Letter pubs.acs.org/JPCL
Pseudocarbynes: Charge-Stabilized Carbon Chains Pilarisetty Tarakeshwar,*,† Peter R. Buseck,†,‡ and Harold W. Kroto§ †
School of Molecular Sciences, Arizona State University, Tempe, Arizona 85287-1604, United States School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-6004, United States § Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, United States ‡
S Supporting Information *
ABSTRACT: Carbyne is the long-sought linear allotrope of carbon. Despite many reports of solid carbyne, the evidence is unconvincing. A recent report of supposed carbyne shows gold clusters in transmission electron microscopy (TEM) images. In order to determine the effects of such clusters, we performed ab initio calculations of uncapped and capped linear carbon chains and their complexes with gold clusters. The results indicate that gold dramatically alters the electron densities of the CC bonds. The resulting charge-stabilization of the carbon chains leads to pseudocarbynes. These findings are corroborated in calculations of the structures of crystals containing isolated carbon chains and those intercalated with gold clusters. Calculated Raman spectra of these pseudocarbynes with gold clusters are in better agreement with experiment than calculated spectra of isolated carbon chains. The current work opens the way toward the design and development of a new class of metal-intercalated carbon compounds.
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can be physically isolated and their interesting properties experimentally harnessed.6,40 Although the experimental evidence for such bundles is questionable, finite-sized carbon chains have been observed: (a) in supersonic-beam-deposited carbon films,41,42 (b) in solid argon at 36 K,43 (c) in break junctions involving graphene,44,45 (d) as chains encapsulated in carbon nanotubes,46,47 and (e) on gold/silver nanoparticles or films.48−50 However, these carbon chains are short-lived (a few seconds) and have not been physically isolated Carbon chains in the solid state were reportedly synthesized using gold either as a substrate or catalyst.51,52 High catalytic activity has been correlated with bilayer clusters containing only ∼10 gold atoms, with dimensions of ∼0.5 nm,53 and stabilized in carbon nanotubes or on sheets.54,55 Although there have been investigations of the interactions of gold clusters with ethene, ethyne, carbon nanotubes, and graphene,56−61 the influence of such clusters on the structure and properties of the carbon chains has not been investigated. Such a study becomes important because of a recent report on the isolation of crystalline carbon chains for which the transmission electron microscopy (TEM) image of the samples shows gold nanoparticles or clusters embedded on the carbon nanorods.52 In order to investigate the structure and electronic properties of finite-sized carbon chains in the presence of small gold clusters, we carried out calculations at the density functional level of theory (DFT) using the Becke gradient-corrected exchange functional and Lee−Yang−Parr correlation functional with three parameters (B3LYP).62−65 The cc-pVDZ basis set
he carbon allotropes have long been the subject of intensive investigation. Interest increased following the discovery of the structures and fascinating properties of fullerenes, carbon nanotubes, and graphene.1−6 However, the simplest allotrope of carbon, the one-dimensional carbyne, has defied attempts to synthesize and characterize it in the solid state. Since its first reported synthesis 50 years ago, the quest for carbyne has been marred by repeated synthetic and spectroscopic setbacks, and the trail is littered with red herrings.7−16 The term carbyne has been used both to represent a compound whose structure contains a carbon atom with three nonbonding electrons and for the crystalline carbon allotrope consisting of bundles of linear carbon chains,17−19 the usage we adopt in this paper. The chains can either have alternating single and triple bonds (CC)n or only double bonds (CC)n.17−19 In contrast, polyynes and oligoynes consist of linear carbon chains end-capped with another element, commonly hydrogen, or organic functional groups.17,18 On the basis of the definition of allotrope,20,21 carbyne contains only carbon with no other element or functional group as essential constituents. Moreover, carbyne would be explosively reactive in the condensed phase because the unpaired electrons at either end of each chain would lead to rapid cross-linking or polymerization.15,16,22 Nonetheless, the quests continue. In the attempt to synthesize carbyne, a common approach is to first produce carbyne-like materials by adding massive end groups to linear carbon chains, with the goal of hindering their reactivity.22−28 There have also been numerous theoretical investigations of linear carbon chains that have either been encapsulated in carbon nanotubes or bundled together.29−39 These results led to the notion that bundles of carbon chains © 2016 American Chemical Society
Received: March 24, 2016 Accepted: April 14, 2016 Published: April 14, 2016 1675
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Figure 1. Calculated Raman spectra of different sizes of sp carbon chains in the (a) singlet and (b) triplet states.
