Fragmentation of Singly Charged Silver/ α,ω-Diaminoalkane Complexes: Competition between the Loss of H ₂ and AgH Molecules

Collision-induced dissociation of complexes, Ag + /NH 2 CH 2 (CH 2 ) n CH 2 NH 2 , where n has values of 0, 1, 2 and 3, show loss of one H 2 molecule at low energies. For the largest complex ( n = 3), a second H 2 molecule is also lost. For complexes with n = 0 and 1, loss of AgH (the only pathway observed in the fragmentation of Ag + /monoamine complexes) also occurs at low collision energies and this becomes the dominant fragmentation pathway at centre-of-mass energies above 2 eV. For complexes with n = 2 and 3, negligible amounts of product ions resulting from loss of only AgH were formed. However, for these complexes, the major products at higher collision energies result from loss of either AgH plus NH 3 or, more likely, H 2 plus AgNH 2 ; the product ions are postulated to be cyclic iminium ions. Density functional theory (DFT) calculations (B3LYP with a DZVP basis set) on Ag + /NH 2 CH 2 (CH 2 ) 2 CH 2 NH 2 showed the loss of H 2 followed by AgNH 2 to have a slightly lower barrier than loss of AgH plus NH 3 . For complexes with n = 2 and 3, there are minor losses of various combinations of AgH, H 2 , 2H 2 and NH 3 . Reaction profiles for the losses of H 2 and AgH from all four Ag + /α,ω-diamine complexes have been examined computationally using DFT calculations. The second step on the reaction profile, loss of either H 2 or AgH from the complex between Ag + and α-amino-γ-imino-propane, Ag + /NH 2 CH 2 CH 2 CHNH, has also been calculated. The highest energy transition state on the profiles to loss of both H 2 and AgH from the smallest complex ( n = 0) are identical; on all the other profiles, the barrier for H 2 loss is lower than that for AgH loss (by 2.5 kJ mol −1 when n = 1, by 13.4 kJ mol −1 when n = 2 and by 33.0 kJ mol −1 when n = 3). Fragmentation of the ligand backbone occurs most extensively for ions derived from the Ag + /NH 2 CH 2 CH 2 CH 2 NH 2 . After loss of H 2 , the product ion, Ag + /NH 2 CH 2 CH 2 CH = NH fragments via a transition state in which the CH 2 –CH 2 bond is breaking, while a 1,5-hydrogen shift from one nitrogen to the other is occurring. Cleavage of the carbon chain of NH 2 CH 2 CH 2 CH = NH 2 + occurs at relatively low energy; DFT calculations provide a mechanism by which H 2 C = NH plus CH 3 CH = NH 2 + are produced.

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