Infrared photodissociation spectroscopy of small organic cations

Abstract

Small organic cations of the form [Cn,Hm]+ (carbocations) and [Cx,Hy,O]+ (carboxonium ions) are produced in a molecular beam via electrical discharge in a pulsed nozzle source. Size-selected ions are investigated using infrared laser photodissociation spectroscopy in the fingerprint region (600-2150 cm-1) and the C−H and O−H stretching region (2400-3800 cm-1). Quantum chemistry is used to predict structures and frequencies to aid assignment of the infrared spectra. We attempted to produce and characterize the infamous 2-norbornyl carbocation [C7,H11] + in the gas phase. Spectral analysis reveals that a previously unanticipated rearrangement occurs. [C7,H9]+ is produced from norbornene, and an unusual structural isomer of the well-known protonated toluene is found. The mass 31 cation [C,H3,O]+ exists in two structural configurations, protonated formaldehyde and the triplet methoxy cation. Cation analogs to formaldehyde [C,H2,O]+ and methanol [C,H4,O]+ both have radically different structural isomers. [C2,H3,O]+ can exist as the remarkably stable acetyl cation, or as its more energetic protonated ketene isomer, depending on the complex reaction pathways involved in the ion source.