Destruction and multiple ionization of PAHs by X-rays in circumnuclear regions of AGNs

The infrared signatures of polycyclic aromatic hydrocarbons (PAHs) are observed in a variety of astrophysical objects, including the circumnuclear medium of active galactic nuclei (AGNs). These are sources of highly energetic photons (0.2 to 10 keV), exposing the PAHs to a harsh environment. In this work, we examined experimentally the photoionization and photostability of naphthalene (C$_{10}$H$_{8}$), anthracene (C$_{14}$H$_{10}$), 2-methyl-anthracene (C$_{14}$H$_{9}$CH$_{3}$) and pyrene (C$_{16}$H$_{10}$) upon interaction with photons of 275, 310 and 2500 eV. The measurements were performed at the Brazilian Synchrotron Light Laboratory using time-of-flight mass-spectrometry (TOF-MS). We determined the absolute photoionization and photodissociation cross sections as a function of the incident photon energy; the production rates of singly, doubly and triply charged ions; and the molecular half-lives in regions surrounding AGNs. Even considering moderate X-ray optical depth values ($\tau = 4.45$) due to attenuation by the dusty torus, the half-lives are not long enough to account for PAH detection. Our results suggest that a more sophisticated interplay between PAHs and dust grains should be present in order to circumvent molecular destruction. We could not see any significant difference in the half-life values by increasing the size of the PAH carbon backbone, N$_C$, from 10 to 16. However, we show that the multiple photoionization rates are significantly greater than the single ones, irrespective of the AGN source. We suggest that an enrichment of multiply charged ions caused by X-rays can occur in AGNs.Comment: 20 pages (appendix: 3 pages), 10 figures, 4 tables. Accepted for publication in the Montly Notices of the Royal Astronomical Society (MNRAS). Accepted 2019 April

Assessing the Potential of Regulating Ecosystem Services as Nature-Based Solutions in Urban Areas

Mounting research assesses the provision of regulating ecosystem services by green infrastructure in urban areas, but the extent to which these services can offer effective nature-based solutions for addressing urban climate change-related challenges is rarely considered. In this chapter, we synthesize knowledge from assessments of urban green infrastructure carried out in Europe and beyond to evaluate the potential contribution of regulating ecosystem services to offset carbon emissions, reduce heat stress and abate air pollution at the metropolitan, city and site scales. Results from this review indicate that the potential of regulating ecosystem services provided by urban green infrastructure to counteract these three climate change-related pressures is often limited and/or uncertain, especially at the city and metropolitan levels. However, their contribution can have a substantially higher impact at site scales such as in street canyons and around green spaces. We note that if regulating ecosystem services are to offer effective nature-based solutions in urban areas, it is critically important that green infrastructure policies target the relevant implementation scale. This calls for a coordination between authorities dealing with urban and environmental policy and for the harmonization of planning and management instruments in a multilevel governance approach. Regulating ecosystem services • Urban green infrastructure • Global climate regulation • Local climate regulation • Air quality regulation • Multi-scale assessmentpublishedVersio

Doubly and Triply Charged Species Formed from Chlorobenzene Reveal Unusual C–Cl Multiple Bonding

In free-radical halogenation of aromatics, singly charged ions are usually formed as intermediates. These stable species can be easily observed by time-of-flight mass spectrometry (TOF-MS). Here we used electron and proton beams to ionize chlorobenzene (C₆H₅Cl) and investigate the ions stability by TOF-MS. Additionally to the singly charged parent ion and its fragments, we find a significant yield of doubly and triply charged parent ions not previously reported. In order to characterize these species, we used high-level theoretical methods based on density functional theory (DFT), coupled-cluster (CC), and generalized valence bond (GVB) to calculate the structure, relative stabilities, and bonding of these dications and trications. The most stable isomers exhibit unusual carbon-chlorine multiple bonding: a terminal CCl double bond in a formyl-like CHCl moiety (<b>1</b>, <i>r</i>_C–Cl = 1.621 Å) and a ketene-like CCCl cumulated species (<b>2</b>, <i>r</i>_C–Cl = 1.542 Å). The calculations suggest that an excited state of <b>2</b> has a nitrile-like CCl triple bond structure

Production of Long-Lived Benzene Dications from Electron Impact in the 20-2000 eV Energy Range Combined with the Search for Global Minimum Structures

In this work, we report a systematic search of metastable C6Hn2+ (n = 1–6) dications from electron impact time-of-flight measurements of several benzene derivatives in combination with global minimum search based on the genetic algorithm. Our theoretical calculations reveal that the C6Hn2+ (n < 6) global minimum structures are completely different from that of the benzene dication, featuring linear carbon chains and/or cyclopropenylium moieties. Experimentally, the doubly charged species were investigated for a wide range of electron impact energies, from 20 to 2000 eV, for benzene and several monosubstituted compounds containing either electron-withdrawing or -donating groups. Furthermore, the C6Hn2+ production, evaluated from the yields of the dications with respect to that of the parent ion (or parent dication), was compared to those obtained from charge exchange in the doubly charged 2E spectra and electron impact experiments available in the literature. The yields of the long-lived benzene dications were contrasted to those analogues formed in chlorobenzene. Moreover, the formation of C6Hn2+ species is strongly dependent on the nature of substituent groups, with electron-withdrawing ones favoring the dication formation

