The Chemical Structure of Carbon Nanothreads Analyzed by

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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 J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03733 • Publication Date (Web): 29 May 2018 Downloaded from http://pubs.acs.org on May 29, 2018

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Journal of the American Chemical Society

The Chemical Structure of Carbon Nanothreads Analyzed by Advanced Solid-State NMR Pu Duan,1 Xiang Li,2,3 Tao Wang,3,4 Bo Chen,5 Stephen J. Juhl,2,3 Daniel Koeplinger,2,3 Vincent H. Crespi,2,3,4,6 John V. Badding,2,3,4,6,* Klaus Schmidt-‐Rohr1,* 1

Department of Chemistry, Brandeis University, Waltham, MA 02453, USA

2 3

Department of Chemistry, Pennsylvania State University, University Park, PA 16802, USA

Materials Research Institute, Pennsylvania State University, University Park, PA 16802, USA

4

Department of Physics, Pennsylvania State University, University Park, PA 16802, USA

5

Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-‐ 1301, USA 6

Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802, USA

ABSTRACT: Carbon nanothreads are a new type of one-‐dimensional sp3-‐carbon nanomaterial formed by slow compres-‐ sion and decompression of benzene. We report characterization of the chemical structure of 13C-‐enriched nanothreads by advanced quantitative, selective, and two-‐dimensional solid-‐state nuclear magnetic resonance (NMR) experiments com-‐ plemented 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 most carbon atoms are bonded to one hy-‐ drogen atom as is expected for enumerated nanothread structures. Characterization of the local bonding structure con-‐ firms the presence of pure fully saturated “degree-‐6” carbon nanothreads previously deduced on the basis of crystal pack-‐ ing considerations from diffraction and transmission electron microscopy. These fully saturated threads comprise be-‐ tween 25% and 50% 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 chemi-‐ cally nonequivalent sites rule out enumerated single-‐site thread structures such as polytwistane or tube (3,0) but are con-‐ sistent with multi-‐site degree-‐6 nanothreads. Approximately a third of the carbon is in “degree-‐4” nanothreads with iso-‐ lated double bonds. The presence of doubly unsaturated degree-‐2 benzene polymers can be ruled out on the basis of 13C-‐ 13 C NMR with spin exchange rates tuned by rotational resonance and 1H decoupling. A small fraction of the sample con-‐ sists 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.

tion because large changes in geometry must occur as the

INTRODUCTION 3

Carbon nanothreads are a novel sp -‐bonded one-‐ dimensional carbon nanomaterial.1,2 They fill the last en-‐ try 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 resilience6,7 while partially satu-‐ rated “degree-‐4” threads4 may act as novel organic con-‐ ductors (Figure 1).1,2 In a remarkable solid state chemical reaction at ~20 GPa, dense high-‐symmetry hexagonal sin-‐ gle crystal packings of carbon nanothreads emerge from within a low-‐symmetry monoclinic polycrystalline ben-‐ zene 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 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 reac-‐

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

van der Waals separations between molecules are re-‐ placed by shorter, kinetically stable covalent carbon-‐ carbon bonds. Indeed, the reaction products formed by

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mechanochemical compression of benzene were de-‐ scribed as amorphous9,10 prior to the discovery that con-‐ trol of reaction kinetics by slow compres-‐ sion/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 new types of nanothreads with heteroatoms in their carbon backbone or substitution of the exterior hy-‐ drogens (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 nanomateri-‐ als 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 proper-‐ ties may thus be possible; for example, incorporation of nitrogen into nanothreads allows for tuning of their pho-‐ toluminescence11 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 sym-‐ metric two-‐dimensional spot patterns that provide com-‐ pelling evidence for hexagonal single crystal packings hundreds of microns across of carbon (and carbon ni-‐ tride)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 spec-‐ troscopy has revealed that nanothreads consist primarily (75–80%) of sp3-‐bonded carbon,1 the details of the chemi-‐ cal 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 de-‐ gree-‐4 nanothreads, which have 4 sp3-‐carbons per ben-‐ zene 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 distin-‐ guished from each other or from mixtures of both degrees merely by geometric packing considerations, although detectable differences in lattice constants are expected in certain cases.2 We observe only (hk0) diffraction spots for carbon nanothreads,2 which indicates that they are either helical, axially disordered, and/or lacking in registry from

