Abstract
The isomeric 3โ and 4โdehydrobenzenesulfonic acid anions b and c were prepared by collision induced dissociation (CID) of the [M โ H]^โ^ ions of isomeric sulfobenzoic acids obtained by negative electrospray ionization (ESI). The CID spectra (MS^3^) of anions b and c are different from each other, and both are different from that of the isomeric benzenesulfonate anion a, obtained from benzenesulfonic acid. The stability of ions b and c shows that 1,2โproton transfer does not take place in this system under the conditions of the CID experiment. Density functional (DFT) calculations at B3LYP/6โ31+G(2d,p) level of theory show that benzenesulfonate anion a is the most stable isomer, and the energies of isomers b and c are higher by more than 65 kcal mol^โ1^. The calculated energies of the transition states involved in the 1,2โhydrogen migration leading to the interconversion of the isomeric anions are very high (>120 kcal mol^โ1^relative to ion a, barrier energies >55 kcal mol^โ1^), much higher than those of transition structures leading to fragmentation. This situation does not allow isomerization of ions b and c to a, under the conditions of the CID experiments. The isomeric 2โdehydrobenzenesulfonic acid anion isomerizes to the benzenesulfonate anion a by a facile proton transfer from the SO~3~H group to the adjacent position 2. The results of this work indicate that the gas phase deprotonation of metaโ and __paraโ__sulfobenzoic acids is a kinetically controlled process. Copyright ยฉ 2005 John Wiley & Sons, Ltd.
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