Coronene Molecules Research Articles

Catalyzed Routes to Molecular Hydrogen Formation and Hydrogen Addition Reactions on Neutral Polycyclic Aromatic Hydrocarbons under Interstellar Conditions

We present first-principle calculations which reveal the existence of low-barrier routes to molecular hydrogen formation on the polycyclic aromatic hydrocarbon (PAH) molecule coronene via Eley-Rideal abstraction reactions and show that such processes could indeed be active under interstellar conditions. The calculations indicate that in regions of low UV flux, coronene, and larger PAHs might be found in superhydrogenated states. Furthermore, the calculations imply that not only edge carbon atoms but also carbon atoms on the inner rings of the coronene molecule can be hydrogenated. Such superhydrogenated PAHs are expected to exhibit significantly changed absorption and emission spectra.

Dehydrogenated coronene cations and interstellar extinction

AbstractDehydrogenated coronene molecules have been proposed as the carrier of the interstellar UV extinction bump and of some diffuse interstellar bands. To test this hypothesis, we have undertaken a combined a) theoretical and b) experimental work on the subject. a) We use the (time–dependent) density functional theory to study the effect of hydrogenation on the UV absorption cross–section of neutral and ionised coronene molecules. b) For the visible and near–UV, these calculations provide useful guidelines to analyse and interpret the electronic spectra obtained with the PIRENEA experimental set-up dedicated to astrochemistry.

Testing the attribution of selected DIBs to dehydrogenated coronene cations

AbstractDehydrogenated coronene molecules have been proposed as the source of the UV-bump in the interstellar extinction curve as well as of some of the diffuse interstellar bands (DIBs). To test this hypothesis we have recently undertaken a combined (a) modelling, and (b) observational work on the subject. (a) In the framework of a global approach to the photophysics of a PAH–like species in space, we used combined theoretical calculated properties, obtained with (time–dependent) density functional theory, and a Monte–Carlo model simulating the time evolution of the population of levels of a given molecule, to obtain the detailed ro–vibrational spectral structure of selected electronic transitions. (b) From the observational point of view, we compare our predictions with observations of the well–known λ6284 and λ5780 DIBs.

Bonding and magnetism in transition metal sandwich structures with the aromatic hydrocarbon coronene C 24H 12 outer layers

Plane wave based ab initio density functional theory has been used to study the chemical bonding and magnetism of transition metal atoms between coronene molecules in sandwich structures M n (C 24H 12) 2 where n = 7 and M = Cr, Fe, Pd. Symmetry conditions permit the metal atoms to occupy either a central site with η 6-coordination as in the metal bis-benzene molecules M 1(C 6H 6) 2 or a lower symmetry edge site with η 2- or η 3-coordination. In some cases at the edge sites the metal atoms crimped the edges of the sandwich towards the smaller separations of the bis-benzene molecules. However, since this effect also depended on local metal atom spin, the sandwich cross-sections could be concave in one symmetry plane and convex in an orthogonal plane. The lowest energy states of the sandwiches had spin S = 5 (Cr), 6 (Fe) and 0 (Pd). The high spin systems occurred with a larger metal–ring spacing so that a concomitantly weaker electric field crossed the metal atom compared to the bis-benzene molecules which have S = 0 (Cr), 2 (Fe) and 0 (Pd) in the ground electronic state. In the spin polarized ground states the unpaired electrons resided exclusively on the metal atoms. In the Pd sandwich the excited states with spin S = 1 and S = 2 had qualitatively similar spin distributions to the Fe and Cr structures, with the difference that there was a much greater leakage of spin density onto the nearby carbon atoms. In all the sandwiches the spin density was not evenly distributed amongst metal atoms, and depended on the environment of the metal as gauged for example by the metal–nearest ring separation.

Coronene-Iodine Coadsorbed Adlayers on Au(111) Surfaces Promoted by Electrochemical Potential Control

A coronene and iodine co-adsorbed adlayer on an Au(111) surface in which iodine atoms surrounded each coronene molecule was found by an in situ scanning tunneling microscopy (STM) study with electrochemical potential control.