Dissociative single and double photoionization of biphenyl (C12H10) by soft X-rays in planetary nebulae

Biphenyl (C12H10), or phenylbenzene, is an important building block of polycyclic aromatic hydrocarbons (PAHs), whose infrared spectral features are present in a variety of galactic and extragalactic sources. In this work, we use synchrotron radiation coupled with time-of-flight spectrometry to study the photoionization and photodissociation processes of biphenyl upon its interaction with soft X-ray photons at energies around the inner-shell C1s resonance. These results are compared with our previous studies with benzene (C6H6) and naphthalene (C10H8), and discussed in the context of four planetary nebulae featuring PAH infrared emission: BD+30○3639, NGC 7027, NGC 5315, and NGC 40. We show that the mass spectrum of biphenyl before the C1s resonance energy is dominated by single photoionization processes leading to C6H+5⁠, C6H4+⋅⁠, and C12H10+⋅⁠, while after the resonance dissociation following multiple photoionization processes is dominant. The release of neutral C6H6 and C6H5⋅ species accounts for one of the most relevant dissociation processes starting from the doubly ionized biphenyl, indicating that heterolytic charge separation of the two phenyl rings is also achieved. By using quantum chemical calculations, we show that the biphenylic structure is a high-lying isomer of the singly and doubly ionized C12H10 species, whose minimum energy geometries are related to the acenaphthene molecule, composed of a C2-bridged naphthalene. Furthermore, we estimate the lifetime of biphenyl for 275 and 310 eV in photon-dominated regions of planetary nebulae. We discuss distinct processes that may enhance its lifetime and those of other small-sized PAHs in such astrophysical environments

Unexpected reversal of stability in strained systems containing one-electron bonds

Ring strain energy is a very well documented feature of neutral cycloalkanes, and influences their structural, thermochemical and reactivity properties. In this work, we apply density functional theory and high-level coupled cluster calculations to describe the geometry and relative stability of C6H12+˙ radical cations, whose cyclic isomers are prototypes of singly-charged cycloalkanes. Molecular ions with the mentioned stoichiometry were produced via electron impact experiments using a gaseous cyclohexane sample (20–2000 eV). From our calculations, in addition to structures that resemble linear and branched alkenes as well as distinct conformers of cyclohexane, we have found low-lying species containing three-, four- and five-membered rings with the presence of an elongated C–C bond. Remarkably, the stability trend of these ring-bearing radical cations is anomalous, and the three-membered species are up to 11.3 kcal mol−1 more stable than the six-membered chair structure. Generalized Valence Bond calculations and the Spin Coupled theory with N electrons and M orbitals were used in conjunction with the Generalized Product Function Energy Partitioning (GPF-EP) method and Interference Energy Analysis (IEA) to describe the chemical bonding in such moieties. Our results confirm that these elongated C–C motifs are one-electron sigma bonds. Our calculations also reveal the effects that drive thermochemical preference of strained systems over their strained-free isomers, and the origin of the unusual stability trend observed for cycloalkane radical cations

Persistent organic pollutants (POPs) in antarctic fish: levels, patterns, changes

Organochlorine compounds were analysed in three fish species of different feeding types from the area of Elephant Island in the Antarctic. In 1996, HCB (means: 15-20 ng/g lipid), p,p'-DDE (5-13 ng/g lipid) and mirex (1-7 ng/g lipid) predominated, while PCBs were minor components (PCB 153: 0.4-2 ng/g lipid). Concentration patterns were species-dependent: PCB 180, PCB 153, mirex, nonachlor III, trans-nonachlor and the toxaphene compound B8-1413 were highest in the bottom invertebrate feeder Gobionotothen gibberifrons and lowest in the krill feeder Champsocephalus gunnari. Levels of p,p'-DDE, PCB 138 and heptachloro-1'-methyl-1,2'-bipyrrole (Q1), a natural bioaccumulative product, were highest in the fish feeder Chaenocephalus aceratus, whereas HCB was present in about equal concentrations in all species. Most compounds were taken up preferentially via the benthic food chain, the chlorinated bipyrrole via the pelagic food chain and HCB from the water. In antarctic fish, biomagnification was generally more important than bioconcentration. Between 1987 and 1996, most POP levels showed significant increases in the benthos feeder and the fish feeder, while they remained nearly constant or increased less in the krill feeder. Hence, the former species represent indicator species for changing POP levels in Antarctica. Ratios (1996/1987) of average concentrations in G. gibberifrons were: PCB 138 0.7, HCB 0.8, B8-1413 1.5, PCB 180 1.7, PCB 153 1.8, p,p'-DDE 2.0, nonachlor III 2.9, trans-nonachlor 3.3, mirex 6.7. By comparison with trends in the northern hemisphere it is concluded that global distribution of HCB is close to equilibrium. Changing levels of other POPs reflect global redistribution and increasing transfer to antarctic waters probably due to recent usage in the southern hemisphere and climate changes