Dissociation of a protonated secondary amine in the gas phase via an ion-neutral complex

Collisionally activated dissociation of conjugate acid ions of neopentyl isopropylamine (1) gives loss of a neutral C5H10 molecule, MH+–C5H10, as the predominant decomposition peak (≥70% of the total fragment ion abundance). Quantitative evaluation of the relative peak intensities from protonated 1 and its deuterated analogues permits an assessment of the contribution of hydrogen transfer from the CH3 of the neopentyl group (γ-position) relative to the CH2 (α-position), as well as the corresponding kinetic isotope effects. The ratio of γ-transfer to α-transfer from the neopentyl group is on the order of 5:1, implying that loss of 2-methyl-1-butene is preferred over loss of 2-methyl-2-butene, despite the fact that the latter C5H10 isomer is >6 kJ mol−1 more stable than the former. An alternative interpretation of the γ-/α-transfer ratio would suppose that all 11 hydrogens in the neopentyl group randomize prior to dissociation. Measured differences between α- and γ-isotope effects argue against hydrogen randomization: the kH/kD for proton transfer from CH2 versus CD2 has a value close to unity, while the deuterium isotope effect for transfer from CH3 versus CD3 exhibits kH/kD = 1.6. Experimental results support a mechanism in which bond fission forms a [tert-amyl cation isopropylamine] ion–neutral complex, which then decomposes via proton transfer from the charged to the neutral partner.