Polycyclic aromatic hydrocarbons interstellar PAHs

It is possible that electrons are not the major carrier of negative charge in dense interstellar clouds. It has been suggested that if a large fractional abundance of polycyclic aromatic hydrocarbons (PAHs) exists throughout dense interstellar clouds (see Section I), then electron sticking reactions of the type,... 

The C60 molecule is difficult to destroy by ultraviolet radiation or by collisions with other particles. While other molecules have serious difficulties to survive in the interstellar medium, the robustness of C60 and of the other fullerenes allows their long survival. The bonds between carbon atoms make them at least as robust against dissociation in the interstellar medium as polycyclic aromatic hydrocarbons (PAHs) can be. 

The polycyclic aromatic hydrocarbons (PAHs) have been inferred to exist in the interstellar dust by the correlation of their general infrared spectral characteristics with observed celestial infrared emission bands [13-15]. 

Naphthalene, anthracene, and similar hydrocarbons are termed polycyclic aromatic hydrocarbons (PAHs) because they are composed of multiple aromatic rings. PAHs have been found in meteorites and identified in the material surrounding dying stars. Scientists have mixed PAHs with water ice in a vacuum at -260°C to simulate the conditions found in interstellar clouds. To simulate radiation emitted by nearby stars, they shined ultraviolet light on the mixture. About ten percent of the PAHs were converted to alcohols, ketones, and esters— molecules that can be used to form compounds that are important in biological systems. 

Abstract The study of the fate of electronically excited radical and radical cation of aromatic hydrocarbons is an emerging topic in modern chemical dynamics. Observations like low quantum yield of fluorescence and photostability are of immediate concern to unravel the mechanism of ultrafast nonradiative internal conversion dynamics in such systems. The radical cations of polycyclic aromatic hydrocarbons (PAHs) have received considerable attention in this context and invited critical measurements of their optical spectroscopy in a laboratory, in striving to understand the enigmatic diffuse interstellar bands (DIBs). 

Interstellar solid-state chemistry can occur within these ices. Laboratory experiments have shown that ices of simple species such as H2O, CO, or NHj can be stimulated by ultraviolet radiation or fast particles (protons, electrons) to form complex molecules, including polycyclic aromatic hydrocarbons (PAHs) containing several benzene-type rings. The detection by astronomers of free interstellar benzene (CsHg) in at least one interstellar region suggests that this solid-state chemistry may be the route by which these molecules are made. 

Carbon chemistry occurs most efficiently in circumstellar and diffuse interstellar clouds. The circumstellar envelopes of carbon-rich stars are the heart of the most complex carbon chemistry that is analogous to soot formation in candle flames or industrial smoke stacks (26). There is evidence that chemical pathways, similar to combustion processes on Earth, form benzene, polycyclic aromatic hydrocarbons (PAHs) and subsequently soot and complex aromatic networks under high temperature conditions in circumstellar regions (27,28). Molecular synthesis occurs in the circumstellar environment on timescales as short as several hundred years (29). Acetylene (C2H2) appears to be the... 

X 10 kg. Analyses of interstellar dust clouds and heavenly bodies fotmd on Earth, such as the Murchison meteorite, showed the presence of a variety of important prebiotic compotmds. such as amino acids, ureas, alcohols. aldehydes, purines, polycyclic aromatic hydrocarbons (PAH), and organic acids,"" highlighting the significance of extraterrestrial carbon delivery to Earth. 

Hammonds M, Pathak A, Sarre PJ. TD-DFT calculations of electronic spectra of hydrogenated protonated polycyclic aromatic hydrocarbon (PAH) molecules implications for the origin of the diffuse interstellar bands Phys Chem Chem Phys 2009 11 4458-64. 

All three pieces have been seen, separately, in interstellar space. The most complex of these is the N (nucleobase) piece. Space telescopes have found huge amounts of cosmic carbon circles, known as polycyclic aromatic hydrocarbons (PAHs). These hexagons of carbon are stable and even form in the soot on an outdoor grill. The carbon circles can be found intermingled with nitrogen, looking like the first draft of a nucleobase. 

Omont (1986) has outlined the evidence for the presence of polycyclic aromatic hydrocarbons in the interstellar medium and given a full acccount of their properties. He pointed out that PAH ions may be more abundant than free electrons in dense clouds. Lepp and Dalgamo (1987) recognized that in this circumstance the recombinatiwi of positive ions occurs through mutual neutralization with PAH ions and have discussed the consequential changes in the chemistry. Omont (1986) gives the rate coeffrcient fen the process to be approximately... 

It is now generally accepted that many of the neutral molecules observed in interstellar clouds are formed in the gas phase by positive ion-molecule reactions which produce a wide variety of molecular positive ions which then dissociatively recombine with electrons to form the observed neutral molecules. The final neutralization step in the conversion of a positive molecular ion to a neutral molecule may also be the transfer of a proton from the ion to another molecule or the transfer of an electron to the molecular ion from a species of low ionization energy (e.g. a metal atom). Very recently, it has been proposed that negatively-charged macromolecules, PAH" (i.e. negatively- charged polycyclic aromatic hydrocarbons, PAH s), may be present in interstellar... 

The interstellar medium may also appear as dark clouds that absorb starlight. The densities are several 10 particles cm . In such clouds polycyclic aromatic hydrocarbons (PAHs), fullerenes, carbon-chains, diamonds etc. are found. 

The electronic properties and stability of PAHs depend not only on the size, but also markedly upon the arrangement of benzenoid rings. According to Clair s aromatic sextet rule, the stability of the isomers of certain polycyclic aromatics increases with the number of sextets. Thus, all-benzenoid polycyclic hydrocarbons (PBAHs) whose structure can be represented by a single Clar formula with no double bonds show extremely high stability, high melting point and low chemical reactivity. Some of them even exist in interstellar space [26]. A typical example is the hexa-peri-hexabenzocoronene (HBC, in Fig. 3.3) which has been studied since... 

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