The origin of the polycyclic aromatic hydrocarbons in meteorites

Adsorption of Polycyclic Aromatic Hydrocarbons and C60 onto Forsterite: C-H Bond Activation by the Schottky Vacancy

Understanding how to catalytically break the C-H bond of aromatic molecules, such as polycyclic aromatic hydrocarbons (PAHs), is currently a big challenge and a subject of study in catalysis, astrochemistry, and planetary science. In the latter, the study of the breakdown reaction of PAHs on mineral surfaces is important to understand if PAHs are linked to prebiotic molecules in regions of star and planet formation. In this work, we employed a periodic density functional theory along with Grimme's D4 (DFT-D4) approach for studying the adsorption of a sample of PAHs (naphthalene, anthracene, fluoranthene, pyrene, coronene, and benzocoronene) and fullerene on the [010] forsterite surface and its defective surfaces (Fe-doped and Ni-doped surfaces and a MgO-Schottky vacancy) for their implications in catalysis and astrochemistry. On the basis of structural and binding energy analysis, large PAHs and fullerene present stronger adsorption on the pristine, Fe-doped, and Ni-doped forsterite surfaces than small PAHs. On a MgO-Schottky vacancy, parallel adsorption of the PAH leads to the chemisorption process (C-Si and/or C-O bonds), whereas perpendicular orientation of the PAH leads to the catalytic breaking of the aromatic C-H bond via a barrierless reaction. Spin density and charge analysis show that C-H dissociation is promoted by electron donation from the vacancy to the PAH. As a result of the undercoordinated Si and O atoms, the vacancy acts as a Frustrated Lewis Pair (FLP) catalyst. Therefore, a MgO-Schottky vacancy [010] forsterite surface proved to have potential catalytic activity for the activation of C-H bond in aromatic molecules.

Planetary Minerals Catalyze Conversion of a Polycyclic Aromatic Hydrocarbon to a Prebiotic Quinone: Implications for Origins of Life

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in astrochemical environments and are disbursed into planetary environments via meteorites and extraterrestrial infall where they may interact with mineral phases to produce quinones important for origins of life. In this study, we assessed the potential of the phyllosilicates montmorillonite (MONT) and kaolinite (KAO), and the enhanced Mojave Mars Simulant (MMS) to convert the PAH anthracene (ANTH) to the biologically important 9,10-anthraquinone (ANTHQ). All studied mineral substrates mediate conversion over the temperature range assessed (25-500°C). Apparent rate curves for conversion were sigmoidal for MONT and KAO, but quadratic for MMS. Conversion efficiency maxima for ANTHQ were 3.06% ± 0.42%, 1.15% ± 0.13%, and 0.56% ± 0.039% for MONT, KAO, and MMS, respectively. We hypothesized that differential substrate binding and compound loss account for the apparent conversion kinetics observed. Apparent loss rate curves for ANTH and ANTHQ were exponential for all substrates, suggesting a pathway for wide distribution of both compounds in warmer prebiotic environments. These findings improve upon our previously reported ANTHQ conversion efficiency on MONT and provide support for a plausible scenario in which PAH-mineral interactions could have produced prebiotically relevant quinones in early Earth environments.

UV Time-Resolved Laser-Induced Fluorescence Spectroscopy of Amino Acids Found in Meteorites: Implications for Space Science and Exploration

Laser-induced fluorescence spectroscopy is a useful laboratory and in situ technique for planetary exploration, with applications in biosignature detection and the search for life on Mars. However, little work has been completed on the utility of fluorescence spectroscopy techniques on asteroid relevant material. In preparation for asteroid sample return missions such as NASA's OSIRIS-REx and JAXA's Hayabusa2, we conducted UV time resolved laser-induced fluorescence spectroscopy (TR-LIF) analysis of 10 amino acids, all of which have been found in the carbonaceous meteorites Murchison and Allende. We present the calculation of decay rates of each amino acid (1.55-3.56 ns) and compare with those of relevant homogeneous minerals (15-70 ns). Moreover, we demonstrate a linear relationship between calculated lifetimes and elemental abundance of nitrogen and carbon (p < 0.025). The quantitative and qualitative fluorescence analyses presented in this work will lead to more reliable identification of organic material within meteorites and asteroids in a time-efficient, minimally destructive way.

