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14 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Cumulene Wires Display Increasing Conductance with Increasing Length

One-dimensional sp-hybridized carbon wires, including cumulenes and polyynes, can be regarded as finite versions of carbynes. They are likely to be good candidates for molecular-scale conducting wires as they are predicted to have a high-conductance. In this study, we first characterize the single-molecule conductance of a series of cumulenes and polyynes with a backbone ranging in length from 4 to 8 carbon atoms, including [7]cumulene, the longest cumulenic carbon wire studied to date for molecular electronics. We observe different length dependence of conductance when comparing these two forms of carbon wires. Polyynes exhibit conductance decays with increasing molecular length, while cumulenes show a conductance increase with increasing molecular length. Their distinct conducting behaviors are attributed to their different bond length alternation, which is supported by theoretical calculations. Furthermore, this study confirms the long-standing theoretical predictions on sp-hybridized carbon wires and demonstrates that cumulenes can form highly conducting molecular wires.

36 MATERIALS SCIENCE↗
NASA NTRS1997

First astronomical detection of the cumulene carbon chain molecule H2C6 in TMC-1

The cumulene carbenes are important components of hydrocarbon chemistry in low-mass star-forming cores. Here we report the first astronomical detection of the long-chain cumulene carbene H2C6 in the interstellar cloud TMC-1, from observations of two of its rotational transitions: J(K,K') = 7(1,7) --> 6(1,6) at 18.8 GHz and 8(1,8) --> 7(1,7) at 21.5 GHz, using NASA's Deep Space Network 70 m antenna at Goldstone, California. In addition we also observed the shorter cumulene carbene H2C4 at the same position. The fractional abundance of H2C6 relative to H2 is about 4.7 x 10(-11) and that of H2C4 is about 4.1 x 10(-9). The abundance of H2C6 is in fairly good agreement with gas-phase chemical models for young molecular cloud cores, but the abundance of H2C4 is significantly larger than predicted.

NASA Discipline Exobiology↗
NASA NTRS1991

Observations of cumulene carbenes, H2CCCC and H2CCC, in TMC-1

Attention is given to the carbon chain molecule H2CCCC, detected in the dark cloud TMC-1 for the first time in the course of a molecular line survey using the Nobeyama 45-m telescope. From nine transitions observed in the frequency region of 17-45 GHz, the total column density of H2CCCC in TMC-1 is derived to be 7.5(+/-2.0) x 10 exp 12/sq cm, which is about half of the value reported in IRC + 10216. Five transitions of a related carbon chain molecule, H2CCC, were also detected in TMC-1. The column density of H2CCC obtained in TMC-1, 2.8(+/-0.9) x 10 exp 12/sq cm, is a factor of three smaller than that of H2CCCC. The ortho-to-para abundance ratios of H2CCCC and H2CCC were found to be 4.2 +/-1.5 and 5.9 +/-2.0, respectively. The chemical reactions of these carbon-chain molecules in dark clouds are discussed.

Kawaguchi, Kentarou↗
DOE OSTI2024

Divalent Titanium via Reductive N−C Coupling of a Ti IV Nitrido with π ‐Acids

The nitrido-ate complex [(PN) 2 Ti(N){μ 2 -K(OEt 2 )}] 2 (1) (PN − =(N-(2-P i Pr 2 -4-methylphenyl)-2,4,6-Me 3 C 6 H 2 ) reductively couples CO and isocyanides in the presence of DME or cryptand (Kryptofix222), to form rare, five-coordinate Ti II complexes having a linear cumulene motif, [K(L)][(PN) 2 Ti(NCE)] (E=O, L=Kryptofix222, (2); E=NAd, L=3 DME, (3); E=N t Bu, L=3 DME, (4); E=NAd, L=Kryptofix222, (5)). Oxidation of 2–5 with [Fc][OTf] afforded an isostructural Ti III center containing a neutral cumulene, [(PN) 2 Ti(NCE)] (E=O, (6); E=NAd (7), N t Bu (8)) and characterization by CW X-band EPR spectroscopy, revealed unpaired electron to be metal centric. Moreover, 1e − reduction of 6 and 7 in the presence of Kryptofix222cleanly reformed corresponding discrete Ti II complexes 2 and 5, which were further characterized by solution magnetization measurements and high-frequency and -field EPR (HFEPR) spectroscopy. Furthermore, oxidation of 7 with [Fc*][B(C 6 F 5 ) 4 ] resulted in a ligand disproportionated Ti IV complex having transoid carbodiimides, [(PN) 2 Ti(NCNAd) 2 ] (9). Comparison of spectroscopic, structural, and computational data for the divalent, trivalent, and tetravalent systems, including their 15 N enriched isotopomers demonstrate these cumulenes to decrease in order of backbonding as Ti II →Ti III →Ti IV and increasing order of π-donation as Ti II →Ti III →Ti IV , thus displaying more covalency in Ti III species. Lastly, we show a synthetic cycle whereby complex 1 can deliver an N-atom to CO and CNAd.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Competition between Hückel’s Rule and Jahn–Teller Distortion in Small Carbon Rings: A Quantum Monte Carlo Study

