Formation of benzene in the interstellar medium

Area of Science:

• Astrochemistry
• Chemical Kinetics
• Interstellar Medium

Background:

• Polycyclic aromatic hydrocarbons (PAHs) are crucial in interstellar medium evolution.
• The formation pathways for simple aromatic molecules like benzene remain largely unknown.
• Understanding benzene formation is key to understanding PAH origins.

Purpose of the Study:

• To elucidate the formation mechanism of the benzene molecule (C(6)H(6)) under interstellar conditions.
• To identify low-temperature synthetic routes for aromatic molecules from acyclic precursors.
• To investigate the role of ethynyl radicals in interstellar aromatic molecule formation.

Main Methods:

• Crossed molecular beam experiments were conducted.
• Electronic structure and statistical calculations were employed.
• Single collision conditions were maintained to isolate the reaction pathway.

Main Results:

• Benzene (C(6)H(6)) is synthesized via the barrierless, exoergic reaction of ethynyl radical (C(2)H) and 1,3-butadiene.
• This reaction represents a general class of barrierless syntheses of aromatic molecules from acyclic precursors.
• Gas-grain astrochemical models indicate this pathway dominates initial aromatic ring formation in cold molecular clouds.
• Subsequent reactions of benzene with ethynyl radicals rapidly form naphthalene-like structures.

Conclusions:

• The ethynyl radical and 1,3-butadiene reaction provides a key low-temperature pathway for benzene formation in space.
• This mechanism explains the presence of aromatic molecules in the interstellar medium.
• The ethynyl-radical mediated pathway efficiently propagates the formation of larger aromatic structures.