An investigation of the structures of isomeric ions derived from acetone, diacetone alcohol, and mesityl oxide

Abstract

An investigation of the structures of isomeric ions formed in ion/molecule reactions occurring in 2-propanone (acetone), 4-hydroxy-4-mefhyl-2-pentanone (diacetone alcohol), and 4-methyl-3-penten-2-one (mesityl oxide) has been performed using a hybrid mass spectrometer of reversed geometry. Ions of m/z 117, which have been observed as products of ion/molecule reactions in acetone and in diacetone alcohol, may have a single structure if an aldol type condensation reaction occurs under acidic conditions in the gas phase; however, high energy collision induced dissociation (CID) results proved the two ions to be isomeric only.Dehydration of the m/z 117 ions produced a species of m/z 99 which, in the case of diacetone alcohol, may have the structure of protonated mesityl oxide. That the m/z 99 ion formed was unique to each system was demonstrated by high energy CID and kinetic energy release (KER) spectrometry. In the acetone system, CID results suggested that m/z 99 was an ion–dipole complex of [Formula: see text]; this hypothesis was tested by clustering [Formula: see text]with (CD3)2CO and comparison of its CID results with those obtained above. In the case of m/z 99 derived from diacetone alcohol or mesityl oxide, the neutral lost via metastable decomposition in the second field-free region was found to be C4H8, presumably 2-methyl propene. While the ion of m/z 99 may also eliminate methane to yield a species of m/z 83, the dominant metastable elimination from this ion is carbon monoxide rather than ethene as shown by collision induced dissociative ionization (CIDI). The ions of m/z 83 observed in the two systems exhibit very similar CID spectra; however, a small variation in the yield of products of m/z 41 and 43 suggests two structures for the ion derived from diacetone alcohol, one of which is identical to that seen in mesityl oxide.