Figure 2. Comparison of the calculated Raman spectra of (a) different sizes of CnH2 (n = 2, 4, 6, 8, 10, 12) polyynes, (b) C10H2 and C10Au2, (c) C10H2 and C10H2Au20.
therefore only considered the latter. Because these uncapped chains contain two unpaired electrons, the calculations were done for both the singlet and triplet states. After employing a scaling factor of 0.97 to account for systematic overestimation resulting from neglect of anharmonic effects, the vibrational frequencies obtained at the B3LYP level of theory are close to those obtained experimentally.71,72 To examine how higherlevel correlation effects influence the calculated frequencies of open-shell systems, we also evaluated the vibrational frequencies at the much higher CCSD(T) {coupled-cluster with single, double, and perturbative triple substitutions} level of theory on the C6 systems.73 The vibrational frequencies
was used to represent light elements like carbon and hydrogen. 66 Gold was represented with a LANL2DZ pseudopotential basis set.67 We focus on the Raman spectra because they are extremely sensitive to the microstructure of carbon allotropes.68,69 In prior studies of linear carbon chains, the calculated Raman frequencies at the B3LYP/cc-pVDZ level of theory were in good agreement with experiment.47,49 Simulations were carried out on uncapped linear carbon chains with lengths from 6 to 12 atoms. Prior calculations suggested that systems containing only double bonds (C C)n are energetically less stable than those containing alternating single and triple bonds (CC)n.70 We 1676
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The Journal of Physical Chemistry Letters predicted at the two levels of theory are nearly identical (Figure S1, Supporting Information), indicating that the B3LYP level of calculation is adequate for investigating these systems. Short linear carbon chains (Cn, n ≤ 10) have an energetic preference for cumulenic bonding (:CC···CC:) with nearly equivalent bond lengths rather than acetylenic bonding (·CC···CC·) with alternating bond lengths.74,75 Increase in a Cn chain length results in a transition from cumulenic to acetylenic structure,75 predicted to occur at n = 48−52.75 Even though the (CC)n motif was used in the initial structures for the Cn chains, upon optimization they exhibit signs of cumulenic bonding. Raman spectra of experimental samples of cumulenic systems exhibit a broad feature in the 1100−1700 cm −1 region.14,19,76−79 Our calculated spectra (Figure 1) of Cn systems display peaks with significant intensities in this region, consistent with experiment. These peaks are absent if the carbon chains are capped with hydrogen or gold atoms (Figure 2). The major difference in the calculated Raman spectra of several CnH2 (n = 2, 4, 6, 8, 10, 12) polyynes (Figure 2a) and uncapped linear carbon chains (Figure 1) is the absence of peaks with appreciable intensity in the 750−2000 cm−1 region. The conduction electrons of gold nanoparticles collectively oscillate to produce a surface plasmon resonance (SPR) at the most commonly used excitation wavelengths (∼514 nm, ∼ 633 nm) in Raman spectra.80 The SPR leads to a dramatic enhancement in the intensity of some of the Raman-active modes of a molecule when it is adsorbed on gold clusters.80 Such surface-enhanced Raman scattering (SERS) has been widely used, for example, to investigate the Raman spectra of cumulenes and polyynes.19,78,79,81 The pronounced enhancement in the intensity of the Raman spectra mentioned above presumably reflects changes in the structure and properties of the carbon chains. Gold can replace the terminal hydrogens of the C10H2 polyynes to produce goldpolyyne organometallic complexes,82 or a C10H2 polyyne can be adsorbed onto a