Investigating the role of curvature on the formation and thermal transformations of soot

In this work, the role of curved polycyclic aromatic hydrocarbons (cPAH) on the initial formation mechanism and thermal transformations of soot was explored. Experimental and computational techniques were used to probe the integration, presence and impact of internal pentagonal rings on the nucleation mechanism of these particulates. A significant charge polarisation was found to occur when an internal pentagonal ring pyramidalises the aromatic network. Phase contrast transmission electron microscopy allowed for the extent of conjugation and degree of curvature to be determined in early soot nanoparticulates with 15 aromatic rings and two pentagons being the median species. The dipole moment of such a species was calculated to be 5.32 debye. The polarity was found to be persistent at flame temperatures with inversion and fluctuations being minimal. Homogeneous nucleation was considered with homodimerisation energies with one or two internal pentagonal rings within cPAH found to be comparable in energy to flat PAH (fPAH) homodimers of similar weight, with more pentagons reducing the binding energy. Ion-induced nucleation was considered with binding energies calculated between chemi-ions and cPAH suggesting small stable clusters at flame temperatures. However, physical and ion-induced nucleation of cPAH were found to be insufficient alone to explain the formation of soot. The impact of curvature on the reactivity of PAH were then studied. Strong crosslinks between σ-radicals and cPAH were found to form at their rim due to decreased aromaticity. Partially saturated rim-based pentagonal rings were also found to form localised π-radicals that allow stacked and bonded complexes to form, suggesting a covalently stabilised soot nucleation. Finally, the curved geometry of highly annealed soot, otherwise known as non-graphitising carbon, was explored using annealed molecular dynamics simulations and a discrete mesh analysis method. Analysis of the angular defect of the meshes revealed an excess of negative curvature. The coexistence of curved and layered ribbon-like structures was found to be possible due to the presence of a small number of non-sp² defects such as screw dislocations and free edges, which will impact the synthesis of novel carbon materials and the oxidation of thermally annealed soot. The incorporation of curvature and pentagonal rings is therefore considered critical for understanding the properties, formation and destruction of combustion generated carbonaceous particles and other carbon materials.This project is supported by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme

Modelling galaxy spectra in presence of interstellar dust. I. The model of ISM and the library of dusty SSPs

In this paper, the first of a series of two devoted to modelling the spectra of galaxies of different morphological type in presence of dust, we present our description of the dust both in the diffuse ISM and the molecular clouds. Our model for the dust takes into account three components, i.e. graphite, silicates and polycyclic aromatic hydrocarbons. We consider and adapt to our aims two prescriptions for the size distribution of the dust grains and two models for the emission of the dusty ISM. We cross-check the emission and extinction models of the ISM by calculating the extinction curves and the emission for the typical environments of the Milky Way and the Large and Small Magellanic Clouds and by comparing the results with the observational data. The final model we have adopted is an hybrid one which stems from combining the analysis of Guhathakurta et al (1989) for the emission of graphite and silicates and Puget et al. (1985) for the PAH emission, and using the distribution law of Weingartner et al. (2001a) and the ionization model for PAHs of Weingartner et al. (2001b). We apply the model to calculate the SED of SSPs of different age and chemical composition, which may be severely affected by dust in young, massive stars while they are still embedded in their parental MCs. We use the "Ray Tracing" method to solve the problem of radiative transfer and to calculate libraries of SSP SEDs. Particular care is paid to model the contribution from PAHs, introducing different abundances of C in the population of very small carbonaceous grains (VSGs) and different ionization states in PAHs. The SEDs of young SSPs are then compared with observational data of star forming regions of four local galaxies successfully reproducing their SEDs from the UV-optical regions to the mid and far infrared.Comment: 24 pages, 29 figure

