Susana Iglesias-Groth

Evidence for the Naphthalene Cation in a Region of the Interstellar Medium with Anomalous Microwave Emission

We report high-resolution spectroscopy of the moderately reddened ( -->AV = 3) early-type star Cernis 52 located in a region of the Perseus molecular cloud complex with anomalous microwave emission. In addition to the presence of the most common diffuse interstellar bands (DIBs) we detect two new interstellar or circumstellar bands coincident to within 0.01% in wavelength with the two strongest bands of the naphthalene cation (C10H -->8+) as measured in gas-phase laboratory spectroscopy at low temperatures and find marginal evidence for the third strongest band. Assuming these features are caused by the naphthalene cation, from the measured intensity and available oscillator strengths we find that 0.008% of the carbon in the cloud could be in the form of this molecule. We expect hydrogen additions to cause hydronaphthalene cations to be abundant in the cloud and to contribute via electric dipole radiation to the anomalous microwave emission. The identification of new interstellar features consistent with transitions of the simplest polycyclic aromatic hydrocarbon adds support to the hypothesis that this type of molecules are the carriers of both diffuse interstellar bands and anomalous microwave emission.

Fullerenes and Buckyonions in the Interstellar Medium

We show that photoabsorption by fullerenes and buckyonions (multishell fullerenes) explain the shape, width, and peak energy of the most prominent feature of interstellar absorption, the UV bump at 2175 A. The predicted optical and near-infrared transitions for these molecules also offer a potential explanation for the long-standing problem of the identity of the diffuse interstellar bands. The implied ubiquitous distribution of fullerenes may also account for the anomalous galactic microwave emission detected by cosmic microwave background experiments. Comparing theoretical cross sections and astronomical data, we estimate a density of fullerenes in the diffuse interstellar medium of 0.1-0.2 parts per million, consistent with the findings in meteorites. Fullerene-based molecules appear to be a major carbon reservoir in the interstellar medium.

Topological lattice descriptors of graphene sheets with fullerene-like nanostructures

Polynomial behaviour of the Wiener index for infinite chemical graphs is subject here to a generalisation to structures with topological dimensionality d T>1. This allows a pure topological analysis of relative chemical stability of graphite lattice portions and fullerene fragments (nanocones) built around a pentagonal face. The Wiener index of the graph acts as a lattice topological potential subject to a minimum principle that is able to discriminate topological structures made of hexagons with different connectivity. A new indicator of graph topological efficiency has been applied in the infinite lattice limit to allow a complete ranking of graph chemical stability. A certain grade of reactivity of the pentagonal ring at the centre of nanocones is also predicted. Our considerations are mainly performed in the dual topological space.

Fullerenes and the 4430 Å Diffuse Interstellar Band

The photoabsorption spectra of single-shell icosahedral fullerenes are investigated in the region of the strongest diffuse interstellar band at 4430 A. According to Huckel and Pariser-Parr-Pople models, several fullerenes (C80, C240, C320, and C540) present transitions in the vicinity of this band. The same semiempirical models also predict a high density of transitions in the UV that may explain the 2175 A bump in the extinction curve of the interstellar medium. It appears that fullerenes could be responsible of these two major features of interstellar absorption. Using the theoretical cross sections and available empirical data on the 4430 A band, we estimate abundances of ~0.05 molecules per million hydrogen atoms for these fullerenes in regions of the interstellar medium with an excess color index of E(B - V) = 1.0.

Electric Dipole Emission by Fulleranes and Galactic Anomalous Microwave Emission

We study the rotation rates and electric dipole emission of hydrogenated icosahedral fullerenes (single and multishell) in various phases of the interstellar medium. Using the formalism of Draine & Lazarian for the rotational dynamics of these molecules in various astrophysical environments, we find effective rotation rates in the range 1-65 GHz with a trend toward lower rotational frequency as the radius of the molecule increases. Owing to the moderately polar nature of the C–H bond, hydrogenated fullerenes (fulleranes) are expected to have a net dipole moment and produce electric dipole radiation. Adopting the same size distribution proposed for fullerenes in the study of the UV extinction bump (2175 A), we predict the dipole electric emission of mixtures of fulleranes for various levels of hydrogenation. We find that these molecules could be the carriers of the anomalous microwave emission recently detected by Watson et al. in the Perseus molecular complex.

Amino acids in comets and meteorites: stability under gamma radiation and preservation of the enantiomeric excess

Amino acids in Solar system bodies may have played a key role in the chemistry that led to the origin of life on Earth. We present laboratory studies testing the stability of amino acids against high energy radiation. All the 20 chiral amino acids in the levo form found in the proteins of the current terrestrial biochemistry were irradiated in the solid state with γ-radiation to a dose of 3.2 MGy, which is the dose equivalent to that produced by radionuclide decay in comets and asteroids in 1.05 x 10 9 yr. For each amino acid the radiolysis degree and the radioracemization degree were measured by differential scanning calorimetry and by optical rotatory dispersion spectroscopy. From these measurements, a radiolysis rate constant k dsc and a radioracemization rate constant k rac were determined for each amino acid and extrapolated to a dose of 14 MGy, which corresponds to the expected total dose delivered by natural radionuclide decay to all the organic molecules present in comets and asteroids in 4.6 x 10 9 yr, the age of the Solar system. It is shown that all the amino acids studied can survive a radiation dose of 14 MGy, although certain fractions of them are lost as a result of radiolytic processes. Similarly, the radioracemization process accompanying the radiolysis does not extinguish the initial enantiomeric enrichment. Knowledge of the radiolysis and radioracemization rate constants may permit the calculation of the original concentrations of the amino acids at the time of the formation of the Solar system, starting from the concentration found today in carbonaceous chondrites. For some amino acids the concentration in the pre-solar nebula could have been up to 6 times higher than that currently observed in meteorites. The preservation of an original enatiomeric excess is also expected. This study adds experimental support to the suggestion that amino acids were formed in the interstellar medium and in chiral excess and then were incorporated into comets and asteroids at the epoch of Solar system formation.