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thread to thread. Nonetheless, uniform “beading” pat-‐ terns parallel to the nanothread axes and ~15 nm in length (i.e., ~100 carbon bond lengths) are observed in TEM im-‐ ages, 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 non-‐topochemical synthesis reaction, and also for improving synthesis pro-‐ tocols 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 compre-‐ hensive structural and quantitative18 compositional analy-‐ sis 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 na-‐ nometer-‐scale composition of carbon nanothreads that uses advanced NMR spectroscopic techniques comple-‐ mented by infrared (IR) spectroscopy.

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Figure 2. C NMR spectra of carbon nanothreads synthe-‐ 13 sized from C-‐enriched benzene. (a) Quantitative multiCP spectrum of all carbons. (b) CH-‐only spectrum obtained by dipolar DEPT, with a spectrum of amorphous polystyrene 13 (PS) C-‐labeled in the backbone CH position shown for ref-‐ erence (dashed line); (c) CH2-‐only spectrum by three-‐spin coherence selection; (d) dipolar dephased multiCP spectrum of nonprotonated and mobile carbon. “ssb” marks a spinning sideband.

Two-‐dimensional 13C-‐13C NMR on organic materials can provide detailed information about bonding and molecu-‐ lar structure.21,22,25-‐27,30 Here we apply it, combined with spectral editing, to 13C enriched nanothreads, which ena-‐ bles us to identify 4-‐atom segments.30 Specifically, on a


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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 di-‐ polar dephasing studies as well as from the 13C and 1H chemical shifts in two-‐dimensional NMR spectra. We distinguish between degree-‐2 polymer (a possible inter-‐ mediate along the addition pathway to degree-‐4 and de-‐ gree-‐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 rates using rotational resonance and 1H decoupling that reveal the characteristic sp3-‐sp2-‐hybridized carbon ratio in the al-‐ kene-‐containing structures. We use long-‐range 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 com-‐ bined experimental observations provide compelling evi-‐ dence for the synthesis of organized carbon nanothreads rather than amorphous carbon.

heteronuclear decoupling.30 Correlation selectively of signals of nonprotonated or mobile carbons with those of neighboring carbons was achieved in a simplified version of EXPANSE NMR30 with multiCP followed by 50-‐ms 13C spin diffusion and recoupled dipolar dephasing before detection. Rotational resonance and 1H decoupling during the spin-‐diffusion period was used to speed up sp2-‐C to sp3-‐C and slow down sp3-‐C to sp3-‐C magnetization transfer, which provides more distinctive spin-‐exchange behav-‐ ior of degree-‐4 nanothread vs. cyclohexadiene-‐containing struc-‐ tures. Spinning sidebands in the ω2 dimension were suppressed by TOSS before detection, while the contribution of the small spinning sideband in the ω1 dimension was subtracted out by assuming it to be equal to the corresponding negative spinning sideband observed on the other side of the diagonal peak. Spin exchange dynamics were simulated in MATLAB for several al-‐ kene-‐containing structural models, as described in detail in the Supporting Information. The exchange-‐matrix formalism was used, with successive multiplications by a single matrix exp(ΠΔt), where Π is the exchange matrix containing the ex-‐ change rates as off-‐diagonal elements.

EXPERIMENTAL Synthesis. As reported previously,2 we compressed polycrystal-‐ line mixtures9,10,34 of 13C enriched benzene in solid phases I and II to 23 GPa in a Paris-‐Edinburgh press over 8 hours at 2–3 GPa/hr from 14 GPa to 19 GPa and 0.6–1.2 GPa/hr from 19 GPa to 23 GPa, held them at pressure for 1 hour, and released them to ambient pressure over 6–8 hours 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 spin-‐ ning (MAS) probehead. The ~1-‐mg sample was center-‐packed between cylindrical glass and hollow KelF spacers. 1H and 13C 90o pulse lengths were 4.2 μs, and 1H TPPM decoupling was applied at |γ B1|/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 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 (2 by dashed squares. MAS frequency: 14 kHz.

