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Hydrogen‐shift isomerism: mass spectrometry of isomeric benzenesulfonate and 2‐, 3‐ and 4‐dehydrobenzenesulfonic acid anions in the gas phase
Author(s) -
BenAri Julius,
Etinger Alex,
Weisz Adrian,
Mandelbaum Asher
Publication year - 2005
Publication title -
journal of mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.475
H-Index - 121
eISSN - 1096-9888
pISSN - 1076-5174
DOI - 10.1002/jms.881
Subject(s) - chemistry , benzenesulfonic acid , deprotonation , isomerization , dissociation (chemistry) , ion , fragmentation (computing) , electrospray ionization , mass spectrometry , medicinal chemistry , mass spectrum , catalysis , organic chemistry , chromatography , computer science , operating system
The isomeric 3‐ and 4‐dehydrobenzenesulfonic acid anions b and c were prepared by collision induced dissociation (CID) of the [M − H] − 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 1,2‐proton transfer does not take place in this system under the conditions of the CID experiment. Density functional (DFT) calculations at B3LYP/6‐31+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 −1 . The calculated energies of the transition states involved in the 1,2‐hydrogen migration leading to the interconversion of the isomeric anions are very high (>120 kcal mol −1 relative to ion a, barrier energies >55 kcal mol −1 ), 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 2‐dehydrobenzenesulfonic 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 meta ‐ and para‐ sulfobenzoic acids is a kinetically controlled process. Copyright © 2005 John Wiley & Sons, Ltd.