Structural properties and energetics of carbon rings are studied with the diffusion Monte Carlo (DMC) method. Our DMC-based geometry optimization reveals that both polyynic C 4n and cumulenic C 4n+2 rings exhibit bond length alternations for n >= 3, which is understood to be due to Jahn-Teller distortions. The bond length alternation even in a cumulenic (4n + 2) carbon ring was experimentally observed in a recently synthesized C 18 molecule. From a comparison of the DMC cohesive energies of C 4n with those of C 4n + 2 , we present a comprehensive picture of the competition between Huckel's rule and Jahn-Teller distortion in small carbon rings; the former is more dominant than the latter for n < 5 where C 4n + 2 rings are more stable than C 4n , while C 4n rings are as stable as C 4n + 2 for n < 5 where dimerization effects due to Jahn-Teller distortion are more important.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1996

First Astronomical Detection of the Carbene Chain H2C6

The cumulene carbenes are important components of hydrocarbon chemistry in low mass star forming cores. Here we report the first astronomical detection of the long chain cumulene carbene H(sub 2)C(sub 6), in the insterstellar cloud TMC1, from observations of two of its rotational transitions:...

interstellar molecules star formation hydrocarbon ↗
DOE OSTI2022

Infrared laser ablation of poly(vinylidene fluoride): The Loss of HF

The mechanism and kinetics of infrared laser ablation of poly(vinylidene fluoride) (PVDF) in vacuum have been investigated to understand how this can be used as a polymer processing technology. The laser heats the surface and initiates decomposition of macromolecules in the molten polymer layer which rapidly leads to production of an ablative flow of polymer decomposition products. There is only a short induction period of up to 2 s before ablation begins. After the initial period, the rate of mass loss increases linearly with the time of laser exposure. Spherical particles with an average diameter of about 100 nm are formed on the surface of the powder coating obtained by laser ablation. Here, the particle distribution ranges from 50 nm to 300 nm. The main gaseous product of ablation is HF from dehydrofluorination. A second important ablation process is the formation of gaseous carbenes generated during secondary reactions. Proposed ablation reactions due to laser-induced thermal dehydrofluorination occur in two stages. In the first stage, a double bond appears in PVDF due to direct loss of HF. Subsequent detachment of HF leads to the appearance of both triple bonds and cumulene bonds.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

A Combined Crossed Molecular Beam and Theoretical Investigation of the Elementary Reaction of Tricarbon (C 3 (X 1 Σ g + )) with Diacetylene (C 4 H 2 (X 1 Σ g + )): Gas Phase Formation of the Heptatriynylidyne Radical ( l -C 7 H(X 2 Π))

An elucidation of the underlying formation pathways to acyclic hydrocarbons such as polyynes (C n H 2 ), cumulenes (C n H 2 ), and linear resonantly stabilized linear radicals (l-C n H) is indispensable to understand the hydrocarbon chemistry in extreme low and high temperature environments. In this study, we exploited the crossed molecular beam technique to investigate the reaction of tricarbon C 3 (X 1 Σ g + ) with diacetylene (butadiyne; HCCCCH; X 1 Σ g + ) at a collision energy of 47 ± 1 kJ mol ⁻1 . The experimental data were merged with ab initio calculations of the singlet C 7 H 2 potential energy surface (PES) revealing that the reaction is initiated via the formation of an initial van der Waals reactant complex in the entrance channel. Subsequent rearrangements lead to various carbene-type and cyclic intermediates via ring-opening, ring-closure, and hydrogen migration processes eventually forming acyclic C 7 H 2 isomers prior to their barrierless unimolecular decomposition to the most stable linear isomer, heptatriynylidyne (C 7 H, X 2 Π) in an overall endoergic reaction (+57 kJ mol ⁻1 ). The reaction exhibits strong similarities to the tricarbon – acetylene (C 3 – C 2 H 2 ). Furthermore, the significant energy threshold suggests that the tricarbon reaction with (poly)acetylenes forming resonantly stabilized linear radicals are open in high-temperature environments such as combustion flames and circumstellar envelopes of carbon stars and planetary nebulae as their descendants; however, these reactions are closed in low-temperature environments as in cold molecular clouds and hydrocarbon-rich atmospheres of planets and their moons such as in Titan.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Engineering phase-frustration-induced flat bands in an aza-triangulene covalent kagome lattice

