High Resolution Leak-Out Spectroscopy of the ν2 Symmetric CH3 Stretching Mode of Protonated Acetonitrile, CH3CNH.

The symmetric CH3 stretching mode ν2 of protonated acetonitrile (CH3CNH+), located around 3.4 μm, has been measured at high resolution in a cryogenic 22-pole ion trap apparatus via leak-out spectroscopy. In addition to this fundamental vibrational mode, several close-lying combination bands interfering through Fermi-resonances were detected in its proximity. Two of those have been assigned as the ν5 + 2ν8 and ν5 + ν7 + ν9 combination bands, and three more likely arising from the four-quanta combination mode, ν7 + 2ν9 + ν10. The formation of ground-state combination differences permits the spectroscopic assignment of 227 out of the 261 detected lines. A thorough treatment of the Fermi resonance system yields spectroscopic constants for three of the bands and approximate spectroscopic constants for all remaining ones.