MECHANISTIC STUDY OF HYDROGEN-DEUTERIUM EXCHANGE BETWEEN [M-1]- IONS OF CHLORINATED BENZENES AND D2O OR ND3

Low-energy collisions between [M - 1]- ions derived from the three isomers of dichlorobenzene and deuterated water and ammonia are found to produce distinctive hydrogen/deuterium (H/D) exchange reaction product patterns. The predominant products observed for p-, m-, and o-dichlorobenzene are 1, 2, and 3 sequential deuterium exchanges, respectively. The reactivity is substantially higher for D2O than ND3. We postulate a mechanism that involves the formation of a five-membered-ring intermediate. The intermediate is thought to be initiated by the attack of ND3 or D2O at the localized negative charge site on the aromatic ring. A successful exchange is followed by the relocation of the charge site to the adjacent carbon. Ion products with higher degrees of deuterium substitution than the expected predominant products of their corresponding isomers are believed to be the results of isomerization of the reactant ions occurring in the ion source. The proposed mechanism fully explains the observed product spectra derived from all the isomers of chlorinated benzenes. The trends for the formation of various H/D exchange products represented by the simulated product-time plots based on the proposed mechanism compare well with the similar trends obtained from the experimental product-pressure plots. The reaction is useful for the mass spectrometric differentiation of chlorobenzene isomers.