π-Conjugated covalent organic frameworks provide a versatile molecular scaffold for the realization of designer quantum nanomaterials. Strong electron–electron correlation within these artificial lattices can give rise to exotic phases of matter. Their experimental realization, however, requires precise control over orbital symmetry, charge localization and band dispersion, all arising from the effective hybridization between molecular linkers and nodes. Here we present a modular strategy for constructing diatomic kagome lattices from aza-[3]triangulene nodes, in which a D 3h -symmetric ground state is stabilized through resonance contributions from a cumulenic linker. First-principles density functional theory and scanning tunnelling spectroscopy reveal that the hybridization of a sixfold-degenerate set of edge-localized Wannier functions in the unit cell gives rise to orbital-phase-frustration-induced non-trivial flat bands. These results establish a general design principle for engineering orbital interactions in organic lattices and open a pathway towards programmable covalent-organic-framework-based quantum materials with correlated electronic ground states.

conjugated polymers↗
NASA NTRS1990

The C9 cluster - Structure and infrared frequencies

A structural characterization is presented for C9, the largest cluster predicted to have a linear ground state. IR laser diode absorption spectroscopy of a supersonic carbon jet is used to measure 51 lines of the nu6 (sigma u) antisymmetric stretch fundamental transition within the linear 1Sigma(g)+ ground electronic state of C9. The measured ground state rotational constant suggests an effective C-C bond length which is in good agreement with ab initio theory and consistent with cumulenic bonding in this cluster.

Heath, J. R.↗
NASA NTRS1995

Organic molecules in the gas phase of dense interstellar clouds

Since a previous Committee on Space Research (COSPAR) review on this subject, the number of molecular species identified by astronomers in dense interstellar clouds or in the envelopes expelled by evolved stars has grown from about eighty to approximately one hundred. Recent detections in stellar envelopes include the radical CP, the second phosphorus-containing astronomical molecule; SiN, the first astronomical molecule with a Si-N bond; and the HCCN radical. In the dense interstellar clouds recent detections or verifications of previous possible identifications include the H3O(+) ion, which is a critical intermediary in the production of H2O and O2; the CCO radical, which is isoelectronic with HCCN; the SO(+) ion, which appears to be diagnostic of shock chemistry; two new isomers of cyanoacetylene, HCCNC and CCCNH; and the two cumulenes H2C3 and H2C4. Some recent work is also described on the mapping of interstellar clouds in multiple molecular transitions in order to separate variations in chemical abundance from gradients in physical parameters.

Irvine, W. M.↗
NASA NTRS1993

Infrared laser spectroscopy of jet-cooled carbon clusters: structure of triplet C6

We report the first structural characterization of the triplet isomer of C6. Forty-one rovibrational/fine structure transitions in the nu 4(sigma u) antisymmetric stretch fundamental of the C6 cluster have been measured by diode laser absorption spectroscopy of a supersonic carbon cluster beam. The observed spectrum is characteristic of a centrosymmetric linear triplet state with cumulene-type bonding. The measured ground state rotational constant B0 = 0.048 479 (10)cm-1 and the effective bond length r(eff) = 1.2868 (1) angstroms are in good agreement with ab initio predictions for the linear triplet (3 sigma g-) state of C6.

Non-NASA Center↗
NASA NTRS1994

Characterization of silicon-carbon clusters by infrared laser spectroscopy: the nu 3(sigma u) band of linear Si2C3

The nu 3(sigma u) fundamental vibration of 1 sigma g+ Si2C3 has been observed using a laser vaporization-supersonic cluster beam-diode laser spectrometer. Forty rovibrational transitions were measured in the range of 1965.8 to 1970.9 cm-1 with a rotational temperature of 10-15 K. A least-squares fit of these transitions yielded the following molecular constants: nu 3(sigma u)=1968.188 31(18) cm-1, B"=0.031 575 1(60) cm-1, and B'=0.031 437 4(57) cm-1. These results are in excellent agreement with recent Fourier transform infrared (FTIR) measurements of Si2C3 trapped in a solid Ar matrix [J. Chem. Phys. 100, 181(1994)] and with ab initio calculations [J. Chem. Phys. 100, 175 (1994)] which suggest cumulenic-like bonding for Si2C3, analogous to the isovalent C5 carbon cluster.

Non-NASA Center↗