Polyyne Chains Research Articles

ChemInform Abstract: Studies of Reactions Relevant to Astrochemistry

AbstractReview: 97 refs.

From GISR 2012: present status and future perspectives of Raman researches in Italy

From GISR 2012: present status and future perspectives of Raman researches in Italy

Tight Binding Model of Quantum Conductance of Cumulenic and Polyynic Carbynes

The linear carbon chains called the carbynes are possibly the most simple carbon molecular systems of interest as the potential materials for nanoelectronics. In a metallic cumulenic form of carbyne, the C atoms form the double bonds (...═C═C═...), but the single and triple bonds alternate in the semiconducting polyynic structure (...–C≡C–C≡C–...). In this work, the ballistic electrical conductance of cumulenic and polyynic carbon chains C40, C20, and C10 is calculated using the π-electron tight-binding methods. The transmission function and dependences of the current on the length of the carbon chain, the type of carbon bonds, material properties of the electrodes, bias voltage, and temperature are obtained. For the calculations of transmission function, we use the Schrödinger equation that meets the specified value of the energy E, the wave functions being the superposition of incident and reflected waves in the region of cathode, and describe the transmitted wave in the anode. For small bias voltages, the problem of calculating the transmission function in the cumulenic and polyynic chains is solved analytically by applying the difference schemes approach which permits us to pass from the tight-binding algebraic equations to the differential equations and to take advantage of their solutions. In the case of large voltages, we apply an iterative technique. The transmission functions τ(E,V) of the cumulenic and polyynic chains of similar composition differ dramatically. The energy regions of very high and low transparency of carbynes for electrons are obtained, and an oscillatory character of the energy dependence of transmission functions is pointed out. Each electronic level of the molecule corresponds to a peak in the energy dependence of transmission function with τ ≈ 1. The peaks are responsible for the resonant electron transfer between the electrodes. The voltage-dependent variations of the τ are initially quite weak, but at the higher voltages their effect is a drastic reduction of the carbyne transparency. One important feature of the current–voltage I–V characteristics is that the current initially increases with growth of the bias voltage, reaches a peak, and then drops to give rise to the negative conductance. On the typical I–V curves of the polyynic chains, there are both the peak and local minimum (valley) at higher voltages. Moreover, there are some oscillations of voltage dependences of conductance due to the discrete nature of the C chain electron energy levels. The π electron model results are compared with ab initio data on conductance of similar chains having only several C atoms. The presence of the negative resistance regions and valley in the curve I–V indicate the possibilities of design of the resonance tunneling devices based on the C chains.

Hydrogen-Terminated Graphene by Laser Vaporization-Controlled Condensation of Graphite Oxide. Observation of Hydrogen-Capped Carbon Chains CnH–, CnH+, and CnH2+ (n = 2–30) in the Gas Phase

We report the development of a facile process for the synthesis of hydrogen-terminated graphene using the laser vaporization-controlled condensation (LVCC) method. The method allows rapid deoxygenation of bulk graphite oxide (GO) in an atmosphere of helium or a helium–hydrogen mixture to produce hydrogen-terminated graphene (HTG) nanosheets without the need for any chemical reducing agents or solvents. Direct laser vaporization/ionization (LVI) of bulk GO in a vacuum coupled with a beam expansion of a carrier gas produces the carbon cluster series Cn with n in the range 4–30, in addition to hydrogenated carbon ions with a significant enhancement in the ion intensity of the n-even ions suggesting the formation of hydrogen-capped polyyne chains corresponding to the H–(C≡C)n–H+ and H–(C≡C)n– formulas. In contrast, LVI of bulk graphite under identical experimental conditions generates mainly the carbon cluster series Cn without any significant hydrogenation in both the positive and negative ion modes. The res...

Carbon Coating Precedes SWCNT Nucleation on Silicon Nanoparticles: Insights from QM/MD Simulations

