Proton transfer reactions of halogenated compounds: Using gas chromatography/Fourier transform ion cyclotron resonance mass spectrometry (GC/FT-ICR MS) and ab initio calculations

We combine modern GC mass spectrometric techniques (GC/FT-ICR MS) and ab initio molecular orbital calculations at G2, G3, and MP2/6-31+G** levels for characterization of disinfection byproducts (DBPs) present in treated drinking water samples. We introduce an additional dimension to GC/MS analysis that utilizes theoretically calculated proton affinities (PAs) and gas-phase basicities (GBs) to elucidate reaction mechanisms. The observed species at m/z = 100.9 (i.e., CH3OCl2+) in our GC/MS experiments is an ion-dipole complex (CHCl2+⋯OH2), formally corresponding to protonated dichloromethanol (G3 calculated PACH2OCl2∼163.3 kcal mol−1PACH2OCl2∼163.3 kcal mol−1) produced in the gas phase either by the association of a water molecule with a CHCl2+ fragment ion from chloroform (present in the treated drinking water sample) or by the elimination of HCl in a condensation reaction between chloroform and protonated water. The calculated PA of chloroform at the G3 level (PACHCl3∼157.8 kcal mol−1PACHCl3∼157.8 kcal mol−1) as well as entropy considerations indicate that a non-dissociative proton transfer (PT) reaction from H3O+ to CHCl3 would be inefficient; however, the observed dissociative PT product ions (e.g., CHCl2+) can be explained by considering the reaction entropy (ΔS). The overall dissociative PT reaction is unfavorable at 298 K and marginally exoergic (“entropy driven”) under our experimental conditions at 360 K. Besides DBPs, we report the presence of the Zundel cation H5O2+ in our mass spectrum. We speculate that the Zundel cation is formed by multiple ion-molecule reactions involving water in the presence of helium carrier gas and GC eluting compounds.

The combined use of GC/FT-ICR mass spectrometry data and ab initio calculations allows deciphering competing pathways for proton transfer reactions to improve unknown identification.Figure optionsDownload high-quality image (59 K)Download as PowerPoint slide