CHEMICAL-IONIZATION MASS-SPECTRA OF SIMPLE 1,3-DIOXOLANES AND THEIR SULFUR ANALOGS RECORDED WITH METHANE, ISOBUTANE, AMMONIA AND ACETONE ASREAGENT GAS

The mass spectral behaviour of nine 1,3-dioxolanes, seven 1,3-dithiola nes and seven 1,3-oxathiolanes was studied under chemical ionization c onditions with ammonia, isobutane, methane, acetone, acetone-d6 or pen tan-3-one as reagent gas. The proton affinity of the first members in each series was not large enough for ammonia to protonate them; instea d, the ionization took place through unstable [M + NH4]+ ions. Isobuta ne, which gave rise to abundant [M + H]+ ions in all cases, was the be st reagent gas for the determination of the molecular mass. Methane ch emical ionization caused extensive fragmentations either through ring cleavage or through the elimination of the largest substituent from ri ng positions 2 as a neutral hydrocarbon. The ketones used as reagent g as reacted to form adduct ions. In the case of dioxolanes and oxathiol anes, the [M + RCO]+ adduct ion decomposed through ring opening and th en, as a consequence of intramolecular nucleophilic substitution, thro ugh the elimination of a neutral carbonyl compound. Resonance-stabiliz ed dioxolanylium and oxathiolanylium ions were obtained for dioxolanes and oxathiolanes, respectively. This reaction was almost non-existent for the dithiolanes.