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, [M35](+) 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. 69-75).