Small carbon clusters: spectroscopy, structure, and energetics

Optical Spectroscopy of Small Carbon Clusters from Electron-Impact Fragmentation and Ionization of Fullerene-C60

A series of cationic molecular fragments (C _n⁺, n = 11, 12, 15, 16, 18, and 21), produced by electron-impact ionization of C₆₀ in the gas phase, were each mass-selected and accumulated in cryogenic Ne matrices. Optical absorption measurements in the UV-vis and IR spectral ranges reveal linear carbon chain structures. In particular, we have observed the known electronic transitions of linear C₁₁, C₁₅, and C₂₁. The NIR transitions of linear C₁₅^-, C₁₆^-, and C₁₈^- have also been detected, indicating that soft-landing of the corresponding cations can also involve charge-changing. Newly observed electronic absorptions at 410.3 and 429.9 nm have been assigned to linear C₁₈ absorptions at 438.2, 443.5, 422.3, and 433.7 nm, to linear C₁₅⁺, and absorption at 395.5 nm, to linear C₁₆. Increasing deposition energy leads to fragmentation upon impact. This is indicated by absorptions of C₁₀ (313, 316.3 nm), when depositing C _n⁺ ( n = 11, 15, 16) as well as C₁₂ (332 nm) or C₁₄ (347.4, 356.6 nm), when depositing C₁₅⁺ or C₁₆⁺, respectively. These were previously assigned to cyclic isomers. We reassign them to linear isomers here on the basis of plausibility arguments. The observations have been supported by time-dependent density functional theory calculations for ring and chain isomers of C _n^+/-/0, 10 ≤ n ≤ 20 up to the vacuum-UV range. The electronic absorptions of carbon chains are at least 1 order of magnitude stronger than all NIR electronic absorptions of C₆₀⁺, which have recently been attributed to several of the diffuse interstellar bands. Considering that fullerene multifragmentation yields long carbon chains that have very strong absorptions both in the UV-vis and IR spectral regions, these systems appear to be good candidates to be observed in regions of space containing fullerenes.

Difficulties and Virtues in Assessing the Potential Energy Surfaces of Carbon Clusters via DMBE Theory: Stationary Points of Cκ (κ = 2-10) at the Focal Point

The previously reported potential energy surfaces (PESs) of ground-state singlet C₃ and triplet C₄ are here utilized as input for the construction of approximate cluster expansions for larger C_κ (κ = 5-10) species. Relying primarily on the double many-body expansion (DMBE) approach, global potentials are obtained by summing the total interaction energies of all atomic subclusters up to four-body terms. The notable capability of the final forms in predicting good estimates of the linear global minima and their thermochemical/structural properties may provide important insights into the structure-determining nature of the (2 + 3 + 4) terms. The main difficulties and virtues in assessing C_κ's via DMBE theory are analyzed and new prospects given for the construction of global reliable PESs for the target species.

Mechanistic Insights into the Formation of Lithium Fluoride Nanotubes

Born-Oppenheimer molecular dynamics (BOMD) and periodic density functional theory (DFT) calculations have been applied for describing the mechanism of formation of lithium fluoride (LiF) nanotubes with cubic, hexagonal, octagonal, decagonal, dodecagonal, and tetradecagonal cross-sections. It has been shown that high energy structures, such as nanowires, nanorings, nanosheets, and nanopolyhedra are transient species for the formation of stable nanotubes. Unprecedented (LiF)_n clusters (n≤12) were also identified, some of them lying less than 10 kcal mol^-[1] above their respective global minima. Such findings indicate that stochastic synthetic techniques, such as laser ablation and chemical vapor deposition, should be combined with a template-driven procedure in order to generate the nanotubes with adequate efficiency. Apart from the stepwise growth of LiF units, the formation of nanotubes was also studied by rolling up a planar square sheet monolayer, which could be hypothetically produced from the exfoliation of the FCC crystal structure. It was shown that both pathways could lead to the formation of alkali halide nanotubes, a still unprecedented set of one-dimensional materials.

