Two competing ionization processes in ESI-MS analysis of N -(1,3-diphenylallyl)benzenamines: formation of the unusual [M−H] + ion versus the regular [M+H] + ion

Abstract

A series of N-(1,3-diphenylallyl)benzenamine derivatives (M) were investigated by electrospray ionization mass spectrometry in the positive-ion mode. Both the anomalous [M−H]⁺ and the regular [M+H]⁺ were observed in the ESI mass spectra. The occurrence of [M−H]⁺ has been supported by accurate mass spectrometry, liquid chromatography mass spectrometry, and tandem mass spectrometry analysis. Calculation results indicated that formation of [M−H]⁺ is attributed to the ion-molecule reaction of M with the protonated ESI solvent molecule (e.g. CH₃OH₂⁺) via hydride abstraction from a tertiary C_sp3−H. The competing ionization processes leading to [M−H]⁺ or [M+H]⁺ were significantly affected by the concentration of formic acid in the electrospray ionization solvent and the proton affinity of the N atom.