gold cluster.56,57 Replacement of the terminal hydrogens by gold leads to an increase in the Raman intensity of the CC stretch around 2100 cm−1 (Figure 2b), but there is no perceptible change in the intensities of the peaks in other regions. However, there is a significant increase in the intensity of the peak at ∼1100 cm−1 if a C10H2 polyyne is adsorbed onto a gold Au20 cluster. Similar effects occur in the calculated Raman spectra of a C10H2 polyyne adsorbed on increasing sizes of gold (Aun, n = 4, 8, 12, 20, 32, 42) clusters (Figure S2, Supporting Information), all of which have been observed in experiments.83−86 The calculations also reveal that the multiple peaks around ∼2100 cm−1 coalesce into a single peak as the size of the gold cluster increases. Calculations of C10H2 with smaller gold clusters were also carried out at the BP86/def2TZVP level,64,87,88 wherein the scalar relativistic effects were considered using the zero-order regular approximation (ZORA).89 Both the B3LYP/(cc-pVDZ+LANL2DZ) and BP86/def2-TZVP spectra exhibit similar trends (Figure S3, Supporting Information). The Raman spectra of C10H2 and its complex with the Au12 and Au20 gold clusters were simulated at excitation wavelengths of 514 and 633 nm (Figure 3a,b) using time-dependent density functional calculations.64,90,91 Although there is little change in the intensity of the peaks around ∼2100 cm−1, the peak around ∼1100 cm−1 only appears in the C10H2−Au12 and C10H2−Au20 complexes. The calculated absorption spectra of the C10H2− Au12 complex, together with the computed electron density
Figure 3. Comparison of the calculated Raman spectra of C10H2, C10H2−Au12, and C10H2−Au20, at (a) 514 nm and (b) 633 nm. Compared to C10H2, the peak around ∼1100 cm−1 (blue arrow) becomes prominent in C10H2−Au12 and C10H2−Au20. (c) Calculated absorption spectra of C10H2−Au12 together with the computed electron density difference between the excited and ground states at ∼514 nm and ∼633 nm. Electron flow is from green to red.
difference between the excited and ground states, indicate that a charge transfer from the filled orbitals of the gold cluster to the C10H2 unoccupied orbitals associated with the C−C single bonds occurs in the excited states at ∼514 nm and ∼633 nm (Figure 3c). A similar resonant charge transfer also occurs in pyridine adsorbed on a gold cluster or surface.90,92 Although the same wavelengths were employed by Pan et al. to obtain the Raman spectra of their samples,52 the role of embedded gold clusters were not considered,52 suggesting a misinterpretation by them. A comparison of the calculated absorption spectra,65 of different C10 species (Figure S4, Supporting Information) indicates that the absorption maxima of pure carbon clusters (C10 singlet and C10 triplet) appear at much higher wavelengths than either a C10H2 polyyne or a C10H2−Au12 complex. Furthermore, the calculated absorption maxima, which are in good agreement with earlier experiments of a C 10 H 2 polyyne,28,93 indicate a pronounced red shift in methanol. The calculations indicate that the binding energies of an isolated C10H2 polyyne with different-sized gold clusters range from 0.5 eV (Au4) to 0.1 eV (Au42). These binding energies are similar to that between an isolated acetylene and a Au7 cluster.56 In particular, the interaction of C10H2 with the larger gold clusters is characteristic of a π-type interaction, wherein 1677
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Figure 4. Contour plots (isovalue: 0.001 au) of electrostatic potential (ESP) mapped onto the electron density of an isolated C10H2 molecule and its complexes with Au20 (central panel) and Au42 (right panel) clusters. ESP values range from −8.0 × 10−3 a.u. (red) to +1.0 × 10−3 a.u. (blue).