Infrared spectroscopy of C3-(H2O)n and C3-(D2O)n complexes in helium droplets

The C₃ molecule is an important species with implications in combustion and astrochemistry, and much of the interest in this molecule is related to its interactions with other species found in these environments. We have utilized helium droplet beam techniques along with a recently developed carbon cluster evaporation source to assemble C₃-(H₂O)_n and C₃-(D₂O)_n complexes with n = 1-2 and to record their rovibrational spectra. We observe only a single isomer of the n = 1 complex, in agreement with theoretical predictions as well as data from earlier matrix isolation studies. The spectra of the n = 1 complex are consistent with the ab initio structure, which involves a nearly linear arrangement of CCC-HO atoms in the complex. The C₃-H₂O spectrum we obtain exhibits slight differences from the analogous C₃-D₂O spectrum, which we assign to a difference in linewidth between the two spectra. We have also examined the n = 2 species and obtained a structure that appears to be distinct from those observed in matrix isolation studies and, to our knowledge, has not been previously observed.

Unraveling the Metastability of C n2+ ( n = 2-4) Clusters

Pure carbon clusters have received considerable attention for a long time. However, fundamental questions, such as what the smallest stable carbon cluster dication is, remain unclear. We investigated the stability and fragmentation behavior of C _n²⁺ ( n = 2-4) dications using state-of-the-art atom probe tomography. These small doubly charged carbon cluster ions were produced by laser-pulsed field evaporation from a tungsten carbide field emitter. Correlation analysis of the fragments detected in coincidence reveals that they only decay to C _n-1⁺ + C⁺. During C₂²⁺ → C⁺ + C⁺, significant kinetic energy release (∼5.75-7.8 eV) is evidenced. Through advanced experimental data processing combined with ab initio calculations and simulations, we show that the field-evaporated diatomic ¹²C₂²⁺ dications are either in weakly bound ³Π_u and ³Σ_g^- states, quickly dissociating under the intense electric field, or in a deeply bound electronic ⁵Σ_u^- state with lifetimes >180 ps.

Magnetically induced current density in triple-layered beryllium–boron clusters

Magnetically induced current densities reveal the double aromatic character of the examined Be–B clusters.

An improved descriptor of cluster stability: application to small carbon clusters

The mass spectra of gas-phase clusters in cluster beams have a rich structure where the relative heights of the peaks compared to peaks corresponding to the clusters of neighboring sizes reveal the stability of the clusters as a function of size N. In an analysis of the published mass spectrum of carbon cluster cations CN+ with N ≤ 16 we have employed the most common descriptor of cluster stability, which is based on the comparison of the total energy of the cluster of size N with the averaged energies of clusters with sizes N + 1 and N - 1. These energies have been obtained from density functional calculations. The comparison between the stability function and the mass spectrum leaves some experimental features unexplained; in particular, the correlation with the detailed variation of the height of the mass peaks as a function of size N is not satisfactory. We then propose a novel stability descriptor which improves the features substantially, in particular the correlation with the detailed variation of the height of the mass peaks. The new stability index is based on the comparison of the atom-evaporation energy of the cluster of size N with the averaged atom-evaporation energies of clusters with sizes N + 1 and N - 1. The substantial improvement achieved is attributed to the fact that evaporation energies are quantities directly connected with the processes controlling the cluster abundances in the beam.

Boron-based binary Be6B102- cluster: three-layered aromatic sandwich, electronic transmutation, and dynamic structural fluxionality

