Fragmentation of singly charged silver/α,Ω-diaminoalkane complexes: competition between the loss of H2 and AgH molecules

Eur. J. Mass Spectrom. 10, 931–940 (2004)
DOI: 10.1255/ejms.688

Fragmentation of singly charged silver/α,Ω-diaminoalkane complexes: competition between the loss of H₂ and AgH molecules

Tujin Shi, Junfang Zhao, Tamer Shoeib, K.W. Michael Siu and Alan C. Hopkinson
Department of Chemistry and Centre for Research in Mass Spectrometry, York University, 4700 Keele Street, Toronto, Ontario, Canada M3J 1P3. E-mail: ach@yorku.ca

ABSTRACT:

Collision-induced dissociation of complexes Ag⁺/NH₂CH₂(CH₂)_nCH₂NH₂, where n has values 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 provided a mechanism by which H₂C=NH plus CH₃CH=NH₂⁺ are produced.

Keywords: H₂ versus AgH elimination, breakdown curves, B3LYP/DZVP calculations

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