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

摘要

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 PACH2OCl2～163.3 kcal mol-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 (PACHCl3～157.8 kcal mol-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 (ΔS). The overall dissociative PT reaction is unfavorable at 298K and marginally exoergic (" entropy driven") under our experimental conditions at 360K. 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.