Unimolecular reactivity of protonated alpha,omega-alkanediamines in the gas phase

Unimolecular deamination of protonated alpha,omega -alkanediamines (1,2-eth anediamine (1), 1,3-propanediamine (2), 1,4-butanediamine (3), and 1,5-pent anediamine (4)) in the gas phase has been examined by tandem mass spectrome try experiments including metastable ions decompositions and collisional ac tivation techniques and molecular orbital calculations up to the G2(MP2,SVP ) level. For all the protonated molecules, only one unimolecular dissociati on channel, leading to the formation of a protonated cyclic amine via an in ternal nucleophilic substitution, was detected. The hydride ion transfer fr om the alpha carbon to the omega position is not competitive with the inter nal nucleophilic substitution. This has been found to be the result of a la rge critical energy for this latter reaction.