Crystal-chemical and carbon-isotopic characteristics of karpatite (C24H12) from the Picacho Peak Area, San Benito County, California: Evidences for the hydrothermal formation

Karpatite from the Picacho Peak Area, San Benito County, California, has been characterized as an exceptionally pure crystal of coronene (C24H12) by infrared absorption analysis, Raman scattering analysis, and differential thermal analysis. Furthermore, the crystal structure of karpatite was determined using a single-crystal X-ray diffraction method for the first time. The mineral crystallizes in the monoclinic system, space group P21/a, with unit-cell dimensions of a = 16.094(9), b = 4.690(3), c = 10.049(8) Å, β = 110.79(2)°, V = 709.9(8) Å3, and Z = 2. The structure was solved and Finally refined to R1 = 3.44% and wR2 = 2.65%, respectively. The coronene molecules in the crystal structure of karpatite are all isolated and the intermolecular distances correspond to van der Waals interactions. The coronene molecules have the high degree of aromaticity and no overcrowded hydrogen atoms, both of which avoid a mixing of other polycyclic aromatic hydrocarbons (PAHs) in karpatite. The corrugated arrangement of coronene molecules constituting karpatite prevents intercalation reactions, accounting for the exceptional purity of this mineral.

Coadsorption‐Induced Reconstruction of Supramolecular Assembly Characteristics

A host supramolecular structure consisting of bis-(2,2':6',2"-terpyridine)-4'-oxyhexadecane (BT-O-C16) is shown to respond to coadsorbed molecules in dramatic ways, as observed by scanning tunneling microscopy (STM) on a highly oriented pyrolytic graphite (HOPG) surface under ambient conditions. Interestingly, the lattice parameter of the triphenylene-filled complex differs significantly from that of the coronene-filled one, although the triphenylene and coronene molecules are nearly the same size. The STM study and density functional theory calculations reveal that intermolecular hydrogen-bond interactions play an essential role in forming the assembly structures. The different electronic properties of coronene and triphenylene molecules are responsible for the difference in lattice parameters and consequently for the difference in filling behaviors in the coronene/BT-O-C16 and triphenylene/BT-O-C16 binary systems.

Vibrations and Thermodynamics of Clusters of Polycyclic Aromatic Hydrocarbon Molecules: The Role of Internal Modes

The vibrational spectra of clusters of coronene molecules are theoretically calculated using a mixed quantum/classical scheme, each molecule being described by a tight-binding Hamiltonian, the intermolecular forces being provided by explicit Lennard-Jones and point charge sites. The normal modes of vibrations are shown to exhibit significant variations upon clustering. In particular, for large clusters intra- and intermolecular modes tend to mix and fill the mid-infrared range. We also calculate the heat capacity of the (C24H12)8 cluster as a function of temperature, emphasizing the isomerizations that take place during melting. Quantum delocalization effects, as obtained from the Pitzer-Gwinn semiclassical approximation, are important enough to wash out all signatures of the structural transitions on the caloric curve. On the basis of a simple two-state model we estimate that clusters containing about 300 molecules are required for melting to be detected on the caloric curve.

Epitaxial Supramolecular Assembly of Fullerenes Formed by Using a Coronene Template on a Au(111) Surface in Solution

