C5H9O2+ IONS - THE CORRELATION BETWEEN THEIR THERMOCHEMISTRY IN ACIDIC SOLUTION AND THEIR CHEMISTRY IN THE GAS-PHASE

Each of a series of C5H8O2 isomeric carboxylic acids and lactones (1-9) was protonated in both concentrated sulfuric acid and trifluoromethanesulfonic acid. The thermally induced transformations of the protonated species were then studied over the temperature range -40 to +160-degrees-C. As a general rule, all the initially generated cations were eventually converted to protonated gamma-valerolactone (1H(O)+) and, finally, to protonated cyclopentenone (10H(O)+). The cations derived from the cyclopropanecarboxylic acids 7 and 8 both underwent ring opening to the unsaturated cation 6H(O)+, which then rearranged to a protonated alpha-lactone. In concentrated sulfuric acid the latter species loses carbon monoxide to afford protonated 2-butanone 11H(O)+. The CIMS spectra of compounds 1-9 were recorded, allowing a correlation between the fragmentation routes in the gas phase and the transformations observed in solution. In this way, the data obtained in strong acids are used to assign reasonable structures to the gas-phase ions.