Chemical Ionization of Fluorophenyl n-Propyl Ethers: Loss of Propene from the Metastable [M + D] + Ions

Abstract

Chemical ionization (CI) mass spectrometry with the reagents D 2O, CD 3OD, and CD 3CN (given in order of increasing proton affinity) has been used to generate metastable [M + D] + ions of a series of mono-, di-, and trifluorophenyl n-propyl ethers and analogs labeled with two deuterium atoms at the β position of the alkyl group. Loss of propene is the main reaction of the [M + D] + ions, whereas dissociation with formation of propyl carbenium ions is of minor importance. The combined results reveal that the deuteron added in the CI process can be incorporated in the propene molecules as well as in the propyl carbenium ions. The extent to which the added deuteron is exchanged with the hydrogen atoms of the propyl group is markedly dependent on the position of the fluorine atom(s) on the ring and the exothermicity of the initial deuteron transfer. For 3-fluorophenyl n-propyl ether, exchange is not observed if D 2O is the CI reagent, and occurs only to a minor extent in the experiments with the CI reagents CD 3OD and CD 3CN. Similar results are obtained for the 3,5-difluoro- and 2,4,6-trifluorophenyl ethers, whereas significant exchange is observed prior to the dissociations of the [M + D] + ions of the 4-fluoro- and 2,6-difluorophenyl n-propyl ethers, irrespective of the nature of the CI reagent. These results are discussed in terms of the occurrence of initial deuteron transfer either to the oxygen atom or the aromatic ring followed by formation of an ion/neutral complex of a fluorine-substituted molecule and a secondary propyl carbenium ion. Initial deuteron transfer to the oxygen atom is suggested to yield complexes that can react by exchange between the added deuteron and the hydrogen atoms of the original propyl group prior to dissociation. By contrast, initial deuteron transfer to the ring is suggested to lead to complexes that react further by loss of propene molecules containing only the hydrogen/deuterium atoms of the original propyl entity.