Siberian Federal University

A comprehensive study of N‑substituted 4‑nitrosopyrazoles was performed using electrospray ionization mass spectrometry (ESI–MS) combined with density functional theory (DFT) calculations. Protonated molecule [M+H]⁺ were successfully detected, and their fragmentation pathways were systematically investigated. DFT calculations revealed preferential protonation at the pyrazole ring nitrogen (N‑1) due to its higher thermodynamic stability compared to oxygen protonation. Key fragmentation pathways involved cleavage of substituent bonds (NO, alkyl groups) while preserving the aromatic pyrazole core. Notably, compounds with bulky phenyl substituents exhibited reduced fragmentation efficiency. Comparative analysis demonstrated that ESI–MS predominantly yields substituent‑driven fragmentation, whereas electron ionization leads to complete ring destruction. These findings provide critical insights into the gas‑phase behaviour of nitrosopyrazoles and establish a foundation for predicting their mass spectrometric patterns