Infrared spectra of gas-phase ions and their use in elucidating reaction mechanisms. Identification of C_7H_7- structural isomers by multiphoton electron detachment using a low-power infrared laser

The ability to distinguish structural isomers of ions on the basis of their vibrational spectra is of considerable interest since a general method would greatly extend the capabilities of mass spectrometry. It is difficult to achieve number densities of gas-phase ions sufficient to observe infrared absorption directly, so indirect methods have been developed by inducing unimolecular or modified bimolecular reactivity as a result of absorption of one or more infrared photons. We have explored the technique of multiphoton dissociation using CW and pulsed infrared lasers to obtain spectra of positive ions. Multiphoton electron detachment (MED) from negative ions was recently reported by Brauman and co-workers using high-power laser pulses from a CO_2-TEA laser. These initial studies have inspired an interesting theoretical treatment of vibrationally induced electron detachment. In this report we present the infrared MED spectra of isomeric C_7H_7- ions obtained with a low-power CW CO_2 laser. This methodology has been used to probe mechanistic details of an ion-molecule reaction in the gas phase for the first time, showing that deprotonation of 2,5-norbornadiene by CH_3O- is accompanied by structural rearrangement to benzyl and cycloheptatrienyl anions.