Unraveling the possible isomers of CH4CN+ and CH2CN+ through FT-IR matrix-isolation spectroscopy of mass-selected ions and DFT theory

Abstract

The positive molecular ions of astrophysical interest — protonated and fragment ions of acetonitrile — are investigated by means of mass-selective deposition in Ne matrices and FT-IR spectroscopy in the region of 500–4000 cm⁻¹. Ionic species are generated by electron impact ionization with parallel fragmentation and ion–molecule reactions of an acetonitrile precursor, CH₃CN (CD₃CN). The ions are deposited in Ne matrices together with an excess of electrons to prevent a buildup of space charge. Spectral assignments are supported by deuterium isotopic measurements and by density functional (DFT/B3LYP) computations. This work resulted in the spectral characterization of protonated acetonitrile CH₃CNH⁺, as well as the identification of three cationic cyanomethyl C₂H₂N⁺ isomers: H₂CCN⁺, H₂CNC⁺ and HCCNH⁺. Although the spectral assignments for two ionic isomers, H₂CCN⁺ and H₂CNC⁺, are somewhat tentative, experimental results give rather strong evidence for the occurrence of the linear HCCNH⁺ ion.