Fragmentation mechanisms of protonated benzylamines. Electrospray ionisation-tandem mass spectrometry study and ab initio molecular orbital calculations

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 H ₂O 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.