Several mass spectrometry (MS) techniques including accurate MS and MS/MS, as well as hydrogen/deuterium (H/D) exchange, were utilized to characterize a pseudo-oxidative reaction by-product (impurity I) in the pharmaceutical synthesis of S-(thiobenzoyl)thioglycolic acid. The negative ion MS/MS data provided complementary structural information to the positive ion MS/MS data. An understanding of the gas-phase Smiles rearrangement upon collision-induced dissociation (CID) in the negative ion MS/MS mode played an important role in structural elucidation of impurity I. The theoretical calculations by density functional theory (DFT) at the B3LYP/6-311G(d,p) level provided insights into the thermochemistry of the Smiles rearrangement reaction. This pseudo-oxidative impurity is proposed to be generated via the base-catalyzed hydrolysis in solution.
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