Investigating the Kinetics of Montmorillonite Clay-Catalyzed Conversion of Anthracene to 9,10-Anthraquinone in the Context of Prebiotic Chemistry

Carbonaceous meteorites contributed polycyclic aromatic hydrocarbons (PAHs) to the organic inventory of the primordial Earth where they may have reacted on catalytic clay mineral surfaces to produce quinones capable of functioning as redox species in emergent biomolecular systems. To address the feasibility of this hypothesis, we assessed the kinetics of anthracene (1) conversion to 9,10-anthraquinone (2) in the presence of montmorillonite clay (MONT) over the temperature range 25 to 250 °C. Apparent rates of conversion were concentration independent and displayed a sigmoidal relationship with temperature, and conversion efficiencies ranged from 0.027 to 0.066%. Conversion was not detectable in the absence of MONT or a sufficiently high oxidation potential (in this case, molecular oxygen (O2)). These results suggest a scenario in which meteoritic 1 and MONT interactions could yield biologically important quinones in prebiotic planetary environments.

Thermal equation of state of solid naphthalene to 13 GPa and 773 K: in situ X-ray diffraction study and first principles calculations

In a wide range of P-T conditions, such fundamental characteristics as compressibility and thermoelastic properties remain unknown for most classes of organic compounds. Here we attempt to clarify this issue by the example of naphthalene as a model representative of polycyclic aromatic hydrocarbons (PAHs). The elastic behavior of solid naphthalene was studied by in situ synchrotron powder X-ray diffraction up to 13 GPa and 773 K and first principles computations to 20 GPa and 773 K. Fitting of the P-V experimental data to Vinet equation of state yielded T 0 = 8.4(3) GPa and T' = 7.2 (3) at V0 = 361 Å(3), whereas the thermal expansion coefficient was found to be extremely low at P > 3 GPa (about 10(-5) K(-1)), in agreement with theoretical estimation. Such a diminishing of thermal effects with the pressure increase clearly demonstrates a specific feature of the high-pressure behavior of molecular crystals like PAHs, associated with a low energy of intermolecular interactions.

Concentrations, gas-particle partitioning, and seasonal variations of polycyclic aromatic hydrocarbons at four sites in Turkey

Ambient air polycyclic aromatic hydrocarbon (PAH) samples were collected at traffic, residential, coastal, and semiurban sites in Bursa, Turkey, between June 2008 and June 2009. For the traffic, residential, coastal, and semiurban sites, the average gas phase total PAH (∑12PAH) concentrations were 113 ± 131, 142 ± 204, 53 ± 73, and 19 ± 34 ng/m(3), respectively, whereas the average particle phase total PAH concentrations were 28 ± 36, 56 ± 85, 24 ± 40, and 11 ± 23 ng/m(3), respectively. Phenanthrene and fluoranthene had the highest concentrations of all of the sampling sites in the gas phase. The PAH concentrations in the heating period were 5-7 times greater than the nonheating period concentrations. Principal component analysis (PCA) was used to investigate the relationship between the levels of PAHs determined in ambient samples and their possible sources. The PCA model shows that coal combustion and vehicle emissions affected PAH emissions. Moreover, the molecular diagnostic ratios indicated that coal-burning and traffic emissions were the dominant PAH sources. The multiple linear regression analysis indicated that the meteorological parameters also affected the ambient PAH concentrations. The sampling site characteristics, meteorological conditions, dispersion, and local sources all affected the concentration levels.

Nucleophilic addition of nitrogen to aryl cations: mimicking Titan chemistry

The reactivity of aryl cations toward molecular nitrogen is studied systematically in an ion trap mass spectrometer at 10(2) Pascal of nitrogen, the pressure of the Titan main haze layer. Nucleophilic addition of dinitrogen occurs and the nature of aryl group has a significant influence on the reactivity, through inductive effects and by changing the ground state spin multiplicity. The products of nitrogen activation, aryldiazonium ions, react with typical nitriles, aromatic amines, and alkynes (compounds that are relevant as possible Titan atmosphere constituents) to form covalently bonded heterocyclic products. Theoretical calculations at the level [DFT(B3LYP)/6-311++G(d,p)] indicate that the N2 addition reaction is exothermic for the singlet aryl cations but endothermic for their triplet spin isomers. The -OH and -NH2 substituted aryl ions are calculated to have triplet ground states, which is consistent with their decreased nitrogen addition reactivity. The energy needed for the generation of the aryl cations from their protonated precursors (ca. 340 kJ/mol starting with protonated aniline) is far less than that required to directly activate the nitrogen triple bond (the lowest energy excited state of N2 lies ca. 600 kJ/mol above the ground state). The formation of aza-aromatics via arene ionization and subsequent reactions provide a conceivable route to the genesis of nitrogen-containing organic molecules in the interstellar medium and Titan haze layers.