| Abstract: | The chemical ionization mass spectra of five isomers of C3H6O (acetone, propionaldehyde, oxetane, propylene oxide and allyl alcohol) have been determined using a variety of reagent gases (H2, D2, N2/H2, CO2/H2 and CO/H2). The [C3H7O]+ ions produced by protonation of these isomers undergo very similar reactions to those reported for analogous [C3H7O]+ metastable ions; however, decomposing ions generated by chemical ionization appear to have somewhat higher internal energies. The results of 2H labelling studies (D2 reagent gas or labelled analogues of C3H6O) indicate that protonation occurs mainly on oxygen and are consistent with previous investigations of metastable oxonium ions. The protonated acetone ion is particularly stable, in agreement with the higher activation energies for fragmentation of this isomer than for other [C3H7O]+ structures. As the calculated heat of protonation of C3H6O is reduced by changing the reagent gas, so the extent to which fragmentation occurs decreases. This is discussed in the context of competition between fragmentation and collisional stabilization of the excited [C3H7O]+* ion. It is concluded that on average a large fraction (approaching 1) of the exothermicity of the protonation reaction resides in the [C3H7O]+* ions produced initially. |