Characteristic properties of the coronene layer formed on Au(111) for the epitaxial growth of various fullerenes are described. The electrochemical behavior of the coronene adlayer prepared by immersing a Au(111) substrate into a benzene solution containing coronene was investigated in 0.1 M HClO4. The as-prepared coronene adlayer on Au(111) revealed a well-defined (4 x 4) structure. Structural changes of the array of coronene molecules induced by potential manipulation were clearly observed by in situ scanning tunneling microscopy (STM). Supramolecularly assembled layers of fullerenes such as C60, C70, C60-C60 dumbbell dimer (C120), C60-C70 cross-dimer (C130), and C60 triangle trimer (C180) were formed on the well-defined coronene adlayer on the Au(111) surface by immersing the coronene-adsorbed Au(111) substrate into benzene solutions containing those molecules. The adlayers thus prepared were characterized by comparison with those which were directly attached to the Au(111) surface. The C60 molecules formed a honeycomb array with an internal structure in each C60 cage on the coronene adlayer, whereas C70 molecules were one-dimensionally arranged with the same orientations. The dimers, C120 and C130 molecules, formed an identical structure with c(11 x 4 radical3)rect symmetry. For the C130 cross-dimer molecule, C60 and C70 cages were clearly recognized at the molecular level. It was difficult to identify the adlayer of the C180 molecule directly attached to Au(111); however, individual C180 molecules could be recognized on the coronene-modified Au(111) surface. Thus, the adlayer structures of those fullerenes were strongly influenced by the underlying coronene adlayer, suggesting that the insertion of a coronene adlayer plays an important role in the formation of supramolecular assemblies of fullerenes.

Ground state structures and photoelectron spectroscopy of [Com(coronene)]− complexes

A synergistic approach involving theory and experiment has been used to study the structure and properties of neutral and negatively charged cobalt-coronene [Com(coronene)] complexes. The calculations are based on density functional theory with generalized gradient approximation for exchange and correlation potential, while the experiments are carried out using photoelectron spectroscopy of mass selected anions. The authors show that the geometries of neutral and anionic Co(coronene) and Co2(coronene) are different from those of the corresponding iron-coronene complexes and that both the Co atom and the dimer prefer to occupy eta2-bridge binding sites. However, the magnetic coupling between the Co atoms remains ferromagnetic as it is between iron atoms supported on a coronene molecule. The accuracy of the theoretical results is established by comparing the calculated vertical detachment energies, and adiabatic electron affinities with their experimental data.

Interplay of electrostatic and van der Waals forces in coronene dimer

AbstractInterplay of electrostatic and van der Waals forces in the coronene dimer is studied. Our results show that the lowest‐energy configuration of the dimer is not necessarily a stack, as it might had been expected a priori. This is a surprising result for the dimer of such a large polycyclic aromatic hydrocarbon (PAH) as coronene (C24H12). The energy of the T‐shaped configuration at all highest feasible levels of density functional theory (DFT) (B3LYP, PBE/6‐31+G(d), D95, cc‐pVDZ, cc‐pVTZ) is lower than the energies of the three plausible stack configurations. To get a better description of the van der Waals interaction, the DFT results were corrected by a phenomenological van der Waals‐type term. This correction gives a slight edge to the parallel‐displaced stack configuration. However, the magnitude of the correction is somewhat arbitrary, depending on the set of parameters used. This makes the definitive conclusion impossible at the currently achievable level of theory. A simple model is proposed that is useful for the qualitative understanding of possible geometries of the coronene dimer and larger coronene clusters. The model represents the coronene dimer as two sets of charged rings interacting via Coulomb and Lennard‐Jones potentials. The model provides an intuitively clear explanation as to why the T‐shaped dimers can be of importance even for some of moderately large PAHs such as coronene, and perhaps for circumcoronene as well. The unexpectedly strong competitiveness of the T‐shaped configuration is connected to the round shape of the coronene molecules. Indeed, rotation or parallel displacement of the non‐round monomers result in significantly smaller Coulomb repulsion as compared with the “face‐to‐face” sandwich configurations. In contrast, for the stacked round molecules rotation and/or displacement are much less effective. Therefore, the round shape of the coronenes leads to an increased role of the electrostatic repulsion in the stack configurations. The proposed model can easily be generalized to other PAH, DNA bases, etc. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Controllable Distribution of Single Molecules and Peptides within Oligomer Template Investigated by STM

This communication provides a facile method for distributing and dispersing molecules within a molecular template. Using the self-assembled template of oligo(phenylene-ethynylene) (OPE), organic molecules such as coronene (COR) and biomolecules such as tripeptide are controllably distributed and dispersed within this molecular template on highly oriented pyrolytic graphite (HOPG) surfaces. COR molecules were controllably distributed into various regular arrays by simply adjusting the molecular molar ratio, while tripeptide molecules were uniformly positioned at the vacancies of the OPE template.

