On the formation of the ·CH2CH2CH=NH2+ distonic radical cation upon ionization of cyclopropylamine and allylamine

Ab initio molecular orbital and density functional theory calculations have been applied to determine the relative stability of the cyclopropylamine 1 and allylamine (CH2=CHCH2NH2+· 2) radical cations and their isomers. It is confirmed that, upon ionization, 1 undergoes barrier-free ring-opening giving the distonic species ·CH2CH2CH=NH2+ 3. 2 also rearranges by a 1,2-H-shift to the more stable 3 (by 70 kJ/mol) which is, however, less stable than the 1-aminopropene ion (CH3–CH=CH–NH2+· 4) by 60 kJ/mol. The transition structure TS 2/3 lies 40 kJ/mol higher in energy than TS 3/4. Although QCISD and B3LYP calculations of isotropic hyperfine coupling constants agree reasonably with observed values, supporting the presence of the distonic 3 in ESR matrix experiments, the exclusive observation of 3, but not 4, is intriguing. This emphasizes the role of the matrix in stabilizing 3.