Boron-based nanoclusters have unique structures, bonding, and dynamic properties, which originate from boron's electron-deficiency. We demonstrate here that pouring in extra electrons can alter such systems fundamentally. A coaxial triple-layered Be6B102- sandwich cluster is designed via global structural searches and quantum chemical calculations. It is well defined as the global minimum, which consists of a slightly elongated B10 monocyclic ring and two Be3 rings, the latter forming a Be6 trigonal-prism albeit without interlayer Be-Be bonding. The B10 ring shows structural and chemical integrity with respect to the Be3 rings, and yet it differs markedly from the free B10 cluster and closely resembles the C10 cluster. The present data testify to the idea of electronic transmutation, in which a B- is equivalent to C and a B10 cluster, upon charge-transfer, is converted to and stabilized as a monocyclic ring analogous to C10. Chemical bonding analyses reveal that the B10 ring in the Be6B102- cluster has 10π and 10σ delocalization and each Be3 ring is held together by 2σ electrons, collectively rendering four-fold π/σ aromaticity. The bonding pattern is in line with the formula of [Be3]4+[B10]10-[Be3]4+, suggesting a highly charged electron-transfer complex. Furthermore, the Be6B102- cluster is dynamically fluxional with dual modes of revolution (orbiting) and rotation (twisting), being structurally robust at least up to a temperature of 1500 K.

Adsorption of Monocyclic Carbon Rings on Graphene: Energetics Revealed via Continuum Modeling

Gas-phase spectroscopic detection of tiny carbon clusters is a recent success story in the area of carbon cluster research. However, experimental production and isolation of these clusters are extremely difficult because of their high reactivity. One possibility to isolate the generated clusters would be to deposit them on graphene and to desorb them for subsequent use. One of the pertinent questions toward realizing this would be the energetics of the adsorption process. Therefore, in this work, the energetics for the adsorption of the monocyclic carbon rings (C _n with n = 10, 12, 14, 16, 18, 20, and 22) on a graphene sheet are investigated using the analytical approaches, developed earlier by Hill and co-workers. The adsorption process here is driven by the noncovalent interactions between the carbon rings and the graphene sheet. The analyses of the interaction energies as a function of both the vertical distance Z and the rotational angle ϕ are performed in order to determine the preferred orientations, equilibrium positions, and binding energies for the adsorption of various carbon rings on graphene. We find that the preferred orientation of the rings with respect to the graphene sheet is the parallel orientation. The results from continuum, discrete-continuum, and discrete models are in good agreement. Further, computations using density functional theory and quantum mechanics/molecular mechanics approaches are performed, and comparisons of the computed energetics with the data from the models are reported. Finally, we highlight the scope and the limitations of the analytical models.

Ab Initio Potential Energy Surfaces of C3 Collision with Proton and Quantum Dynamics of Rotational Transition

New ab initio potential energy surfaces have been generated for the ground state and low-lying excited states of the H⁺ + C₃ system using the multireference configuration interaction (MRCI) method with Dunning's augmented correlation consistent polarized valence quadruple-zeta (aug-cc-pVQZ) basis sets. The ground state and low-lying excited states show avoided crossing, indicating nonadiabatic coupling. The anisotropy of the ground-state surface has been analyzed by computing the multipolar expansion coefficients with the frozen C-C equilibrium bond length. The asymptotic potential has been merged with the interaction potential by spline fit. This potential is then used in the full close coupling calculations of rotational excitation in C₃ collision with the proton for rotational levels j = 0, 2, 4, 6, 8 at very low collision energy. By averaging the cross sections over a Boltzmann distribution of velocities of the incoming atom, we obtain and discuss corresponding rate coefficients of C₃ collision with the proton in the interstellar medium.

Cn (n=2-4): current status

The major aspects of the C₂, C₃ and C₄ elemental carbon clusters are surveyed. For C₂, a brief analysis of its current status is presented. Regarding C₃, the most recent results obtained in our group are reviewed with emphasis on modelling its potential energy surface which is particularly complicated due to the presence of multiple conical intersections. As for C₄, the most stable isomeric forms of both triplet and singlet spin states and their possible interconversion pathways are examined afresh by means of accurate ab initio calculations. The main strategies for modelling the ground triplet C₄ potential are also discussed. Starting from a truncated cluster expansion and a previously reported DMBE form for C₃, an approximate four-body term is calibrated from the ab initio energies. The final six-dimensional global DMBE form so obtained reproduces all known topographical aspects while providing an accurate description of the C₄ linear-rhombic isomerization pathway. It is therefore commended for both spectroscopic and reaction dynamics studies.This article is part of the theme issue 'Modern theoretical chemistry'.