The chemisorption of dibenzo[a,j]coronene on Si(001)-2×1

Adsorption structures of the dibenzo[a,j]coronene (C(32)H(16)) molecule on the clean Si(001)-2 X 1 surface were investigated by scanning tunneling microscopy (STM) in conjunction with electronic structure calculations. The dibenzo[a,j]coronene molecules were found to adsorb on three different sites: one major adsorption site and two minor adsorption sites. The formation of four to eight Si-C covalent bonds is responsible for the different surface bonding structures observed. Bond strain effects due to out-of-plane bending of the molecule play a significant role in governing the surface bond energies. The geometries of the three adsorption sites were established by comparison of the experimental and simulated STM images. By applying an electrical pulse, the molecule can be made to hop from one site to another site without breaking the dibenzo[a,j]coronene molecular structure.

Effects of intermolecular interactions on the controlled assembly of organic monolayers: an STM study

AbstractRepresentative types of intermolecular interactions were seen to affect self‐assembled molecular architectures, as studied by scanning tunneling microscopy (STM) under ambient conditions. As an example, the ππ interaction is attributed to the formation of chiral domains consisting of 4‐octadecylnitrobenzene (ONB). It is further demonstrated that the ONB assembly can serve as a buffer layer to immobilize coronene molecules on large areas of two‐dimensional close‐packed assemblies. Assemblies of 1,4‐dicarboxylic benzoic acid (DBA) and 4,4′‐bipyridine (Bpy) are associated with two different hydrogen‐bonded, two‐dimensional supramolecular networks. The results of theoretical calculations using density functional theory (DFT) are in qualitative agreement with the observed STM images. Copyright © 2006 John Wiley & Sons, Ltd.

Coronene cluster experiments: Stability and thermodynamics

Neutral coronene clusters are produced in a gas aggregation source at 77 K. They are excited either by a UV-laser in vacuum or thermally by a heat bath of helium. Fragmentations of clusters and molecules are investigated. Two well-separated thermodynamic regimes have been found for the evaporation of coronene from the cluster and the fragmentation of the coronene molecule. The stability of the cluster has been measured and the distribution of inner energies on inner- and intermolecular modes has been deduced.

A Plasmon Resonance in Dehydrogenated Coronene (C 24 H x ) and Its Cations and the Origin of the Interstellar Extinction Band at 217.5 Nanometers

Analysis of the wavelength dependence of extinction in the ultraviolet is shown to provide additional insight into the physical properties of the carrier of the interstellar absorption feature at 217.5 nm (5.7 eV). In particular, it is found that this band has a cutoff at both long and short wavelengths, supporting the assignment of this feature to a plasmon band. The photon energies corresponding to these cutoffs are highly diagnostic of the carrier of this feature and suggest that the absorber is a seven-ring aromatic molecule. Comparison with laboratory data, together with theoretical calculations of the plasmon resonance in small dehydrogenated aromatic-ring structures, supports the assignment of this feature to the π-π* plasmon resonance in dehydrogenated coronene molecules, C24Hx and its cations, where x ≤ 3. It is shown that this assignment is consistent with available observational data on the 217.5 nm feature, including such properties as a constant central wavelength, variable width, its relation to E(1250-V)/E(B-V), and the carbon abundance in the interstellar medium. The π and π* energy levels in the 217.5 nm carrier, derived from an analysis of interstellar extinction, have been used to construct its energy-level diagram, enabling comparison with other observational data

The chemisorption of coronene on Si(001)-2×1

Coronene (C24H12) adsorption on the clean Si(001)-2 x 1 surface was investigated by scanning tunneling microscopy and by density-functional calculations. The coronene adsorbed randomly at 25 degrees C on the surface and did not form two-dimensional islands. The scanning tunneling microscopy measurements revealed three adsorption sites for the coronene molecule on the Si(001) surface at low coverage. The major adsorption configuration involves coronene bonding to four underlying Si atoms spaced two lattice spacings apart in a dimer row. The two minor adsorption configurations involve asymmetrical bonding of a coronene molecule between Si dimer rows and form surface species with a mirror plane symmetry to their chiral neighbor species. The two minor bonding arrangements are stabilized by a type-C defect on the Si(001) surface.

