Infrared spectroscopy of the methanol cation and its methylene-oxonium isomer

The carbenium ion with nominal formula [C,H₄,O]⁺ is produced from methanol or ethylene glycol in a pulsed-discharge supersonic expansion source. The ion is mass selected, and its infrared spectrum is measured from 2000 to 4000 cm⁻¹ using laser photodissociation spectroscopy and the method of rare gas atom tagging. Computational chemistry predicts two isomers, the methanol and methylene-oxonium cations. Predicted vibrational spectra based on scaled harmonic and reduced dimensional treatments are compared to the experimental spectra. The methanol cation is the only isomer produced when methanol is used as a precursor. When ethylene glycol is used as the precursor, methylene-oxonium is produced in addition to the methanol cation. Theoretical results at the CCSD(T)/cc-pVTZ level show that methylene-oxonium is lower in energy than methanol cation by 6.4 kcal/mol, and is in fact the global minimum isomer on the [C,H₄,O]⁺ potential surface. Methanol cation is trapped behind an isomerization barrier in our source, providing a convenient method to produce and characterize this transient species. Analysis of the spectrum of the methanol cation provides evidence for strong CH stretch vibration/torsion coupling in this molecular ion