Reaction Kinetics of CN + Toluene and Its Implication on the Production of Aromatic Nitriles in the Taurus Molecular Cloud and Titan’s Atmosphere

Abstract

Reactions between cyano radicals and aromatic hydrocarbons are believed to be important pathways for the formation of aromatic nitriles in the interstellar medium (ISM) including those identified in the Taurus molecular cloud (TMC-1). Aromatic nitriles might participate in the formation of polycyclic aromatic nitrogen-containing hydrocarbons (PANHs) in Titan's atmosphere. Here, ab initio kinetic simulations reveal a high efficiency of ∼10 ^−10 cm ^3 s ^−1 and the competition of the different products of the CN + toluene reaction at 30–1800 K and 10 ^−7 –100 atm. In the star-forming region of the TMC-1 environment, the product yields of benzonitrile and tolunitriles for CN reacting with toluene are approximately 17% and 83%, respectively. Detections of the main products, tolunitriles, can serve as proxies for the undetected toluene in the ISM due to their much larger dipole moments. Competition between bimolecular and unimolecular products is extremely intense in the warmer and denser PANH-forming region of Titan's stratosphere. Computational results show that the fractions of tolunitriles, adducts, and benzonitrile are 19%–68%, 15%–64%, and 17%, respectively, at 150–200 K and 0.0001–0.001 atm (Titan's stratosphere). Then, benzonitrile and tolunitriles may contribute to the formation of PANHs by consecutive C _2 H additions. The kinetic information of aromatic nitriles for the CN + toluene reaction calculated here helps to explain the formation mechanism of polycyclic aromatic hydrocarbons or PANHs under different interstellar environments and constrains corresponding astrochemical models.

Keywords

• Astrochemistry
• Interstellar medium
• Titan
• Interstellar clouds
• Computational methods
• Interstellar molecules