Nucleation of single-walled carbon nanotubes (SWCNTs) from C2 molecules adsorbed on silicon nanoparticles (SiNPs) has been investigated using quantum chemical molecular dynamics (QM/MD) simulations. The SWCNT nucleation mechanism was observed to be strikingly different from that of traditional, transition metal catalyzed SWCNT growth. Most notably, neither a bulk Si carbide phase nor the precipitation of carbon from the nanoparticle bulk was observed to precede SWCNT nucleation on SiNPs. Instead, the intermediate stage during SWCNT nucleation featured the formation of a “carbon-coated” Si surface, i.e., one covered with networks of sp2-hybridized carbon. In addition, the QM/MD simulations indicate that the growth of these sp2-carbon networks was dependent exclusively on the dynamic motion of the polyyne chains formed on the catalyst surface. Analysis of the SiNP phase during SWCNT nucleation also indicated that nucleation proceeded while the Si catalyst remained in the solid phase. Thus, it is concluded that the SWCNT nucleation mechanism presented here was consistent with a vapor–solid–solid mechanism, rather than a vapor–liquid–solid mechanism. This conclusion correlates with recent findings concerning SWCNT nucleation on SiO2 catalyst nanoparticles (Page et al. J. Am. Chem. Soc. 2011, 133, 621).

Studies of reactions relevant to astrochemistry

We review experimental studies of bimolecular exchange reactions between neutral species that can occur in dense molecular clouds: kinetics experiments performed with the so-called CRESU technique which measure global reaction rates and dynamics measurements performed with crossed molecular beam apparatuses which bring information on the products and branching ratios. Special attention is devoted to reactions of the carbon atom which may contribute to the synthesis of long polyyne or polyene chains. Results on the reactions of C(3PJ) atoms with methylacetylene, allene, ethylene and acetylene, with CRESU (in terms of rate coefficients) and crossed-beam experiments (in terms of integral and differential reaction cross sections) are discussed. Emphasis is placed on the interest to use these complementary methods in conjunction to obtain detailed rate coefficients. It is also shown that crossed-beam experiments can furnish sensitive tests to the state-to-state cross sections and rates for rotational energy transfer by inelastic collisions obtained by theoretical methods, as exemplified with CO + H2 collisions.

First total synthesis of Debilisone C

Total synthesis of Debilisone C, a lactone containing conjugated endiyne has been achieved. The adopted strategy involves the stereoselective construction of the five membered lactone ring and the C20 endiyne chain followed by regioselective coupling of both the parts. The lactone was obtained from the oxidative cleavage of the hydroxy olefin which was derived from the benzyl protected S-epichlorohydrine by regioselective epoxide opening with allyl trimethyl silane in presence of a Lewis acid at -78 °C. Takai olefination, Pd(0)/Ag(1) catalyzed cross-coupling reaction and selective substitution of trimethylsilyl groups were successfully utilized to establish the C20 polyyne chain. The final coupling of both the parts have been carried out by alkyl coupling using LDA in THF-DMPU (1 : 1).

Synthesis of Polyyne Rotaxanes

Active-metal templating has been used to synthesize rotaxanes consisting of a phenanthroline-based macrocycle threaded around a C8, C12, or C20 polyyne chain. The crystal structure of the C12 rotaxane has been determined. In the rhenium(I) carbonyl complex of this rotaxane, with Re(CO)(3)Cl coordinated to the phenanthroline macrocycle, the proximity of the polyyne chain quenches the luminescence of the rhenium. These rotaxanes offer a new approach to controlling the environment and interactions of a polyyne chain.

Preparation of Polyynes by the Laser Ablation of Graphite in Water and Organic Solvents

Polyynes were formed by laser ablation of a graphite target in deionized water (H2O and D2O) and various organic solvents such as acetonitrile, n-hexane, and c-hexane and were identified by analyzing ultraviolet (UV) absorption and Raman spectra. We assigned the major UV absorption peaks that coincided with the electronic transitions corresponding to linear polyyne chains. The UV absorption peak intensities of a polyyne solution decreased as the holding temperature of the solution increased. Also, the absorption spectra of polyynes obtained by laser ablation of a graphite target at different volume fractions of H2O and D2O were examined.

Bent polyynes: ring geometry studied by Raman and IR spectroscopy

AbstractInfrared (IR) and Raman spectroscopy have been used to examine the vibrational characteristics of a series of three macrocyclic tetraynes in comparison with an acyclic analog. By changing the length of the alkyl tether of the macrocycles, varying degrees of bending of the tetrayne moiety can be achieved, and the joint use of IR and Raman spectroscopies provides an avenue to probe the impact of bending on the sp‐chain. The spectroscopic data show a general trend toward decreasing activation of Raman bands in the IR spectra, and vice versa, as bending of the polyyne chain is decreased. Density functional theory calculations provide a detailed rationalization of the experimental observations. Copyright © 2011 John Wiley & Sons, Ltd.

