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

Abstract

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., CH 3OCl 2 +) in our GC/MS experiments is an ion-dipole complex (CHCl 2 +⋯OH 2), formally corresponding to protonated dichloromethanol (G3 calculated P A C H 2 OC l 2 ∼ 163.3 kcal mo l − 1 ) produced in the gas phase either by the association of a water molecule with a CHCl 2 + 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 ( P A CHC l 3 ∼ 157.8 kcal mo l − 1 ) as well as entropy considerations indicate that a non-dissociative proton transfer (PT) reaction from H 3O + to CHCl 3 would be inefficient; however, the observed dissociative PT product ions ( e.g., CHCl 2 +) can be explained by considering the reaction entropy (Δ S). 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 H 5O 2 + 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.