Destruction of interstellar methyl cyanide (CH3CN) via collisions with He+ ions

TL;DR

This study combines experimental and theoretical methods to accurately determine the reaction rates and pathways for the destruction of methyl cyanide (CH3CN) by He+ ions in interstellar environments, impacting astrochemical models.

Contribution

It provides new cross sections, branching ratios, and reaction rate coefficients for CH3CN destruction by He+ ions, improving upon previous models and databases.

Findings

Reaction rate at 10 K is nearly ten times smaller than previous estimates.

Charge exchange leads to dissociative fragments including HCCN+/CCNH+, HCNH+, and CH2+.

Results suggest methyl cyanide in disks forms mainly via gas-phase reactions.

Abstract

Methyl cyanide is one of the simplest interstellar complex organic molecules, widely detected in young solar analogues, shocked regions, protoplanetary disks and comets. CH3CN can be considered a key species to explore the chemical connections between planet forming disks and comets. For such comparison to be meaningful kinetics data for the reactions leading to CH3CN formation and destruction must be updated. We focus on the destruction of methyl cyanide through collisions with He+. A combined experimental and theoretical methodology is employed to obtain cross sections (CSs) and branching ratios (BRs) as a function of collision energy, from which reaction rate coefficients $k(T)$ are calculated in the temperature range from 10 to 300 K. CSs and BRs are measured using a guided ion beam set-up. A theoretical treatment based on an analytical formulation of the potential energy surfaces (PESs) for the charge exchange process is developed. The method employs a Landau Zener model to obtain reaction probabilities at crossings between the entrance and exit PESs, and an adiabatic centrifugal sudden approximation to calculate CSs and k(T). Rates and BRs differ from those predicted from widely-used capture models. In particular, the rate coefficient at 10 K is estimated to be almost one order of magnitude smaller than what reported in the KIDA database. As for BRs, the charge exchange is completely dissociative and the most abundant fragments are HCCN+/CCNH+, HCNH+ and CH2+. Our results, combined with a revised chemical network for formation of CH3CN, support the hypothesis that methyl cyanide in protoplanetary disks could be mostly the product of gas-phase processes rather than grain chemistry, as currently proposed. These findings are expected to have implications in the comparison of the abundance ratios of N-bearing molecules observed in disks with cometary abundance ratios