  We report on a form of gas-phase anion action spectroscopy based on infrared
multiple photon electron detachment and subsequent capture of the free
electrons by a neutral electron scavenger in a Fourier Transform Ion Cyclotron
Resonance (FTICR) mass spectrometer. This method allows one to obtain
background-free spectra of strongly bound anions, for which no dissociation
channels are observed. The first gas-phase spectra of acetate and propionate
are presented using SF6 as electron scavenger and a free electron laser as
source of intense and tunable infrared radiation. To validate the method, we
compare infrared spectra obtained through multiple photon electron
detachment/attachment and multiple photon dissociation for the benzoate anion.
In addition, different electron acceptors are used, comparing both associative
and dissociative electron capture. The relative energies of dissociation (by
CO2 loss) and electron detachment are investigated for all three anions by DFT
and CCSD(T) methods. DFT calculations are also employed to predict vibrational
frequencies, which provide a good fit to the infrared spectra observed. The
frequencies of the symmetric and antisymmetric carboxylate stretching modes for
the aliphatic carboxylates are compared to those previously observed in
condensed-phase IR spectra and to those reported for gas-phase benzoate,
showing a strong influence of the solution environment and a slight substituent
effect on the antisymmetric stretch.