the gold atom interacts with the π electron cloud of the polyyne.56 The gold cluster dramatically alters the electron densities of the CC bonds of C10H2 (Figure 4), showing that it is essential for stabilizing the carbon chains. Self-consistent density functional theory calculations were also carried out to examine the effect of gold on the structure of crystalline bundles of carbon chains.94 Atomic-orbital basis sets were used for the valence electrons and norm-conserving pseudopotentials for the atomic cores.94 The pseudopotentials were constructed using the Trouiller−Martins scheme,95 and the vdW functional was employed for the exchange-correlation potential.96 We employed a double-ζ plus polarization (DZP) basis set. The numerical integrals were performed and projected on a real-space grid with an equivalent cutoff of 200 Ry. Energy minimizations on all the structures were carried out using the conjugate gradient method with a force convergence of 0.004 eV/Å. Several sets of a and c values have been reported for the hexagonal lattice parameters of crystalline carbon chains.8,12,52 The a values range from 4.76 to 9.92 Å and the c values from 7.4 to 15.36 Å.8,12,52 Using the most recently reported parameters (a = 5.78 Å, c = 9.92 Å)52 resulted in a kinked structure (Figure 5a). Calculations in which the lattice parameters were relaxed in the geometry optimizations yielded a linear structure (Figure 5b). A model of kinked carbon chains based on the structure of graphite was proposed to explain the huge variations in the observed lattice parameters.12,52 We suggest that use of accurately determined lattice parameters of a genuine crystalline sample of carbyne, should it exist, would lead to a linear structure. However, we are strongly skeptical since we have yet to see convincing evidence of the occurrence of solid carbyne.9,16,97,98 The effects of gold on the structures of crystals containing carbon chains is both profound and relevant. Kinking appears to increase proportionally to the number of gold atoms adjacent to the chains (Figure 5c,d) and can be attributed to changes in the charge-density profiles of the carbon chains as a result of interactions with the gold atoms (Figure 6). A similar kinking was reported in a recent theoretical study of adsorption of carbon chains on metal surfaces.36 Optimization of the structure of a crystal with carbon chains intercalated with Au8 clusters led to the formation of chains of gold along the carbon chains (Figure S5, Supporting Information). An earlier calculation of carbon nanotubes intercalated with small gold clusters resulted in a similar formation of a gold chain alongside the nanotube axis.59 Implications of our results include: (a) uncapped finite-sized carbon chains exhibit a cumulenic character, indicated by the appearance of a peak in the 1100−1700 cm−1 region of the Raman spectra; (b) although spectra of isolated hydrogencapped carbon chains contain a strong peak around ∼2100 cm−1, no peaks appear in the 750−2100 cm−1 region; (c)
Figure 5. Optimized 2 × 2 × 2 structures of hypothetical crystals of carbon chains based on an (a) fixed lattice, (b) variable lattice, and (c,d) similar carbon chains with increasing concentrations of gold atoms between the carbon chains. Unit cells of these pseudocarbyne crystals are indicated by light black lines.
interaction of polyynes with small gold clusters leads to a peak around ∼1100 cm−1, which arises from a C−C stretch; (d) the electron-density profiles indicate that gold atoms have a significant interaction with the polyyne, which we suggest leads to the electronic stabilization of polyynes in crystals; and (e) intercalated gold atoms or clusters in crystals consisting largely of carbon chains leads to kinking that results from interaction of the electronic densities of the carbon chains and gold. On the basis of the above, the evidence suggests that the material synthesized and characterized by Pan et al.52 is more consistent with C/Au clusters than with pure or even hydrogen-capped carbon chains. Crystals such as in Figure 5 are plausible explanations for their experimental results. The current work opens the possibility of a range of chargestabilized carbon chains having different stoichiometries, with gold and also other noble metals, but whose occurrence 1678
