Structural Isomerization of the Gas-Phase 2-Norbornyl Cation Revealed with Infrared Spectroscopy and Computational Chemistry

In an attempt to produce the 2-norbornyl cation (2NB(+)) in the gas phase, protonation of norbornene was accomplished in a pulsed discharge ion source coupled with a supersonic molecular beam. The C7H11+ cation was size-selected in a time-of-flight mass spectrometer and investigated with infrared laser photodissociation spectroscopy using the method of tagging with argon. The resulting vibrational spectrum, containing sharp bands in the C-H stretching and fingerprint regions, was compared to that predicted by computational chemistry. However, the measured spectrum did not match that of 2NB(+), prompting a detailed computational study of other possible isomers of C7H11+. This study finds five isomers more stable than 2NB(+). The spectrum obtained corresponds to the 1,3-dimethylcyclopentenyl cation, the global minimum-energy structure for C7H11+, which is produced through an unanticipated ring-opening rearrangement path.