Ionization and fragmentation pathways induced by ionizing agents are key to understanding the formation of complex molecules in astrophysical environments. Acetonitrile (CH 3 CN), the simplest organic nitrile, is an important molecule present in the interstellar medium. In this work, DFT and MP2 calculations were performed in order to obtain the low energy structures of the most relevant cations formed from electron-stimulated ion desorption of CH 3 CN ices. Selected reaction pathways and spectroscopic properties were also calculated. Our results indicate that the most stable acetonitrile cation structure is CH 2 CNH + and that hydrogenation can occur successively without isomerization steps until its complete saturation. Moreover, the stability of distinct cluster families formed from the interaction of acetonitrile with small fragments, such as CH n + , C 2 H n + , and CH n CNH + , is discussed in terms of their respective binding energies. Some of these molecular clusters are stabilized by hydrogen bonds, leading to species whose infrared features are characterized by a strong redshift of the N− H stretching mode. Finally, the rotational spectra of CH 3 CN and protonated acetonitrile, CH 3 CNH + , were simulated using distinct computational protocols based on DFT, MP2, and CCSD(T) considering centrifugal distortion, vibrational−rotational coupling, and vibrational anharmonicity corrections. By adopting an empirical scaling procedure for calculating spectroscopic parameters, we were able to estimate the rotational frequencies of CH 3 CNH + with an expected average error below 1 MHz for J values up to 10.

The ion-distribution from condensed pyridine due to 2 keV electron impact shows hydrogenated fragments and clusters with m/z ≤ 320 u and shifts towards higher masses compared to the gas-phase fragmentation. The formation of a bond between the pyridine and a carbenium ion is crucial for the stability of the selected cluster ions.