Vibrational and NMR properties of polyynes

The bond-stretching phonon modes of linear polyynes with hydrogen atom termination at the both ends are calculated as a function of chain length within the density functional theory. The frequency of one of two particular Raman active phonon modes monotonically decreases with the increase of polyyne chain length while that of the other one shows an oscillating behavior, consistent with previous Raman measurements. The relative Raman intensity of the two phonon modes are evaluated by optimized geometries for ground states and excited states. We also present a nuclear magnetic resonance (NMR) calculation for spin–spin coupling constants as a function of distance between hydrogen and carbon-13 nuclei and, within carbon-13 nuclei, up to the polyyne center of symmetry. We compare the calculated results with recent NMR experiments.

Bending Effect of sp-Hybridized Carbon (Carbyne) Chains on Their Structures and Properties

Density functional theory (DFT) calculations were exploited to evaluate the bending of polyynic and polycumulenic atomic carbon chains. It was found that the chains can be easily bent with a small arc−chord ratio. Furthermore, the bending of the chains caused the variation of bond lengths, leading to the change in the chain lengths; namely, the chain was first shortened slightly and then elongated with increasing arc−chord ratio. Furthermore, the bending decreased all bond angles as an oscillation manner, in which the larger decrease of bond angles occurred on even-ordinal-number carbon atoms than on corresponding odd-ordinal-number carbons, and the largest decrease was at the middle of the chain. With increasing the bending, the HOMO−LUMO energy gap of the polyynic chain was decreased, whereas the gap of the polycumulenic chain remained almost unchanged. This indicates that bending can increase the electric conductance of the polyynic chain, whereas the conductance of the polycumulenic chain is not affec...

Thermal annealing of SiC nanoparticles induces SWNT nucleation: evidence for a catalyst-independent VSS mechanism

Density-functional tight-binding molecular dynamics (DFTB/MD) methods were employed to demonstrate single-walled carbon nanotube (SWNT) nucleation resulting from thermal annealing of SiC nanoparticles. SWNT nucleation in this case is preceded by a change of the SiC structure from a crystalline one, to one in which silicon and carbon are segregated. This structural transformation ultimately resulted in the formation of extended polyyne chains on the SiC nanoparticle surface. These polyyne chains subsequently coalesced, forming an extended sp(2)-hybridized carbon cap on the SiC nanoparticle. The kinetics of this process were enhanced significantly at higher temperatures (2500 K), compared to lower temperatures (1200 K) and so directly correlated to the surface premelting behavior of the nanoparticle structure. Analysis of the SiC nanoparticle Lindemann index between 1000 and 3000 K indicated that SWNT nucleation at temperatures below 2600 K occurred in the solid, or quasi-solid, phase. Thus, the traditional vapor-liquid-solid mechanism of SWNT growth does not apply in the case of SiC nanoparticles. Instead, we propose that this example of SWNT nucleation constitutes evidence of a vapor-solid-solid process. This conclusion complements our recent observations regarding SWNT nucleation on SiO(2) nanoparticles (A. J. Page, K. R. S. Chandrakumar, S. Irle and K. Morokuma, J. Am. Chem. Soc., 2011, 133, 621-628). In addition, similarities between the atomistic SWNT nucleation mechanisms on SiC and SiO(2) catalysts provide the first evidence of a catalyst-independent SWNT nucleation mechanism with respect to 'non-traditional' SWNT catalyst species.

SWNT Nucleation from Carbon-Coated SiO2 Nanoparticles via a Vapor−Solid−Solid Mechanism

Since the discovery of single-walled carbon nanotubes (SWNTs) in the early 1990s, the most commonly accepted model of SWNT growth on traditional catalysts (i.e., transition metals including Fe, Co, Ni, etc.) is the vapor-liquid-solid (VLS) mechanism. In more recent years, the synthesis of SWNTs on nontraditional catalysts, such as SiO(2), has also been reported. The precise atomistic mechanism explaining SWNT growth on nontraditional catalysts, however, remains unknown. In this work, CH(4) chemical vapor deposition (CVD) and single-walled carbon nanotube (SWNT) nucleation on SiO(2) nanoparticles have been investigated using quantum-chemical molecular dynamics (QM/MD) methods. Upon supply of CH(x) species to the surface of a model SiO(2) nanoparticle, CO was produced as the main chemical product of the CH(4) CVD process, in agreement with a recent experimental investigation [Bachmatiuk et al., ACS Nano 2009, 3, 4098]. The production of CO occurred simultaneously with the carbothermal reduction of the SiO(2) nanoparticle. However, this reduction, and the formation of amorphous SiC, was restricted to the nanoparticle surface, with the core of the SiO(2) nanoparticle remaining oxygen-rich. In cases of high carbon concentration, SWNT nucleation then followed, and was driven by the formation of isolated sp(2)-carbon networks via the gradual coalescence of adjacent polyyne chains. These simulations indicate that the carbon saturation of the SiO(2) surface was a necessary prerequisite for SWNT nucleation. These simulations also indicate that a vapor-solid-solid mechanism, rather than a VLS mechanism, is responsible for SWNT nucleation on SiO(2). Fundamental differences between SWNT nucleation on nontraditional and traditional catalysts are therefore observed.

