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 Csp3−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.