Most favorable cumulenic structures in iron-capped linear carbon chains are short singlet odd-carbon dications: a theoretical view

DFT computations suggest that the odd iron-capped linear-carbon dications exhibit large ΔE_S–T values and more cumulenic structures than short even-carbon chains.

Revisit of the Saito-Dresselhaus-Dresselhaus C2 ingestion model: on the mechanism of atomic-carbon-participated fullerene growth

We introduce a mechanistic study based on a controversial fullerene bottom-up growth model proposed by R. Saito, G. Dresselhaus, and M. S. Dresselhaus. The so-called SDD C₂ addition model has been dismissed as chemically inadmissible but here we prove that it is feasible via successive atomic-carbon-participated addition and migration reactions. Kinetic calculations on the formation of isolated pentagon rule (IPR)-obeying C₇₀ and Y₃N@C₈₀ are carried out by employing the SDD model for the first time. A stepwise mechanism is proposed with a considerably low barrier of ca. 2 eV which is about 3 eV lower than a conventional isomerization-containing fullerene growth pathway.

Lowest bending mode of 13C-substituted C3 and an experimentally derived structure

The ν₂ lowest bending mode of linear C₃ and of all its ¹³C-substituted isotopologues was recorded using a terahertz-supersonic jet spectrometer in combination with a laser ablation source. Sixty-five ro-vibrational transitions between 1.8 and 1.9 THz have been assigned to linear C12C12C12, C12C12C13, C12C13C12, C13C13C12, C13C12C13, and C13C13C13. For each isotopologue, molecular parameters were obtained and the C-C-bond length was derived experimentally. All results are in excellent agreement with recent ab initio calculations [B. Schröder and P. Sebald, J. Chem. Phys. 144, 044307 (2016)]. The new measurements explain why the interstellar search for singly substituted C12C12C13 has failed so far. A spectral line list with recommended transition frequencies based on global data fits is given to foster future interstellar detections.

M3C: A Computational Approach To Describe Statistical Fragmentation of Excited Molecules and Clusters

The Microcanonical Metropolis Monte Carlo method, based on a random sampling of the density of states, is revisited for the study of molecular fragmentation in the gas phase (isolated molecules, atomic and molecular clusters, complex biomolecules, etc.). A random walk or uniform random sampling in the configurational space (atomic positions) and a uniform random sampling of the relative orientation, vibrational energy, and chemical composition of the fragments is used to estimate the density of states of the system, which is continuously updated as the random sampling populates individual states. The validity and usefulness of the method is demonstrated by applying it to evaluate the caloric curve of a weakly bound rare gas cluster (Ar₁₃), to interpret the fragmentation of highly excited small neutral and singly positively charged carbon clusters (C_n, n = 5,7,9 and C_n⁺, n = 4,5) and to simulate the mass spectrum of the acetylene molecule (C₂H₂).

Mid infrared emission spectroscopy of carbon plasma

Mid infrared time-resolved emission spectra were recorded from laser-induced carbon plasma. These spectra constitute the first study of carbon materials LIB spectroscopy in the mid infrared range. The carbon plasma was induced using a Q-switched Nd: YAG laser. The laser beam was focused to high purity graphite pellets mounted on a translation stage. Mid infrared emission from the plasma in an atmospheric pressure background gas was detected by a cooled HgCdTe detector in the range 4.4-11.6μm, using long-pass filters. LIB spectra were taken in argon, helium and also in air. Despite a gate delay of 10μs was used there were strong backgrounds in the spectra. Superimposed on this background broad and noisy emission bands were observed, the form and position of which depended somewhat on the ambient gas. The spectra were digitally smoothed and background corrected. In argon, for instance, strong bands were observed around 4.8, 6.0 and 7.5μm. Using atomic spectral data by NIST it could be concluded that carbon, argon, helium and nitrogen lines from neutral and ionized atoms are very weak in this spectral region. The width of the infrared bands supports molecular origin. The infrared emission bands were thus compared to vibrational features of carbon molecules (excluding C2) of various sizes on the basis of previous carbon cluster infrared absorption and emission spectroscopic analyses in the literature and quantum chemical calculations. Some general considerations are given about the present results.

