Chemical ionization mass spectra of simple 1,3-dioxolanes and their sulphur analogues recorded with methane,isobutane, ammonia and acetone as reagent gas

The mass spectral behaviour of nine 1,3-dioxolanes, seven 1,3-dithiolanes and seven 1,3-oxathiolanes was studied under chemical ionization conditions with ammonia, isobutane, methane, acetone, acetone-d₆ or pentan-3-one as reagent gas. The proton affinity of the first members in each series was not large enough for ammonia to protonate them; instead, the ionization took place through unstable M + NH₄]⁺ ions. Isobutane, which gave rise to abundant M + H]⁺ ions in all cases, was the best reagent gas for the determination of the molecular mass. Methane chemical ionization caused extensive fragmentations either through ring cleavage or through the elimination of the largest substituent from ring positions 2 as a neutral hydrocarbon. The ketones used as reagent gas reacted to form adduct ions. In the case of dioxolanes and oxathiolanes, the M + RCO]⁺ adduct ion decomposed through ring opening and then, as a consequence of intramolecular nucleophilic substitution, through the elimination of a neutral carbonyl compound. Resonance-stabilized dioxolanylium and oxathiolanylium ions were obtained for dioxolanes and oxathiolanes, respectively. This reaction was almost non-existent for the dithiolanes.