Analysis of diarylmethylamine compounds using electrospray mass spectrometry: formation mechanisms of radical ions and dehydro cations

Abstract

A series of diarylmethylamine compounds were analyzed using electrospray ionization quadrupole time-of-flight mass spectrometry (ESI-QTOF-MS). [M]⁺˙ and [M − H]⁺ were both observed, but showed different abundances. A possible mechanism for the formation of [M]⁺˙ and [M − H]⁺ was proposed to explicate the rule for the ratio change of I_[M]⁺˙/I_[M−H]⁺. The [M]⁺˙ has two structures, which can interconvert into each other in the gas phase. The substituted groups on the benzene rings play a crucial role in the transfer between the two structures. Electron withdrawing groups can prevent the formation of carbocations, thus nitro-containing diarylmethylamines remained mainly as structure I and were detected as [M]⁺˙. On the contrary, electron donating groups help to stabilize carbocations. This makes structure I transfer to structure II, and structure II prefers to further generate [M − H]⁺ by loss of an H radical. Nuclear magnetic resonance and D-labelled MS experiments indicate that the 1-C–H bond has strong activity.