Wheel-like, elongated, circular, and linear geometries in boron-based CnB7−n(n = 0–7) clusters: structural transitions and aromaticity

Boron–carbon mixed clusters C_nB_7−n(n= 0–7) assume wheel-like, elongated, circular, and linear geometries, dictated by (π and σ) aromaticity and antiaromaticity.

Fullerene and nanotube growth: new insights using first principles and molecular dynamics

Shortly after the discovery of fullerenes, many researchers pointed out that carbon nanotubes could be considered as elongated fullerenes. However, the detailed formation mechanism for both structures has been a topic of debate for several years, and consequently it has been difficult to draw a clear connection between the two systems. While the synthesis conditions appear to be different for both fullerenes and nanotubes, here, we demonstrate that it is highly likely that, at an initial growth stage, single-walled carbon nanotubes begin to grow from a hemisphere-like fullerene cap. More importantly, by analysing the minimum-energy path, it is shown that the insertion of C2 fragments drives the transformation of this fullerene cap into an elongated structure that leads to the formation of very short carbon nanotubes.This article is part of the themed issue 'Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene'.

The Jahn-Teller plus pseudo-Jahn-Teller vibronic problem in the C3 radical and its topological implications

The combined Jahn-Teller plus pseudo-Jahn-Teller [(E'+A1')⊗e'] problem is discussed for the tricarbon radical (C3) by means of ab initio calculations at the multireference configuration interaction level of theory. For the (1)E' electronic state arising from a e'(2) valence configuration, three additional symmetry-equivalent C2v seams are found to lie in close proximity to the D3h symmetry-required seam over the entire range of the breathing coordinate here considered. As the perimeter of the molecule increases, the C2v disjoint seams approach the D3h one almost linearly and ultimately coalesce with it at Q1 = 5.005 a0, thence forming an intersection node or confluence. By further increasing the size of the molecular triangle, the C2v seams get rotated by ±π in the g-h plane. A three-state vibronic Hamiltonian is also proposed to model locally the title system and shown to accurately mimic the calculated data over the region close to the minimum energy crossing point. No net geometric phase effect is observed when the associated electronic wave functions are adiabatically transported along closed paths encircling the four singularity points. For all paths enclosing the intersection node, the sign reversal criterion is shown to be not fulfilled, even for infinitesimal loops. The results so obtained are expected to be valid for other ring systems experiencing similar topological attributes.

High-level theoretical rovibrational spectroscopy beyond fc-CCSD(T): The C3 molecule

An accurate local (near-equilibrium) potential energy surface (PES) is reported for the C3 molecule in its electronic ground state (X̃(1)Σg (+)). Special care has been taken in the convergence of the potential relative to high-order correlation effects, core-valence correlation, basis set size, and scalar relativity. Based on the aforementioned PES, several rovibrational states of all (12)C and (13)C substituted isotopologues have been investigated, and spectroscopic parameters based on term energies up to J = 30 have been calculated. Available experimental vibrational term energies are reproduced to better than 1 cm(-1) and rotational constants show relative errors of not more than 0.01%. The equilibrium bond length has been determined in a mixed experimental/theoretical approach to be 1.294 07(10) Å in excellent agreement with the ab initio composite value of 1.293 97 Å. Theoretical band intensities based on a newly developed electric dipole moment function also suggest that the infrared active (1, 1(1), 0)←(0, 0(0), 0) combination band might be observable by high-resolution spectroscopy.

Pseudocarbynes: Charge-Stabilized Carbon Chains

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 C≡C 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.

