Sequential ortho effects: characterization of novel [M − 35]+ fragment ions in the mass spectra of 2-alkyl-4,6-dinitrophenols

AbstractHigh-resolution mass spectrometry (HRMS), hybrid tandem mass spectrometry (MS/MS) (EBqQ), and photoelectron-photoion coincidence (PEPICO) experiments were conducted to examine a possible ortho-ortho effect resulting in a novel [M − 35]+ fragment ion in 2-alkyl-4,6-dinitrophenols. For compounds having ethyl or larger alkyl substituents, [M − 35]+ was observed only when [M − 18]+ ions were present, with the ortho nitro group being involved in the reaction to [M − 35]+. For [M − 18]+ and [M − 35]+, HRMS results were consistent with losses of H2O and H2O + OH, respectively, whereas MS/MS results indicated a sequential reaction due to metastable dissociations. The appearance energy determined by PEPICO for [M − 35]+ was found to be greater than the appearance energy for [M − 18]+, thus supporting a sequential reaction