Void induced coronene C23H12++ was suggested to be a possible carrier of the astronomically observed polycyclic aromatic hydrocarbon (PAH), which shows unique molecular structure with carbon two pentagons connected with five hexagons. Well observed astronomical infrared spectrum from 3-15 micron could be almost reproduced based on density functional theory. However, there remain several discrepancies with observed spectra, especially on 11-15 micron band weaker intensity. Observed 11.2 micron intensity is comparable to 7.6-7.8 micron one. Methyl-modified molecule C24H14++ revealed that calculated peak height of 11.4 micron show fairly large intensity up to 70-90% compared with that of 7.6-7.8 micron band. Also, nitrogen atom was substituted to peripheral C-H site of void coronene to be C22H11N1++. Pentagon site substituted case show 60% peak height. This molecule also reproduced well 12-15 micron peak position and relative intensity. Vibration mode analysis demonstrated that 11.3 micron mode comes from C-H out of plane bending. Heavy nitrogen plays as like an anchor role for molecule vibration.
Aims. Due to the limitations of current computational technology, the fragmentation and isomerization products of vibrationally-excited polycyclic aromatic hydrocarbon (PAH) molecules and their derivatives are poorly studied. In this work, we investigate the intermediate products of PAHs and their derivatives as well as the gas-phase reactions relevant to the interstellar medium, with coronene as a case study. Methods. Based on the semi-empirical method of PM3 as implemented in the CP2K program, molecular dynamics simulations are performed to model the major processes (e.g., vibrations, fragme…
The electronic and optical properties of various polycyclic aromatic hydrocarbons (PAHs) with lower symmetry, namely, benzo[ghi]perylene (C$_{22}$H$_{12}$), benzo[a]coronene (C$_{28}$H$_{14}$), naphtho[2,3a]coronene (C$_{32}$H$_{16}$), anthra[2,3a]coronene (C$_{36}$H$_{18}$), and naphtho[8,1,2-abc]coronene (C$_{30}$H$_{14}$) were investigated. For the purpose, we performed electron-correlated calculations using screened, and standard parameters in the $π$-electron Pariser-Parr-Pople (PPP) Hamiltonian, and the correlation effects were included, both for ground and excited states, using the mult…
Dimerization of polycyclic aromatic hydrocarbons (PAHs) is an important, yet poorly understood, step in the on-surface synthesis of graphene (nanoribbon), soot formation, and growth of carbonaceous dust grains in the interstellar medium (ISM). The on-surface synthesis of graphene and the growth of carbonaceous dust grains in the ISM require the chemical dimerization in which chemical bonds are formed between PAH monomers. An accurate and cheap method of exploring structure rearrangements is needed to reveal the mechanism of chemical dimerization on surfaces. This work has investigated the chem…
Infrared (IR) absorption spectra of individual polycyclic aromatic hydrocarbons (PAHs) containing methyl (-CH3), methylene (>CH2), or diamond-like *CH groups and IR spectra of mixtures of methylated and hydrogenated PAHs prepared by gas phase condensation were measured at room temperature (as grains in pellets) and at low temperature (isolated in Ne matrices). In addition, the PAH blends were subjected to an in-depth molecular structure analysis by means of high-performance liquid chromatography, nuclear magnetic resonance spectroscopy, and matrix-assisted laser desorption/ionization time-of-f…
While powerful techniques exist to accurately account for anharmonicity in vibrational molecular spectroscopy, they are computationally very expensive and cannot be routinely employed for large species and/or at non- zero vibrational temperatures. Motivated by the study of Polycyclic Aromatic Hydrocarbon (PAH) emission in space, we developed a new code, which takes into account all modes and can describe all IR transitions including bands becoming active due to resonances as well as overtones, combination and difference bands. In this article, we describe the methodology that was implemented a…
Astronomical dust molecule of carbon-rich nebula-Lin49 and nebula-Tc1 could be identified to be polycyclic-pure-carbon C23 by the quantum-chemical calculation. Two driving forces were assumed. One is high speed proton attack on coronene-C24H12, which created void-induced C23H12. Another is high energy photon irradiation, which brought deep photo-ionization and finally caused dehydrogenation to be C23. Infrared spectrum calculation show that a set of ionized C23 (neutral, mono, and di-cation) could reproduce observed many peaks of 28 bands at wavelength from 6 to 38 micrometer. Previously predi…
Failure of hydrogenation in protecting polycyclic aromatic hydrocarbons from fragmentation
最近对天然和氢化晕苯阳离子C $_{24}$ H $_{12+m}^+$ $m=0-7$中的软X射线吸收的研究得出结论，额外的氢原子保护（星际）多环芳烃（PAH）分子免于碎裂[Reitsma等人，物理修订版信函113，053002（2014）]。本实验与快速（30-200 eV）He原子和芘（C $_{16}$ H $_{10+m}^+$，$m=0$，6和16）之间的碰撞和模拟没有参考的激发方法表明相反。我们发现，芘分子的绝对碳骨架断裂截面不减少，但增加的氢化程度。
By applying quantum chemical calculation, interstellar infrared spectrum was categorized to three classes. Type-A show unusual feature of strong peaks at 11.3,12.9, and 14.0 micrometer. Usually observed 6.2, 7.7, and 8.6 micrometer bands are weak or not recognized. Typical examples are NGC1316 and NGC4589. Such spectrum could be identified for the first time by pure carbon molecule (C23)2+ (dication) having two carbon pentagons combined with five hexagons. Calculated spectrum coincided well at 11.3, 13.0, and 14.0 micrometer. Also we could find more coincidence at 5.2, 5.6, 7.6, 8.8,10.6,15.7 …
Interstellar infrared observation shows featured spectrum due to polycyclic aromatic hydrocarbon (PAH)at wavelength 3.3,6.2,7.6,7.8,8.6,and 11.3 micrometer,which are ubiquitously observed in many astronomical dust clouds and galaxies. Our previous first principles calculation revieled that viod induced coronene (C23H12)2+ and circumcoronene (C53H18)1+ could reproduce such spectrum very well. In this study, quantum-mechanic origin was studied through atomic configuration change and atomic vibration mode analysis. By a high speed particle attack, carbon void would be introduced in PAH. Molecular…
Benzene (C6H6) and polycyclic hydrocarbons such as naphthalene (C10H8), anthracene (C14H10) and coronene (C24H12) are well known aromatic organic compounds. We study the substitution of Li replacing all H atoms in these hydrocarbons using density functional method. The vertical ionization energy (VIE) of such lithiated species, i.e., C6Li6, C10Li8, C14Li10 and C24Li12 ranges 4.24-4.50 eV, which is lower than the IE of Li atom. Thus, these species may behave as superalkalis, due to their lower IE than alkali metal. However, these lithiated species possess planar and closed-shell structure, unli…
The formation of polycyclic aromatic hydrocarbons (PAHs) and their methyl derivatives occurs mainly in the dust shells of asymptotic giant branch (AGB) stars. The bands at 3.3 and 3.4 $μ$m, observed in infrared emission spectra of several objects, are attributed C-H vibrational modes in aromatic and aliphatic structures, respectively. In general, the feature at 3.3 $μ$m is more intense than the 3.4 $μ$m. Photoionization and photodissociation processes of toluene, the precursor of methylated PAHs, were studied using synchrotron radiation at soft X-ray energies around the carbon K edge with time…