Molecular Structure and Bonding in Plutonium Carbides: A Theoretical Study of PuC3

The most relevant species of plutonium tricarbide were characterized using theoretical methods. The global minimum is predicted to be a fan structure where the plutonium atom is bonded to a quasi-linear C3 unit. A rhombic isomer, shown to be a bicyclic species with transannular C-C bonding, lies about 39 kJ/mol above the fan isomer. A linear PuCCC isomer and a three-membered ring CPuC2 isomer were found to be higher in energy (150 and 195 kJ/mol, respectively, above the predicted global minimum). The possible processes for the formation of these species are discussed, and the IR spectra were predicted to help in possible experimental detection. The nature of the Pu-C interaction has been analyzed in terms of a topological analysis of the electronic density, showing that Pu-C bonding is essentially ionic with a certain degree of covalent character.

Highly Unsaturated Platinum and Palladium Carbenes PtC3 and PdC3 Isolated and Characterized in the Gas Phase

Carbenes of platinum and palladium, PtC3 and PdC3 , were generated in the gas phase through laser vaporization of a metal target in the presence of a low concentration of a hydrocarbon precursor undergoing supersonic expansion. Rotational spectroscopy and ab initio calculations confirm that both molecules are linear. The geometry of PtC3 was accurately determined by fitting to the experimental moments of inertia of twenty-six isotopologues. The results are consistent with the proposal of an autogenic isolobal relationship between O, Au(+) , and Pt atoms.

Highly Unsaturated Platinum and Palladium Carbenes PtC3 and PdC3 Isolated and Characterized in the Gas Phase

Carbenes of platinum and palladium, PtC3 and PdC3, were generated in the gas phase through laser vaporization of a metal target in the presence of a low concentration of a hydrocarbon precursor undergoing supersonic expansion. Rotational spectroscopy and ab initio calculations confirm that both molecules are linear. The geometry of PtC3 was accurately determined by fitting to the experimental moments of inertia of twenty-six isotopologues. The results are consistent with the proposal of an autogenic isolobal relationship between O, Au+, and Pt atoms.

The Quadruple Bonding in C2 Reproduces the Properties of the Molecule

Ever since Lewis depicted the triple bond for acetylene, triple bonding has been considered as the highest limit of multiple bonding for main elements. Here we show that C2 is bonded by a quadruple bond that can be distinctly characterized by valence-bond (VB) calculations. We demonstrate that the quadruply-bonded structure determines the key observables of the molecule, and accounts by itself for about 90% of the molecule's bond dissociation energy, and for its bond lengths and its force constant. The quadruply-bonded structure is made of two strong π bonds, one strong σ bond and a weaker fourth σ-type bond, the bond strength of which is estimated as 17-21 kcal mol(-1). Alternative VB structures with double bonds; either two π bonds or one π bond and one σ bond lie at 129.5 and 106.1 kcal mol(-1), respectively, above the quadruply-bonded structure, and they collapse to the latter structure given freedom to improve their double bonding by dative σ bonding. The usefulness of the quadruply-bonded model is underscored by "predicting" the properties of the (3)Σ+u state. C2's very high reactivity is rooted in its fourth weak bond. Thus, carbon and first-row main elements are open to quadruple bonding!

Self-assembly of endohedral metallofullerenes: a decisive role of cooling gas and metal-carbon bonding

The endohedral metallofullerene (EMF) self-assembly process in Sc/carbon vapor in the presence and absence of an inert cooling gas (helium) is systematically investigated using quantum chemical molecular dynamics simulations. It is revealed that the presence of He atoms accelerates the formation of pentagons and hexagons and reduces the size of the self-assembled carbon cages in comparison with analogous He-free simulations. As a result, the Sc/C/He system simulations produce a larger number of successful trajectories (i.e. leading to Sc-EMFs) with more realistic cage-size distribution than simulations of the Sc/C system. The main Sc encapsulation mechanism involves nucleation of several hexagons and pentagons with Sc atoms already at the early stages of carbon vapor condensation. In such proto-cages, both Sc-C σ-bonds and coordination bonds between Sc atoms and the π-system of the carbon network are present. Sc atoms are thus rather labile and can move along the carbon network, but the overall bonding is sufficiently strong to prevent dissociation even at temperatures around 2000 kelvin. Further growth of the fullerene cage results in the encapsulation of one or two Sc atoms within the fullerene. In agreement with experimental studies, an extension of the simulations to Fe and Ti as the metal component showed that Fe-EMFs are not formed at all, whereas Ti is prone to form Ti-EMFs with small cage sizes, including Ti@C28-Td and Ti@C30-C2v(3).