RESULTS AND DISCUSSION In this section, we first compare the one-‐dimensional C NMR spectrum of carbon nanothreads with that of amorphous polymers and carbon materials. Then we dis-‐ cuss 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. 13

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


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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-‐pressure polymerization of benzenes. The extra hydrogens in CH2/CH3 groups likely come from ar-‐ omatic substitution and sigmatropic H-‐shifts (see the analysis of aromatic linkers below).

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

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 com-‐ pared to that of a specific site in an amorphous polymer (see Figure 2b), the spread of frequencies is far less pro-‐ nounced than in the shell of nanodiamond24 or in amor-‐ phous carbons.19,40 Hydrogen-‐rich tetrahedral amorphous carbon (ta-‐C:H)40 or diamond-‐like amorphous carbon films,19 specifically those made from cyclohexane chemi-‐ cally vapor deposited with dc plasma assistance,40 are the closest analogues to carbon nanothreads but, like all amorphous carbon, show much wider spectral bands (~30 ppm width) than carbon nanothreads (~12 ppm). This confirms the higher degree of order in carbon nanothreads, consistent with the diffraction peaks1 that prove a crystal-‐like superstructure. Methyl or methylene groups. A small shoulder is consistently observed1 around 27 ppm in the 13C spectrum, see Figure 2a. While the chemical shift might suggest CH3 groups, its disappearance after dipolar dephasing, see Figure 2d, shows that this signal cannot be assigned to rotating CH3 groups, which would remain at >57% inten-‐ sity. The minimal remaining signal imposes an upper lim-‐ it 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 con-‐ sistent with IR bands of CH2 or CH3 groups observed be-‐ tween 1350 and 1500 cm-‐1 and near 2900 cm–1 (Figure S1). Assignment to CH2 is possible since chemical shift esti-‐ mations for methylenes in a carbon nanothread environ-‐ ment, 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 short-‐range 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.

Multiple sites in degree-‐6 nanothreads. The broad-‐ ening 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) chemi-‐ cal 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 elec-‐ trons can be excluded in carbon nanothreads by analyzing the T1H and T1C relaxation. Figure S3 shows that both re-‐ laxation processes are relatively slow and have exponen-‐ tial character, unlike the characteristically fast 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 only peaks on the diagonal. The multi-‐site degree-‐6 car-‐ bon nanothreads recently enumerated5 can account for the quasi-‐continuous 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. Quantum-‐chemical calculations within density functional theory can further identify nanothread structures consistent with the observed posi-‐ tions 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 sp2-‐carbons. The smaller peak in the 13C NMR spec-‐ trum 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 im-‐ portant 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 dif-‐ ferent types of sp2-‐hybridized carbon present, one of which is associated with degree-‐4 nanothreads. Further-‐ more, 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 aro-‐ matic sp2-‐carbon.


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Figure 2d shows a 13C NMR spectrum detected after 1H decoupling has been gated off for 68 µs. The dipolar fields of 1H 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.

remains near 130 ppm in the dipolar-‐dephased spectrum (Figure 2d). Therefore, the (130 ppm, 40 ppm) cross peak cannot be assigned to an alkyl-‐linked aromatic ring be-‐ cause 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 C-‐H resonance to alkenes. The 1H chemical shift below 6 ppm also indicates that the double bonds are not conjugated, consistent with expec-‐ tations for degree-‐4 nanothreads and some degree-‐2 pol-‐ ymers (Figure 1).