On the Way to Graphene: The Bottom-Up Approach to Very Large PAHs Using the Scholl Reaction

The Scholl reaction was adopted to synthesize the very large polycyclic aromatic hydrocarbons (VLPAHs) hexabenzocoronene (HBC), quaterrylene and dicoronylene as well as their nonalternant isomers. These molecules can be considered subsections of a graphene sheet since the radial diameter of HBC is 13.5 Å and the length of both quaterrylene and dicoronylene is about 17.9 Å. Improved synthetic procedures of synthesis are presented, and the resulting VLPAHs were isolated and recognized from their characteristic electronic absorption spectra. The precursor PAHs used in the Scholls synthesis where studied by differential scanning calorimetry (DSC), and the thermal behavior of the resulting VLPAHs was studied by thermogravimetric analysis and differential thermal analysis (TGA-DTA). The VLPAHs hexabenzocoronene (HBC), quaterrylene and dicoronylene show a trend to carbonization (growth of the laminar molecules into stacked and large graphene sheets) when heated slowly to high temperature in a nitrogen atmosphere.

Electronic absorption spectroscopy of polycyclic aromatic hydrocarbons (PAHs) radical cations generated in oleum: A superacid medium

Oleum (fuming sulphuric acid), a well known superacid, was used as medium for the generation of the radical cation of a series of selected PAHs. The resulting radical cation spectra were studied by electronic absorption spectroscopy. Not only common PAHs like naphthalene, anthracene, tetracene, pentacene, perylene, pyrene, benzo[a]pyrene, phenanthrene and picene were studied but also the less common and very large PAHs relevant also for the astrochemical research, like coronene, hexabenzocoronene, quaterrylene, dicoronylene and a coronene oligomer. A correlation between the first ionization potential (IP1) of the PAHs studied and the energy to the so-called A-type band of the radical cations observed in oleum has led to the equation IP1=1.30EA+4.39 (in eV) which permits to estimate the energy of the PAHs radical cation transition (EA) in the VIS-NIR knowing the relative ionization potential or vice versa.

Synthesis and FT‐IR Spectroscopy of Perdeuterofullerane: C60D36 Evidences of Isotope Effect in the Stability of C60D36

Perdeuterofullerane, C60D36 was synthesized in toluene solution from DCl and Zn. The FT‐IR spectrum of C60D36 was compared with that of C60H36, and the C‐H stretching and bending shift due to deuteration was measured so that Ω H/ Ω D = 1.36. The thermal stability of C60D36 was studied under N2 by thermogravimetric and differential thermal analysis. It has been found that the substitution of hydrogen with deuterium in fullerene leads to a significant increase in thermal stability which was attributed to isotope effect. In particular, the onset of decomposition of C60H36 occurred at 412°C in comparison to 465°C measured on C60D36. The maximum decomposition rate of C60H36 was found at 501°C and was shifted to 549°C in C60D36. Perdeuterofullerane was found much more stable also to air oxidation. It oxidized slowly when exposed to air and some days were needed to reach a degree of oxidation, which was reached in 1 hour by C60H36.

The Chemical Composition of Cernis 52 (BD+31° 640)

We present an abundance analysis of the star Cernis 52 in whose spectrum we recently reported the naphthalene cation in absorption at 6707.4 A. This star is on a line of sight to the Perseus molecular complex. The analysis of high-resolution spectra using a χ2-minimization procedure and a grid of synthetic spectra provides the stellar parameters and the abundances of O, Mg, Si, S, Ca, and Fe. The stellar parameters of this star are found to be T eff = 8350 ± 200 K, log(g/cm s2)=4.2 ± 0.4 dex. We derived a metallicity of [Fe/H] = –0.01 ± 0.15. These stellar parameters are consistent with a star of ~2 M ☉ in a pre-main-sequence evolutionary stage. The stellar spectrum is significantly veiled in the spectral range λλ5150-6730 A up to almost 55% of the total flux at 5150 A and decreasing toward longer wavelengths. Using Johnson-Cousins and Two Micron All Sky Survey photometric data, we determine a distance to Cernis 52 of 231+135 –85 pc considering the error bars of the stellar parameters. This determination places the star at a similar distance to the young cluster IC 348. This together with its radial velocity, vr = 13.7 ± 1 km s–1, its proper motion and probable young age support Cernis 52 as a likely member of IC 348. We determine a rotational velocity of vsin i = 65 ± 5 km s–1 for this star. We confirm that the stellar resonance line of Li I at 6707.8 A is unable to fit the broad feature at 6707.4 A. This feature should have a interstellar origin and could possibly form in the dark cloud L1470 surrounding all the cluster IC 348 at about the same distance.