Charge Transport Through A Single Molecule of Dibenzo[a,j]coronene on Si(001)-2 * 1: Possible Coulomb Blockade

Charge transport through single dibenzo[a,j]coronene (C32H16) molecules (DBC) chemisorbed on the Si(001)-2 × 1 surface was investigated by current-voltage (I-V) measurement with scanning tunneling microscopy (STM) at 25°C. The I-V curves taken from two different adsorption structures of the DBC are found to be significantly different. The major DBC species shows periodic steps in the {I-V} curve, as expected from a possible Coulomb blockade, while the minor DBC species shows a similar I-V curve to the Si substrate. This indicates that the adsorption structure of the chemisorbed DBC molecule to the Si surface governs its conductivity behavior. [DOI: 10.1380/ejssnt.2006.588]

Localized Deposition of Coronene Molecules on Si(001)-2×1 Using a Scanning Tunneling Microscope Tip Source

The deposition of coronene molecules from scanning tunneling microscope (STM) tips onto a clean Si(001)-2x1 surface at 25 degrees C was investigated. The STM tips, contaminated with coronene, were found to deposit coronene molecules on the clean Si(001) surface, allowing patterns to be generated. Covalent Si-C chemical bonds, formed between the coronene molecules and the Si substrate, froze the flip-flop motion of the adjacent Si-Si dimers on the substrate. In most cases, the mode of coronene bonding to Si(001) is independent of whether deposition occurs from the gas phase or from the STM tip. Despite the covalent chemical bonds formed between the coronene molecule and the Si substrate, the STM tip can drag the coronene laterally on the Si substrate without inducing a chemical change in the molecule. Sharp spikes observed in the tunneling current during the coronene deposition reflect the abrupt decrease of the tip-substrate distance at the instant of transport of the molecule from tip to surface.

A search for a strong physisorption site for H2 in Li-doped porous carbons

The mechanism of hydrogen absorption between two coronene molecules has been studied by first principle calculations. Examination of different sites for H(2) molecule confirmed the classical picture of physisorption. We have also considered molecular hydrogen adsorption in a charged carbon structure achieved by doping with lithium at a density corresponding to the intercalate compound LiC(6). We have performed different types of calculations [Hartree-Fock and density functional theory (DFT)] for various atomic basis sets using CRYSTAL98, GAUSSIAN98, and DMOL3 codes. B3LYP-DFT (B3LYP-three-parameter functional of Backe, Lee, Yang and Parr) energy minimization calculations unravel that there is a stable adsorption site for molecular hydrogen in Li-doped sp(2) carbon structure. These calculations also give an insight into the atomic configurations of interlayer species (H(2) and Li) as the interlayer spacing increases. It can be shown that large changes occur in the positions and electronic properties of interlayer species. Hydrogen molecule does not show any tendency for dissociation and adopts a position in the interlayer void that is deeply related to that of lithium ions. We have evidenced a rather large charge transfer from lithium and capping hydrogen species on neighboring slab carbon atoms that induce the stabilization of molecular hydrogen. We have also found that rotating one carbon layer with respect to the other one (at constant interlayer distance) does not change the adsorption energy to a large extent. The best adsorption site is about five times deeper than the physisorption site found in the undoped case and occurs at an interlayer separation of 5.5+/-0.5 A. The corresponding atomic configuration consists in a hydrogen molecule standing (nearly) perpendicular to the plane surface surrounded by the three lithium ions in a configuration close to that of the LiC(6) intercalation compound.