An Aromatic Universe–A Physical Chemistry Perspective

Abstract

This Perspective presents recent advances in our knowledge of the fundamental elementary mechanisms involved in the low-and high-temperature molecular mass growth processes to polycyclic aromatic hydrocarbons in combustion systems and in extraterrestrial environments (hydrocarbon-rich atmospheres of planets and their moons, cold molecular clouds, circumstellar envelopes). Molecular beam studies combined with electronic structure calculations extracted five key elementary mechanisms: Hydrogen Abstraction−Acetylene Addition, Hydrogen Abstraction−Vinylacetylene Addition, Phenyl Addition−DehydroCyclization, Radical−Radical Reactions, and Methylidyne Addition− Cyclization−Aromatization. These studies, summarized here, provide compelling evidence that key classes of aromatic molecules can be synthesized in extreme environments covering low temperatures in molecular clouds (10 K) and hydrocarbon-rich atmospheres of planets and their moons (35−150 K) to high-temperature environments like circumstellar envelopes of carbon-rich Asymptotic Giant Branch Stars stars and combustion systems at temperatures above 1400 K thus shedding light on the aromatic universe we live in.

The gas-phase infrared spectra of the 2-methylallyl radical and its high-temperature reaction products

“…2 A recent review summarizes the various mechanisms that are thought to form PAHs. 3 However, the process is far from understood, because the complex chemistry of the relevant systems is difficult to disentangle. Resonance-stabilized radicals (RSR) are of particular relevance, because they can accumulate in a reactive environment and contribute to molecular growth.…”

Neutral Dissociation of Pyridine Evoked by Irradiation of Ionized Atomic and Molecular Hydrogen Beams

“…Ion–molecule interactions lead to the chemical transformation of simple and complex compounds in the atmospheres of Earth, planets, and the interstellar medium [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ]. Some collisional reactions are pertinent to combustion processes [ 8 , 9 ].…”

Experimental Observation of Hydrocarbon Growth by Resonance‐Stabilized Radical–Radical Chain Reaction

“…[2] Better understanding of elementary growth pathways could open new opportunities for producing nanoparticles and specialized materials. [1b, 3] In addition to these high-temperature pyrolysis environments, an unexplained abundance of PAHs in the low-temperature interstellar medium (ISM) [4] has led to a search in the physical chemistry community [5] for barrierless radical-driven PAH growth mechanisms.…”

“…The HACA (hydrogen-abstraction-C 2 H 2 -addition) mechanism [6] is widely employed to describe hydrocarbon molecular weight growth in combustion [1b] and to a lesser extent in astrochemistry. [4,5,7] In addition to HACA, the PAC (phenyl-addition-dehydrocyclization) mechanism [8] provides a pathway for addition of multiple rings in a single step, and methyl addition pathways [9] can help explain the prevalence of odd-carbon PAHs. For low-temperature environments, the HAVA (hydrogen-abstraction-vinylacetylene-addition) mechanism [7,10] involves barrierless addition of vinylacetylene and 1,3-butadiene to PAH radicals, and the MACA (methylidyne-addition-cyclization-aromatization) mechanism [11] describes barrierless methylidyne addition to stable PAHs.…”

“…Furthermore, PAH growth in stellar ejecta may also be driven by low-temperature pathways, as the HACA mechanism seems to require PAHs to pass through a critical temperature region in the circumstellar envelopes of asymptotic giant branch stars many, many times to explain observed PAH ejection. [13] Electronic structure calculations and molecular beam studies have provided insight into low-temperature growth pathways, [5] primarily based on the HAVA [7,10] and MACA [11] mechanisms which involve specific feedstocks (vinylacetylene, 1,3-butadiene, and methylidyne). Barrierless radical-radical chain reactions provide another low-temperature pathway for PAH formation that does not require repeated H-abstraction.…”