CHEMICAL-IONIZATION OF PHENYL N-PROPYL ETHER AND METHYL-SUBSTITUTED ANALOGS - PROPENE LOSS INITIATED BY COMPETING PROTON-TRANSFER TO THE OXYGEN-ATOM AND THE AROMATIC RING

The mechanism of propene loss from protonated phenyl n-propyl ether an d a series of mono-, di-, and trimethylphenyl n-propyl ethers has been examined by chemical ionization (CI) mass spectrometry in combination with tandem mass spectrometry experiments. The role of initial proton transfer to the oxygen atom and the aromatic ring, respectively, has been probed with the use of deuterated CI reagents, D2O, CD3OD, and CD 3CN (given in order of increasing proton affinity), in combination wit h deuterium labeling of the beta position of the n-propyl group or the phenyl ring. The metastable [M + D](+) ions of phenyl n-propyl ether- formed with D2O as the CI reagent-eliminate C3H5D and C3H6 in a ratio of 10:90, which indicates that the added deuteron is incorporated to a minor extent in the expelled neutral species. In the experiments with CD3OD as the CI reagent, the ratio between the losses of C3H5D and C3 H6 from the metastable [M + D](+) ions of phenyl n-propyl ether is 18: 82, whereas the ratio becomes 27:73 with CD3CN as the reagent. A simil ar trend in the tendency to expel a propene molecule that contains the added deuteron is observed for the metastable [M + D](+) ions of phen yl n-propyl ether labeled at the beta position of the alkyl group. Inc orporation of a hydrogen atom that originates from the aromatic ring i n the expelled propene molecule is of negligible importance as reveale d by the minor loss of C3H5D from the metastable [M + H](+) ions of C6 D5OCH2CH2CH3 irrespective of whether H2O, CH3OH, or CH3CN is the CI re agent. The combined results for the [M + D](+) ions of phenyl n-propyl ether and deuterium-labeled analogs are suggested to be in line with a model that assumes that propene loss occurs not only from species fo rmed by deuteron transfer to the oxygen atom, but also from ions gener ated by deuteron transfer to the ring. This is substantiated by the re sults for the methyl-substituted ethers, which reveal that the positio n as well as the number of methyl groups bonded to the ring exert a ma rked effect on the relative importances of the losses of C3H5D and C3H 6 from the metastable [M + D](+) ions of the unlabeled methyl-substitu ted species.