Synthesis, photophysics and excited state structure of 1,8-di(p-tolyl)-1,3,5,7-octatetrayne

The Cadiot–Chodkiewicz type C2-elongation of p-tolylacetylene followed by dimerization of the resulted diyne p-CH3C6H4(CC)2TMS (1) gave thermally stable octatetrayne p-CH3C6H4(CC)4-p-C6H4CH3 (2) as light yellow powder in 75% yield. Compound 2 was characterized by spectroscopic methods and X-ray crystallography. Careful analysis of the crystal data revealed high degree of chain linearity with a potential for 1,n-topochemical polymerization. Next, photophysical properties of 2 were studied in details by experimental and advanced theoretical methods (ab initio HF as well as DFT calculations involving both the ground and excited state geometry optimization). These properties are similar to the properties of the parent polyyne C6H5(CC)4C6H5 previously described in literature (in particular, the lowest electronic excited singlet state (S1) of 2 is a dark state). This was confirmed by the experimental facts, namely, few progressions of the stretching mode (ag) of the polyyne chain, starting from different “false origins” lying below the second excited (bright) state (S2), were observed in the fluorescence excitation spectrum. The red shift of the electronic spectra (155cm−1 and 300cm−1 for absorption and fluorescence, respectively) and an increase of an energy gap between S1 and S2 states by 450cm−1 are the effects of methylation of C6H5(CC)4C6H5 to 2. Theoretical results showed that in the excited state of 2 a shortening of single bonds and elongation of triple bonds occurred. This is in accordance with an observation of a long progression of the stretching vibration mode in the experimental absorption spectrum of 2. Besides that, stretching vibration of the polyyne chain is also active in the fluorescence spectra.

Polyyne synthesis and amorphous carbon nano-particle formation by femtosecond irradiation of benzene

A range of carbon nanoparticles together with polyyne chains are formed when liquid benzene is irradiated with 120 fs pulses of intensity ≈ 1 × 10 14 Wcm −2. The morphology and composition of these nanoparticles are reported. Atomic force microscopy (AFM) shows irradiation result in the formation of spherical, ellipsoidal and toroidal nanoparticles with sizes between 1 and 50 nm. Surface enhanced Raman spectroscopy (SERS) indicates that the composition of these particles is primarily amorphous carbon reinforced by polyyne chains with 8–14 carbon atoms. Time of flight mass spectroscopy (TOF-MS) has been used to determine the precursor molecules created in the irradiation of benzene and shows the importance of ring-opening reactions in particle formation. A simple chemical mechanism is proposed to explain the appearance of polyyne chains in irradiated benzene.

ChemInform Abstract: Toward Carbyne: Synthesis and Stability of Really Long Polyynes

AbstractReview: 81 refs.

Mechanisms of Single-Walled Carbon Nanotube Nucleation, Growth, and Healing Determined Using QM/MD Methods

