Collision-Induced Dissociation Study of Isomeric Tetrahydrocarbazole Carboxylic Acids Relevant to Corrosive Petroleum Crudes

Abstract

Positional isomers of carboxylic acid-substituted tetrahydrocarbazole (Schemes 1A and 1B) were studied as model compounds for the N1O2 class of neutral nitrogen species in heavy crude oils with elemental composition CcHhNnOo (where n is 1 and o is 2). These species can contribute to the corrosion of the petroleum pipelines and storage tanks. We combined positive electrospray ionization collision-induced dissociation tandem mass spectrometry (+ESI CID MS/MS) with theoretical methods to analyze the model compounds. Each protonated precursor ion at m/z 216 exhibited distinct dissociation behavior at a laboratory collision energy, Elab, of 15 eV. The isomers with the COOH group at C-1 or C-3 in the cyclohexane ring generated the m/z 170 ion as a major fragment via consecutive losses of CO and H2O with distinct relative abundances. When the COOH group is located at C-6 in the benzene ring, the m/z 172 ion was a significant fragment formed via CO2 loss from the precursor ion. The principal fragment was the m/z 198 ion formed via H2O loss from the precursor ion when the COOH group is located at C-8 in the benzene ring. Theoretical analyses of each fragmentation mechanism revealed that multistep proton transfer was crucial for the formation of most products. For some products, formation was favored by entropy, in addition to sufficient internal energy. These results contribute to a broader foundation for investigating a wider range of structurally related carbazole-based carboxylic acids that are relevant to corrosion. Ultimately, understanding their fragmentation chemistries contributes valuable insights into developing strategies to mitigate crude oil corrosion and catalyst fouling.