Fragmentation Chemistry of Tetrahydrocarbazole Analogs Relevant to Nitrogen Compounds in Heavy Crude Oils

Heavy crude oil’s heterocyclic nitrogen compounds influence its catalytic refining process. Identifying these compounds at the molecular level can aid in developing effective handling strategies and indicate the presence of other debilitating compounds. Rules derived from the study of model compounds are of great use for the identification of these compounds, as well as similar compounds. We focused on the tetrahydrocarbazole template, which is the first to investigate the tetrahydrocarbazole core in the context of crude oil analysis. We selected the N1 class compound 1,2,3,4-tetrahydrocarbazole and its analogs, featuring C-1 substituents including a carboxylic acid group (−COOH), an oxo group (═O), an alcohol group (−OH), and an amine group (−NH2). These correspond to the N1O2, N1O1, and N2 model compounds, respectively. Each model compound was ionized through positive electrospray ionization (+ESI) and dissociated through collision-induced dissociation tandem mass spectrometry (CID MS/MS). The various functionalized model compounds exhibited distinct fragmentation patterns, enabling their identification. Quantum-chemical calculations using density functional theory (DFT) were performed to obtain the lowest-energy optimized structures for each precursor ion. Notably, the positive charge consistently remained localized on the nitrogen atom of the pyrrole ring, regardless of the protonation site. The key product ions observed in +ESI CID MS/MS were identified, and their corresponding structures were also generated. We observed that the products associated with the dominant peaks were formed through pathways ranging from a single step to multiple steps. Additionally, in the presence of electron-withdrawing groups such as ═O and −COOH, multiple isomerization steps were required to form the major products. In contrast, with electron-donating groups such as −OH and −NH2, either no isomerization or only a single isomerization step yielded the major product. All the ring-opened products were formed via isomerization, for which proton mobilization plays a crucial role.