Wiener Index Calculation on the Benzenoid System: A Review Article

The Weiner index is considered one of the basic descriptors of fixed interconnection networks because it provides the average distance between any two nodes of the network. Many methods have been used by researchers to calculate the value of the Wiener index. starting from the brute force method to the invention of an algorithm to calculate the Wiener index without calculating the distance matrix. The application of the Wiener index is found in the molecular structure of organic compounds, especially the benzenoid system. The value of the Wiener index of a molecule is closely related to its physical and chemical properties. This paper will show a comprehensive bibliometric survey of peer-reviewed articles referring to the Wiener index of benzenoid. The Wiener index values of several benzenoid compounds using cubic polynomial are also reported. The Wiener index of benzenoid supports much of the research and provides productive citations for citing the study.   Keywords: Wiener index, benzenoid, distance matrix, chemical properties, cubic polynomial, topological

Growth of unsaturated, cyclic, and polycyclic aromatic hydrocarbons: Reactions under the conditions of the interstellar medium

Hydrocarbons, in particular polycyclic aromatic hydrocarbons (PAHs), have been long discussed to be carriers of interstellar infrared (IR) emission and ultraviolet (UV) absorption features. Yet, their origin in dense phases of the interstellar medium (ISM), such as molecular clouds, remains unclear. In this work, growth mechanisms based on ion-molecule reactions between cationic PAHs/hydrocarbons and methyne (CH) were investigated. The reaction type and the precursor were derived and selected from known chemical and physical properties of the ISM. These chemical reactions were characterised by calculating branching ratios (based on cross sections) and capture rate coefficients, minimum reaction paths, reaction enthalpies, thermal equilibrium constants, and microcanonic isomerisation and radiative deactivation rate coefficients. In order to cope with the variety of reaction parameters, a hierarchic workflow scheme was set up. First, the reaction potential energy surface was sampled by molecular dynamics simulations. Then, minimum energy paths of the most probable reaction channels were investigated. Finally, molecular and kinetic properties of stationary points were calculated. The quantum chemical level of theory was increased at each step from DFTB (tight-binding density-functional), to DFT, and finally to post-Hartree-Fock methods. Results on CH based hydrocarbon growth showed the transition from non-cyclic hydrocarbons to cyclic and aromatic structures and from cyclic to polycyclic aromatic hydrocarbons. Additionally, the reactive collisions between hydrocarbons and CH were found to produce sufficient energy for isomerisation and fragmentation processes even at ultra low temperatures. In all, the results indicate that methyne might be a proper precursor for the formation of large interstellar PAHs.Kohlenwasserstoffe, insbesondere polyzyklische Kohlenwasserstoffe (engl. PAHs), werden seit einigen Jahren als Mitverursacher interstellar IR-Emissions- und UV-Absorptionsbanden angesehen und diskutiert. Dabei ist die Herkunft dieser Moleküle in den dichten Phasen des interstellaren Mediums (ISM) aber noch nicht aufgeklärt. In dieser Arbeit wurden daher die Bildungsmechanismen, welche auf Ion-Molekül-Reaktionen zwischen kationischen PAHs und Kohlenwasserstoffen und dem Molekül CH beruhen, untersucht. Sowohl der Reaktionstyp als auch der Präkursor wurden anhand von bekannten physikalischen und chemischen Eigenschaften des ISM abgeleitet und ausgewählt. Die Analyse der chemischen Reaktionen basierte auf Berechnungen zur Produktzusammensetzung und Einfangsratenkoeffizienten (welche wiederum aus berechneten Reaktionsquerschnitten hervorgingen) Minimumenergiepfade (MEP), Reaktionsenthalpien, thermische Gleichgewichtskonstanten und mikrokanonische Isomerisierungs- und Strahlungsdeaktivierungs-Ratenkoeffizienten. Um der Vielzahl an Reaktionsparameter gerecht zu werden, wurden die Berechnungsmethoden entsprechend eines hierarischen Fließschemas kombiniert. Hierzu wurden zuerst durch Molekulardynamik-Simulationen die Reaktionspotentialenergieflächen abgerastert. Auf der nächsten Stufe wurden statistisch bedeutsame Reaktionskanäle bezüglich ihrer Minimumenergiepfade untersucht. Den Abschluss bildete die Berechnung molekularer und kinetischer Charakteristika stationärer Punkte auf einem MEP. Entsprechend dieses Schemas wurde die quantenchemische Genauigkeit auf jeder Stufe von approximativer DFT über DFT zu post-Hartree-Fock verändert. Die Ergebnisse des CH-basierten Kohlenwasserstoffwachstums zeigten einen Übergang von nichtzyklischen zu zyklischen and aromatischen Strukturen, sowie von zyklischen zu polyzyklischen Kohlenwasserstoffen. Außerdem zeigte sich, dass reaktive Kollisionen zwischen Kohlenwasserstoffen und CH auch bei Tiefsttemperaturen immer ausreichend Energie für Isomerisierungs- und Fragmentationsprozesse liefert. Die Ergebnisse dieser Arbeit lassen den Schluss zu, dass CH ein geeigneter Präkursor für die Bildung großer interstellarer PAH ist