2

Figure 5. Dynamics of magnetization transfer from sp -‐ 3 hybridized carbon at 132 ppm (mostly alkene) to sp -‐ hybridized C at their rotational resonance with and without 1 3 H decoupling, and among sp -‐hybridized C for reference, as obtained from a series of 2D exchange spectra, see Figure S6. 3 (a) Intensity ratio of sp -‐C exchange peaks to alkene diagonal signals, as a function of spin-‐exchange time. Filled symbols 1 (thick solid line): with H decoupling; open symbols (dashed line): without decoupling during spin exchange. (b) Decrease 3 1 of sp -‐hybridized C diagonal peak; H decoupling slows down 3 3 the spin exchange and constrains the sp -‐sp exchange rate 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).

Alkenes. Cross peaks between =CH resonating at 130 ppm and sp3–CH at 40 ppm in the DQ/SQ (double quan-‐ tum/single quantum) spectrum (Figure 4) can be unam-‐ biguously attributed to alkenes, which are present in both degree-‐2 polymers and degree-‐4 nanothreads.4 The bond-‐ ing of both carbons to hydrogen is clearly shown by dipo-‐ lar dephasing, as no significant signal of nonprotonated C

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

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 degree-‐4 nanothreads. Alkene CH linked to sp3-‐ hybridized CH occurs in both degree-‐2 polymers and de-‐ gree-‐4 nanothreads (Figure 1). However, the sp3:sp2 car-‐ bon ratio is four-‐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).


5


The cross section at the alkene 13C chemical shift shows the 4:2 ratio characteristic of degree-‐4 nanothreads. Simi-‐ larly, the sp3-‐:sp2-‐ carbon ratio can be probed through 2D 13 C-‐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 out the presence of degree-‐2 polymers. Thus, if there are any degree-‐2 polymers formed along the pathway to nanothreads (possibly by “para polymeriza-‐ tion” of a diradical)4 they do not survive in the final reac-‐ tion products. Degree-‐2 polymers were calculated to have the highest energies among structures with degree of sat-‐ uration from 0 to 6,4 which might be why they were not observed in the product sample. Degree-‐4 nanothreads vs. dispersed cyclohexadi-‐ ene. 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 cy-‐ clohexadiene 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 ex-‐ change processes for alkene and diene structures can be made distinct, see Figure 5a. Adjusting the spinning fre-‐ quency to rotational resonance between sp2-‐ and sp3-‐ hybridized C speeds up exchange from sp2-‐ to sp3-‐ hybridized carbons. The rotational resonance condition is fulfilled when the spinning frequency is equal to the chemical-‐shift frequency difference between the coupled sites of interest.42 For the alkene–sp3-‐CH spin pairs, the spinning frequency was therefore chosen as (131 ppm – 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 four-‐fold slow-‐down of the decrease of the sp3 diago-‐ nal peak documented in Figure 5b (see also Figure S6b). This makes the 4:2 ratio of sp3:sp2-‐hybridized carbons apparent. The different exchange rates are indicated by arrows of different lengths in the structural cartoons in Figure 5a. Simulations of the spin exchange process un-‐ der these conditions for the different models (see the Supporting Information for details) indeed provide dis-‐ tinctly 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 experi-‐ mental 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 cyclohexadiene. Aromatic rings linked to alkyls. A shoulder consist-‐ ently observed at 145 ppm1 is resolved as a peak after dipo-‐ lar 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 car-‐ bon. 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 13C-‐13C NMR (Figure 6a).

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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

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

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 benzene rings. Further cor-‐ roboration 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 < 140 ppm in solid-‐state NMR spectra.43 It is important to establish whether the alkyl-‐linked ar-‐ omatic rings are just pendant groups (monosubstituted benzenes) or mostly linkers connecting alkyl segments (multiply substituted benzenes); this can be probed by two-‐dimensional NMR. In a pendant ring, the nonproto-‐ nated: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 in-‐ dicates 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 independently. Magnetization quickly transfers to several sp3-‐hybridized 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 para-‐substitution, but the IR spectrum reveals a more complex picture.44 It also shows signals indicative of mono-‐substitution, ortho or meta-‐disubstitution, and


