Fragmentation Mechanisms of Protonated Benzylamines. Electrospray Ionisation-Tandem Mass Spectrometry Study and ab Initio Molecular Orbital Calculations

Abstract

Our research into neurotransmitters in a biological fluid presented an opportunity to investigate the fragmentations under low collision energy characterising benzyl-amines protonated under electrospray ionisation (ESI) conditions in a triple quadrupole mass spectrometer. In this work we present the breakdown graphs of protonated 3,4-dihydroxybenzylamine, DHBAH+, and 3-methoxy, 4-hydroxybenzylamine, HMBAH+, at various source temperatures and various pressures in the collision cell, the collision energy varying from 0 to 46 eV in the laboratory frame. Both parent ions eliminate first NH3 at very low collision energy. The fragmentations of [MH – NH3]+ occur at high collision energy and are quite different for DHBAH+ and HMBAH+: formation of [MH – NH3 – H2O – CO]+ for the former; formation of the radical cation [MH – NH3 – CH3]+• for the latter. These fragmentations are interpreted by means of ab initio calculations up to the B3LYP/6-311+G(2d,2p) level of theory. The successive losses of H2O and CO involve first the rearrangement in two steps of benzylic ions formed by loss of NH3 into tropylium ions. The transition states associated with this rearrangement are very high in energy (about 400 kJ mol−1 above MH+) explaining (i) the absence of an ion corresponding to [DHBAH – NH3 – H2O]+. The determining steps associated with the losses of H2O and with H2O + CO are located lower in energy than the transition states associated with the isomerisation of benzylic ions into tropylium ions; explaining (ii) the formation of the radical cation [MH – NH3 – CH3]+•. The homolytic cleavage of CH3–O requires less energy than does the rearrangement.