frequency difference between the coupled sites of interest.42 For the alkene−sp3-CH spin pairs, the spinning frequency was therefore chosen as (131−41 ppm) 0.1 kHz/ ppm = 9 kHz. Furthermore, 1H decoupling greatly slows down sp3−sp3 exchange out of the cyclohexadiene rings, as proved by the 4fold slow-down of the decrease of the sp3 diagonal peak documented in Figure 5b (see also Figure S6b). This makes the 4:2 ratio of sp3:sp2-hybridized carbons apparent. The different exchange rate constants are indicated by arrows of different lengths in the structural cartoons in Figure 5a. Simulations of the spin exchange process under these conditions for the different models (see the Supporting Information for details) indeed provide distinctly different curves (Figure 5a) with a sloping plateau extrapolating to 0.5 = 2:4 for 1,4-cyclohexadiene, and even lower for 1,3-cyclohexadiene, where the CH units are clustered (i.e., conjugated double bonds). The experimental data obtained from diagonal and cross peaks in a series of 2D spectra (Figure S6 and 5) are in much better agreement with the presence of linked degree-4 nanothreads rather than dispersed cyclohexadienes. Aromatic Rings Linked to Alkyls. A shoulder consistently observed at 145 ppm1 is resolved as a peak after dipolar dephasing, see Figure 2d. Its intensity decreases by less than 15% after 68 μs without 1H decoupling, which shows that this signal is due to a carbon not bonded to H. The chemical shift is typical of a CH-substituted aromatic carbon. An alternative assignment would be a substituted alkene; the double bonds would have to be conjugated since the 145-ppm carbon is bonded to at least two sp2-hybridized CH sites, according to 13 C−13C NMR (Figure 6a). This assignment can be excluded since IR spectroscopy shows prominent signals of substituted benzene rings between 700 and 850 cm−1 (Figure 7 and Table S1). The 1H chemical shift of 6.8 ppm in the 1H−13C spectrum after dipolar dephasing in Figure 6b is also more consistent with aromatic than alkene C−H. The 2D spectrum and its cross section shown in Figure 6b also shows that the aromatic carbon is near alkyl 1H, indicating that the 145-ppm aromatic carbon is mostly bonded to alkyl CH, which is confirmed by a cross peak to alkyl CH in the DQ/ SQ spectrum (Figure 4, upper left corner). There is no (145 ppm, 145 ppm) peak indicative of a linkage between two such

bonds are not conjugated, consistent with expectations for degree-4 nanothreads and some degree-2 polymers (Figure 1). Degree-4 Nanothreads vs Degree-2 Polymers. Next, we seek to distinguish between the cyclohexadiene rings of degree-2 polymers and the less concentrated alkenes in degree4 nanothreads. Alkene CH linked to sp3-hybridized CH occurs in both degree-2 polymers and degree-4 nanothreads (Figure 1). However, the sp3:sp2 carbon ratio is 4-fold different, namely 2:4 in degree-2 polymers and 4:2 in degree-4 nanothreads. The ratio of protons in sp3- and sp2-hybridized CH groups near the alkene carbons can be determined in 1H−13C HetCor NMR with a short 1H spin diffusion time of 0.2 ms (Figure S5). The cross section at the alkene 13C chemical shift shows the 4:2 ratio characteristic of degree-4 nanothreads. Similarly, the sp3-:sp2-carbon ratio can be probed through 2D 13C−13C exchange experiments (Figures 5 and S6). The fast increase of the sp3:sp2 signal ratio past 1:2 to beyond 1.0 in Figure 5a rules

Figure 5. Dynamics of magnetization transfer from sp2-hybridized carbon at 132 ppm (mostly alkene) to sp3-hybridized C at their rotational resonance with and without 1H decoupling, and among sp3hybridized C for reference, as obtained from a series of 2D exchange spectra, see Figure S6. (a) Intensity ratio of sp3-C exchange peaks to alkene diagonal signals, as a function of spin-exchange time. Filled symbols (thick solid line): with 1H decoupling; open symbols (dashed line): without decoupling during spin exchange. (b) Decrease of sp3hybridized C diagonal peak; 1H decoupling slows down the spin exchange and constrains the sp3-sp3 exchange rate constant used in the simulations in (a). The fit curves in (a) compare the dynamics predicted in degree-4 polymer (upper curves) and two different cyclohexadienes in an alkyl matrix (lower curves in (a)). 7662

