Infrared Spectroscopy of Gas Phase Ions: Using Infrared Multi-photon Spectroscopy to Investigate Ionization Sites and Hydrogen Bonding in Gaseous Ions

Abstract

In this work a novel instrument was developed to obtain infrared multiphoton dissociation (IRMPD) spectra of gas phase ions. This was done by coupling a tunable infrared laser with a Fourier transform mass spectrometer (FTMS). Gas phase ions were trapped in the FTMS cell and irradiated with infrared light for a period of time, typically 1-60 seconds, and the fragmentation of the parent ions was observed. Fragmentation was plotted vs. infrared wavelength to generate infrared action spectra. Experimental spectra were compared to computational spectra to gain insight into the gas phase structure of the irradiated ions. Some species were then derivatized to experimentally assign infrared absorption bands. The first objective was the determination of the ionization site of the (M+H)+ ion of p-aminobenzoic acid (1) formed during positive mode electrospray ionization (ESI) from either methanol/water or acetonitrile solutions. The two most likely protonation sites are at the amino or carboxyl functional groups. Protonated p-aminobenzoic acid ions were irradiated for 20 seconds from 2800-4000 cm-1. Computational predictions for both amino-protonated and carboxy-protonated ions were generated. When methanol/water was used as the ESI solvent there was an excellent match between experimental and the carboxy-protonated computational spectra, indicating that the carboxy-protonated structure was generated. When acetonitrile was used as the ESI solvent the experimental spectrum is identical to the carboxy-protonated structure, however, only 70% of the ions fragment. This indicates that a 70 : 30 ratio of carboxy : amino protonated structures were generated. A series of poly-hydroxy hydrocarbons (polyols) were investigated via IRMPD. Of particular interest is the extent of intramolecular hydrogen bonding in polyols containing as many as seven hydroxyl groups (i.e. 2), as well as the binding characteristics of a series of anions to triol 3. The infrared action spectrum of polyol 2 clustered with chloride anion shows the presence of several hydroxyl groups with varying hydrogen bond strengths indicating that a second, and even third, solvation shell forms around the chloride anion. Chlorine and sulfur anions prefer a geometry with the maximum of three hydrogen bonds while oxygen anions are presumably too small and thus only utilizes two of the three possible hydrogen bonds. Finally, the ionic structure of proline clustered to chloride anion was examined by IRMPD. Whether proline exists in its zwitterionic or neutral structure is not well understood. An exhaustive computational analysis was conducted on multiple neutral and zwitterionic conformers and the computed spectra were compared to the experimental infrared action spectrum. Furthermore, infrared action spectra of proline-d2 and proline-d7 were obtained to empirically assign infrared absorption features. It was determined that while the observed IRMPD spectrum suggests the neutral structure was generated as the sole ion, a more in depth study of the photokinetic data and computations indicate that both the neutral and zwitterionic structures are generated in approximately equal proportions.