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

High-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 H 2O and H 2O + 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.