DOI: 10.1021/jacs.8b03733 J. Am. Chem. Soc. 2018, 140, 7658−7666

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benzenes); this can be probed by two-dimensional NMR. In a pendant ring, the nonprotonated:protonated carbon ratio would be 1:5, in a linker 2:4. A slice through a 13C−13C exchange spectrum (after dipolar dephasing) at 145 ppm shows about two aromatic C−H per nonprotonated aromatic C (Figure 6a). This indicates that, per ring, there are on average two linkages to alkyl carbon sites. The deconvolution of the sp2-hybridized carbon signal (Figure 8d) confirms this ratio

Figure 8. Selective 13C NMR spectra of 13C-enriched nanothreads. (a) Spectrum of nonprotonated carbons, after dipolar dephasing. The band at 40 ppm is a residual artifact from the intense CH peak. (b) Spectrum of alkene and nearby alkyl CH, from 1H−13C spectrum at a 1 H chemical shift of 5.5 ppm. (c) Spectrum of aromatic carbons and nearby alkyl CH. (d) Quantification of alkene and aromatic components in a quantitative 13C NMR spectrum by deconvolution of the sp2-hybridized carbon signal based on the experimental spectra in (a−c). Dashed line: Weighted superposition of the component spectra.

Figure 6. (a) 13C−13C exchange spectrum after dipolar dephasing at 14 kHz MAS, with total sideband suppression during detection. Spinexchange time: 50 ms (including 25 ms DARR). Vertical cross sections at 145 and 129 ppm are shown on the right. (b) 1H−13C HetCor spectrum with 0.5 ms cross-polarization and after dipolar dephasing, showing proximity of multiple alkyl CH groups to the average aromatic ring.

independently. Magnetization quickly transfers to several sp3hybridized CH groups, according to the strong cross peaks with alkyl-CH in both 13C−13C and 1H−13C spectra (Figure 6). The nonprotonated:protonated aromatic carbon ratio of 2:4 ratio seen in NMR data might suggest exclusively parasubstitution, but the IR spectrum reveals a more complex picture.44 It also shows signals indicative of monosubstitution, ortho or meta-disubstitution, and 1,3,5-trisubstitution. To balance the small (1:5) nonprotonated:protonated aromatic carbon ratio of monosubstituted rings, a similar amount of 1,3,5-trisubstituted aromatic rings needs to be invoked. The identification of alkyl-substituted benzenes (as well as CH2 discussed above) in the sample also indicates hydrogen transfer occurring in the high-pressure transformation of benzene to nanothreads. Possible H-transfer via aromatic substitution and sigmatropic H-shift mechanisms are shown in Figure S7. The amounts of CH2 and nonprotonated aromatic carbons match (3−4%, see below). Free Benzene and Linked Aromatics. After dipolar dephasing, a sharp peak partially remains at 129 ppm (see Figure 8a), indicating a nonprotonated carbon, or a mobile molecule or segment. Nonprotonated carbon signals are observed at this chemical shift only in large fused ring systems or with COOH substitution,45 both of which are absent from our sample. Furthermore, 1H−13C correlation shows that this sharp 13C peak is associated with a similarly narrow 1H signal, see Figure 6b. Since the chemical shift agrees to within 1 ppm

Figure 7. IR spectrum of carbon nanothreads (black curve), with empirical band assignment to vibrations of mono-, disubstituted and 1,3,5-trisubstituted benzene rings. Wavenumbers of the ranges shown are listed in Table S1.44

benzene rings. Further corroboration of the assignment of the 145 ppm peak to a linkage to alkyl CH comes from the chemical shift itself: Two aromatic carbons linked by a single bond resonate at