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

Abstract

Collisionally activated dissociation of conjugate acid ions of neopentyl isopropylamine ( 1) gives loss of a neutral C 5H 10 molecule, MH +–C 5H 10, 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 CH 3 of the neopentyl group (γ-position) relative to the CH 2 (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 C 5H 10 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 k H/ k D for proton transfer from CH 2 versus CD 2 has a value close to unity, while the deuterium isotope effect for transfer from CH 3 versus CD 3 exhibits k H/ k D = 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.