The chemical characterisation of the unresolved complex mixture (UCM) observed in the gas chromatograms of biodegraded petroleums.

This research was undertaken to elucidate the chemical composition of the unresolved complex mixture (UCM) found in heavily biodegraded crude oils. The isolation of total hydrocarbons fraction by alumina column chromatography of the deasphalted, biodegraded crude oil (West Sak, from the Shublich formation, Alaska), was subjected to a variety of techniques, including chromatographic methods such as column chromatography and TLC to obtain aliphatic and aromatic sub-fractions. Molecular inclusion techniques such as urea, thiourea adduction and 13X molecular sieving were applied to gain some general indication on the UCM component sizes and the distribution of cyclic components, which was also aided by elemental analysis. All fractions obtained by the above-mentioned methods were analysed by gas chromatography (GC) and some by combined gas chromatography-mass spectrometry (GC-MS). Chemical oxidative degradation using chromic acid was employed to derive more specific structural data on the various units present in the UCM. The chemical oxidation products obtained were analysed by GC and GC-MS as full methyl esters. Spectroscopic/spectrometrie techniques such as FT-IR, FT-NMR (1H, 13C), UV, EI-MS and CI-MS were also employed to elucidate the general chemical nature of the UCM components. It is apparent from NMR and IR studies that the UCM is mainly aliphatic (ca. 90%) and comprises a significant proportion ofcyclic systems with probably up to four and five fused rings. In contrast, TLC resulted in a ca. 50:50 mixture between aliphatic and aromatic components in the UCM. An average empirical formula of C4H7 was obtained from the elemental analysis implying that mixture of di- and tri-cyclic components are present in the UCM. Quantification data of the adducted UCM by urea (ca. 50 + 25%) reflected the presence of a significant proportion of branched/cyclic systems with n-alkyl chains attached at one end. With thiourea, it was found that ca. 65% of the UCM was confined to the thiourea adduct, suggesting that branched and/or cyclic structures with sizes up to four fused rings are important contributors to the UCM structure. Aliphatic/aromatic class fractionation with 13X molecular sieve indicated that ca. 70% of the UCM components are aliphatic and ca. 30% are aromatic. Oxidation product data suggested that cyclic systems present are substituted with alkyl chains. The maximum n-alkyl chains would appear not to exceed C20 in length and their frequency distribution seems to maximise around C18. Molecular ion information was not obtained from EI-MS and CI-MS, although evidence was obtained to support a predominantly aliphatic mixed cyclic/acyclic composition.<p

Combustion generated fine carbonaceous particles

Soot is of importance for its contribution to atmospheric particles with their adverse health impacts and for its contributions to heat transfer in furnaces and combustors, to luminosity from candles, and to smoke that hinders escape from buildings during fires and that impacts global warming or cooling. The different chapters of the book adress comprehensively the different aspects from fundamental approaches to applications in technical combustion devices

Carbon in the Galaxy: Studies from Earth and Space

Presented here is the text of the invited papers presented during a meeting entitled, Carbon in the Galaxy: Studies from Earth and Space, that was held at NASA Ames Research Center on November 5 and 6, 1987. For completeness, abstracts from all of the poster papers and the text of a paper summarizing what was learned during the course of the meeting are also included. The underlying premise for the meeting was that there is much to be gained by bringing together scientists from very different disciplines, all of whom study carbon in different ways for different reasons. The interchanges took place during the meeting and the contents of the enclosed papers validate that premise