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Figure 6. Profiles of (a) the electron density (ρ(r), isovalue: 0.08 e/Å3) and (b) the charge density difference (Δρ(r), isovalue: ± 0.004 e/Å3) of the gold−carbon chain complex in Figure 5d. The red regions around the carbon atoms show charge enhancement, and the green regions show depletion. They are, respectively, contracted and distended near the carbon atoms (black arrows) close to gold. (10) Whittaker, A. G. Carbyne Forms of Carbon: Evidence for Their Existence. Science 1985, 229, 485−486. (11) Smith, P. P. K.; Buseck, P. R. Reply to Carbyne Forms of Carbon: Do They Exist? Science 1985, 229, 486−487. (12) Heimann, R. B.; Kleiman, J.; Salansky, N. M. A Unified Structural Approach to Linear Carbon Polytypes. Nature 1983, 306, 164−167. (13) Lagow, R. J.; Kampa, J. J.; Wei, H.-C.; Battle, S. L.; Genge, J. W.; Laude, D. A.; Harper, C. J.; Bau, R.; Stevens, R. C.; Haw, J. F.; Munson, E. Synthesis of Linear Acetylenic Carbon: The sp Carbon Allotrope. Science 1995, 267, 362−367. (14) Kastner, J.; Kuzmany, H.; Kavan, L.; Dousek, F. P.; Kürti, J. Reductive Preparation of Carbyne with High Yield. An in Situ Raman Scattering Study. Macromolecules 1995, 28, 344−353. (15) Baughman, R. H. Dangerously Seeking Linear Carbon. Science 2006, 312, 1009−1010. (16) Kroto, H. Carbyne and Other Myths about Carbon. Chem. World 2010, 7, 37. (17) Chalifoux, W. A.; Tykwinski, R. R. Synthesis of Polyynes to Model the sp-Carbon Allotrope Carbyne. Nat. Chem. 2010, 2, 967− 971. (18) Jevric, M.; Nielsen, M. B. Synthetic Strategies for Oligoynes. Asian J. Org. Chem. 2015, 4, 286−295. (19) Casari, C. S.; Tommasini, M.; Tykwinski, R. R.; Milani, A. Carbon-atom Wires: 1-D Systems with Tunable Properties. Nanoscale 2016, 8, 4414−4435. (20) Jensen, W. B. The Origin of the Term Allotrope. J. Chem. Educ. 2006, 83, 838−839. (21) IUPAC Nomenclature of Inorganic Chemistry; Connelly, N. G.; Damhus, T., Eds.; RSC Publishing: Cambridge, U.K., 2005; p 49. (22) Johnson, T. R.; Walton, D. R. M. Silylation as a Protective Method in Acetylene Chemistry. Tetrahedron 1972, 28, 5221−5236. (23) Eastmond, R.; Johnson, T. R.; Walton, D. R. M. Base-Catalyzed Cleavage of Silyl-Substituted Polyynes. Attenuation of Hydrocarbon Acidity and Transmission of Substituent Electrical Effects in LongChain Conjugated Polyacetylenes. J. Organomet. Chem. 1973, 50, 87− 92. (24) Rubin, Y.; Lin, S. S.; Knobler, C. B.; Anthony, J.; Boldi, A. M.; Diederich, F. Solution-Spray Flash Vacuum Pyrolysis: A New Method for the Synthesis of Linear Poliynes with Odd Numbers of C-C Bonds from Substituted 3,4-Dialkynyl-3-cyclobutene1-,2-diones. J. Am. Chem. Soc. 1991, 113, 6943−6949. (25) Gibtner, T.; Hampel, F.; Gisselbrecht, J.-P.; Hirsch, A. End-Cap Stabilized Oligoynes: Model Compounds for the Linear sp Carbon Allotrope Carbyne. Chem. - Eur. J. 2002, 8, 408−432. (26) Hlavatý, J.; Kavan, L.; Kubišta, J. Carbonaceous Materials from End-capped Alkynes. Carbon 2002, 40, 345−349. (27) Eisler, S.; Slepkov, A. D.; Elliott, E.; Luu, T.; McDonald, R.; Hegmann, F. A.; Tykwinski, R. R. Polyynes as a Model for Carbyne: Synthesis, Physical Properties, and Nonlinear Optical Response. J. Am. Chem. Soc. 2005, 127, 2666−2676.
remains to be tested experimentally. Such pseudocarbynes could prove to have interesting properties. However, the bottom line is that carbyne, the linear carbon allotrope, remains as elusive and theoretical as ever.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b00671. Calculated Raman spectra of several C10H2−Aun gold cluster complexes at different levels of theory, absorption spectra of several C10 species, and the optimized structure of Au8 gold cluster intercalated carbon chains. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Dr. Péter Németh for helpful discussions. REFERENCES
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NOTE ADDED IN PROOF A recent study reports the production of exceptionally long and stable carbon chains encapsulated in carbon nanotubes.99 Based on the Raman spectra, it is evident that a nanotube is essential for stabilizing the encapsulated carbon chain. Therefore, such an encapsulated chain provides another example of a pseudocarbyne.
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