C5H7O2+ ions: The correlation between their thermochemistry in acidic solution and their chemistry in the gas phase

Each of a series of C5H6O2 isomeric carboxylic acid and unsaturated lactone s (1-7) was protonated in both concentrated sulfuric acid and trifluorometh anesulfonic acid. The thermally induced transformations of the protonated s pecies were then studied over a temperature range of -30 to +160 degrees C. In the case of alpha,beta-unsaturated lactones, protonation took place on the carbonyl oxygen and gave the corresponding stable beta-protonated speci es. Conversely, unconjugated lactones and acetylenic acid 7 were converted even at low temperature into the diprotonated ketoacid 8H(2)o(+2) by the ac id-catalyzed addition of water to the C-protonated precursor. Upon being he ated at 160 degrees C, this acid gave protonated 1,3-cyclopentanedione. In the absence of water, decarbonylation followed by polymerization was observ ed in lactones 4 and 5. The CIMS spectra of compounds 1-7 were recorded usi ng methane, ammonia, and moist air as reagent gases to determine the correl ation between the fragmentation routes in the gas phase and the transformat ions observed in solution. Ammonia and moist air enabled us to determine th e different proton affinities of these compounds. The data obtained in stro ng acids were used to assign reasonable structures to the gas-phase ions.