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

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.