Since their discovery in the early 1990s, single-walled carbon nanotubes (SWNTs) have spawned previously unimaginable commercial and industrial technologies. Their versatility stems from their unique electronic, physical/chemical, and mechanical properties, which set them apart from traditional materials. Many researchers have investigated SWNT growth mechanisms in the years since their discovery. The most prevalent of these is the vapor-liquid-solid (VLS) mechanism, which is based on experimental observations. Within the VLS mechanism, researchers assume that the formation of a SWNT starts with co-condensation of carbon and metal atoms from vapor to form liquid metal carbide. Once the liquid reaches supersaturation, the solid phase nanotubes begin to grow. The growth process is partitioned into three distinct stages: nucleation of a carbon "cap-precursor," "cap-to-tube" transformation, and continued SWNT growth. In recent years, molecular dynamics (MD) simulations have come to the fore with respect to SWNT growth. MD simulations lead to spatial and temporal resolutions of these processes that are superior to those possible using current experimental techniques, and so provide valuable information regarding the growth process that researchers cannot obtain experimentally. In this Account, we review our own recent efforts to simulate SWNT nucleation, growth, and healing phenomena on transition-metal catalysts using quantum mechanical molecular dynamics (QM/MD) methods. In particular, we have validated each stage of the SWNT condensation mechanism using a self-consistent-charge density-functional tight-binding (SCC-DFTB) methodology. With respect to the nucleation of a SWNT cap-precursor (stage 1), we have shown that the presence of a transition-metal carbide particle is not a necessary prerequisite for SWNT nucleation, contrary to conventional experimental presumptions. The formation and coalescence of polyyne chains on the metal surface occur first, followed by the formation of the SWNT cap-precursor, "ring condensation", and the creation of an sp(2)-hybridized carbon structure. In our simulations, the nucleation process takes approximately 400 ps. This first step occurs over a much longer time scale than the second stage of SWNT condensation (approximately 50 ps). We therefore observe SWNT nucleation to be akin to the rate-limiting step of the SWNT formation process. In addition to the QM/MD simulation of various stages of SWNT nucleation, growth, and healing processes, we have determined the effects of temperature, catalyst composition, and catalyst size on the kinetics and mechanism of SWNT growth. With respect to temperature dependence, we observe a "sweet-spot" with respect to the efficiency of SWNT growth. In addition, Ni-catalyzed SWNT growth is observed to be 70-100% faster compared to Fe-catalyzed SWNT growth, depending on the catalyst particle size. We also observe a noticeable increase in SWNT growth rates using smaller catalyst particles. Finally, we review our recent QM/MD investigation of SWNT healing. In particular, we recount mechanisms by which adatom defects, monovacancy defects, and a "5-7 defect" are removed from a nascent SWNT. The effectiveness of these healing mechanisms depends on the rate at which carbon moieties are incorporated into the growing SWNT. Explicitly, we observe that healing is promoted using a slower carbon supply rate. From this rudimentary control of SWNT healing, we propose a route towards chirality-controlled SWNT growth.

Vibrational properties of sp carbon atomic wires in cluster‐assembled carbon films

AbstractLinear chains made by a single row of sp‐hybridized carbon are predicted to display fascinating mechano‐electronic properties connected with their termination and stabilization inside realistic carbon structures. The present work describes how the computed vibrational properties of cumulenic and polyynic carbon chains allow one to interpret the carbynic features observed in Raman spectra of cluster‐assembled sp–sp2 films. The overall picture is consistent with the measured decay of the components induced by air or oxygen exposure.magnified image The electronic properties of carbon atomic chains attached to the edges of graphitic fragments can be controlled by a relative twist of the terminating planes.

Comparison of single-walled carbon nanotube growth from Fe and Ni nanoparticles using quantum chemical molecular dynamics methods

Metal-catalyzed SWCNT growth has been modeled using quantum chemical molecular dynamics (QM/MD) in conjunction with feeding of carbon atoms to C 40–Fe 55 and C 40–Ni 55 model complexes at 1500 K. The rate of Fe 55-catalyzed SWCNT growth determined in this work was 19% slower than the Fe 38-catalyzed growth rate. Conversely, Ni 55-catalyzed SWCNT growth exhibited a growth rate 69% larger than of Fe 55-catalyzed SWCNT growth, a fact consistent with excellent performance of Ni in laser evaporation and carbon-arc experiments. Ni 55-catalyzed growth was preceded by the formation of extended polyyne chains at the base of the SWCNT, and so differed fundamentally from Fe 55-catalyzed growth. These polyyne chains usually persisted for 10–30 ps. Subsequent polyyne ring condensation resulted in carbon polygon addition at the SWCNT base. The relative stabilities of the C n carbon cluster moieties on the Fe 55 and Ni 55 surfaces were consistent with the relative strengths of the Fe–C, Ni–C and C–C interactions. The presence of smaller carbon moieties on the Fe 55 surface led to the dissemination of surface iron atoms, and subsequent diffusion of short C n units through the subsurface region of the catalyst particle. Conversely, the Ni 55 catalyst particle was observed to be more stable, remaining intact to a greater extent.