Gas-Phase Spectroscopic Detection and Structural Elucidation of Carbon-Rich Group 14 Binary Clusters: Linear GeC3Ge

Guided by high-level quantum-chemical calculations at the CCSD(T) level of theory, the first polyatomic germanium-carbon cluster, linear Ge2C3, has been observed at high spectral resolution in the gas phase through its remarkably complex fundamental antisymmetric C-C stretching mode ν3 located at 1932 cm(-1). The observation of a total of six isotopic species permits the derivation of a highly accurate value for the equilibrium Ge-C bond length. The present study suggests that many more Ge-C species might be detectable in the future using a combination of laser-ablation techniques for production and high-resolution infrared and/or microwave techniques for spectroscopic detection.

C2 in a Box: Determining Its Intrinsic Bond Strength for the X(1)Σg(+) Ground State

The intrinsic bond strength of C2 in its (1)Σg(+) ground state is determined from its stretching force constant utilizing MR-CISD+Q(8,8), MR-AQCC(8,8), and single-determinant coupled cluster calculations with triple and quadruple excitations. By referencing the CC stretching force constant to its local counterparts of ethane, ethylene, and acetylene, an intrinsic bond strength half way between that of a double bond and a triple bond is obtained. Diabatic MR-CISD+Q results do not change this. Confinement of C2 and suitable reference molecules in a noble gas cage leads to compression, polarization, and charge transfer effects, which are quantified by the local CC stretching force constants and differences of correlated electron densities. These results are in line with two π bonds and a partial σ bond. Bond orders and bond dissociation energies of small hydrocarbons do not support quadruple bonding in C2.

Carbon rings: a DFT study on geometry, aromaticity, intermolecular carbon–carbon interactions and stability

Strong complimentary electrostatic interactions lead to large number of C⋯C interactions in doubly antiaromatic C_4N dimers while the delocalized system of π electrons in doubly aromatic C_4N+2 dimers suggest very few intermolecular C⋯C interactions.

Puzzles in bonding and spectroscopy: the case of dicarbon

The unstable molecule C₂ has been of interest since its identification as the source of the "Swan band" features observable in the spectra offlames, carbon arcs, white dwarf stars, and comets, and it continues to serve as a focal point for experimental and theoretical discovery. Recent spectroscopic work has identified a quintet state of the molecule for the first time, while new insights into the bond order of C₂ in its ground state have been provided by sophisticated computational methods based on valence bond theory. This article gives a review of spectroscopic and computational work on C₂ including both historical background and the most recent discoveries.

Thermal Isomerizations of Diethynyl Cyclobutadienes and Implications for Fullerene Formation

The mechanism by which carbon condenses to form PAHs or fullerenes is a problem that has garnered considerable theoretical and experimental attention. The ring-coalescence and annealing model for the formation of C(60) involves a [2 + 2] cycloaddition reaction of a cyclopolyyne to form a tetraalkynyl cyclobuta-1,3-diene intermediate, followed by a Bergman cycloaromatization reaction of the enediyne moiety. Intramolecular trapping of the incipient p-benzyne diradical across a diyne moiety of the macrocyclic ring affords an aromatic ring that must undergo further intramolecular reactions via polyradical intermediates to produce a condensed graphitic structure or fullerene. Computational studies of a model system for the intriguing tetraalkynylcyclobuta-1,3-diene intermediate, however, reveal that the corresponding p-benzyne diradical lies in a shallow minimum with a very low barrier to ring opening to cyclooctadienediyne. This pathway has not been previously considered in the mechanism for carbon condensation.