Fragmentation of Singly Charged Silver/ α,ω-Diaminoalkane Complexes: Competition between the Loss of H2 and AgH Molecules

Abstract

Collision-induced dissociation of complexes, Ag+/NH2CH2(CH2) nCH2NH2, where n has values of 0, 1, 2 and 3, show loss of one H2 molecule at low energies. For the largest complex ( n = 3), a second H2 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 NH3 or, more likely, H2 plus AgNH2; the product ions are postulated to be cyclic iminium ions. Density functional theory (DFT) calculations (B3LYP with a DZVP basis set) on Ag+/NH2CH2(CH2)2CH2NH2 showed the loss of H2 followed by AgNH2 to have a slightly lower barrier than loss of AgH plus NH3. For complexes with n = 2 and 3, there are minor losses of various combinations of AgH, H2, 2H2 and NH3. Reaction profiles for the losses of H2 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 H2 or AgH from the complex between Ag+ and α-amino-γ-imino-propane, Ag+/NH2CH2CH2CHNH, has also been calculated. The highest energy transition state on the profiles to loss of both H2 and AgH from the smallest complex ( n = 0) are identical; on all the other profiles, the barrier for H2 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+/NH2CH2CH2CH2NH2. After loss of H2, the product ion, Ag+/NH2CH2CH2CH = NH fragments via a transition state in which the CH2–CH2 bond is breaking, while a 1,5-hydrogen shift from one nitrogen to the other is occurring. Cleavage of the carbon chain of NH2CH2CH2CH = NH2+ occurs at relatively low energy; DFT calculations provide a mechanism by which H2C = NH plus CH3CH = NH2+ are produced.