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Infrared Spectroscopy of the Mass 31 Cation: Protonated Formaldehyde vs Methoxy

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journal contribution
posted on 20.09.2012, 00:00 by J. D. Mosley, T. C. Cheng, A. B. McCoy, M. A. Duncan
Pulsed discharges containing methanol or ethanol produce ions having the nominal formula [C,H3,O]+, i.e. m/z = 31. Similar ions resulting from electron impact ionization in mass spectrometers are long recognized to have either the CH2OH+ protonated formaldehyde or CH3O+ methoxy cation structures. The H2OCH+ oxonio-methylene structure has also been suggested by computational chemistry. To investigate these structures, ions are expanded in a supersonic beam, mass-selected in a time-of-flight spectrometer, and studied with infrared laser photodissociation spectroscopy. Sharp bands in the O–H and C–H stretching and fingerprint regions are compared to computational predictions for the three isomeric structures and their vibrational spectra. Protonated formaldehyde is the most abundant isomer, but methoxy is also formed with significant abundance. The branching ratio of these two ion species varies with precursors and formation conditions.