Action spectroscopy of gas-phase carboxylate anions by multiple photon IR electron detachment/attachment

TL;DR

This paper introduces a novel gas-phase anion action spectroscopy method using infrared multiple photon electron detachment and electron capture, enabling background-free spectra of strongly bound carboxylate anions with validation against other techniques.

Contribution

The study presents the first gas-phase spectra of acetate and propionate using a new method involving electron scavengers and tunable IR radiation, validated through comparison with dissociation spectra and theoretical calculations.

Findings

Successful acquisition of background-free IR spectra of acetate and propionate.

Validation of the method through comparison with dissociation spectra.

DFT calculations accurately predict vibrational frequencies and elucidate environmental effects.

Abstract

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.