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1,3,5-‐trisubstitution. To balance the small (1:5) nonproto-‐ nated:protonated aromatic carbon ratio of monosubsti-‐ tuted rings, a similar amount of 1,3,5-‐trisubstituted aro-‐ matic rings needs to be invoked. The identification of alkyl-‐substituted benzenes (as well as CH2 discussed above) in the sample also indicates hy-‐ drogen transfer occurring in the high-‐pressure transfor-‐ mation of benzene to nanothreads. Possible H-‐transfer via aromatic substitution and sigmatropic H-‐shift mecha-‐ nisms are shown in Figure S7. The amounts of CH2 and nonprotonated aromatic carbons match (3-‐4%, see be-‐ low).

anisotropy shows that the mobility of the ring must be anisotropic. The strong asymmetry of the sideband pat-‐ tern is characteristic of fast rotational jumps around the ring’s six-‐fold axis. This motion by itself in solid benzene at 223 K results in Δσ ≈ 180 ppm;46 the additional 33% nar-‐ rowing must be ascribed to large-‐amplitude wobbling of the rotation axis. Unreacted benzenes might be embed-‐ ded sideways between threads (particularly where on-‐ thread defects may form small voids in thread packing) or between the ends of two co-‐axial threads. The anisotropi-‐ cally mobile benzene is quite intimately associated with the alkyl-‐rich matrix, showing 13C-‐13C cross peaks within 50 ms, see Figure 6a, and full equilibration of magnetiza-‐ tion from benzene with that of other carbon sites within 1 s (see Figure 9 below). In the DQ/SQ spectrum of Figure 4, a diagonal peak is observed faintly near (135, 135) ppm. It must be assigned to directly linked aromatic rings. Indeed, extractable naphthalene and diphenyl have been observed by gas chromatography – mass spectrometry,47 and out-‐of-‐plane deformation of the benzene rings at lower pressures was speculated to be related to such oligomerizations.48 Their contribution can be assessed based on the nonprotonated aromatic signal between 135 and 140 ppm. It is relatively small, accounting for < 3% of all carbons.

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Figure 8. Selective C 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 near-‐ 1 13 1 by alkyl CH, from H-‐ C spectrum at a H chemical shift of 5.5 ppm. (c) Spectrum of aromatic carbons and nearby alkyl CH. (d) Quantification of alkene and aromatic components 13 in a quantitative C NMR spectrum by deconvolution of the 2 sp -‐hybridized carbon signal based on the experimental spec-‐ tra in (a) – (c). Dashed line: Weighted superposition of the component spectra.

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 mo-‐ bile 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 similar-‐ ly narrow 1H signal, see Figure 6b. Since the chemical shift agrees to within 1 ppm with the reported value for neat benzene, we assign it to mobile benzene. A pronounced “self-‐correlation” peak is observed at 129 ppm on the di-‐ agonal in the sheared DQ/SQ spectrum of Figure 4, which proves that at least two carbons of the same chemical shift are bonded to each other, as is true for benzene. This benzene signal accounts for 5% of all carbon (see below). The benzene carbons have a large chemical-‐shift ani-‐ sotropy (CSA) of Δσ ≈ 120 ppm, according to fast dephas-‐ ing under CSA recoupling (Figure S8) and the sideband pattern observed under 3-‐kHz MAS (Figure S9). This shift

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Figure 9. (a) 0.5-‐s C-‐ C spin-‐exchange spectrum at rota-‐ tional resonance (with total suppression of spinning side-‐ bands before detection) and (b) horizontal slices from the 3 2 sp -‐ (solid green line) and the sp -‐ (dashed red line) carbon signal maximum. The spectra have been scaled to match the 2 1 sp -‐carbon bands. 30-‐kHz H decoupling was applied during the 0.5-‐s mixing time. (c) Corresponding slices from a spec-‐ trum with 1-‐s mixing time (including 0.2 s of 30-‐kHz decou-‐ pling). The difference spectrum in c) (thick blue lines) can 3 2 be attributed to sp -‐C not in

The Chemical Structure of Carbon Nanothreads Analyzed by

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